R34 GTT RB rough idle problem with weird RPM gauge fluctuations

[Joni Boi]

Hey guys. I've got a rough idle issue on my R34 GTT RB30/25DET Neo. I've worked on my RB for years and this is the first time where I don't have a fkin clue why it's happening, its baffiling me. 

Here's a video of it in action:

Long story short: My daily R34 decided to randomly start doing this weird RPM idle fluctuation thing and it wants to die on me sometimes and I've got a higher than normal pressure in my fuel line before the fuel rail which I'm thinking can be contributing to this issue.

 

 

The long story:

 

This is my daily and the car has always performed really well for me up n till now. 

It was tuned on haltech platinum pro (running on board map sensor) 4 months ago and did 440hp (on 98ron) and has been driving really well, no issues what so ever. Then one day (about a month ago) I came home from work, fuelled up, popped into the shops and then came back, got in the car to head home, started it and it started idling really rough. Like, I would start it and then it would instantly die on me. So I had to rev it to keep the engine alive and I had to baby it home. And it would also do this weird RPM fluctuations where the rpm needle on the cluster goes up fast then down fast like its glitching weirdly (can see on the video).

Note: nothing has been changed on the engine since the tune nor has the ecu been plugged into anything since the tune. So this problem has occurred randomly and is still the same today. Also to note: this happened when the car was at full operating temp (was probably a 30 degree day). And the same idle issue will happen on cold starts. I've attached my complete list of mods here. I haven't been driving the car since.

So I've plugged the haltech ecu into my laptop to check for any error codes, nothing. I let the engine idle and I watched the sensors on the haltech ecu manager, nothing odd, and no errors. Temps all look good. TPS voltage is good. I've got the haltech hooked up to a wide-band O2 sensor (pro sport gauge kit) and the afr starts off rich on cold start then eventually it goes to 14.2 like normal, so afr seems fine and normal to me. Since I've changed the spark plugs, it now won't die instanly on me but it still idles weirdly. 

The weird thing is, the RPM on my cluster fluctuates up and down but the RPM on the haltech shows normal-ish (no fluctuations).

Note: the video was taken after I have done all the checks below.

 

So here's the list of things that I've checked off and done on my list to try and eliminate possible causes:

• Compression test: perfectly fine like it was before the tune (phew!😅).

• Changed spark plugs to new ones: this helped a bit with the rough idle, old plugs were all black (they were about 3000kms old).

• Did the old quick coil plug test by un-pluging one by one while engine is idling to hear for changes in rpm: rpm/idle changed as they should on each one.

• Changed fuel filter: still the same.

• Checked TPS sensor on haltech and it reads correcly. 0.45v no throttle and 4v WOT.

• Cleaned IACV with a new gasket: nothing changed.

• Swapped the IACV with another known working one: nothing changed.

• Swapped CAS to a known working one. Nothing changed.

• Unplugged the water temp sensor: I know it’s not this because when I unplug it, it would show some weird temp on the haltech software and when i plug it in it shows normal.

• I've tried attaching the R35 PRP coil plug harness's ground cable to somewhere els on the head: nothing changed

• I've tried putting a jumper lead on the negative battery terminal and then attaching it to the head and block to see if its a grounding issue: nothing changed. 

• I've checked the timing marks on the cams and crank all line up.

• Changed the fuel pump horn relay (constant 12v mod) to a new one: nothing changed.

• I've taken my haltech ecu out and then tested it into my brother's r34 with his map to check if the ecu is still fine, and it is.

 

Now here's where it gets a bit interesting (and let me know if it's irrelevant);

I checked the fuel pressure for the first time ever by tee'ing in a pressure gauge between the fuel filter and damper that enters into the fuel rail (can see on the video). When I prime the fuel pump, it goes to 43ish psi, which it should be according to the R34 workshop manual. Then when I go to start and idle the engine, it stays on 50psi. The workshop manual says that it should be about 32psi. So I immediately thought its my fuel pressure regulator and I changed it to a known working one. And still... it's idling at 50psi fuel pressure. When I take off the vac line on the FPR, nothing really changes. So then I changed the fuel damper that sits on the fuel rail to a known working one: still the same 50psi. 

Then I hooked up a long hose onto the exit of the fuel rail straight to the fuel tank flap to see if there are any blockages: it dropped a little bit by 2psi... So no real restriction in the return line.

So now Im wondering, if there might be a blockage in the fuel rail or my 640cc xspurt injectors are maybe blocked? 🤷‍♂️

 

The question I have is; is this fuel pressure on idle (50psi) normal for an RB that has a Walbro 225 fuel pump with the constant 12v mod, with upgraded injectors + standalone ecu on a stock fuel rail and stock fpr? And would this high pressure be my main culprit for my shithouse of an idle? 

 

The other interesting thing I found/tested, was; i swapped out the cluster to my old stock nissan one to see if the RPM fluctuation thing also appears on that one, and it did the same thing weird RPM fluctuation on my old nissan cluster. So something weird electrically is going on that is contributing to this idle issue (maybe?).

The fuel pressure also doesn’t seem normal, so I wanted to check in with you guys before i go pulling the fuel rail off (which is such a bitch to do!). What do you guys think this might be? Can I test some other stuff? Does the haltech platinum pro ecu have a diagnosing feature of electrical components that I can try? It can't be the tune/map because it ran perfectly fine for 3 months after the tune and no one has touched the map since. 

 

The next thing I’m tempted to try is injector cleaner which I've never tried before. Can I pour it straight into the feeding fuel line instead of in the tank? Can injector cleaners damage stuff when not diluted in the tank?

mod list.txt

[joshuaho96]

32 minutes ago, Joni Boi said:

Hey guys. I've got a rough idle issue on my R34 GTT RB30/25DET Neo. I've worked on my RB for years and this is the first time where I don't have a fkin clue why it's happening, its baffiling me. 

Here's a video of it in action:

 

Long story short: My daily R34 decided to randomly start doing this weird RPM idle fluctuation thing and it wants to die on me sometimes and I've got a higher than normal pressure in my fuel line before the fuel rail which I'm thinking can be contributing to this issue.

 

 

The long story:

 

This is my daily and the car has always performed really well for me up n till now. 

It was tuned on haltech platinum pro (running on board map sensor) 4 months ago and did 440hp (on 98ron) and has been driving really well, no issues what so ever. Then one day (about a month ago) I came home from work, fuelled up, popped into the shops and then came back, got in the car to head home, started it and it started idling really rough. Like, I would start it and then it would instantly die on me. So I had to rev it to keep the engine alive and I had to baby it home. And it would also do this weird RPM fluctuations where the rpm needle on the cluster goes up fast then down fast like its glitching weirdly (can see on the video).

Note: nothing has been changed on the engine since the tune nor has the ecu been plugged into anything since the tune. So this problem has occurred randomly and is still the same today. Also to note: this happened when the car was at full operating temp (was probably a 30 degree day). And the same idle issue will happen on cold starts. I've attached my complete list of mods here. I haven't been driving the car since.

So I've plugged the haltech ecu into my laptop to check for any error codes, nothing. I let the engine idle and I watched the sensors on the haltech ecu manager, nothing odd, and no errors. Temps all look good. TPS voltage is good. I've got the haltech hooked up to a wide-band O2 sensor (pro sport gauge kit) and the afr starts off rich on cold start then eventually it goes to 14.2 like normal, so afr seems fine and normal to me. Since I've changed the spark plugs, it now won't die instanly on me but it still idles weirdly. 

The weird thing is, the RPM on my cluster fluctuates up and down but the RPM on the haltech shows normal-ish (no fluctuations).

Note: the video was taken after I have done all the checks below.

 

So here's the list of things that I've checked off and done on my list to try and eliminate possible causes:

• Compression test: perfectly fine like it was before the tune (phew!😅).

• Changed spark plugs to new ones: this helped a bit with the rough idle, old plugs were all black (they were about 3000kms old).

• Did the old quick coil plug test by un-pluging one by one while engine is idling to hear for changes in rpm: rpm/idle changed as they should on each one.

• Changed fuel filter: still the same.

• Checked TPS sensor on haltech and it reads correcly. 0.45v no throttle and 4v WOT.

• Cleaned IACV with a new gasket: nothing changed.

• Swapped the IACV with another known working one: nothing changed.

• Swapped CAS to a known working one. Nothing changed.

• Unplugged the water temp sensor: I know it’s not this because when I unplug it, it would show some weird temp on the haltech software and when i plug it in it shows normal.

• I've tried attaching the R35 PRP coil plug harness's ground cable to somewhere els on the head: nothing changed

• I've tried putting a jumper lead on the negative battery terminal and then attaching it to the head and block to see if its a grounding issue: nothing changed. 

• I've checked the timing marks on the cams and crank all line up.

• Changed the fuel pump horn relay (constant 12v mod) to a new one: nothing changed.

• I've taken my haltech ecu out and then tested it into my brother's r34 with his map to check if the ecu is still fine, and it is.

 

Now here's where it gets a bit interesting (and let me know if it's irrelevant);

I checked the fuel pressure for the first time ever by tee'ing in a pressure gauge between the fuel filter and damper that enters into the fuel rail (can see on the video). When I prime the fuel pump, it goes to 43ish psi, which it should be according to the R34 workshop manual. Then when I go to start and idle the engine, it stays on 50psi. The workshop manual says that it should be about 32psi. So I immediately thought its my fuel pressure regulator and I changed it to a known working one. And still... it's idling at 50psi fuel pressure. When I take off the vac line on the FPR, nothing really changes. So then I changed the fuel damper that sits on the fuel rail to a known working one: still the same 50psi. 

Then I hooked up a long hose onto the exit of the fuel rail straight to the fuel tank flap to see if there are any blockages: it dropped a little bit by 2psi... So no real restriction in the return line.

So now Im wondering, if there might be a blockage in the fuel rail or my 640cc xspurt injectors are maybe blocked? 🤷‍♂️

 

The question I have is; is this fuel pressure on idle (50psi) normal for an RB that has a Walbro 225 fuel pump with the constant 12v mod, with upgraded injectors + standalone ecu on a stock fuel rail and stock fpr? And would this high pressure be my main culprit for my shithouse of an idle? 

 

The other interesting thing I found/tested, was; i swapped out the cluster to my old stock nissan one to see if the RPM fluctuation thing also appears on that one, and it did the same thing weird RPM fluctuation on my old nissan cluster. So something weird electrically is going on that is contributing to this idle issue (maybe?).

The fuel pressure also doesn’t seem normal, so I wanted to check in with you guys before i go pulling the fuel rail off (which is such a bitch to do!). What do you guys think this might be? Can I test some other stuff? Does the haltech platinum pro ecu have a diagnosing feature of electrical components that I can try? It can't be the tune/map because it ran perfectly fine for 3 months after the tune and no one has touched the map since. 

 

The next thing I’m tempted to try is injector cleaner which I've never tried before. Can I pour it straight into the feeding fuel line instead of in the tank? Can injector cleaners damage stuff when not diluted in the tank?

 

mod list.txt 1008 B · 1 download

If your idle fuel pressure suddenly changed for any reason it would cause the problem you're seeing. Running rich is as bad as lean. The tachometer issue is likely a red herring. Pull the cluster out and resolder the joints to fix that one. You can also flow test your injectors.

[GTSBoy]

^^What he said.

The fuel pressure wants to be about 43 psi with just priming, or with the engine running but the reg sense line pulled off (and plugged to stop it being a vacuum leak). It should obviously be about 13 psi less than that with the engine running and the sense line connected, because that's going to be close to manifold vacuum. So, somewhere in the 30 psi territory.

45 is PSI is too much, and very likely wrong. It is remotely possible that that is the fuel pressure it was set to when it was tuned, if you have an adjustable reg, which I couldn't see. But I suspect that it is not correct, because it was showing very similar pressure at both prime and running. It looks like the sense line is disconnected or blocked (or the reg is stuff, or something else weird).

The tacho fluctuating like that is either a fault with the tacho, the wiring from the ECU, or the ECU's own output. The engine was clearly not changing speed like the tacho was.

[MBS206]

I need to clarify something.

The weird actual idle of the engine, AND the RPM gauge being really weird and wonky both started happening at exactly the same time?

 

If so, I'm going against both of you above, and saying they're likely related somehow.

 

Unfortunately, the video was filmed in psychopath mode (portrait), which makes it next to impossible to get much detail from it even at full screen size.

 

My bet is that if the gauge has started going weird, AND the engine went rough, both at the same time, they're related, and it'll be a weird electrical issues. Most likely something back feeding / putting noise in a circuit that shouldn't be there. Quite possibly, a connection going wrong, or a power feed being really wack/out of sorts.

[Joni Boi]

5 hours ago, joshuaho96 said:

If your idle fuel pressure suddenly changed for any reason it would cause the problem you're seeing. Running rich is as bad as lean. The tachometer issue is likely a red herring. Pull the cluster out and resolder the joints to fix that one. You can also flow test your injectors.

This is the first time I've checked my fuel pressure and I've got no idea what the pressure was before this. So I'm not sure if this is 100% the true cause of my idling problems atm, but this high pressure is my first clue to something that is "out of spec" according to the workshop manual. Also, you can see in the video, the haltech tries to get the afr target and it does maintain it at 14.2. So I would assume the afr is fine?

I have to dissagree with the cluster comment. I don't think it's a bad solder joint. There's nothing wrong with my Nismo cluster because I swapped out the cluster to my old nissan one and it does the same rpm glitchy thing. So to me, thats the signal coming from the ecu that is farked. I bet if i were to put both those clusters in a working r34 gtt, it would work as it should. 

 

5 hours ago, GTSBoy said:

The fuel pressure wants to be about 43 psi with just priming, or with the engine running but the reg sense line pulled off (and plugged to stop it being a vacuum leak). It should obviously be about 13 psi less than that with the engine running and the sense line connected, because that's going to be close to manifold vacuum. So, somewhere in the 30 psi territory.

I just took this video now to show what happens to the fuel pressure when I remove the vac lines on both the FPR and damper. It remains at 50psi:

 

Can definitly feel theres vacuum. No aftermarket reg, just stock one.

5 hours ago, GTSBoy said:

The fluctuating like that is either a fault with the tacho, the wiring from the ECU, or the ECU's own output. The engine was clearly not changing speed like the tacho was.

I reckon its the signal coming from the ecu. So correct me if im wrong, cas send signal to ecu to show rpm of engine (and thats what we see on the haltech software) then ecu sends out ignition and injector pulses ect ect, and then spits out a signal for the rpm gauge on the cluster? Makes me wonder if something in the injectors electrically (solenoids?) or something is causing some weird feedback to the cluster signal. 

1 hour ago, MBS206 said:

I need to clarify something.

The weird actual idle of the engine, AND the RPM gauge being really weird and wonky both started happening at exactly the same time?

Yes! They did. And I also think they are related/connected to this idle issue. 

1 hour ago, MBS206 said:

Unfortunately, the video was filmed in psychopath mode (portrait), which makes it next to impossible to get much detail from it even at full screen size.

Haha sorry about it. I just filmed it with my phone. Let me know if you want me to get something els specific on vid.

Yeah it's really odd. I would like to try and diagnose the injector wiring next. Don't know if that requires an oscilloscope? 

[GTSBoy]

The dampener won't make any difference. And blocking the detached hose with your finger is only about eliminating the vacuum leak which will cause the idle speed to increase (which you can hear happening when you block/unblock it).

But the reg? It's not doing anything at all. Nada.

[Joni Boi]

5 hours ago, GTSBoy said:

The dampener won't make any difference. And blocking the detached hose with your finger is only about eliminating the vacuum leak which will cause the idle speed to increase (which you can hear happening when you block/unblock it).

Yeah, I was just trying to show on camera that the manifold is pulling enough vacuum through that hose to pull the diaphram in the stock fpr but couldn't really hear it on film. 

5 hours ago, GTSBoy said:

But the reg? It's not doing anything at all. Nada.

Yeah, it's strange. Fuel should be by-passing through the reg to exit back into the tank thus reducing the fuel pressure:

Sorry for my awful drawing here (I like to make diagrams to understand it better 🤪)

The (temporary) fuel gauge is plumbed between the fuel filter and dampner. I've already eliminated the possible cause of restriction that the return line might have had and it's not that because I placed a long hose from the FPR (exit from the fuel rail), by-passing the 2nd dampner and return line, then straight into the filler fuel cap (as seen on my first vid). And it was still at 50 psi. Which means that I have a fuel restriction between the 1st dampner and the FPR. 

Possible causes:

• Blockage/restiction in the 1st dampner: I know it's not this because I've swapped it out to a known working one from a spare complete neo engine sitting in the shed.
• Blockage/restiction in the Fuel Pressure Regulator (FPR): Also, can't be this because I swapped it out to a known working one from the other engine. 
• Blockage/restriction in the fuel rail and fuel rail's lines: this could probably be the case. Maybe red rubber grease got in there when I changed the injectors (with new seals) a year ago? Who knows. The only way to know is if I take the bastard off and inspect it (or swap it to the other spare one), which I think I will just have to do. 
• Injectors causing a restriction in fuel causing high pressure? Could this be? I wouldn't think so? Because even if injectors were completely blocked, fuel would just by-pass them through to the FPR and exit? Let me know if I'm wrong though. Because if that's the case, it could explain some weird electrical shit where the injectors aren't opening enough or something. 
• Air bubbles in fuel rail? Can this be a thing? Could air get trapped inside the fuel rail and cause high pressure? Shouldn't though right?
• Faulty pressure gauge? I just grabbed this old pressure gauge that was sitting in our toolbox at work. I'm going to buy another gauge and see if I get the same result just incase. But it reads the priming correctly at 40ish psi. idk...

The spare rb25det neo engine that we have in the shed came out of my brother's R34 that was working perfectly fine before we pulled it out to slap a rb30/25det neo in there. It also has the same xsprut 640cc injectors on the fuel rail still. So I might have to swap them out and see what it does. 😮‍💨

[Duncan]

Can you bypass the damper as well, it can also fail

[Joni Boi]

15 minutes ago, Duncan said:

Can you bypass the damper as well, it can also fail

I would need one of these adpaters to by-pass the factory dampner:

Which I don't have here atm. So I'm just going to have to bite the bullet and take out the fuel rail and inspec it. And I'll put the other spare one in, do a fuel pressure test with top half of the stock manifold off.

(^from google images)

If the pressure is still high, I can plumb the gauge maybe in there (circled in red) to check for the pressure after the dampner. Can't do it with the top half of the manifold in the way.

[GTSBoy]

10:1 it's just the reg is seized.

[joshuaho96]

17 hours ago, Joni Boi said:

Yeah, I was just trying to show on camera that the manifold is pulling enough vacuum through that hose to pull the diaphram in the stock fpr but couldn't really hear it on film. 

Yeah, it's strange. Fuel should be by-passing through the reg to exit back into the tank thus reducing the fuel pressure:

Sorry for my awful drawing here (I like to make diagrams to understand it better 🤪)

The (temporary) fuel gauge is plumbed between the fuel filter and dampner. I've already eliminated the possible cause of restriction that the return line might have had and it's not that because I placed a long hose from the FPR (exit from the fuel rail), by-passing the 2nd dampner and return line, then straight into the filler fuel cap (as seen on my first vid). And it was still at 50 psi. Which means that I have a fuel restriction between the 1st dampner and the FPR. 

Possible causes:

• Blockage/restiction in the 1st dampner: I know it's not this because I've swapped it out to a known working one from a spare complete neo engine sitting in the shed.
• Blockage/restiction in the Fuel Pressure Regulator (FPR): Also, can't be this because I swapped it out to a known working one from the other engine. 
• Blockage/restriction in the fuel rail and fuel rail's lines: this could probably be the case. Maybe red rubber grease got in there when I changed the injectors (with new seals) a year ago? Who knows. The only way to know is if I take the bastard off and inspect it (or swap it to the other spare one), which I think I will just have to do. 
• Injectors causing a restriction in fuel causing high pressure? Could this be? I wouldn't think so? Because even if injectors were completely blocked, fuel would just by-pass them through to the FPR and exit? Let me know if I'm wrong though. Because if that's the case, it could explain some weird electrical shit where the injectors aren't opening enough or something. 
• Air bubbles in fuel rail? Can this be a thing? Could air get trapped inside the fuel rail and cause high pressure? Shouldn't though right?
• Faulty pressure gauge? I just grabbed this old pressure gauge that was sitting in our toolbox at work. I'm going to buy another gauge and see if I get the same result just incase. But it reads the priming correctly at 40ish psi. idk...

The spare rb25det neo engine that we have in the shed came out of my brother's R34 that was working perfectly fine before we pulled it out to slap a rb30/25det neo in there. It also has the same xsprut 640cc injectors on the fuel rail still. So I might have to swap them out and see what it does. 😮‍💨

I forgot we're talking about an R34. So probably not the R33 dreaded tachometer failure. It might be related then. You might need an oscilloscope to diagnose this stuff unfortunately. Scope the tachometer signal and see what it looks like vs a clean normal tach signal, check grounds, etc.

Constant +12V to the fuel pump is always going to cause high fuel pressure at idle. The OEM setup uses the FPCM to slow down the pump to hit 3 bar at idle. Your tuner likely worked around this by adjusting fueling around idle and low load but it's very fragile as you are discovering. Any weirdness will cause running issues.

Air can get into the fuel rail, either because of a leak or because of fuel boiling from heat. Hopefully neither though. Injectors pulsing + pressure building at the pump + fuel return will purge all of this. My experience is this is primarily an issue if your fuel system doesn't hold pressure + you're getting the fuel extremely hot from a warm restart and an engine bay that traps too much heat. A stock fuel system has all of this already engineered out.

 

[tylink720]

I had to get an aftermarket pressure regulator when I switched to a walbro 255 in my R34. I couldn't get the idle pressure to come down without it, and it was causing a rougher idle as well, although not to the point of stalling.

[luke gtr]

TL;DR

Not sure if checked, but confirm no blockage in the vac/boost reference line to the ecu. 

Have had rich and fluctuating idle from a blocked line and ecu could read that it was under vacuum(no MAF).

[MBS206]

You know something we've all over looked?

The fact he said "fuelled up, went to the shops, then came back out and it started doing this"

 

Go get some fresh gas from a totally different servo, jerry rig a fuel pump, and run the new tank supply to the fuel input at the motor.

 

There's every possibility you've got really shit fuel, thats possibly full of water etc.

[GTSBoy]

Yeah, I'd agree, but there does also seem to be a physical symptom with the fuel pressure. So I lean towards coincidence and shenanigans wrt the fuel up.

Doesn't mean the proposal is a bad thing. It's an obvious thing to try and might sidestep half (or maybe even all) of the issue.

BTW, for all those posting about having high idle fuel pressure after installing something as small as a Walbro 255.....what? I mean, I have the same pump, running off the standard R32 FPCM, and my fuel pressure is fine.

[MBS206]

1 hour ago, GTSBoy said:

Yeah, I'd agree, but there does also seem to be a physical symptom with the fuel pressure. So I lean towards coincidence and shenanigans wrt the fuel up.

Doesn't mean the proposal is a bad thing. It's an obvious thing to try and might sidestep half (or maybe even all) of the issue.

BTW, for all those posting about having high idle fuel pressure after installing something as small as a Walbro 255.....what? I mean, I have the same pump, running off the standard R32 FPCM, and my fuel pressure is fine.

Does the R32 FPCM slow the pump down at idle?

If it does, what happens to your fuel pressure at idle if you give it constant 12V.

My thoughts on fuel being the issue also won't explain the dash at the same time.

[Duncan]

The FPCM does exactly that....but I've got the walbro 525 in the stagea with standard lines and reg, no FPCM, no base fuel pressure issues. 

same with the race car, it has standard reg and lines with a bosch 044, no FPCM and it has no base pressure issues either

[GTSBoy]

Yeah, I was going to say that the std reg should be able to handle much more than stock pump capacity at full blast at idle without losing control of pressure, but there's a limit to how many words I can type during a meeting.

[tylink720]

Possibly my oem fuel reg was already shot when I put the 255 pump in, but yeah it wouldn't come down lower than 50psi I think. Drilled out, then bypassed the venturi, didn't help either.

edit: its also hardwired, no variable voltage thing anymore

Edited October 17 by tylink720

[MBS206]

14 hours ago, Duncan said:

The FPCM does exactly that....but I've got the walbro 525 in the stagea with standard lines and reg, no FPCM, no base fuel pressure issues. 

same with the race car, it has standard reg and lines with a bosch 044, no FPCM and it has no base pressure issues either

I'm glad you could confirm those Duncan!

I couldn't remember if when I had my 044 in the 33 if I still had the standard fuel pressure reg, or my SARD reg in it.