RB25DET coughs/splutters/misfires/stalls when idling, fine otherwise, any ideas?

[Bozz]

Its as if it has a lumpy cam, when brought above about 2000rpm it runs perfectly, has good boost and power, everything sounds spot on.

When it falls back to idle it runs like poo.

What its not -

ECU (swap tested)

Oxygen sensor (swap tested)

AFM (swap tested)

The ECU reports code 55 so it thinks everything is fine.

Its not (to the best of my knowledge) a vacuum leak. Crimped every hose and tested. The crankcase is pressurised, opening the oil filler alters the idle a fair bit.

Tried crimping the fuel return hose to see if the injectors were sticky. No difference.

Got a thread on PF about the exact same thing here:

http://board.performanceforums.com/forums/...9579#post679579

I'm running out of ideas.....

[95SKY]

could be the airflow meter or a crook plug maybe. best bet take to a mechanic they will soo track it down. were bouts you from i could recommend someone good.

[Bozz]

Originally posted by 95SKY

could be the airflow meter or a crook plug maybe. best bet take to a mechanic they will soo track it down. were bouts you from i could recommend someone good.

I've already said I've replaced the airflow meter, read my initial post.

The whole idea of asking on here is to help me get ideas so I dont have to take it to a mechanic.

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Update - I've compared all pins from this engine (series 1) with all pins from a series 2 RB25DET while idling. All voltages and waveforms are identical. Its definitely going to be an ignitor or injector causing the problem.

[Zensoku]

AAC valve?

(auxiliary air control valve)

This flows air to the engine to control the idle when the throttle body butterfly is closed.

It's a longshot, but if it isn't the AFM, ECU, O2 sensor, then it'd have to be either that or a bad coil.

[red17]

Timing? I know my mechanic had trouble setting my timing to stock setting, kept dying and idling rough. Mines still a tad rough on idle, but im gonna put up with it.

[Gradenko]

Bozz, have you tried cleaning the aac valve? That and cleaning the tps solved my lumpy cam idle when I first got the car. Also, any of the plugs especially blacker than the others? I had a stuck injector that fouled plugs instantly and had similar symptoms, although more severe (running on 5 cylinders severe).

[knore]

find all your earths .. engine battery etc

take them off, get a wire brush .. clean up both the area it joins onto chassie and plug.. should fix it

[Bozz]

Its not earths, I just did the wiring and have triple checked everything.

Its not the AAC valve, unplugging it and replugging it in makes a huge difference in idle. Exact same effect on engine with no problems.

Timing will not make an engine run like this.

Its also not injectors, swap tested.

Its a strange one indeed, keep the ideas coming

Now I think its something like a cam out of time

[Zensoku]

...here are a few more ideas...

Is your Speed sensor wire been snipped to remove the speed cut? Because i know that this has some weird results on idle control if the ECU has no idea of what speed the car is doin.

Another thought is if you have an aftermarket BOV, perhaps the spring pressure is way too soft and is open on idle which has the result of the AFM measuring "x" amount of air passing through the inlet only to find it leaking out into the atmosphere.

What is your fuel pressure like? have you tested that? Perhaps you have a faulty fuel regulator. or even the vacuum line from the reg to the manifold is leaking.

It can't be a cam out of timing, otherwise it would drive very badly across the whole rev range. It sounds like either a crook coilpack or a stuffed O2 sensor, as the ECU goes into closed-loop when it idles and looks at the O2 sensor signal when there is 0% throttle reading from the TPS. Perhaps the TPS doesn't even go to 0% when you take you foot off the throttle. Maybe the throttle cable/linkages need adjusting?

The reason why it would drive OK is because when the TPS is sending it's signals to the ECU (open loop) and referencing this to it's internal program maps and not even looking at what the O2 sensor has to correct it.

I'm stumped, if it isn't anything above...

Let us know how you go.

[Bozz]

Originally posted by Zensoku

...here are a few more ideas...

Is your Speed sensor wire been snipped to remove the speed cut? Because i know that this has some weird results on idle control if the ECU has no idea of what speed the car is doin.

Another thought is if you have an aftermarket BOV, perhaps the spring pressure is way too soft and is open on idle which has the result of the AFM measuring "x" amount of air passing through the inlet only to find it leaking out into the atmosphere.

What is your fuel pressure like? have you tested that? Perhaps you have a faulty fuel regulator. or even the vacuum line from the reg to the manifold is leaking.

It can't be a cam out of timing, otherwise it would drive very badly across the whole rev range. It sounds like either a crook coilpack or a stuffed O2 sensor, as the ECU goes into closed-loop when it idles and looks at the O2 sensor signal when there is 0% throttle reading from the TPS. Perhaps the TPS doesn't even go to 0% when you take you foot off the throttle. Maybe the throttle cable/linkages need adjusting?

The reason why it would drive OK is because when the TPS is sending it's signals to the ECU (open loop) and referencing this to it's internal program maps and not even looking at what the O2 sensor has to correct it.

I'm stumped, if it isn't anything above...

Let us know how you go.

Pin 53 isn't connected since its a conversion. I've never had issues with not connecting this pin before though, I might try earthing it via a 1k resistor or something to ensure its not getting spurious signal interference.

No aftermarket BOV, no vacuum leaks anywhere that I can see, hear or test.

Fuel pressure I haven't checked. Its quite possible its the fuel pressure regulator. Vacuum or atmo pressure on the rising rate reg makes no difference in idle. Also tried crimping the fuel return line to bump up the pressure and that makes no difference in idle. I'd dare say it could be too high a fuel pressure making it rich at idle.

Its not the o2 sensor - swap tested.

I just assumed it wouldn't be a coil pack since it should break down at high RPM/high speed. Its a few cylinders randomly misfiring, not just one.

[Zensoku]

hmmmm. :nowigetit

if the ECU is stock with the original loom and sensors/actuators, except for the mentioned rising rate regulator, then i think you'd be right on the money with the engine running extremely rich and upsetting the idle. The ECU would be forever trying to correct itself to a stable air/fuel ratio from what the o2 sensor would be measuring. This would especially be the case if you have a big Bosch pump such as the 040.

Stock ECU + rising rate reg with high base pressure + aftermarket pump = more fuel flowing through each injector on every pulse width.

My next suggestion would be getting a wide-band lambda (o2) sensor hooked up to look at the air/fuel ratios at idle and load. It'd be worth getting the base fuel pressure set to standard Nissan specs if you're still using the stock Nissan ECU. (Can't "chip" it to fix the injector pulse widths either!)

I'm no tuning expert, but i've heard of such stories from a guy i know that works for Chiptorque. He says he often sees guys bringing in cars to tune, which have read magazine articles and took a DIY approach with fuel pumps and adj regs and even injector upgrades and wondering why it drives like sh!t....:uh-huh:

[Bozz]

I think I worded my post badly above

It's the factory fuel pressure regulator, I just call it a rising rate reg because its got a vacuum/boost connection to the plenum hence the fuel pressure will vary depending on plenum pressure.

Now need fuel pressure tester.

[Zensoku]

The factory fuel pressure regulator works just like any other manufacturer's... I also have a Commodore V6, dead stock, as my daily driver. (please don't laugh! at least i get to appreciate the modern Japanese cars alot more)

This also has a vacuum line connected to the inlet plenum. All EFI fuel regs vary the pressure, dependant on vacuum. Or in turbo/supercharged cars, boost.

There are two aftermarket types:

Ones that are purely adjustable up and down only. This only moves the whole pressure versus vacuum/boost curve up and down.

The other type is the rising rate adjustable regs which you can actually skew the whole pressure curve in ratios. you can have a ratio of 8:1 for example, which would multiply "x" amount of fuel pressure for every pound of boost.

Basically more boost being produced, more fuel pressure. these are hard to setup perfectly, apparently.

[nismo_simo]

It sounds like the same problem I had with my same engine! I replaced 4 air flow meters of different cars and only one fixed the prob. But Tony from Revolution racegear in Southport fixed the problem by adjusting the idle control somehow as I put on a type3 turbosmart BOV on. I didn't see him do it, but you could call him or maybe go see him to get it fixed, as it fixed mine.

[nismo_simo]

It sounds like the same problem I had with my same engine! I replaced 4 air flow meters of different cars and only one fixed the prob. But Tony from Revolution racegear in Southport fixed the problem by adjusting the idle control somehow as I put on a type3 turbosmart BOV on. I didn't see him do it, but you could call him or maybe go see him to get it fixed, as it fixed mine.

[Bozz]

Cheers for that explanation Zensoku.

BTW - I've got nothing against Commodores (or most other cars), only P platers and homies in them (or any car) that think they're a dorifto king in a packed carpark.

[Bozz]

nismo simo - the problem is, on another RB25DET next to this car, I can unplug the o2 sensor, AAC valve, AFM, VVT solenoid and the TPS and it doesn't idle like this, it only idles high at about 1500rpm with no misfiring. The idle hasn't been adjusted before so by attempting to do so would be masking up the original problem

[Zensoku]

phew, thank gawd! at least there is another mature person on here that doesn't start bagging Commodore drivers, as soon as it is mentioned. Like you, i have nothing against people who drive any type of car. Working in automotive retail, i see all types of cars which cater for different driver's needs. in my case, it is purely transport (i haven't been tempted to perform one single mod! that's what the other car is for)

i'd be covering all bases by checking everything on a dyno. i know it is expensive at around $80+ an hour. but things like a/f ratios, fuel pressure, vacuum/boost pressure, RPM, timing, lambda analysing, injector duty cycle, inlet/water/oil/exhaust temps can all be monitored in a controlled environment. That way you can easily eliminate things you thought may have been a problem.

failing that, get a hold of a 100psi fuel gauge and check fuel pressure at idle with the vacuum hose on and off (should be a change) Don't forget to idle the engine and remove the fuel pump fuse! This will let the car stall to deplete the fuel pressure in the lines before you fit the fuel pressure gauge! or you will get a nasty surprise!

Since i only have an R32 service manual, here are the Nissan fuel pressure specs for an RB25DE:

When ignition switch is ON: Approx. 3.0 kg/cm2 (42.67003 psi)

During idling: Approx. 2.55 kg/cm2 (36.26953 psi)

Idling with vacuum hose removed: Approx. 3.0 kg/cm2 (42.67003 psi)

not sure if it is the same for RB25DET. i'd imagine it would be, but can anyone else confirm?

[nismo_simo]

Bozz...... How many klms do you get to a full tank? if it is 300 or less then you know definately that the o2 sensor is cactus. As for the other stuff you have tried, good luck with finding the official cause of the problem. Here is Revolution Racegears phone number (07) 5571 0800, speak to Tony and say Simon from Coffs told you to call, it's up to you, but it fixed my skyline and now it hauls ass! I'm just about to change the EMS to a microtech LTX12 and upgrade turbo to 600hp capable with big injectors and malpassi, plus 100hp NOS up it's guts will send my car well over 400hp at the wheels..good luck!:burnout:

[Bozz]

Thanks for the advice nismo_simo, i'll give a bit more info -

The o2 sensor I swapped it with is 6,000km old from my car (R31 RB25DET) that's and running absolutely perfectly with 600km per tank (50 litres) on the highway and about 500km/tank around town. It made no difference whatsoever. The o2 sensor sends a 0-1 volt variation to the ECU which regularly trims the mixture settings around the feedback it receives. At idle, the sensor can only swap around two or three times per second because its not hot enough from all the exhaust gas. At full noise it should alternate hundreds of times per second. This one responds perfectly and exactly how it should.

This is a conversion and I haven't driven it yet. I'm unable to find out how the engine ran in the previous car before it got removed.

To cut a long story short, it aint the o2 sensor

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Back to bagging other cars, I also forgot to mention that its not just homies and dorifto wannabe's that I hate, I also passionately dislike most people who *tastelessly* rice their econoboxes then somehow attempt to prove how superior they are.