Rb26 Power Loss, Bov Recirc Issue?

[flavzz]

Hey all,

I'm chasing some power issues on my 26.

Sorry for the long winded essay so if you dont want to read the background skip to the question at the bottom I have just included the info below so if I have missed something people may be able to spot the problem.

The symptoms are;

Low responsiveness/sluggishness, overall power output lower than normal. Running rich, backfires between gears, some flutter/surging when travelling uphills especially. The car still makes decent power from 3-3500rpm but lacks some punch. I should also note that I have been battling these issues since buying the car so I dont have a baseline to compare it to which makes it hard. Car made 140kw on the dyno which is probably at least 40-50kw lower than where it should be comparing it to other stock 26's with full 3 inch exhaust. Compression is 150 across all cylinders and it doesn't blow smoke or misbehave otherwise. I have done the air flow meter resolder since the dyno run and feels like it picked up a little power since then so maybe its at 150kw now. Certainly runs smoother too. Also I have been diagnosing the problem with another SAU member who knows much more about RB's than me.

Car specs

Stock RB26

3 inch front pipes and 3inch Apexi cat back system

Originally thought maybe one of the turbo's had given up so we;

Measured boost on the turbo side using a boost gauge and some vacuum line plugged under the Twin Turbo pipe. This showed good strong boost but we noticed at the same time the reading on the stock boost gauge(which obviously measures pressure in the plenum) trailed the turbo side significantly(as much as 50%), suggesting to us that on the inlet plenum side it may be losing boost somewhere. So it seems the turbo's are making good boost but im losing it somewhere between the turbos and the plenum.

Therefore if the intercooler looks ok(which it does) then we thought maybe it was a blow off valve issue, so the front bar was removed and all intercooler and hose connections inspected, no obvious leaks were apparent. Blow off valves taken off and inspected and they looked in great condition, opening and shutting perfectly. The last thing we could think of in the system is if the pressure lines to keep the blow off valves shut under boost arent working properly then boost is being wasted back into the turbo side and not into the plenum. This could account for the rich running, backfiring, flutter/surging etc and therefore the power loss I think. We tried to trace the lines from the BOV's to the plenum with not much success besides seeing anything under a GTR plenum is challenging at the best of times.

The next thing we did to test this theory is block off the blow off valves with some hard rubber and test drive to remove the recirculation problem from the equation. On the test drive the car seemed much more responsive and stronger throughout the rev range. Also boost came on much earlier as indicated on the stock gauge. So we returned the car to original condition and it seemed to return to the usual behaviour. Therefore is this my problem and where to go from here if these return pressure lines arent working correctly?

Question:

Where is the correct location for the lines from the top of the BOV's to connect to the plenum (I want to check they are in the correct place)? Is there another suitable location to plumb them into the plenum to test the BOV's shutting properly under boost? I have the diagram here (The salmon colour in the top left corner) but I cant tell where they connect, it seems to have them spliced to the charcoal cannister. Is this correct, and could there be a leak be in this area?

I understand there is a lot to take in here but I think we have narrowed the problem down to the boost recirculating back before entering the plenum. I may be completely wrong but this is where I'm at. Cheers in advance if you have any helpful advice.

Edited August 25, 2013 by flavzz

[Ben C34]

Any where in the plenum would work fine for the bovs.

[XRATED]

I would suggest you remove the origional vac line from the bov's and replace the line with new vac hose. The vac/boost port for the bov's is under the pleunum next to the IAC motor.

Failing that, just block the bov's off with blanking plates sanwiched between the bov and the mounting flange. You don't have to ru them... I don't on my 2.8.

Honsetly, I don't think the BOV's are your issue... unless they're damaged or not connected correctly. Plus a constant recirc will not make the car run rich that air has allready been measured.

I think you have a leak somwhere... possibly in your bov return pipe work.

[scotty nm35]

Have you tried replacing the afm's temporarily? It sounds like one may have needed the soldering treatment, perhaps it is still shagged?

I assume your intake is stock?

[flavzz]

Yes intake is stock. Haven't tried any other AFMs yet. Permanent blanking is an option but I would rather keep it all running as designed.

Also I have replaced the vac lines from BOV to the chassis nipples with new vac line. But I could still replace the line to the plenum.

Edited August 25, 2013 by flavzz

[GTSBoy]

You need to work out if you have a leak or if you have a blockage in the cooler pipes. If there is a significant restriction in the cooler pipes you suffer a large pressure drop as the air flows past it. Depending on where the boost source is tapped for boost control, the turbos might be working real hard to overcome it too.

[flavzz]

Thanks for the replies so far. Is there any way to check for a blockage besides taking the whole system apart? I will try get a leak test done to identify all possible losses but nothing is audible upon driving it.

The only other thing of relevance is when I took it to the dyno the timing was retarded to 10*. For what reason I'm not sure, so they advanced it using the CAS back to 20* stock value. But now the CAS setting is about as far to the right as it can go which my mate said is unusual to see on a RB26. It should be pretty neutral in its stock setting yeah? Is it worth checking if the base timing of the cam gears is set up correctly?

[niZmO_Man]

Yeah the CAS sits at about middle. Timing belt might have jumped a tooth, or someone bodged it up when putting it together.

[GTSBoy]

Or the timing marks on the pulley are out of place due to slippage/impending death.

[flavzz]

I will get it checked when the timing belt gets changed soon.

Any confirmation on if the vac line from the BOV's is split into any other line before the plenum such as the charcoal cannister or is that separate?

[GTSBoy]

Charcoal canister signal and purge lines should be entirely separate from the BOV signal line.

[flavzz]

Cheers

[flavzz]

Yeah the CAS sits at about middle. Timing belt might have jumped a tooth, or someone bodged it up when putting it together.

This is something my mate and I have discussed as we have been troubleshooting the problem. We have also discussed the fact it could be a combination of leaks and or timing issues too but I think there must be one major contributor to the problem.

He had this to say to me so I thought it was worth adding to this thread.

"There is nothing glaringly obvious or busted, and aside from making sure that the pressure line to the BOV is hooked up, and there are no boost leaks, there isn't a whole lot left to make sure it's functioning really. Which is a good thing!

1) I don't think it's a AFM problem - it's too consistent! They are usually glitchy problems, as you've experienced!

2) I don't believe there are any major leaks. Maybe a bit getting out here or there, but other than a leaking bov pressure line, nothing else would have a dramatic effect in performance.

3) What's left - timing. If the timing is out, by a few teeth, then the engine is going to be way down on efficiency, and make less power. The position of the CAS kinda points towards that as a pretty good cause. Think about it, the engine- it's a pump - if the exhaust valve is closing late, then a shitload of air that was supposed to be compressed gets out before it can be compressed. The computer pumps X amount of fuel in, but some of that air has escaped combustion - runs a bit rich/pops on lifting off the throttle etc. If one cam has jumped a tooth, then its out in relation to the crank and the other cam. All three have got to be synced up perfect. That's my bet!"

Any thoughts on this theory? It sounds pretty close to me.

Edited August 26, 2013 by flavzz

[GTSBoy]

It's a valid theory, except for the error in saying that a late closing exhaust valve affect trapping efficiency. That would be the inlet valve that has that effect. A late closing exhaust valve is also a late opening exhaust valve, which is a bad enough thing in its own right. The exhaust valve should open before BDC in order to allow the gas to start getting the hell out of Dodge before the piston starts to rise and help push it out. If you delay that then there is excessive pressure in the cylinder when the pistion starts rising (which costs mechanically) and then you don't get as good a clear out of exhaust (leaving more in the cylinder to contaminate the fresh charge, which is bad in so many different ways, especially on a turbo).

Best course of action would be to;

• make sure that the timing marks are accurate (turn the engine to TDC with #1 plug out and feel for TDC with a screwdriver (or preferably a dial indicator)),
• then assuming the above checks out, dismantle the timing cover and check the valve timing on both cams. Hopefully the timing marks in there are clear enough to prove what's going on.

If the valve timing is shown to be wrong, correct as required. If not, then you get to start looking for rags in intercooler pipes and so on.

[XRATED]

oooooorrr take to someone who knows what they're doing.

[flavzz]

It's a valid theory, except for the error in saying that a late closing exhaust valve affect trapping efficiency. That would be the inlet valve that has that effect. A late closing exhaust valve is also a late opening exhaust valve, which is a bad enough thing in its own right. The exhaust valve should open before BDC in order to allow the gas to start getting the hell out of Dodge before the piston starts to rise and help push it out. If you delay that then there is excessive pressure in the cylinder when the pistion starts rising (which costs mechanically) and then you don't get as good a clear out of exhaust (leaving more in the cylinder to contaminate the fresh charge, which is bad in so many different ways, especially on a turbo).

Best course of action would be to;

• make sure that the timing marks are accurate (turn the engine to TDC with #1 plug out and feel for TDC with a screwdriver (or preferably a dial indicator)),
• then assuming the above checks out, dismantle the timing cover and check the valve timing on both cams. Hopefully the timing marks in there are clear enough to prove what's going on.

If the valve timing is shown to be wrong, correct as required. If not, then you get to start looking for rags in intercooler pipes and so on.

Thanks mate, I'm with you. Definite chance the problem lies within here. Will test for boost leaks too.

oooooorrr take to someone who knows what they're doing.

I have no problem in doing that but part of my enjoyment in owning such a car is gettin to know every part of it. I (mostly) enjoy trying to troubleshoot issues. Once I have reached a level where I feel I am out of ideas I will happily take it somewhere. But if I took it straight to a shop every time I had a problem I would have empty pockets

[niZmO_Man]

If I saw that the timing isn't right with the CAS around middle position, I would have set the timing properly first. Eliminate that, then concentrate on other areas.

[flavzz]

Yes the new plan is to check all the timing is set correctly and move on from there. Thanks guys, I will report back once this is done with hopefully some good news.

[flavzz]

Bit of an update to this thread. A week ago I had the timing belt/water pump changed which all went well. This helped things a little in terms of getting rid of the backfiring on deceleration and between gears but the problem with the crank angle sensor position still remained and power unchanged. They said timing had not slipped a tooth which was good. Unfortunately he could not consult with the ECU as it was sending signals intermittently. I had already acquired an R32GTR ecu ready for a Nistune board so we swapped over the ECU's and this time it communicated fine.

With the CAS in the fully advanced position the timing was not more than 15degrees. Therefore swapped the CAS over to a second hand item and in the neutral setting the timing was back to stock just less than 20* which was much more normal. So a faulty CAS was the main problem.

Ran it up on the dyno keeping in mind this was on a dyno dynamics dyno where as the first run earlier in the year of 140kw was on a mainline dyno. The car made 183rwkw which is not far off where it should be for a stock rb26 so I was stoked with the result. The difference is very noticeable and the car feels much better to drive and quicker especially above 4krpm. Will try get out for a hills drive soon to test it out properly. Overall a great result. Shaun and the team at Boostworx did a great job and he explains everything he does in detail which I love.

Edited September 25, 2013 by flavzz

[ZED X]

Interesting read.

I cant say i have the same issue with mine, however my CAS is in the same full right position as yours. This a proven 20deg with a timing light which i thought was really strange. Mind you this hasnt actually caused any issues and my car runs beautify.

I've also inspected for a skipped tooth and that hasn't occurred. Might try swapping out the CAS as you've done and see if this changes anything. Only for curiosity's sake