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Strange idle issue after head gasket job. Vacuum leak?
Hey everyone, so I just finished putting my car back together. I had coolant leaking into cylinder 2, so I pulled the head, replaced the gasket, had the head decked (wasn't level), had the exhaust valve guides & all valve seals replaced.
Well, the car doesn't start very enthusiastically... It cranks for a while & eventually sputters to life. It idles fairly rough & when you rev it to around 2k & let it drop, it bogs to around 500 rpm, barely catches itself & comes back. Taking it for a test drive, it actually feels great under acceleration & gets to full boost, right around 15psi w/ a 16% SC pulley. Unfortunately, if you push the clutch in & try to coast after that spirited acceleration, the engine drops dead & you're just rolling w/ a dead motor.
I'm also not getting any codes thrown, which is pretty strange IMO. Now, in the car's initial disassembly, I did disconnect the green SC inlet gasket. Even though I just replaced it with a new one around 2 months ago when I did the SC oil service, I'm thinking this may be where I shot myself in the foot. The problem with testing this gasket is that it's so buried under everything else there's no way to spray brake cleaner at the seam to see if the idle changes.
As a part of the troubleshooting process, I borrowed the MAP & T-MAP sensors from our other R53 to see if this changed the behavior at all... it didn't.
Lastly, when I was looking at it this morning, I noticed the arm to the BPV moving on its own while sitting at idle. Not sure if this is normal, just thought I'd throw it in there.
Does this sound like I'm on the right track? Car makes full boost but has a possible vacuum leak - the leak would have to be after the TB but before the SC, correct? Wouldn't something like this normally throw a code?
If anyone has any other thoughts on this I'd love to hear em!
Posdible vacuum leak to the FPR? If you have a leak, in closed loop mode (idling/coasting) the car would run very poorly or not at all. However there is the possibility that under high throttle levels in open loop mode, the car could run fine. Just some food for thought. Where is your boost gauge teed off from? If its from the FPR hose on the hot side of the SC horn i'd be interested to see what type of vacuum reading you get.
My car sits at about 16 in hg at idle and builds to about 15.5 lbs boost on a 15% pulley for reference.
Posdible vacuum leak to the FPR? If you have a leak, in closed loop mode (idling/coasting) the car would run very poorly or not at all. However there is the possibility that under high throttle levels in open loop mode, the car could run fine. Just some food for thought. Where is your boost gauge teed off from? If its from the FPR hose on the hot side of the SC horn i'd be interested to see what type of vacuum reading you get.
My car sits at about 16 in hg at idle and builds to about 15.5 lbs boost on a 15% pulley for reference.
Yep, my gauge is tapped off the fpr line. Right now I'm reading 10 in hg at idle. I think you're onto something with the closed loop / open loop difference. It runs like garbage / dies in closed loop but fine in open. I've checked all those lines pretty extensively & sprayed them with brake cleaner with no change in idle. Is there something else I can check? I do have access to Dash Command to look at different sensor outputs if that helps.
Well here is the diagram of the FPR system, not much to it really:
From how you are describing the issue it seems like there is more of an issue with the vacuum supply rather than any part of the FPR assembly, as it functions fine while the car is on boost. Do you know how many in Hg you were running prior to all of this service?
Well here is the diagram of the FPR system, not much to it really:
From how you are describing the issue it seems like there is more of an issue with the vacuum supply rather than any part of the FPR assembly, as it functions fine while the car is on boost. Do you know how many in Hg you were running prior to all of this service?
I'm not sure what my vacuum was at idle prior to the operation unfortunately. I'll certainly triple-check the FPR line tonight when I get home though. Maybe I'll take the gauge tap out of the equation & run the FPR direct to see if there's a change. Anything I can check to isolate the issue to vacuum or fuel pressure?
Finally! Threw a code on a test drive this evening, 0172 - mixture control - system too rich. Due to a vacuum leak perhaps?
Yeah that code is for "System Too Rich (Bank 1)" which is odd given your description of the bogging. It would seem almost as if there wasn't enough fuel present since it was cutting out at low RPMs, although I suppose it could be flooding. What type of driving conditions triggered the code?
I would guess the leak is somewhere else than the FPR. That doesn't typically cause the car to run terribly at idle. Have a smoke test done and you'll find your culprit.
Yeah that code is for "System Too Rich (Bank 1)" which is odd given your description of the bogging. It would seem almost as if there wasn't enough fuel present since it was cutting out at low RPMs, although I suppose it could be flooding. What type of driving conditions triggered the code?
I believe the code came up as I was driving and let off the throttle while going down hill, still in gear.
Small update to this, I replaced the green gasket but still have the same issue, though not as bad..... it doesn't bog quite as bad and it now pulls about 13 in hg when sitting at idle, though the needle waivers. So, it seems that I'm probably on the right track. I'm thinking I may have made a bad call going with the victor Reinz gasket rather than OEM. Also thinking there may be an issue with the actual SC inlet pipe.
If, by some chance this issue is not being cause by a vacuum leak, what else could it be?
Last edited by silence2-38554; Nov 6, 2015 at 12:00 PM.
