cresus

Hi everyone, I'm at my wits end right now. I've got an LS1 from a '99 Vette that I can't seem to get running right. I scanned with HPTuners and my fuel trims max out at +25% and the MAP is around 33 kPa which seems to indicate I have a vacuum leak. I have looked pretty much everywhere except pulling the intake to make sure it's not cracked. It runs really rough and backfires in the exhaust (also have a periodic misfire DTC P0300). This is a stock engine except I have shorty headers on it. Am I looking at this the right way? Is there any place I should especially look for vacuum leaks on these engines? Both banks are lean which seems to me that it isn't a intake gasket problem.

nitrousjunky

The slightest header leak will do that, especially if its in the vicinity of the o2 sensor. My 02 Camaro did that and thats what it was.

XtraCajunSS

iTrader: (2)

^^^^ Header leaks X 2... Also, an intake manifold leak or cracked PCV hose will do the same.

Shane

cresus

I would be very surprised if it was a header leak. The engine had the same problem before I put the new exhaust in and I wouldn't expect both sides to be doing this. I'll double check it though. The hose after the PCV valve doesn't fit very tightly (that funny little elbow where the heater strip wraps the pcv valve). Is that supposed to have a hose clamp on it? Would a crack in the hose downstream of the valve do anything?

Also, the other thing that made me think it wasn't a header leak was the vacuum level. I've been looking trying to figure out what normal range manifold pressures are for an LS1 but I would bet the vacuum is supposed to be stronger than 10". Even if the O2's were getting tricked with more air from a leak there's still the MAP. Thanks again.

XtraCajunSS

iTrader: (2)

Low 30's MAP is fairly normal. I would think your intake gaskets are the most likely culprit. Any crack or leak in any part of the PCV system can cause this issue. Basically any air leak after the MAF and before the O2's will cause your fuel trims to be jacked...

Shane

bigboar

iTrader: (1)

by any chance have you checked your fuel pressure? might not be, but my 01 ls was running badly, it threw o2 codes and a misfire code. I looked for leaks everywhere before smeone told me to check my pressure. I checked it and ended up having a fuel pump that was bad. My fuel trims were both max lean also. New fuel pump and car is running perfectly.

cresus

I got hopeful the other day and thought maybe this was it. Checked and it's 60psi.