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2005 LS2
p0172 p0175
Fuel pressure is 61psi at idle and while bliping the throttle. Pressure immediately drops to 54psi key off, but holds that for over 1hr. I have replaced both stock air filters (K&N), MAF sensor (Delphi), both upstream 02s (NTK). I have also cleaned the TB and MAP sensor.
I took it to a local shop. They couldn't find the issue, but suspected a faulty ecm and recommended me to a dealership before buying a replacment.
Any ideas? Thank you!
I highly doubt a dealership will be able to diagnose this so don’t even bother…HIGHLY doubt it’s your ECM so your local shop was only guessing….you gather the data and then make a decision !!…they only do brake and oil changes most likely…diagnosing requires a different skill set…your fuel pressure is OK, and cleaning your MAP sensor really doesn’t accomplish anything…looking at your freeze frame your MAF looks a little high but you were only running the car for less than 2 minutes…at “hot unloaded idle” it should be no less than the liter displacement of the engine (6 grams/second)…we don’t use pounds/minute…your manifold vacuum is only 16 inches of vacuum…29-13 inches which is a little low but again when fully warmed up seeing about 19-21 inches is good…are your upstream O2’s switching between 200-800 mv’s when engine is warmed up ??…I would make sure your EVAP purge valve is not leaking gas fumes into the intake manifold because that can cause negative fuel trims…I’d disconnect the EVAP line from the intake manifold and see if the long terms improve…if your scan tool has the ability to do a fuel injector balance test and is not a code reader I would do that because you have a fuel pressure gauge…I’ve seen negative fuel trims with cats that were starting to clog…do your fuel trims get better off idle ??…may be a bad MAF sensor and just because yours is new doesn’t mean it’s good…look at your MAP ignition on engine off…if you are at sea level you should see 29.9 inches mercury…I’d find a diagnostic shop in your area if you think you can’t diagnose this yourself.
I highly doubt a dealership will be able to diagnose this so don’t even bother…HIGHLY doubt it’s your ECM so your local shop was only guessing….you gather the data and then make a decision !!…they only do brake and oil changes most likely…diagnosing requires a different skill set…your fuel pressure is OK, and cleaning your MAP sensor really doesn’t accomplish anything…looking at your freeze frame your MAF looks a little high but you were only running the car for less than 2 minutes…at “hot unloaded idle” it should be no less than the liter displacement of the engine (6 grams/second)…we don’t use pounds/minute…your manifold vacuum is only 16 inches of vacuum…29-13 inches which is a little low but again when fully warmed up seeing about 19-21 inches is good…are your upstream O2’s switching between 200-800 mv’s when engine is warmed up ??…I would make sure your EVAP purge valve is not leaking gas fumes into the intake manifold because that can cause negative fuel trims…I’d disconnect the EVAP line from the intake manifold and see if the long terms improve…if your scan tool has the ability to do a fuel injector balance test and is not a code reader I would do that because you have a fuel pressure gauge…I’ve seen negative fuel trims with cats that were starting to clog…do your fuel trims get better off idle ??…may be a bad MAF sensor and just because yours is new doesn’t mean it’s good…look at your MAP ignition on engine off…if you are at sea level you should see 29.9 inches mercury…I’d find a diagnostic shop in your area if you think you can’t diagnose this yourself.
Thank you for the quick and informative response. The car has 160k miles. Could I have a bad MAP sensor? I've sprayed carb cleaner in suspected areas for air leaks but found nothing. My 02 sensors are oscillating correctly. Fuel trims get better as Im accelerating but immediately go back to being rich when I let off the gas. I took off the evap purge valve and made sure it wasn't stuck open, I will try disconnecting the evap line and watch my fuel trims tonight. I got the MAF sensor from Oriellys it's not the GM original replacement but it is Delphi.
As I mentioned ignition on engine off look at your MAP data PID and it should read 29.9 inches of Hg at sea level…that will tell you if it is skewed…also look at MAF when the engine is warmed up and no load…minimum of 6 grams per second…could be cat that is becoming clogged…dirty MAF has negative fuel trims at idle but go positive under load.
As I mentioned ignition on engine off look at your MAP data PID and it should read 29.9 inches of Hg at sea level…that will tell you if it is skewed…also look at MAF when the engine is warmed up and no load…minimum of 6 grams per second…could be cat that is becoming clogged…dirty MAF has negative fuel trims at idle but go positive under load.
Baro reads 28.64 my local weather says 29.9. My maf averages 7.5-8 g/sec warmed up ideling. Is the maf reading too high? I'll run a vaccum test today to check on my cats. Thanks for your help!
Check valve cover gaskets, valve cover bolt gaskets, oil cap o-ring, vacuum lines, intake couplers, throttle body gasket, intake manifold gaskets, etc.
These kinds of leaks are a bitch to diagnose and find.
Should I go ahead and purchase a smoke machine to test for leaks? I recently replaced the valve cover gasket along with the bolt gaskets. I'll make sure to check the rest you listed.
Yeah it's been kicking my *** haha. Thanks for the help
Should I go ahead and purchase a smoke machine to test for leaks? I recently replaced the valve cover gasket along with the bolt gaskets. I'll make sure to check the rest you listed.
Yeah it's been kicking my *** haha. Thanks for the help
Not a bad idea to have a smoke machine.
I wonder. Did your problem start around the same time as the valve cover and bolt grommet replacement? If so, there is a possibility that this is causing the problem. Not a bad idea to check to make sure the gaskets are properly seated and the bolts are tight. Pretty easy for a gasket to get out of place when trying to install.
I wonder. Did your problem start around the same time as the valve cover and bolt grommet replacement? If so, there is a possibility that this is causing the problem. Not a bad idea to check to make sure the gaskets are properly seated and the bolts are tight. Pretty easy for a gasket to get out of place when trying to install.
No I bought the car from a private seller with the check engine light on. I noticed a build up around the valve covers, so I decided to replace the gaskets along with the other parts.
Should I go ahead and purchase a smoke machine to test for leaks? I recently replaced the valve cover gasket along with the bolt gaskets. I'll make sure to check the rest you listed.
Yeah it's been kicking my *** haha. Thanks for the help
With a vacuum leak your fuel trims will be positive…your fuel trims are negative !!
Baro reads 28.64 my local weather says 29.9. My maf averages 7.5-8 g/sec warmed up ideling. Is the maf reading too high? I'll run a vaccum test today to check on my cats. Thanks for your help!
A vacuum test is not the test to do to check your cats…you need a pressure gauge that goes from 0-10 psi…you would screw the gauge into the upstream O2 sensor….if 8 grams that’s ok…did you remove the EVAP purge line that goes to the intake manifold and retest ??..you should do the injector balance test also…rich conditions are rare and the last one I diagnosed were a few bad injectors flowing to much fuel.
A vacuum test is not the test to do to check your cats…you need a pressure gauge that goes from 0-10 psi…you would screw the gauge into the upstream O2 sensor….if 8 grams that’s ok…did you remove the EVAP purge line that goes to the intake manifold and retest ??..you should do the injector balance test also…rich conditions are rare and the last one I diagnosed were a few bad injectors flowing to much fuel.
Okay cool, very informative I appreciate the guidance. I'll run that test right now! I removed the evap line from the intake manifold and saw no change from the fuel trims. I'll have to get a better scanner to do the injector balance test. I don't know the prior history of the car it could be cheap injectors
A vacuum test is not the test to do to check your cats…you need a pressure gauge that goes from 0-10 psi…you would screw the gauge into the upstream O2 sensor….if 8 grams that’s ok…did you remove the EVAP purge line that goes to the intake manifold and retest ??..you should do the injector balance test also…rich conditions are rare and the last one I diagnosed were a few bad injectors flowing to much fuel.
Max is 2 psi at 2000 RPM….you should check it at 2000 RPM
I had someone rev it to 1500-2000 RPM and hold it there for a second and it still did not go above 1 psi on both banks. LTFT are at -20. Fuel pressure is ok at 61 psi no loss of pressure, cats are ok less than 1 psi of back pressure, MAF reads a little high 7.5-8 g/sec, MAP sensor is ok confirmed by BARO pressure 28.6, 02s oscillating between 150-745mv. Is it safe to say my main concern should be injectors?
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