5.3l Mystery vacuum leak!

[chevyman70]

So, I've had a problem with my 5.3l swap. It now reads it has a vacuum leak and codes p0171 and p0174 appear. I replaced MAF, intake manifold gasket, fuel pump, new filter, valve cover gasket, pcv, pcv grommet, and replaced vacuum hoses. Ltft reads 10-18% during iddle on both banks, 25% during acceleration and part throttle. When I plug the PCV, the fuel trims drop to 0% during idle, part throttle, 10% during acceleration. So my question is, what would cause the pcv to act as a vacuum leak?

[BR3W CITY]

If its truly a vacuum problem, it might be worth taking some starting fluid and spraying the intake manifold. On occasion, the plastic manifolds can crack if they were dropped, or torqued down really hard.

Otherwise, its possible that a tune related issue is triggering what seems to be a leak, however you have to troubleshoot it down to that point.

[chevyman70]

I have done that as well. I don't see/hear any difference. I have even tried two sets of gaskets, felpro, and victor reinz. same results. I even replaced intake manifold bolts.

[chevyman70]

if the pcv is a controlled vacuum leak, how does the computer know its a vacuum pulling from the pcv?

[mattcrp1]

If you have access to a scanner that will allow data logging, monitor ltft, stft and o2 while spraying down the suspected area's, as well as clamping the pcv line. This can give an instant reading when the condition changes.

[ls1nova71]

I have heard there was a problem with the valve cover, the one with the tube in it not the one with the PCV valve, but I dont recall the specifics of I, but it had to do with the baffling in it not sealing correctly and replacing the valve cover would fix the problem. Might try looking it up, not sure if it was a factory service bulletin or not.

[chevyman70]

ill try that

[ls1nova71]

Originally Posted by ls1nova71

I have heard there was a problem with the valve cover, the one with the tube in it not the one with the PCV valve, but I dont recall the specifics of I, but it had to do with the baffling in it not sealing correctly and replacing the valve cover would fix the problem. Might try looking it up, not sure if it was a factory service bulletin or not.

Im an idiot.... I really shouldnt post things when its late and I'm tired. Thinking back today, I think that was the fix gor a buddy's truck that was using a lot of oil. Sorry.

[chevyman70]

Originally Posted by mattcrp1

If you have access to a scanner that will allow data logging, monitor ltft, stft and o2 while spraying down the suspected area's, as well as clamping the pcv line. This can give an instant reading when the condition changes.

After spraying several cans of carb/choke cleaner, I did not notice any changes to the LTFt, STFT, or significant change to O2's. I drove it around with the pvc plugged, and I noticed the LTFT would jump to 25% just by slightly taping on the throttle. As soon as i let go of the throttle, it will drop around 8-10%. Stft, starts going to the negative as soon as I let go of the throttle.

[BR3W CITY]

Since your in Cali, are you running an EVAP and is it plugged in? The EVAP system is a giant controlled vacuum leak.

[chevyman70]

the Evap is removed.