Questions and Answers

GMC Z71 Running Way Too Rich
Q. Dear Vincent, I have a 1997 GMC Z71 truck, 5.7 liter V-8. I have been having a problem with engine codes, particularly P0175, P0172 and P0300.

Some history: About a year ago, the check engine light came on sporadically and was off for some time, then about 6 months ago the light came on again and I had the vehicle checked, the result was that I had the code P0175. I took it to the repair shop and the O2 sensor was changed but minutes after living the shop on a long trip, the code came back on and staid on.

When I returned from the trip I purchased a code reader and gave me the codes mentioned above, all three of them. The engine has been running sluggish; sometimes the engine stops when stopping to go in reverse or at a stop. I have a strong smell coming from the muffler, the engine hesitates and is hard to start every time.

On the muffler I have a thick black crust that I believe is the result of bad carburation. Also, at the same time, the oil pressure has dropped, especially when idling the engine when before the gauge did not move. At idle the pressure is about 20 psi. At idle, the engine light will come on and off indicating the code P0300 and will go steady when the RPMs go up or I move the vehicle. My RPMs stay about 650 to 700 at idle.

My vehicle has 142,000 miles and before then it has never had any problem. I have changed the oil every 3,000 miles and have kept good preventative maintenance on the vehicle. The timing chain has never been changed but it has not given me any rattling or any noise to prompt me to do it.

The only rattling that I do get is when driving the vehicle over a small bump and the noise comes from underneath the vehicle about the middle of it. I have not been able to pinpoint the area of the noise.

I was wondering if the problem is the EGR valve and probably now, the catalytic converter because of the smell. The spark plugs, wires, rotor and cap have about 6,000 miles on them. Is there any way the EGR can be tested?

I would appreciate any help that I can get. I would like to fix the vehicle myself before spending too much on repairs at the dealer.

Thank you very much for any help you can give me.
Jaime

A. Those codes can all be related to a rich running engine. Sounds to me like you may have a leaking fuel pressure regulator which is under the intake plenum. A fuel pressure test like the service manual describes should lead you to a fix. This could be leaking so bad that it's thinning out the oil, so get it looked at pronto!