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Codes P0172 and P0175

My check engine light turns on and off occasionally. Had codes checked twice and P0172 and P0175 (rich bank 1 and rich bank 2) are the only ones Autozone says are set. Both times I checked codes, both were set.

This is a 1996 Chevy K1500 with 5.7L motor. I checked fuel pressure and running is 55psi and key-on-motor-off is 63psi. Spec per Autozone is

60-62psi key on motor off so I think that is close enough but comments appreciated. Truck has plenty of power, appears to run fine, maybe idles a little rougher than new but idles good but is a bit harder to start (cranks longer than it used to) but when checking fuel pressure, I noticed with key off, the pressure drifted slowly down, probably to 0 if I waited a few minutes. When I connected the gauge, no fuel sprayed out but there was when I disconnected.

Looking for possible causes and next step. What I read and previous advice says these codes could be:

  1. O2 sensor but unlikely both sides go bad at the same time so probably OK.

  1. MAF dirty or going bad. - What I read, dirty and bad tend to make for lean mixture

  2. Fuel pressure - my test looks like it is OK. Looks like a check valve may leak back to tank or maybe an injector is leaking. Any way to test which? Also, would a leaking injector set rich codes on both sides??

  1. something esle.

Again, I appreciate any and all feedback. If I take it in, how would the shop diagnose?

Reply to
bob
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How DO you effectively check for a clogged cat? The traditional way of checking for clogged exhaust by disconnecting the pipe at the manifold is way too crude. Is there an easy way to look at exhaust manifold pressure and compare it to a number in the book?

--scott

Reply to
Scott Dorsey

The first thing I'd check for is a clogged cat. Then I'd take it out on the road and see where the trim's are off and by how much. Is it pinging bad when you get on it?

Reply to
Steve Austin

Steve, No pinging and really runs fine. Only thing is idle may be a little rough.

From what I read, I want to pull the vac line off of the fuel pressure regulator but not sure where it is on this motor. There is no "fuel rail" as such. I found other post with very similar symptoms and he had fuel dripping from the vacuum port of his fuel pressure regulator. Sounded simple to test if I can find it.

Not sure how to test for clogged cat either. Will see if there are good puffs coming out of tail pipe this evening. Dang light is out right now so something is pushing it just to to the edge of the fuel trim.

Reply to
bob

Depends on what you consider easy.

Easiest is when there is a back pressure tube going to an EGR modulator or DPFE, put a compound pressure/vacuum gauge on and read the back pressure.

A little harder is to remove an upstream O2 sensor and use either a fabbed 18mm adaptor or an 18mm compression gauge fitting in the o2 sensor hole and hook your compound gauge onto it.

About as hard is to drill a hole in the exhaust pipe ahead of the cat and insert a probe which is connected to your compound gauge. The probe can be as simple as a needle valve thing like you'd use to inflate a football.

High tech when there are left and right bank O2 sensors, left and right bank cats, a MAF sensor and data stream access is to test drive and monitor fuel trims, a plugged cat will cause negative fuel trims on that side while the not plugged side will cause positive fuel trims. When graphed, the trims will go almost exactly opposite each other.

Old school is to measure intake manifold vacuum at idle, raise the RPM to 2000-2500, vacuum should be 2 inches higher than it was at idle.

Back yard is to hit the cat with a mallet and see if it rattles.

Obvious is when the air switching valve for the air pump is melted into a blob.

No start is when the engine won't start until you remove the upstream O2 sensor (a Saturn favorite!).

General rule of thumb when measuring with a gauge is you don't want to see more than 3 PSI on snap throttle.

Reply to
aarcuda69062

Fuel pressure regulator is inside the intake manifold. Remove upper plenum (the plastic one) and there it is on the side of the fuel spider. (I think I mentioned this a week or so ago) Look for areas that are washed clean.

I posted seven different techniques earlier.

Reply to
aarcuda69062

Actually, the first thing I'd do is plug in my brick. I'd do an idle and 2000 rpm MAP compare before I hit the road. See aarcuda's reply for better checks.

Reply to
Steve Austin

Aarcuda, Many thanks for posting the advice. The light is out right now (maybe the snake oil helped) but if it comes back on, I will pull the upper plenum. May dump some sea foam in at next tank as well though since if that helps, it is much easier.

Again, thanks to all who have posted responses. Bobby

Reply to
bob

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