Hey all, I've been hunting an issue for two weeks now, and I think someone here will be able to connect dots before I throw parts at it.

Code 173, HEGO senses Running Rich. This is the only KOER code I can pull. KOEO is 111. Pulled for codes multiple times after resetting and running again. I verified the resistance and voltage from the o2 harness and sensor. Even then, I replaced the sensor anyway just because it had been 50,000 Miles. Took out the PCM - does not look damaged. Exhaust smells slightly rich, so I trust the code.

Each time I shut the truck off, the code disappears, and will not appear again for a few minutes into the next drive. Occasionally, the code will not come on during a short trip (5 miles or so).

Symptom: Bucks and misses at part throttle cruise, acceleration very hesitant, occasionally bucking at the same time. Full throttle give plenty of power. Idle during the warmup hunts for a long time and stumbles quite a bit. Once warm, idle is steady about 800RPM.

Things I've done looking for the solution:

Basic tune-up: New Plugs, Rotor, Cap, Thermostat, air filter, fuel filter, oil Change. Cleaned contacts on the ECT during thermo change. Kept the dist. wires as they seemed in good condition. Ran a can of seafoam through the brake vacuum and another can in the rear tank. Checked all lines for vacuum leaks.

Fuel pressure test. Stable at 50PSI during start. Increases to about 55 at 1500 RPM or so when driving around. I did NOT test after shutdown for very long to see if pressure held. Didn't think to do it, and I suppose i do.

When running, I pulled the vacuum line off the FPR. No leaks or fuel smell. Cleaned electrical contacts and internals of the IAC valve. Seats properly.

EGR Valve works properly and holds a vacuum. EGR solenoid was leaking and sending vacuum when cold at idle, so I replaced it. New solenoid appears to work correctly. No vacuum when the solenoid is unplugged and inducing throttle. Works when it is plugged in.

Pulled out PCV valve, checked operation (good).

Have not checked MAP sensor, need new multi-meter that can read Hz.

So here's the two abnormalities I've noticed:

1. Tested voltage on the ACT sensor. With the engine warmed up and running, I was getting a steady .85v - .9v, which according to the oldfuelinjection library, correlates with a very high temperature (165F). I'm wondering if this could be part of the problem. I was going to test it cold this morning, but a contact on my cheapo multi-meter broke during the last test of the day.

2. Engine at operating temperature, I increased the throttle with my hand and tried to induce the stumble while looking at the EGR. So in park, I could run the throttle up to 1500 RPM, and the EGR valve would open. Higher throttle = more EGR open. It was my understanding that it shouldn't open unless I'm in cruise. When I disconnected the EGR solenoid, this didn't happen, leading me to think that the computer is telling the EGR to open.

 

Update for today.

Did another fuel pressure test - showed 50 when running, 40 within 20 minutes after shutdown and holding pretty steady. Over an hour later, 20 PSI. Is there a good guideline for how much pressure loss is acceptable?

Also checked resistance on all the injectors - 15.2 all around.

Retested the ACT Sensor with a new multimeter that had a temp setting. Voltage indicated .9, temp at 107. According to the tables I can find, that's about 50 degrees high.