mmanna

Hi all,
Have been chasing this problem on and off for 10+yrs... Have a 91 3/4 ton suburban with a 5.7 and a 4L80E. It has 158,000 on the clock. Here is the story...Started to idle high, enough that it would push you though stop lights. Replaced TBI with a reman unit. Ok for a short while then it started running rough when cold, ok when warmed up. Changed the ECT senor (no change), new ECT sensor pigtail (no change), new MAP sensor (ran worse, put old one back on). New EGR valve (no change put the old one back on), new ignition module (no change). Fuel pressure is 13psi, new fuel filter, new cap, new rotor, new wires, new plugs, new coil, timing set to 3deg BTC as if i set it to 0 it wants to die. The TPS is set to .55v. I have a scan tool that will read OBD1 and it shows that the coolant temp sensor is accurate per the computer. Integrator was at 147 so that says it is running rich, which it is. Note this is when cold, once it goes into closed loop it is fine but will buck a little at 3/4 throttle but clear up right away. Have set the IAC several times and just rebuilt the TBI again with reman injectors (nothing is leaking fuel). Scan tool said it was at 12 deg timing, if that means anything (no spark knocking). I have checked for a vacuum leak and nothing was found. It definitely is running rich. It will hunt for an idle getting richer and richer until it just dies (when cold). If you jab the throttle it will clean up and then start the process again. No unusual sounds, no hissing or nothing. The oem exhaust is on the truck and there are no modifications to the truck. It is all OEM.

No codes in the computer.

What do you think?

Thanks

Sabino56

Well, this is at the edge of my experience so consider maybe half of what I suggest may be correct.
You’ve checked fuel pressure and timing (disconnect that connector when checking?) which is first thing to do.
If you can log data, then at idle some things I’d check
-that you are in the correct BLM (ie. the ECM thinks you are idling with foot off throttle) - the TPS output just has to be a little high for ECM to go into another cell.
- While you’re there- check that ECM is reading TPS correctly. Is should show 0% throttle
- Check to see that you are getting into closed loop. There is a flag which you should see go active after reaching temp, some time, some O2 output
- look at O2 output while in closed loop. It should be varying between about 700mv and 400mv if closed loop is working
- if closed loop is working, you should be able to see a rich/lean flag switch in sync with the O2 sensor
- you should be able to see the INT counts go up and down varying the injectors driving the engine rich/lean based on O2 output

if it never gets into closed loop, I think it defaults to some base injector pulse which is not supposed to be too rich, shouldn’t kill the engine.
It’s like you’re getting into closed loop and following a lazy O2 sensor off the cliff? I didn’t see you say you replaced the O2 sensor? You can have a bad or lazy O2 which doesn’t set a code.

Good luck

mmanna

Hi,
Thank you for the response. I did replace the O2 a while back but it has less then a 5000 miles on it. The computer sees the TPS at .55 volts which is idle and it does go up from there. It does go into closed loop.

Ok, i understand what you are suggesting. Once in closed loop it runs just fine, so when the O2 sensor is running the mix it is ok. It is only when everything is cold is the problem. I changed out the MAP sensor this morning for another new one. Seems to be running better but hard to tell. It is 94F outside and goes into closed loop within a minute or two... I am going to run out and see if i can see any O2 voltage data. Don't remember if i can. Being an OBD1 system the amount of available data is pretty limited.

Thanks for helping, i do appreciate it.

Sabino56

I have a 93 C1500 5.7l . I use TunerPro RT for diagnostics and it will show O2 varying, INT, flags, etc. I don't know about a 91 but I'd be surprised the basics like O2 sensor, INT counts, etc. aren't available - but then again, I'm often surprised.
Here's example, this was during a hot drive so it's more confusing as it's moving between BLM cells - only a little time around beginning of trace where it was at idle and easy to see O2 varying and INT changing. btw - O2 there was bad. It was too slow to react and @ idle it'd eventually drop out of closed loop because it didn't vary far enough, fast enough. This condition didn't cause a code - just ran worse and failed emissions. It would make sense to me that something messing up in this area could cause a rich condition. Seems less likely it would make you idle fast - that sounds more like the idle control valve which you already messed with

mmanna

Sabino56,
Thank you for your help, i do appreciate it. I could not get much data from the scanner but since O2 sensors are so cheap for these trucks i just replaced it. $13....shipped for an AC/Delco unit. Ran like a top how but still hunting when cold. I read else where that the computer does not manage idle with the IAC until it hits a certain temp so i went back for a third time looking for a vacuum leak. The engine was fine but then remembered i could only get the air out the heating vents and no hiss when you move the selector. Checked the HVAC lines and found a broken vacuum ac line. Fixed it and now all is well. Funny in the old days i would have heard it leaking....

So i think my issue was a cascade effect. A bunch of things together. So if it seems like it has a vacuum leak it probably does have one, even if you can't find it... Thanks again for your help. I do appreciate it. TIme will tell if all is well. Now i have to find a place to get the super small vacuum hose. It isn't much bigger then a piece of cooked spaghetti....

Sabino56

Nice to hear you fixed it! Thanks for getting back with resolution.