I've been having this issue for a month and the mechanic was unable to diagnose it. The car starts without any trouble and idles alright (a little high) but when applying a little throttle in reverse or drive, the RPMs drop and the engine stalls. Once past the dead spot of around 1000 RPM, the car runs great, no trouble at higher speeds. I think I narrowed it down to a communication issue between the TPS and ECU. When the engine side of the TPS is unplugged, the problem totally goes away and the car runs great. I would leave it unplugged, but I've heard that can cause other issues later on. Here is what I have done:

• Tried three different TPS and calibrated all to about .82 volts.
• New IAC.
• Cleaned throttle body and replaced the gasket.
• New spark plugs, distributor, rotor, cap and wires.
• New fuel filter.
• Checked wire resistance to the ECU.
• Cleaned the dipstick ground terminal.
• New O2 sensor.
• Re-soldered sensor grounds.

Any other ideas to help me pin point this issue before I pull all my hair out? Thanks!

 

What meter reading do you get when performing Tip 5?

Have you made sure your intake manifold nuts aren't loose?

Is your EGR functional?

 

You've done a good job so far. These are sticky things to figure out and sometimes there could be a single hidden issue, or several problems. At this point I think a guy needs to start going back to the basics. I would recommend:

1) Test and know your vacuum. A vacuum tester is pretty cheap, but I think you could get one from Oreileys. Is it steady and good? What are the readings and when? Mine tends to run a little low at idle, but bumps up off idle. Vacuum leaks can cause lots of problems owers blame on ECMs. And yes, that includes the intake manifold, hoses, canister, brake booster, throttle body base gaskets, valve cover grommet, low compression ..and all that.
2) Where does your temp sensor ohm out at? Double-check it's good. I had an errant temp sensor on my pick-up that was tricking the engine into thinking it was a different temp than it was ....throwing off the fuel and shift algorithms. It ran ok, but sort of like you are describing.
3) Did your mechanic do a compression test. Probably not a problem, but good to know what it is in case it's causing low vacuum.
4) Distributor: Is there play in the distributor shaft? Is the pickup in there good? Errant distributor readings will flake out the ECM. It'll have to take the high and low and find somewhere in the middle. That makes a difference. On a pick-up I once had the star-shaped distributor pulse unit had deteriorated from age and moisture. It caused dirveability issues. It looked normal until I noticed a small crack in one of the star-shaped ends (and it broke off when I touched it). Also plugs, I've replaced with new spark plugs before and got a bad one. Wires too. Wouldn't hurt to ohm them out.
5) Fuel pressure ...what's that look like at idle and driving?
6) IAC and air sensor? I assume someone fully cleaned the IAC area. How about the air sensor? I pulled mine out and it was all carboned up from 30 years of never been looked at. Yours might be too. Hard to imagine mine even read at all. Also inspect for excessive shaft play at the throttle body (losing vacuum).
7) EGR they get terribly boogered up underneath. Some of your problems sound like occur when the EGR gets boogered up or isn't working right. Remove and clean go online and read the test proceedure. There's also a diaphragm that and the vacuum has to work.

Well, those are the back-to basics. A lot of those are fairly quick to test. Some take a little longer. Everyone seems to want to point their fingers at the ECM right away, but with all the hoses and other components, it's kind of amazing these engines run as trouble-free as long as they do.

Best of luck!

 

Hey Cruiser54 and Jeepwalker - thanks a ton for the fantastic advice already! I am determined to get to the bottom of this!

Curiser54 - Initially the ground from the TPS to the negative battery terminal, I was reading 3.5 ohms. I re-soldered the brown and white wires before the C101 and now I am reading .9 ohms, so I think those grounds are good. I will double check on the intake manifold bolts today, but kind of put that on the back burner because the mechanic said there was no vacuum leak (that was my original thought when I brought it in, but I will check again cause I trust my own results more then theirs). Lastly, I tried running through your EGR test (tip 21) and it passed the opening test, but I wasn't able to figure out how to manually depress the diaphragm on the closing test. Do I take needle nose pliers to that rod that moves during the opening test? I think I will open that up, clean it all out and replace the gasket next.

Jeepwalker-
1. I have not picked up a vacuum or fuel presser tester yet, but those were definitely on my radar. The mechanic said those were good, but like you said, I may just start from the basics and fully confirm everything they said was 'fine'. I will take some butane gas to all those vacuum connections to 100% confirm.
2. Is that the coolant temp sensor on the radiator or the air temp sensor on the intake you're referring to? To test that, would I just back-probe the the negative terminal to the battery just like the TPS? I will check on that today.
3. I will be sure to triple check everything vacuum related and will do some research on the compression test.
4. The distributor is brand new (the mechanic said to replace it so I did it myself after getting everything TDC and it started right up first try. Saved myself about $400 - gotta love learning to do it yourself!!). That said, I will be sure to check on that to ensure that it seated properly and that everything is snug.
5. I will check fuel pressure while I'm digging into vacuum issues.
6. The IAC is brand new as well. Put a new one in when I cleaned out the throttle body. Do I need to do anything else manually with that to ensure that it is seated properly? Not really sure how that valve closes off, but assumed it was fine since it was new.
7. I am thinking that the ERG may be a part of the culprit since I read a few others have had that problem. I think after vacuum and fuel pressure tests, I am off to cleaning and possibly replacing that.

Thanks again for all the insight!! I will be sure to keep you posted on my findings. Hopefully it can help save someone else a headache!

 

Good ....good! New distributor probably rules that out. Remove the EGR and look under it. The whole passage could be clogged up with carbon deposits, or the EGR can be clogged up by the pintle valve. I'd be surprised if both weren't (unless the mechanic cleaned it). Usually a guy can reuse the gasket, or apply a little RTV. Don't grab onto the shaft with pliers, you might goof it up. On the intake there's the air flow sensor. Remove and clean.

The IAC could also be carboned up underneath. It sounds like you cleaned all that out with some carb cleaner and a brush, right? Yeah it has to be adjusted right, but sounds like you're on top of that. The temp sensor is on the side of the engine, isn't it? What you do is when the vehicle is completely cool (several hrs of not being driven), ohm out the sensor and compare the reading to the outside temp. Should match. Some temp sensors live forever, some 'drift' over time and need replacing.

Cruiser has invaluable tips ...keep doing his suggestions and once you find the issue, you'll have renewed a lot of deferred maintenance items which will keep it running trouble-free for a long time.

Might want to study this:
https://comancheclub.com/topic/36875...n-manual-here/

 

OK I have an update after taking a couple of days off to sit on it. I took off the EGR valve and fully cleaned it and replaced the gasket and it seems to be working well. I cleaned all the connections that I could find with electrical cleaner and dielectric grease. I couldn't find any manifold bolts that were loose, but I couldn't really get too far in there without removing the fuel rail and heat shield. That said, still no change. I assume since the problem totally goes away once the TPS is unplugged, the issue must be electric instead of mechanical. I still need to clean the MAF sensor and the connections to the ECU itself, but short of putting in a new ECU, I am kind of out of ideas. The temp sensor seemed to be reading about 20 degrees higher than expected, not sure if that could be contributing. I believe the IAC is fine, but I am not sure if I need to manually adjust it differently. Everything I read says they don't need to be adjusted manually. Any other ideas would be greatly appreciated! Otherwise, I may try to track down a new ECU and will post any updates if I find one. Thanks all!

 

Have you backprobed and read any of the sensor readings at the ECU? Especially the TPS? Checked the wires at the ECU for corrosion or bad connection?

Not sayin' it can't be the ECU, but the problems are rarely the ECU. But you never know, right?

 

Hey I figured I would give an update in case anyone else comes here looking for advice. The problem went away somewhat randomly. I figured since I did a ton of work, it just got better as I drove it, but it's somewhat unsatisfying since I would have liked to know the exact cause. That said, the last things I did were clean the grounds, re-solder old wires, clean off all relay and other electrical connections and clean the ERG valve. I'm sure that did the trick, but the problems were still happening right after when I test drove it, so who knows. Anyways, thanks a ton for all the help and now I'm off to fixing the next problem!

 

Thanks for the update!!

And, congrats for sticking with it.