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Good afternoon,

I am new here so forgive me if this is posted in the wrong place or already addressed somewhere else. I've been working on my daughter's 95 S10 2.2 manual truck. One day it wouldn't start for her leaving work, but cranked and had spark. Couldn't test fuel pressure but smell gas at tailpipe. We towed it home the next day after showing up with a fuel pressure gauge and nowhere to connect it, it wouldn't start then either. Checked codes, nothing.

I suspected fuel pump or circuit, and tested pressure and it was fine--55 psi deadhead, 44 connected, 38 psi after 10 min shut off. After cycling pump a few times to purge air out of filter (I had removed it to connect the gauge), on a whim I cranked it and it started right up.

Next suspicion was ignition switch, and possibly that my repeated cycling without starting had briefly cleaned it up. Now with the new ignition switch, the test drive went great. When my daughter was ready to head home after dinner (about an hour later) it would crank and fire intermittently. Pushing the gas got it to surge up but then drop off, and I had to shuffle it out of driveway to get the other car out, it didn't have enough power to get back in the drive.

My neighbor (a good mechanic and the former owner) heard my difficulties and the next day suggested the Idle Air Control, that they can become flakey and stick. He had another one and offered I could switch it. But when I went to try it later that day it started fine. I never swapped it as the problem wasn't occurring. I disconnected the IAC while running and was surprised that no codes showed or check engine light.

Yesterday I came to the hypothesis that it is a heat soak thing. I went home and fired it right up, tapped on various sensors / controls to see if there was something loose, jiggled wiring, plug wires, ignition module, etc with no stumble. I have jiggled wires around the ECM as well with no symptoms. I then drove my usual 5 minute test drive route, and parked it, all running great. 25 minutes later I started it and it wouldn't stay running. It's like it is trying to shut at idle, or missing a cylinder or 2 or not firing an injector or 2 If I step on the gas it picks up, but holding partial throttle it repeats the problem at the new position and stalls out so I have to keep modulating the gas pedal to keep it running. No codes again. With nothing else to go on, I tried to swap the IAC but it wasn't the right one and I stopped for the night.

Truck is basically all original, and has recent new wires and plugs. Has about 200k on it but a new motor at 173k.

Where should I be looking?

Should I be concerned that disconnecting the IAC didn't cause a code? Does this point to the ECM?

Thanks in advance for your help.
 

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time to get cereal
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It's been a while but I don't think the truck should even start without the IAC valve hooked up. At least I don't think my '01 does.
 

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Discussion Starter #3
It's been a while but I don't think the truck should even start without the IAC valve hooked up. At least I don't think my '01 does.
Just to clarify, the time when I disconnected the IAC was when it was idling fine (cold) and was already running. Electrical disconnect only. 8^)

Thanks,
Rob
 

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Discussion Starter #4
Yesterday I came to the hypothesis that it is a heat soak thing. I went home and fired it right up, tapped on various sensors / controls to see if there was something loose, jiggled wiring, plug wires, ignition module, etc with no stumble. I have jiggled wires around the ECM as well with no symptoms. I then drove my usual 5 minute test drive route, and parked it, all running great. 25 minutes later I started it and it wouldn't stay running. It's like it is trying to shut at idle, or missing a cylinder or 2 or not firing an injector or 2 If I step on the gas it picks up, but holding partial throttle it repeats the problem at the new position and stalls out so I have to keep modulating the gas pedal to keep it running. No codes again. With nothing else to go on, I tried to swap the IAC but it wasn't the right one and I stopped for the night.
New information: Problem is not confined to heat soak. I tried to start it cold late last week for a little more diagnostic work, intending to drive my test drive route and let it heat soak, but then I found it cranked but did not start. It does makes fuel pump priming sounds. After looking at a list of codes you can get on a 1995, I chose 3 to test to see if I could create a code. I repeated the IAC disconnect, but no code showed. (should be code 35, IAC Error). I reconnected that and disconnected the oxygen sensor. No code. Should be code 13, Oxygen 02S Sensor Circuit (Open Circuit). I reconnected this and disconnected the throttle position sensor. That one should give me a code 21 or 22 (Throttle Position sensor circuit high or low). Again, no codes.

Oddly enough the check engine light seems to do what it should, comes on with key to run, then blinks off, then comes on indefinitely until cranking. With pins 5 and 6 grounded, it flashes code 12 (diagnostics working) all day long. (Ok, I lose patience after 7 repetitions)

So my question now is if there's anything else I should look for as a cause of an ECM problem, before I use a new one as a test light or smoke generator. 8^) What else is there to check out in wiring, sensors, etc before I just replace it?
 

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Discussion Starter #5
Talking to myself here I guess. Is there another part of this S10 Forum that would get more responses?

It started fine when I went back to my diagnostics last night. Tried one more sensor, Engine coolant temperature. (ECT) Disconnected, checked codes, nothing. This is the one sensor it should be looking at all the time even if it's ignoring the rest until it reaches temp for closed loop. Nada, just flashing "12" repeatedly like it's all good.

Took ECM down from under dash, tapped on it while running, wiggled wiring around it, couldn't create the stumble. Pulled the ECM, no corrosion, no damaged pins, or burnt smell. Inside it all looks fine for all I can tell, which is not much. It is a remanufactured ECM and all the usual labels are gone, only rebuilder labels. My mechanic neighbor who used to own it said it was replaced before he bought it, as he didn't change it. So that makes it at least 15 years ago.

I am sending it to Module Experts after discussing with Adam there. Major pointer to ECM for me is no codes when disconnecting major engine controls. Wish I felt really confident that this is all, and also that there's nothing else to worry about. Especially anything else that might hurt a freshly repaired ECM.

I'll let you know next week what I find out. If anyone's out there.
 

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Without having a 95 service manual handy to look at, I do know that some of the codes require more than one cycle to show and some only show after the PCM does a test of that function.
 

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Discussion Starter #8
Module Experts reports that the ECM is bad, and that problem was in the main processor chip and is not removable / therefore ECM not repairable. So they are providing a replacement remanufactured. Repair cost applies to reman. Total is more than just buying one, but I did at least get a test to confirm problem. No programming required and a lifetime warranty. I wish the process had gone faster but this should get it done.

Ironically other options to buy one were not great, Rock Auto does not ship to my address due to a tax collection issue, though they had one on hand. Another online source listed it at a good price but wouldn't ship for a couple weeks, which would be about now.

Overall, testing to determine if this unit had failed, and lifetime warranty won out over other options that might be faster/ cheaper.

Stay tuned.
 
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