Rough running, stalling, check engine light, and can't pull codes.

[sdryluth]

Hi there, I have a 1990 K1500 with the 5.7 TBI engine that i'm trying to diagnose.

Sometimes it will run just fine, especially when the engine isn't warm yet. When it's up to temp it will often go into this phase where it runs very rough and stalls. It especially likes to stall when coming to a stop, but in its rough running phase it will often get so bad that when I step on the gas the engine just sputters and doesn't do anything. Sometimes if I really try to fight it and keep the gas down when its misbehaving it seems to backfire with a loud bang from under the hood followed by a strong burnt oil smell.

When its in its happy, running nicely phase there is no check engine light on. When it starts to run poorly the check engine light will flash and/or come on and stay on.

The injectors were leaking quite a bit so I replaced them along with the pressure regulator diaphragm and also cleaned up the idle air control valve. This made no difference to the running issue at all.

The really weird thing is I can't seem to pull codes. I tried shorting the pins on the diag connector but the check engine light just stays on solid with key on engine off and refuses to start flashing to give me the codes. It won't even do the initial series of flashes to indicate that its in code checking mode.

Any ideas what I'm dealing with here?

 

[Schurkey]

Failed wiring between the diagnostic connector and the ECM?

Failed ECM?

Are all the fuses good? Are you sure?

 

[sdryluth]

The wiring to the diagnostic connector is fine so I guess it must be an issue with the PCM or the wiring to the check engine light?

I'm also not sure if the issue being able to check codes is the same or even related to the engine running poorly, but it does make it a lot harder to diagnose. Ignoring the inability to check codes issue, any ideas on what would cause just the poor running problems?

 

[Schurkey]

The wiring to the diagnostic connector is fine so I guess it must be an issue with the PCM or the wiring to the check engine light?

HOW do you know the diagnostic connector wiring is "fine"?

If the ECM can turn the light on, I'm thinking a wiring problem to the light is possible, but really unlikely.

I'm also not sure if the issue being able to check codes is the same or even related to the engine running poorly, but it does make it a lot harder to diagnose. Ignoring the inability to check codes issue, any ideas on what would cause just the poor running problems?

So many possibilities that it doesn't make sense to consider them until you can narrow things down with the scan tool.

 

[sdryluth]

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The above image shows the connections leading to the ALDL port on the 1990 K1500. I checked Pin A and found it has good connection to ground. I checked Pin B and found it has good connection back to the ECM connector. I think this means it should at least be capable of putting into code flashing mode.

I was still suspect of the check engine light circuit because of the weird way the light was flickering. Sometimes it even would come on like halfway lit and then slowly fade in or out. I still am not sure what was causing this but the issue seems to be intermittent and after some testing I managed to get it to flash out the codes.

The only code stored in the computer was 43. I got multiple sequences of 12 - 43 - 12 so I know that 43 is the only one stored.

I took a look at the distributor and found that it was loose and able to be rotated. I set the timing properly and tightened it back down so its properly held in place.

Weirdly this didn't seem to solve the issue or change anything. It runs perfectly 25% of the time and horribly 75% of the time and sets the code 43.

 

[sdryluth]

I did a bit more work trying to figure out the check engine light issue. That diagram I posted before shows that the pin the ECM grounds to turn the CEL on also runs down to Pin D on the ALDL connector. I hooked the scope up to that pin and found that when the CEL starts to flicker, its because that wire is being pulled to ground off and on very quickly.

I then checked at the connector on the ECM and found that it is what's randomly pulling that pin to ground. I tried shorting A and B again to go into code flashing mode while it was doing this weird flickering light thing, and the ECM was still just freaking out and very rapidly grounding and ungrounding the CEL wire, often so quickly that it made the CEL appear only half lit up.

What is going on with it do you think? I feel like this points towards the ECM being faulty, but I don't think I have ever heard of this failure mode on one of these. Do you think it's also related to my other issues?

 

[Schurkey]

Assure the grounds on the ECM itself are good.

If they are...I'm leaning towards a faulty ECM.

IF you could get a stable ECM--Diagnostic Connector connection, hook up a scan tool instead of looking for "Codes". Verify the sensors, look at the computer outputs See what changes when it runs good vs. running poorly.

In the meantime, follow the service manual diagnostic procedure for Code 43. Maybe what's causing the poor running is the knock sensor is providing false signal that retards the timing.

 

[sdryluth]

I borrowed a scan tool which in theory should have been able to pull live data. It didn't work, but I'm unsure if its an issue with the tool or the truck. I'm inclined to think its an issue with the truck, probably the ECM given the other problems.

I took this video of the weird flickering thing the check engine light does. This is with key on engine off, and the ALDL pins A and B are NOT connected. From my understanding it shouldn't be flashing, let alone flickering weirdly like that.

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One other thing is that the poor running, hard to start, and stalling issues seem to happen at the same time that the check engine light is flickering, and when its running fine the check engine light is operating as normal. I'm unsure which is causing which. I can't find any record anywhere online of someone having the check engine light flicker like this on a GM product with this style of ECM. I'm going to pull the ECM out tomorrow and have a look to see if it has any sort of weird damage.

 

[Schurkey]

Fully-charge the battery. You should have 12.6--12.7 volts fully-charged, AFTER the battery charger has been disconnected for an hour or so. Might be 13+ volts directly after charging due to "surface charge" that will dissipate with a little time.

 

[sdryluth]

The battery was at 12.6 volts, it was just slow to crank because it's -10 degrees here.

Your comment brought up an interesting point though, the volt gauge is reading lower than I would expect for 12.6 volts. I took a look at the battery connections and found this.

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The ground cable going from the battery to the chassis is cut. The cable going from the battery to the engine is fine, but obviously having the engine properly grounded but the chassis not can cause some extremely weird issues. I suspect the flickering check engine light and my other weird problems were being caused by this, but I haven't actually fixed it yet so I don't know for sure. I need to go get some wire to repair it with and then we will find out.