Is Mechanic Right in Suspecting Bad ECM? 1987 R10

[Delaney]

Hi all,

I'll preface this by saying I'm very very new and am learning as I go, so dumbed-down explanations are appreciated!

I have a 1987 R10 5.0L. Ever since I got it, it will occasionally die while idling or cruising-- every once in a while I think it's died while I am just slightly pressing the accelerator.
It has happened while cold and warmed up. I've driven it 30 minutes with nothing happening, for 30 minutes and it dies right as I park, stalled while switching into reverse/park, and I've had it stall immediately after starting up. So it seems to be pretty random except that it really only happens while cruising/idling. It just kind of winds down, but it'll start back up again.
Also seems to idle a bit rough, going up and down.

The following parts have been replaced:
- Radiator replacement
- Ignition system (distributor cap, ignition coil)
- Coolant temp sensor
- Idle control module / idle air control
- Fuel filter
- EGR valve
- There was a fire under the dash at some point, so a ton of electrical work has been done and wires replaced.

The EGR valve and fuel filter are the repairs I did myself after getting it. Didn't fix it and I finally brought it into a mechanic that did the CTS and IAC replacement a couple months ago, before I got the truck. I did so because I saw that it'd been having the same issue (stalls while idling) back then, so I wondered if it was the same issue or something they'd missed.

They have it about a week and a half. They turned it back over saying the following:
No codes stored in the ECM. ECM has good power and ground. The truck still has fire/spark when it dies. They think it isn't the ignition side or the power side, the alternator's good, there's no parasitic drain, the ICM, MAP sensor, and throttle control all seem good-- at first they thought it might be one of the sensors talking to the idle control or the idle control itself, but then they noticed that the fuel injectors are like 'skipping' every once in a while. In a random-seeming pattern, not a regular one. So they used a lab scope and found that the ECM is not always grounding the drivers, and while running, the ground circuits are not always completed (they measured 2V left over that wasn't grounding? Not sure that makes sense).

They did not actually manage to view this with the scope while the engine stalled to confirm it's what's happening.

But they concluded that the ECM's drivers are bad and it needs a new ECM.

Does this sound right to y'all? I guess I'm skeptical as a failing ECM seems like it's usually not the real problem, so I'm wondering if it's more likely to be the mixture solenoid or something-- but If it were that, wouldn't the ECM catch it? Any other suggestions? Thanks!

[VileZambonie]

Validate your fuel pressure. What is fuel integrator and block learn? The ignition pickup coil and module act as the cam/crank signal. They don't last forever. I doubt it's the ECM, probably time for a new distributor.

[MIKE S]

If for some reason you decide to change the ECM,don’t forget to take the prom chip out of your old one and put it into the replacement. It will not come with one. The prom is specific to your vehicle and just not anyone will work.   

[Delaney]

Validate your fuel pressure. What is fuel integrator and block learn? The ignition pickup coil and module act as the cam/crank signal. They don't last forever. I doubt it's the ECM, probably time for a new distributor.

VileZambonie, could you explain a little more what you mean?
You're suggesting I check the fuel pressure using a gauge hooked up to the fuel filter, right? Is that to see if the fuel pump is failing?
Not sure what you mean by "What is fuel integrator and block learn."
The distributor cap, ignition coil, and caps have been replaced but the mechanic is suspicious of the distributor cuz it was an autozone/junkyard part. He said the ignition side of things is good though.
Sorry, I really am new to this so breaking it down is extremely helpful!

[VileZambonie]

Integrator and block learn are the prior versions of short and long term fuel trims. Instead of firing the parts cannon, invest in a scan tool that reads OBDI for your 87. If you haven't validated fuel pressure when hunting down any TBI drivability problem, you are again loading up the parts cannon for a fun dollar bill distribution. You have replaced secondary ignition components which have nothing to do with the control side of the ignition system. The pickup coil and ignition module are on the control side which is what is required for primary ignition control and injector pulse. 

[Delaney]

I ended up replacing the ECM and it hasn't died since, so looks like it was the ECM after all!

The passenger side fuel pump is also working again, when it seemed before like it had failed, so that's neat.

Next I'll be replacing the dash bulbs so I can read OBDI codes myself, as the check engine light doesn't come on at all right now, even when starting up the truck. Before replacing the ECM it wasn't throwing any codes at all (had to get a mechanic to scan them). I'm wondering if it will give me codes now, as it's still got plenty of issues-- seems to be running rich and have poor MPG, doesn't really like going above 55 MPH. Anyhow, thanks for the input!

[Delaney]

Hi all,

Got a 1987 R10 5.0L a couple months ago. Ever since I got it, the SES light does not come on, even at start up. It's misfiring atm so I'd like to be able to check codes without going to a mechanic who has an OBDI reader.

The gauge lights do work (mostly), and the 'seatbelt' light does work, but that's it. Never seen another light come on.
So, this is kind of a dumb simple question, but-- what lights *should* come on at ACC? I'm trying to figure out if it's just an old/burnt bulb or something else.
If it's missing a bunch of lights, I guess I'll check the gray wire from the fuse to the instrument cluster?
And if it's only missing the SES light-- does anyone have a diagram of where the actual bulbs are located in the back of the cluster? I was able to reach in and pull one without taking off the cluster, but I don't know if I was checking the right one. The one I did pull, just next to and below the big wiring harness thing, *looked* okay.

I'm also very new at all this so sorry if I'm not using the correct words.

[VileZambonie]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=37738.0

[Delaney]

Thanks, VileZambonie.

Is there anything else that CKT 419 runs through? The engine runs, and the seat belt light comes on, but only that light. And I just replaced a faulty ECM.

[Delaney]

Obligatory 'I'm very new at this and very confused' warning.

Got an 87 R10 with 5.0L V8.

Some background: there was a fire under the dash at some point before I had it, so it's had a lot of wiring and electrical work done. Definitely still needs more.
The truck was stalling while idling, and I couldn't figure out the problem till I ended up taking it to a mechanic, who said the ECM had worn drivers and wasn't fully grounding the circuit to the fuel injectors.
So, I replaced the ECM a few weeks ago, and it hasn't stalled since.
BUT, the truck is getting like....5 MPG. Ugh.
I wanted to start solving THAT issue by pulling codes, but the SES light doesn't work. So, I've been trying to resolve that first.
I began by checking the ECM side of things.
The SES light runs on CKT 419, a brown-white wire that runs to the A5 pin in the ECM harness (again, very new at this so sorry if I'm using the wrong terms, but I hope you can understand me).
So, I unplugged the ECM and used a voltmeter to check the pin. I got the 0.L reading, so I thought, okay, guess the wire is bad or something? (actually not sure how to interpret that-- here's where my confusion starts).
BUT, then I started checking all the pins and found that loads of them are giving me the same reading! Not all, but I'd say like...30%.
Pins that seem to be ungrounded are:

A5
A6
A8
A9
A10
A11
C2
C3
C4
C5
C6
C9
C10
C11
C13
C14
D2
D7

A couple others, like B1, give a reading with high resistance (3).

So....what all does this mean? Do I have a bunch of ungrounded circuits? Or does this not mean anything? Or am I testing all this wrong?
If there is a problem with a bunch being ungrounded, how do I address that? Pretty mystified out here.

[Captkaos]

Going to back to the start and ask a few questions...
Does the engine currently run?  Assuming it does but wanted clarification.
Does it run well or is there a problem?  (outside of poor mileage)  Missing, bogging blowing smoke etc
Have you check the bulb behind the CEL (check engine Light) to see if it is working?



Edit: I merged these topics since they were the same diagnostic search

[Delaney]

Thanks for the reply. Yes, it runs.
In addition to the poor MPG, I think it's running rich (fuel smell), and it has an uneven, up-and-down idle when I first start it, but it then idles normal if I rev the engine or drive a little bit.
In addition to the ECM, I recently replaced the EGR valve and fuel filter.
There seem to be two bulb sockets behind the instrument cluster-- I've looked for diagrams showing which bulb does what, but couldn't find anything. I've replaced one bulb, and am waiting on a socket so I can replace the other.
There might be a third bulb under those two, though-- and I'm unclear on whether there ought to be a wire (the brown-shite CKT 419 wire) hooked up directly to one of the bulbs or the printed circuit board. Photos would be really helpful tbh!

[Captkaos]

did you look at the factory wiring diagrams on this forum from GM? http://forum.73-87chevytrucks.com/smforum/index.php?topic=30115.0