ECM swap

[Jimmy85]

So I’ve been hunting down a miss or vacuum leak for the better part of 6 months now. Engine idles a little high, stumbles and sputters a little off acceleration. Fuel mileage is affected. After finding a bad egr, cracks in a few plug wires, knock sensor not sending the right output and a few other new parts, I still have these flat spots. I tried carb cleaner looking for vacuum leaks with no luck. Its like the timing isn’t advancing. I do get a code 42 & 32 after short highway stints.
Yesterday afternoon I tried a spare ECM that I have and I think I have the problem solved! Engine idles perfect, acceleration is crisp and I’m riding high last night. Even went out for a test drive last night to confirm. All smiles. But this morning……..truck won’t even start. Few minutes of testing and determine no spark. I reinstall the original ECM, truck starts back up again. Al beit with the same usual issues. I get where I’m going and decide to reinstall the spare ECM again for grins. Truck fires right up and is idles perfect, no issues. I give it a few hours and truck with spare ECM won’t fire again. I swap back in original, starts again.
I am disconnecting the battery when I switching out the ECMs. Is there any other procedure I should be following? Is my spare ECM crap too?

 

[dyeager535]

The ECM's almost never fail. Seriously. Their failure rate is probably one of the lowest of any component of the vehicle.

That you have differing problems when you swap them out, to include the problem going away, I'd look very close at the ECM connector itself. Check the pins for corrosion, broken wire, bad insulation, pins that aren't locked in place, etc. This is good advice for all the wiring both underhood and under-dash. Grounds, firewall passthroughs, and so on.

I chased a gremlin for awhile that turned out to be an ECM connector pin that the lock had failed on. Luckily I had a lot of spare components so swapping stuff out cost me nothing but time, but generally, throwing parts at it is going to cost a ton.

 

[Jimmy85]

After some toggling between the two ECMs the not starting with the spare ecm hasn’t happened anymore. Thinking maybe there was some corrosion on the pins. I will be inspecting the wiring a little more as some of the previous symptoms are starting to return.

 

[dyeager535]

I just put dielectric grease on everything now. Disregarding the pros and cons people talk about dielectric grease, my truck sits so long, and deals with so much condensation and humidity, I feel any possible cons of dielectric are offset by the likelihood that corrosion is going to be my problem.

 

[imiceman44]

The ECM's almost never fail. Seriously. Their failure rate is probably one of the lowest of any component of the vehicle.

That you have differing problems when you swap them out, to include the problem going away, I'd look very close at the ECM connector itself. Check the pins for corrosion, broken wire, bad insulation, pins that aren't locked in place, etc. This is good advice for all the wiring both underhood and under-dash. Grounds, firewall passthroughs, and so on.

I chased a gremlin for awhile that turned out to be an ECM connector pin that the lock had failed on. Luckily I had a lot of spare components so swapping stuff out cost me nothing but time, but generally, throwing parts at it is going to cost a ton.

I have to disagree, ECMs do fail, but mostly from corrosion.
I have had 3 trucks that came from the coast and had problems that were solved by replacing the ECM with a clean one or in one instance opened the case and cleaned up the corrosion and sprayed contact cleaner and put it back together.

 

[Jimmy85]

Well I have either regressed or my actual issue is slapping me in the face now. Voltage to the fuel pump is only little over 8 volts now. While bringing the truck in the shop to inspect wires, the fuel pump started acting erratic and fuel pressure is now under 1psi. Barely idles now. At least I got it inside.
I’ve followed the wires feeding the tank to the firewall, no knicks or breaks. I have +12 volts to the relay. (I should have tested the relay, will do that tomorrow). Cleaned some grounds. Looks like I’ll be printing out some schematics tonight.

 

[Jimmy85]

Finally got some time to dig into my voltage issue and I don’t know if I should revive older threads on low voltage to fuel pumps or start a new one. I need a little direction now, maybe I can get some help on this thread.
I found the issue is on the pink or hot side of the circuit. I ran a wire off the positive terminal of the battery and used the ground in the connector, have 12v that way. Then I used the jumper off the fuel relay and have 12v at the connector. So I believe it’s not a grounding issue. I went ahead and opened the wiring harness on checking the joints that connect oil pressure switch & fuel relay. Everything looked and had less than 1 ohm of resistance between connectors & joints.
Now I’m a little lost on where to look and test. Or I need more wiring diagrams to chase. Any thoughts would be appreciated.

 

[imiceman44]

So by providing an alternate positive source it works?
And you're trying to figure out where that original line is interrupted?

Finally got some time to dig into my voltage issue and I don’t know if I should revive older threads on low voltage to fuel pumps or start a new one. I need a little direction now, maybe I can get some help on this thread.
I found the issue is on the pink or hot side of the circuit. I ran a wire off the positive terminal of the battery and used the ground in the connector, have 12v that way. Then I used the jumper off the fuel relay and have 12v at the connector. So I believe it’s not a grounding issue. I went ahead and opened the wiring harness on checking the joints that connect oil pressure switch & fuel relay. Everything looked and had less than 1 ohm of resistance between connectors & joints.
Now I’m a little lost on where to look and test. Or I need more wiring diagrams to chase. Any thoughts would be appreciated.

 

[Jimmy85]

Correct. When applying an alternate positive I can get 12 volts to the pump and get the fuel pressure back above 9 pounds again. What can be causing the lose on the positive side? And it not being in harness on the engine.

 

[imiceman44]

Correct. When applying an alternate positive I can get 12 volts to the pump and get the fuel pressure back above 9 pounds again. What can be causing the lose on the positive side? And it not being in harness on the engine.

Well I guess follow the wire all the way to the source

 

[Jimmy85]

That's the question, what is the source?!
I've traced the wires to bulk head on the firewall, but get lost under the dash. I can't find any wiring diagrams of inside the cab from the bulk head.

 

[dyeager535]

Check voltage at the ECM B fuse, and make sure it's good. That feeds a few things, oil pressure switch included. Since the oil pressure switch would keep it running if the relay was having problems, IF the pressure switch is working properly, that whole circuit isn't seeing 12v.

The manual shows the positive going out of the cab on that circuit (440, FWIW) out through the fuse panel firewall connector, and then a five way splice to feed the ECM (2x), the relay, the pressure switch, and of course the fused 12v from the panel.

Never use another wiring diagram when the GM one is available. http://67-72chevytrucks.com/vboard/showpost.php?p=5621043&postcount=1

Fuel pump wiring is a bit hidden, but I think you'll find it under power distribution?

 

[Jimmy85]

I have book marked that site and will digging in there tonight!
I have to eliminate the wiring to and from the OP & fuel pump relay. After testing from each connector to the ecm/junctions/each other and to the firewall, getting less the an ohm of resistance and 12 volts at each connector, I can't find any issues with the wiring harness. I tested the orange (440), the green/white (445 or 465, I can't tell) and the ground (120), all passed.

 

[Jimmy85]

But I still have the issue of the voltage heading out of the bulk head at on 8 and half volts. I did check voltage on each side of the fuse. Swapped it with another. Checked voltage of a few others there, all above 12v. I feel I'm getting closer!

 

[Jimmy85]

I'm guessing I need to following the pink (30) to where ever the fuel gauge leads me.

 

[dyeager535]

There isn't anything I can think of on the truck that should be less than 12V.

If it's 12V on one side of the panel, and 8V on the other, there is something wrong inside the panel. I can't tell which pin you are checking in your photo, but 120 is the one.

I think if you dig further into the manual it will show you the fuse panel internals. Most of it is run off of shunts/busbars(?). Not a lot to go wrong inside the panel, but of course, who knows.

Fuel gauge has nothing to do with the fuel pump. Same connector, but there is no control shared by either. It's a single wire running from the sending unit to the gauge.

 

[formercruiserhead]

I had similar issues last summer. Diagnosed and replaced faulty parts to just have something else go weird. Would run great for 30mins or sometimes 5 mins and die in the driveway. At one point I would shut off the truck and the injectors would keep firing till the line was dry. Changed out the ecm with one a friend had and symptoms went away. Ended up swapping between 3 used ecm's that would all have some random issue. Bought a new ecm and now haven't had an issue since. I think they are all really old at this point and it they had been sitting for awhile they may have gotten corroded internally and cause sporadic issues. I had the knock sensor code as well almost every time I drove it. ended up being a bad IAC. Replaced the IAC no more knock code.

 

[imiceman44]

I had similar issues last summer. Diagnosed and replaced faulty parts to just have something else go weird. Would run great for 30mins or sometimes 5 mins and die in the driveway. At one point I would shut off the truck and the injectors would keep firing till the line was dry. Changed out the ecm with one a friend had and symptoms went away. Ended up swapping between 3 used ecm's that would all have some random issue. Bought a new ecm and now haven't had an issue since. I think they are all really old at this point and it they had been sitting for awhile they may have gotten corroded internally and cause sporadic issues. I had the knock sensor code as well almost every time I drove it. ended up being a bad IAC. Replaced the IAC no more knock code.

I had similar problems with a 1999 van I bought that sat on the coast 4 years. I opened the ecm case and it was full of white fuzz.
I cleaned it really well and sprayed some break cleaner then contact cleaner and it's been running fine for a couple of years

 

[formercruiserhead]

As far as your voltage issue goes. I have had high resistance in wires before. Replaced fuel pump multiple times before we figured out that the supply wire had high resistance and was slowly damaging the fuel pumps. After replacing the wire no more fuel pumps going bad. Since you have had weird intermittent issues with the ECM (like me) it could be another corrosion/stuff is just old issue.

 

[Blue85]

It would be interesting to see if that 8.5V changed when things were switched on and off. You can get bizarre things when a circuit or fuse is blown and things end up in series. IIRC, there is a fuse that blows and puts the dome light and underdash light in series for a strange dim glow. Those little buzzers that tell you the lights are on connect between the light circuit and switched ignition. So when you turn the key off the buzzer sends current through who-knows-what, maybe everything on switched ignition.