1988 LeSabre #3 Injector operates intermittently, runs rough

[propilot74]

A few weeks ago my 1988 LeSabre 3800 began to run rough/misfire intermittently. One minute its running smooth as silk, the next its chugging and shaking like crazy.
I checked for ignition problems, all cylinders checked out fine for spark regardless of how the engine was running.
I listened to each injector with a stethoscope, and determined that #3 injector was not "clicking" when the engine began to run rough; as soon as it smoothed out, the injector was clicking again.
Since the car is old, I changed all 6 injectors this morning, assuming #3 was bad, and the rest would shortly follow.
I took it for a drive and after a few miles, the problem reappeared. I can only conclude there must be some sort of electrical issue with the #3 injector circuit.
Does anybody have any ideas how to troubleshoot/fix this? Right now I'm at a loss.

 

[Spriteman]

How long does it run rough when it happens?

If there is enough time - you could check the voltage from the PCM to make sure it's pulsing correctly. At least then you would know if it's an electrical issue or not. If the failure happens too quick then you could buy a cheap Noid light that would show when the voltage stops from the PCM without having to break out the meter.


First idea - clean both sides of the fuel injector connector.

Second - trace the ground for that injector and clean where it attaches.


If it still occurs after that you could have a problem with your PCM sending voltage to the injector.


That could mean that it's the PCM or the wiring harness.

Hope this gives you something to go on.

 

[propilot74]

Thanks for your reply Spriteman, to answer your questions:
When it happens, it can run rough from anywhere between 2seconds and a few minutes, it varies.

The connectors on all the injectors look factory clean (harness and injector)

Your second idea has me a bit baffled. You say "trace the ground for that injector and clean where it attaches". There are 2 wires that connect to the injector; my understanding is one is a constant common +12v to all 6 injectors, and the other is an individual pulsed ground from the PCM. I'm not aware of any other ground wire.

This leads me to suspect either the wire running from the PCM to that injector has an intermittent short to ground, or a internal break, or the PCM is on its way out.
The problem is going to be how can I inspect the entire length of the wire to find a possible damaged spot (especially when its intermittent).

I can buy a used PCM and see if that solves the problem, but I already spent $$$ on injectors I really didn't need, and don't want to keep throwing money at "maybes".
If you have any ideas as to the best way to troubleshoot the wire in the harness, or test the PCM, I would appreciate it.

Thanks!

 

[Spriteman]

Do you have access to a scanner that shows live running data? You could hook that up while you're driving and see if when it's driving rough whether or not the PCM is commanding the injector to fire. If it is that means there is a wiring problem - if not, then either the PCM is bad or something is preventing the PCM to command the #3 from firing.

 

[propilot74]

Ok, so I pulled the PS pump to access the engine harness where it's squished between the pump reservoir and the valve cover/engine block. There was nothing protecting the individual wires in this area, the original harness tape has perished a long time ago. Sure enough I found the wire for #3 injector was chafed and had exposed copper, so I taped it up and covered the whole thing with new plastic conduit. Haven't been for a long test drive yet, but I'm pretty sure that's fixed the problem.

The damaged wire.


Bundle after repair.

Thanks to everyone who offered their help!

 

[propilot74]

Update : just went for a 20 minute drive and the problem continues. I fixed the exposed wire so I'm now wondering if the short may have damaged the PCM somehow; although that doesn't make a lot of sense since its an intermittent problem. Maybe I'll have to open up the harness again and reinspect the wire; I stopped looking after I found the chafed spot. I don't see how it could ground out anymore because I have everything wrapped in plastic conduit now. I won't have time to look at it till later in the week. Will update at that time.