Mechanics

STUTTERING PERFORMANCE

1992 Pontiac Sunbird

Engine Performance problem
1992 Pontiac Sunbird 4 cyl Front Wheel Drive Automatic 122300 miles

I've been driving this car after replacing the cylinder head for about a week now. The car had been sitting for some time before I bought it, so I was happy once I finally worked out the timing and got the car driving nicely.

I notice that when stopped at lights, it seems to idle pretty low, but it hasn't necessarily been a " problem.&Quot; Then yesterday, coming down a little slope to a stoplight, I stopped the car, and the engine died on me. I started it right back up and proceeded to drive.

a little later (maybe a few miles at most) the car started stuttering. Gaining and losing power in little blips. I had to feather the gas a little just to make sure it didn't die at the lights.

I got out of the car once I reached my destination and left it running and popped the hood. Nothing was visibly wrong (though I see my coolant is leaking from a weld in an OEM part. That's something I'll have to fix later) so I started jiggling things. I SWEAR as soon as I jiggled the portion of the main engine harness that goes behind the engine, the engine died instantly.

Any ideas as to what is going on here? It's been driving fine for a week now and just out of nowhere starts up with this strange bucking/power glitch thing.
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RyanVee
May 8, 2010.



This is exactly the kind of " bugs" I built into donated cars for my students to troubleshoot. The two things to look for are a spot on the harness where it has rubbed through over a sharp metal bracket or is laying against a hot engine or exhaust part, and look inside any electrical connectors for stretched or corroded pins. Pay special attention to anything you had apart when the head work was done.

The first step in all of Chrysler's troubleshooting charts, but it applies to any car, is to check for " pushed out pins" in the connectors. That means two mating terminals that instead of sliding together, they butted end-to-end and one pushed the other one out of the connector.

Some things to help narrow it down include, 1) you know it's not related to the charging system because the engine will run for a couple of hours on just the battery, 2) it's not related to a single injector or ignition coil. Whatever it is, the circuit affects something in common to ALL of the injectors or ALL of the coils, 3) assuming it started again, it didn't blow a fuse. If it's a common power wire for coil(s), injector(s), or something similar, the wire opened up, (broken) rather than shorted to ground, and 4) if it is a power or signal wire related to a sensor, the Engine Computer should have detected it and memorized a diagnostic fault code and turned on the Check Engine light. You can have codes read at many auto parts stores. That should only be necessary if the visual inspection doesn't reveal the problem.

Caradiodoc

Caradiodoc
May 8, 2010.
You know I still have not been able to figure out exactly what the issue is. I can drive the car for a week without any problems at all, then it'll out of the blue stutter and quit and not wanna start again.

If I shove my arm down behind the engine and jiggle the big main wiring loom the car will sputter OR stop instantly. Very strange. And of course, you know. It's in a spot where it would be almost impossible to get a meter or a light or anything to check it out.

I'm at wits end with this thing!

Tiny
RyanVee
Jun 29, 2010.
Well, I can share a few thoughts. If the Check Engine light is not coming on, you can probably rule out things that are monitored by the Engine Computer. For the most part, that leaves fuel delivery and the charging system. Also, before 1996, cylinder misfires aren't detected either so you could have a loss of spark problem due to a computer output circuit. The ignition coil and wiring to it would be likely suspects.

What you might have to do is connect a wire to a known point in a circuit and run it inside the car where you can connect it to your voltmeter so you can monitor it while driving. You might find the supply voltage disappearing to the fuel pump, ignition coil, or even a sensor.

I'm not real familiar with GM service manuals but the Chrysler ones do a real good job of showing where wiring harnesses run. I'm a big proponent of manufacturer's service manuals over the aftermarket stuff and especially any data on a computer. Harness diagrams usually show the locations of connectors and splices. You can use that information to figure out which wires are running behind the engine. That will give you some ideas on which wires to monitor with your voltmeter. I made a box once with a dozen leds and resistors that I could connect to a dozen different places to monitor. Only used it once and I accidentally found the problem while trying to make the connections.

If you find the intermittent circuit this way, you might be able to run a new wire. One thing I always pounded into my students was it is not acceptable, (on a customer's car), to just run a new wire for a couple of reasons. If they don't find the break or place it is grounded, why did it happen and will it happen again? If a wire got cut from rubbing on a sharp metal bracket, which circuit will be affected next when the next wire in the harness rubs through? Maybe your power locks stop working intermittently now, but that next wire might kill the engine in the bad part of town or out on a deserted highway. Finding this type of problem will result in repairing the mounting hardware that let the harness fall onto the metal bracket or hot exhaust manifold before it can affect the next wire. The second thing is if the problem is intermittent, and the wire is grounding out, running a new wire won't remove the intermittent short. You have to cut the original wire out of the circuit at both ends. In some case, there could be a whole bunch of wires that connect at a hidden splice. Every one of those wires would have to be cut out of the circuit before running all new wires. It is almost always faster and easier to find the location of the short. If a splice is corroded, it might cause a loss of power to only one item. Think of four tail lights / front running lights all connected together in a splice. Suppose the right rear light stops working because its wire corroded off of the splice. You could run a new wire from the switch to that one light and all four would work. But eventually that splice would corrode some more and the second light would stop working. Now you have to do the job all over again for the second light. Had you found the corroded splice originally, the second problem never would have occurred.

While I talked a lot about splices, loose, stretched, or corroded connector pins are a lot more common. That's what I would look for first when you wiggle stuff.

Caradiodoc

Caradiodoc
Jun 29, 2010.