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A bunch of mechanics are stuck

I have a 1994 Chevy 1500 with a 4.3 that’s been having issues. Within the past year it’s had fuel pump, fuel filter, and spark plug wires replaced. Within the past 3 months it’s had plugs, new throttle body, gaskets, injectors, air filter all replaced. I cleaned EGR valve and tested the EGR valve vacuum solenoid. Checked the distributor cap and rotor, checked for fuel, for spark, for pulse. Everything was good. In a last ditch effort, I replaced the computer (wasn’t convinced it would solve it but as I said, last ditch effort. Not a single mechanic I know, even in my shop, could give me any other ideas what may be causing the issue.) And no, replacing the computer did not help.

The problem it’s having is it will run fine for a few minutes and then it will act as if it’s either being starved for fuel, so the injectors will spray more than they need to, and then they’ll back down and spray a little, then spray a lot again, then back and forth like that until it just stops, then it dies. Then when you go to start it back up the injectors don’t spit any fuel. Try starting it a little later and it will start up just fine and run fine for a few min, then the issues start again. The most confusing part about it is there will be times that it runs perfectly fine. Just a few weeks ago I was able to drive it around town. Then two days later I went to drive it to work and it started doing this again.

Any advice or suggestions?

Not a single mechanic I know can think of anymore possible solutions. I’m a mechanic, the guys I work with in the shop, my other friend mechanics, even my teachers can’t think of what more I can try to get this damn thing running…

Bad fuel pressure regulator?

Perhaps, but this sounds like a characteristic vacuum leak symptomology. Perhaps even a leak upstream from one of the upstream oxygen sensors affecting the integrity of the O2 sensor reading.

Have you tried putting a vacuum gage on it? Or, optionally, vacuum line being dirt-cheap by the foot, replacing the lines (one by one, so you don’t cross any)?

Have you checked to see if the spark timing is stable? If not, and it’s a distributor based system, check to see if the distributor shaft has any lateral play in it. Lateral play in the distributor shaft can cause wandering of the spark timing, and that can manifest itself as wandering engine operation. If it’s a COP system, check the crank position sensor signal… A bad CPS can cause wandering operation.

Those are all off the top of my aged head. I’ll ponder the symptoms and if I think of other things I’ll post. I’m sure others here will also post ideas, as there’s guys here far more knowledgeable than I.

I’m probably going to be of little or no help at all, but what about diagnostic codes in all of this?
Anything present?

If you’re a mechanic I think you have a basic understanding of what it takes to make an engine run, and run properly.

What kind of test equipment do you have available? I guarantee the answers are readily available if you can have the proper equipment hooked to the car when it is acting up. You need a scan tool that shows real time data of all inputs and outputs, a vacuum gauge, fuel pressure gauge, and a labscope or graphing multimeter, a timing light.

Is the car in open loop or closed loop control when this is happening. What does your fuel trim show? What’s the commanded timing advance vs. actual? Are the injectors being told to vary their pulse width as you describe?

This is a very basic system, I’m surprised a handful of mechanics can’t figure this out.

Have you replaced the ignition module yet? The ecm uses the signal from it to drive the injectors. I just did one on the same truck for a porter where I work. Same issues you are having, when the module starts to fail it sends bad signals to the ecm and it can’t properly control the injectors.

Mods/asemaster I may have just flagged your post on accident, fat fingers on the phone. If I did can a mod remove the flag?

I’ve personally had one truck that had the problem SteveC76 pointed out. The thermal grease was all dried up and caked, causing the module to periodically overheat. I also had one with the throttle shaft bores worn out that would do something similar. That one was hard to track down with the old spray trick due to the proximity to the intake. I re-bushed the TB and fixed it…

@SteveC76‌ you can remove your own flags by clicking on them again. But it doesn’t look like you flagged anything here.

If you have a MAF, have you cleaned it yet?

It sounds like a classic ignition failure. It used to be caused by a bad ignition module. The question would be “what’s in your distributor”? If the answer is almost nothing then I have no suggestions. Is the 4.3 the V-6 with the fuel spider issues? Can’t remember.

I remember a customer that had a problem kinda like yours. I found that the pickup filter inside his fuel tank had gotten debris on it. The sock inside with tighten up a times when trying to suck fuel and other times not have such a problem. Took a long time to find it but, it worked great after replacing the sock. You should be able to access it at the tanks pickup tube.

We all anticipated the solution would be something really simple and that would make us feel stupid. Sure enough, every guy that has helped me on this truck now feels like an idiot for not thinking about the ignition module. Replaced it last night and so far so good with it not dying. It’s been quite temperamental though, and will sometimes run perfectly fine for hours after replacing something and then suddenly start dying again. I’ll be keeping all your suggestions close in the chance this isn’t the true solution.

Thanks for the help all! Any other ideas and suggestions are appreciated. Need to get this thing running.

Any other ideas and suggestions are appreciated.
Find new mechanics.

“Any other ideas and suggestions are appreciated.”

Live in a world where everything is obvious and nothing is controlled by little black boxes with 17 wires.

I would look for a vacuum leak. Also the MAF may be suffering an “in range failure” and misreporting the air flow. If you can look at the datastream coming from the PCM, also check what the IAT (intake air temperature) sensor reads and if it seems to be pretty close to the ambient temp outside.

Thanks for posting back and certainly shouldn’t beat yourself up over it. Heck, I’ve had to re-learn a number of lessons I forgot over the years. Then when you solve it a second time, you kick yourself. That’s a really common failure mode for these trucks. You did use the thermal grease on the new one right?

Edit: This will teach me to not post before reading the whole thread! lol … good call by SteveC and TwinTurbo for calling out the ignition module. Not something I would normally have thought of with these symptoms. Unless it occurred only in damp weather, hot temperatures, etc.


This is a tough one. What I thought of as I was reading your post is there’s something amiss in the idle control system. The computer tries to maintain the correct idle rpm, but for some reason it can’t, the rpm drops, so it injects more fuel to compensate, at which point it decides (due to the O2 sensor reading), the mixture is too rich, so it cuts back on fuel. Etc etc., you end up with an engine with an rpm that goes up and down and generally misbehaves.

It can be caused by the idle-servo motor not working correctly, air leaks allowing unmetered air past the MAF, faulty injectors, faulty injector programming (the ECM), faulty ignition or ignition timing, valve timing, sticking EGR.

hmmm … It sounds like the mechanics may have been thinking the same thing, replacing the throttle body, ECM, injectors, EGR.

I admit this is a total guess, but I think you’ve still got some problem with the air metering system. There’s either air getting into the engine and not being measured, or the sensor which measures the air flow (MAF/MAP) is faulty.

The experts here sometimes say to spray carb cleaner (I think that’s what they say to use) in the air intake system area and see if there’s any bump in idle speed. If so, there’s an air leak. Next step would be to investigate, one by one, all the way air can get into the engine. Not by accident, by design. The idle air control, on cars of this era there’s often other means, vacuum switches which get enabled to bump the idle speed when the headlights are on, the rear window defroster is on, or the power steering pressure is high, etc.

If your engine has an air bleed screw to adjust the idle speed, if that is mis-adjusted, this can be the result. Make sure it is at the nominal position.

One more idea, make sure the engine is running at the proper temperature (i.e. the thermostat is working correctly), and the ECM is measuring the actual temperature of the coolant.

Edit: Another one more idea, also on cars of this era, sometimes there is a separate cold start injector. If your car has one, do the recommended diagnostics to make sure it is working properly, and especially no leaking.

Sounds like the “SPIDER” to me… Ive replaced so many of these failing units I lost count…WELL KNOWN Problemo on these vehicles. Look up " Chevy 4.3 Vortec SPIDER " on the internet and you will see the OCEANS of people who have had to replace this part on their 4.3 Vortec motors…Ive done quite a few…

Blackbird

I’ve successfully saved a spider, by running the engine on injector cleaner, which was hooked up directly to the test port

One injector had failed the injector balance test, so I knew it was partially plugged. It’s been several years, and that truck is still running great

I believe GM’ policy was to first attempt a cleaning. If it worked, great. If it didn’t, the spider was supposed to be replaced with the updated part