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Random misfire on my 1997 GMC Sonoma, 4.3L Vortec engine. When engine is cold, misfires do not happen much, as it warms up it gets very bad. Misfires happen at any rpm. Here are the things I have tried / points of information.
1. Checked for vacuum leaks with carb cleaner: none found
2. Observed that the fuel trims, short and long, are both very close to 0%, going no higher than 7% in either direction.
3. Replaced the spark plug wires as they were questionable (the good fuel trim values lead me to believe it is ignition related)
4. Measured fuel pressure. With the key on, engine off, the pressure primed was about 55-58 psi. spec says it should be 60-66, but I don’t know if this small amount of discrepancy would cause an issue. It uses one of those spider injector systems. Fuel pressure also remained constant for 10 or so minutes, indicating there are no internal leaks. It did drop off (very slowly!) eventually but I don’t believe this is of major cause for concern.
5. I just bought this truck from a guy in town, and he said he just replaced the fuel pump. Fuel filter was done about 5000 miles ago if I remember correctly.
6. Observed spark plugs in cylinder numbers 1,2,3, and 4 (5 and 6 were tough to get to) and they had a little bit of a white soot like appearance, but I believe this is not bad. They all looked the same.
7. Did a compression test on cylinder number 2, showed 150 PSI.
8. MAF live data looks good, (g/s reading matches spec).
9. Coolant temp sensor giving good data to computer, It isn’t jumping around or anything. Stays relatively constant around 90 C, indicating that the thermostat is working.
10. MAP sensor data appears to be ok. Startsoff at 95 or so kPA and drops to 33 kPA on start up and at idle. I still need to put my vacuum gauge on it though.
11. Cap and rotor were done less than 10,000 miles ago and look good. I cleaned the contacts just in case and did a resistance check to check for carbon traces. All good.I still want to check the distributor gear, as well as maybe do compression tests on more than 1 cylinder, but I figured since it is a random misfire, it must be affecting all cylinders, such as a skipped timing chain or something. I also want to check the EGR gasket and valve to see if I am getting leaks there.
Any thoughts????
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I’d want to throw an expensive scan tool at it that can access and show you the misfire counter. GM was a little lackluster with the P030X codes back then but their misfire counter was always spectacular. If there’s a chance you know somebody who’d trust you with their scan tool, get your hands on it and look at that misfire counter to see if you can narrow down a particular offending cylinder. If that same scan tool has the counter, it should also have the injector kill test. You can use that as your cylinder contribution test.
I do like you chasing the distributor as a potential problem. An easy thing to look at would be the cam retard offset if you can get that scan tool. That can give you an idea where the timing is without pulling things apart. IIRC, the offset shouldn’t be more than 10-12°. I could be wrong, but that’s still a reasonable range. If you also happen to have code P1345 stored, you could probably just throw in a new distributor blindly and come out a winner.
I believe if memory serves, the ignition advance was about 20 or so degrees. Definitely no less than 15.
I was going to pop off the distributor and then I realized that a worn gear doesn’t really fit with it getting much worse as the engine is hot. Here are a few more things I did.
1. Compression test of 3 more cylinders, all very good.
2. Removed EGR valve and plugged up vacuum port to see if the car ran better. It didn’t. This rules out the EGR valve as well as exhaust back pressure issues (Since the exhaust side of the EGR was just venting out).
3. This one is REALLY interesting. I looked at the MAF sensor data and every time there was a misfire, the MAF reading spiked upwards. I don’t know if the misfire is because of this spike, or the spike is because of the misfire.
4. Cleaned MAF, no change.I think maybe it might be the MAF?? The one on there is very cheap and looks old, but I don’t know for sure…
EDIT:
Also, the fuel pressure WILL bleed down if I leave it on there for very long. But it hasn’t ever bled down below 40 PSI. Don’t know if this is because of the valve in the tank, or a spider injector leak problem.
The cam retard offset is a different PID than the ignition advance. Ignition advance is variable while the cam retard offset is generally going to be a constant value within a couple degrees. If you don’t have access to that PID, just move on. If you’re committed to pulling the distributor, get your timing marks set up appropriately before you pull the distributor.
I had trouble with my distributor when installing a new aluminum housed unit. The rotor just wouldn’t line up with the timing mark on the distributor housing. (It’s a non adjustable distributor so I’d think this mattered.) It always looked to be at least one tooth off in either direction of the timing mark and I couldn’t do anything about it. Somewhere in some service info, it said that there could be some deviation in alignment so I ran it as is. It ran without issue for 6000 miles. Then I developed misfire problems and resolved them for another 1500, and then I kept having repeatable symptoms with more than one cause after that and I gave up after a while and lived with the misfire. My truck broke down in March and I was forced to deal with it. That’s when I learned about that cam retard offset PID. I was pretty far out compared to the recommended spec posted on Identifix. I notched the clamp on my distributor to make it adjustable and just zeroed the offset and my truck has been running surprisingly well since.
If you can feel your misfires reasonably easy while cruising, check to see if they go away under wide open throttle. @ WOT, the computer will put enough advance on the timing that the misfires can stop. That’ll also serve as an indicator that there is indeed a problem in the distributor. Looking at my own distributors that sat in my engine, I can tell you that the gears do get eaten up and I hear bushing wear in the housing shaft is also rampant.
One other thing, if you want to try and eliminate the MAF as a suspect, you should be able to run the engine with the MAF disconnected entirely. If misfires go away, so does the old MAF.
Hopefully, this will narrow down your problem source. Good luck.
Thanks for the reply!
It is difficult for me to feel the misfires when I am cruising / at higher RPMs. I mainly just go by sound at that point.
Today I unplugged the MAF sensor and the misfires seemed to remain the same, so that rules that out.
I think next I will pull the distributor and take a look. I feel as though I have ruled everything else out at this point. However I have never replaced the spark plugs. But I really do not think it is likely that enough spark plugs went to cause a random misfire. I also don’t have a way to test the ignition control module next to the coil.
There is also that slightly low fuel pressure…. I am trying to avoid replacing the spider injector assembly, but at this point I am running out of things to test. The pressure bleeds off very very slowly when I shut it off. Don’t know if this is a valve in the spider injector assembly, or the one way valve in the tank.
If the plugs are worn worse than you thought or the gaps are too wide, they can cause random misfires anywhere and everywhere. Make sure you’ve got the right plugs in there too. It’s always been my understanding that they should be iridium plugs (AC Delco 41-993 or 19256067), but I’ve also been told on occasion they should be platinum plugs. I’ve been running the iridium plugs without issue.
Now that I’m thinking about it, coils are known to be a little on the weak side for our engines. You can get an inline spark tester and dial it out to 30 or40kV and attach it to any plug wire you like. If you have a nice blue/white spark at that high a kV, you can probably rule out the coil as a potential problem.
You likely won’t need to worry about the ignition module. The fact that the truck is running kinda dictates that it’s fine. Ideally, you’d want an oscilloscope to test the ignition module anyway. It’s the only way for you to see how the primary coil is firing.
I’m not inclined to worry about that fuel pressure as you are. Your fuel trims you mentioned earlier suggest fuel isn’t a problem.
Help me out a bit. You said it’s difficult to feel the misfires at cruising and higher RPM. Is the misfiring worse at idle?
I did come up with one thought about a fuel problem. The poppet nozzles clog up often enough on the spider injectors. Before I refitted to the MPI from the original CSFI, I tested fuel pressure drops on my injectors and two of them were plugged solid. I actually used a high pressure nitrogen pump to blast out all my poppets and I probably didn’t need to change my spider assembly for it. You may just need to run some fuel injector cleaner in a couple tanks or go to a shop that can run your engine on a motor-vac system to flush the poppets. Keep this in mind though only for after you can rule out something in the ignition system.
I’ll replace the plugs, it is never a bad thing to do. I will also test spark. Recall I DID do a resistance check on the coil. There were actually 3 resistance checks in the service manual, and it passed all of them.
I work in a laboratory and I was able to bring home a lab oscilloscope to look at the sensor waveforms. The only one I have looked at thus far has been the crank sensor. I thought maybe the signal would periodically go bad and cause a misfire, but it seemed to be good. One thing that was weird is that it wasn’t a true square wave. It looked funny, like a decaying voltage at the top and bottom of each square. (See attached image!)
Today I drove it and I definitely could feel the misfires when I was driving it. It has just been so long since I drove it any real distance that I didn’t remember correctly. I already put one bottle of injector cleaner in it, but from what I hear, the stuff you buy off the shelf in an autoparts store generally isn’t strong enough to do much good. Maybe I need multiple bottles over many tanks as you suggest.. I also forgot to test at WOT.
I was lucky I could find these still laying around. They’re from my 98 and I have similar traces somewhere from my old 99 Blazer. The first one is a correlation between the cam and crank sensor. The second is a standalone of the crank sensor signal circuit. I can’t explain it, but the three little blips on the cam sensor trace is normal. I used the Blazer as a known good to compare against my S10.
Yours looks like an AC pattern on your scope. I think your engine should be the same as mine in that it would have a hall effect crank and cam sensor. Did you have three wires or two wires at your crank sensor?
My crank sensor is 3 wire. Which I think should be a hall effect, right? My signal does not look the same as yours.
Three wires makes yours a hall effect. If it was a PM signal generator, you’d only have two wires. If you’ve still got that scope handy, can you scope all three wires simultaneously? It would be interesting to see how the power and ground is behaving at the same time.
I just had another thought. Did you probe directly at the crank sensor or were you at the PCM end of the signal wire?
I will do that! It might only have 2 channels so I might have to do signal+Power and then signal +ground.
I was probing at the PCM, not directly on the sensor
If you can help it, probe directly at the crank sensor if or as needed. If you see the same traces at either end of a circuit, you’ll at least be able to rule out some wiring faults. It would probably be a good idea to get down there and make sure the CKP is seated all the way into the timing cover anyway.
Wife’s car broke down yesterday so I had to fix that. Consequently I only had time yesterday to do the spark plugs on the truck. Some of them had a bit of blackish residue, and one had a bit of oil on it, but for the most part, they seemed good. I am not an expert on spark plug analysis. I will try to attach a pic later.
Anyway, during the spark plug changing I pulled the fuel pump fuse and did some compression tests. I noticed that when I started it up after replacing the plugs, there was a loud *tick tick tick!* in the top end that wasn’t there when I pulled the truck into the shop. It sounded like it was coming from under the intake in the back (where the spider assembly is). After 5 or so minutes it went away. Don’t know what to make of it. Air in the lines from pulling the fuel pump fuse?? Will try to look at the waveforms today.
A HA! I got it! After measuring all the waveforms from the crank (which looked a lot better, I think I had a bad connection last time) and the ignition coil, I decided to simply turn off the lights and see if I can see spark arcing where it isn’t supposed to. And Low and behold, I am getting arcing between the coil and the laminated core around the coil, as well as the main ignition wire going from the plug to the distributor (which is a new wire!).
I removed the coil and gave it a wash and used contact cleaner on the connections, as well as cleaned up where it bolts to the engine to see if maybe there was a bad connection or something driving up the voltage. After doing all that, I am still getting arcing, but mainly on the wire going to the distributor. I wonder if I should just replace the coil. Tomorrow I am going to redo the resistance checks (which it passed btw), but both when it is cold and when it is hot. I would bet that as the coil gets hotter, it will fail the resistance spec tests.
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