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Hello I have a 92 gmc sonoma 2.8 5spd and I am having a "Miss" in my engine that is driving me insane. I've been looking for about 2 or 3 weeks now and have found absolutely nothing wrong, check engine light isn't coming on, there is no arcing anywhere on the wires, spark plugs are good, dizzy is good, ignition coil is good, I've checked all the connectors I thought might have an effect. It isn't missing constantly but when it starts I can hardly get my truck to move out out of my driveway.

The odd thing (at least to me anyway) is that when it is missing, the engine itself shows no signs of it. No shaking or anything. It is more like the engine is shutting off momentarily. I've checked everything I know of to check, read quite a few forum posts, I had a few of my friends who have been working with engines for 40+ years look at it, and I am still just as lost as I was to begin with.

As for when it seems to happen, temperature seems to make no difference what so ever, sometimes it does it as soon as I start it up in the morning, sometimes it doesn't do it at all. I have noticed that it shows up on my tach as a spike and otherwise erratic movement of the needle. Hitting a bump does seem to make it miss so I am almost certain it is an electrical issue somewhere, but I don't have a clue as to where it might be. I cannot seem to recreate it by shaking or wiggling things.

If anyone here has any ideas as to what it might be, I would greatly appreciate it. I am truely lost at this point.
 

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Hello I have a 92 gmc sonoma 2.8 5spd and I am having a "Miss" in my engine that is driving me insane. I've been looking for about 2 or 3 weeks now and have found absolutely nothing wrong, check engine light isn't coming on, there is no arcing anywhere on the wires, spark plugs are good, dizzy is good, ignition coil is good, I've checked all the connectors I thought might have an effect. It isn't missing constantly but when it starts I can hardly get my truck to move out out of my driveway.

The odd thing (at least to me anyway) is that when it is missing, the engine itself shows no signs of it. No shaking or anything. It is more like the engine is shutting off momentarily. I've checked everything I know of to check, read quite a few forum posts, I had a few of my friends who have been working with engines for 40+ years look at it, and I am still just as lost as I was to begin with.

As for when it seems to happen, temperature seems to make no difference what so ever, sometimes it does it as soon as I start it up in the morning, sometimes it doesn't do it at all. I have noticed that it shows up on my tach as a spike and otherwise erratic movement of the needle. Hitting a bump does seem to make it miss so I am almost certain it is an electrical issue somewhere, but I don't have a clue as to where it might be. I cannot seem to recreate it by shaking or wiggling things.

If anyone here has any ideas as to what it might be, I would greatly appreciate it. I am truely lost at this point.
any other symptoms? Backfiring? poor combustion smelling exhaust?
If not then I think you may be onto something with it being electrical. I would bet in the timing/ignition. Have you checked the rotor? Maybe the spark control thing inside the distributor(I never remember its correct name).
 

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ignition module... yea could be that or what jcricket said. I had a miss for 6 months til I figured out in was a head gasket... no signs but the misfire... until I saw a plug wet 6 months later. Just for giggles do a compression test. Hope it doesnt get that far for you.
 

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ignition module... yea could be that or what jcricket said. I had a miss for 6 months til I figured out in was a head gasket... no signs but the misfire... until I saw a plug wet 6 months later. Just for giggles do a compression test. Hope it doesnt get that far for you.
I really hope I don't have that problem, as much as I hate troubleshooting wires I'd rather do that than replace a head gasket, what worries me is that I did notice a plug was wet when I pulled it, I thought it just a bit of gas so I really didn't check into too much. What else did it do? Did it miss when you hit bumps or anything like that?
 

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any other symptoms? Backfiring? poor combustion smelling exhaust?
If not then I think you may be onto something with it being electrical. I would bet in the timing/ignition. Have you checked the rotor? Maybe the spark control thing inside the distributor(I never remember its correct name).
I thought that too, but from what I've checked everything looks normal. Dizzy and rotor look like they did the day I put them in. The best way I can think to describe the miss is like the engine is actually cutting off. No weird smells or smoke or water or anything coming from the exhaust. Smells like normal. I am so totally lost on this. I have no backfire. Other than the miss or whatever it might be, everything seems perfectly normal. Whats throwing me off is that the engine LOOKS silky smooth. No shaking or vibration or anything at all.
Is there anywhere electrical wise that you'd recommend looking?
 

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So this is miss is really more like the engine shutting off? Not like a single cylinder not firing? Correct?

If so, I think you have either(and this is a far stretch) a failing ignition switch( I saw this once on a Mercedes),Possibly a failing coil or spark modulator, or bad wiring between the coil and the distributor(not the spark rotor wire). Of course this is old school thinking. Add in the computer , which it could be, and I get lost....................

Maybe someone else can chime in. Are they thermal safety sensors on the motor to shut it down? How about oil pressure sensors? If the computer has these and senses a fail it might just do this too.

just a thought
 

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So this is miss is really more like the engine shutting off? Not like a single cylinder not firing? Correct?

If so, I think you have either(and this is a far stretch) a failing ignition switch( I saw this once on a Mercedes),Possibly a failing coil or spark modulator, or bad wiring between the coil and the distributor(not the spark rotor wire). Of course this is old school thinking. Add in the computer , which it could be, and I get lost....................

Maybe someone else can chime in. Are they thermal safety sensors on the motor to shut it down? How about oil pressure sensors? If the computer has these and senses a fail it might just do this too.

just a thought
See I'm oldschool too lol, my last truck had very little wiring and no computer at all. I actually have a video showing what the tach is doing, you can't hear the miss in the video (the miss is actually hard to hear at all) but you can dang sure feel it going down the road. I'm going to check the computer wires tomorrow and see if anything is messed up. I have checked nearly every electrical thing concerning the engine that I can think of now.

It doesn't seem to matter whether the truck is warmed up or not. I know people have suggested head gasket, and I while I can't rule that out yet I wouldn't imagine it would show up on my tachometer like it does, nor would I think it'd be affected by bumps in the road, but I very well could be wrong on that.
 

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Would you think it is more than one cylinder causing this? If not, then disconnect one cylinder spark plug wire at a time. Is it possibly just a bad connection on a plug, maybe two?

This is what I would do. I would re-examining everything. I would be trying everything systematically and in baby steps.

I build Hi-Fi, stereo vacuum tube amplifiers for a hobby. When I get to where you are, I take a break, and then make a list. I go through everything again. Normally, it is a loose wire, a bad connection. I usually find it on accident by bumping something. I would also do a compression check, but I think you may be right, it is electrical. Pull the plugs and see if any of them are excessively wet or oily. Your approach to eliminate things is how I do it too. I re-ask my self, how do I know what I have done is a good test? Is there another test that I can try for a "?"

good luck and keep us posted.
 

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I just re-read your OP.
"It shows up as a spike on my tach" I assume an RPM increase? Maybe look at this. I would think this points to the fuel system or what ever is controlling it.
What do you think?
 

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I recently sold my 99 S10, and the deal breaker (almost) was a slight miss. It didn't happen all of the time, but when it did, the truck was useless. I sort of knew what it was though...
Turned out to be 1 bad plug wire. The truck would idle smoothly and skip once in a while. I looked at the wires, they looked to be in good condition.
So, I waited until it was dark out, and ran the engine. Sure enough, I saw a spark jump on #2 cylinder - only ONE time!!. It was not audible from outside with the hood closed.
I replaced the wires and the problem went away. #2 plug wire had a tiny cut in it at the spark plug boot.


Now this going over a bump thing....
Almost all of the connections on this truck are weatherpack connectors, so they don't come loose. They also don't corrode easily.
I'm not suspicious of your ignition switch, usually the dash lights will flash (charge light or brake warning light) if it's turning off and on. It is a possibility though.
Has this ever been apart? Like engine out of truck?
Also, check you ECU connection - the big plug. If anyone has ever messed with that, it is possible that the clip holding the plug tight may have been installed incorrectly. (unlikely, but grabbing a straw)
Oh, and for the hell of it - check the connectors on your TBI injectors. If they are faulty, this could cause this problem too.
 

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Discussion Starter #11
I recently sold my 99 S10, and the deal breaker (almost) was a slight miss. It didn't happen all of the time, but when it did, the truck was useless. I sort of knew what it was though...
Turned out to be 1 bad plug wire. The truck would idle smoothly and skip once in a while. I looked at the wires, they looked to be in good condition.
So, I waited until it was dark out, and ran the engine. Sure enough, I saw a spark jump on #2 cylinder - only ONE time!!. It was not audible from outside with the hood closed.
I replaced the wires and the problem went away. #2 plug wire had a tiny cut in it at the spark plug boot.


Now this going over a bump thing....
Almost all of the connections on this truck are weatherpack connectors, so they don't come loose. They also don't corrode easily.
I'm not suspicious of your ignition switch, usually the dash lights will flash (charge light or brake warning light) if it's turning off and on. It is a possibility though.
Has this ever been apart? Like engine out of truck?
Also, check you ECU connection - the big plug. If anyone has ever messed with that, it is possible that the clip holding the plug tight may have been installed incorrectly. (unlikely, but grabbing a straw)
Oh, and for the hell of it - check the connectors on your TBI injectors. If they are faulty, this could cause this problem too.
I've checked my plug wires repeatedly actually. The last set I had arced a lot at the boots, nearly every single one of them actually, but I have since fixed that problem and it didn't seem to help in the slightest. As far as I know, the truck has never had the engine out, at least not since I've had it. I've checked the tbi connectors a lot, they also seem fine. I'm going to recheck everything though. It is nearly undriveable in its current state, and being as jerky as it is now I can't imagine thats good for wear and tear on my drive trains. I know its not good for my tires lol. I am definitely checking the ecu though, is it in the dash on the passenger side? I've never personally messed with it.
 

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I just re-read your OP.
"It shows up as a spike on my tach" I assume an RPM increase? Maybe look at this. I would think this points to the fuel system or what ever is controlling it.
What do you think?
Yeah its just spiking all over the place. I could be running at 1k rpm and it may spike to 5k rpm without an actual increase in engine rpm. I'm actually getting to the point of I don't know how to check some of these things. I'm going to go check and make sure its not like the fuel pump cutting out or anything. Is it possible an o2 sensor or maf sensor could cause any of them? As far as sensors go, I've not actually checked any of those yet.
 

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Yeah its just spiking all over the place. I could be running at 1k rpm and it may spike to 5k rpm without an actual increase in engine rpm. I'm actually getting to the point of I don't know how to check some of these things. I'm going to go check and make sure its not like the fuel pump cutting out or anything. Is it possible an o2 sensor or maf sensor could cause any of them? As far as sensors go, I've not actually checked any of those yet.
If your tach jumps to 5k but the motor does not, then I think you are seeing the problem. Something in the coil, distributor or spark control module is goofy. Essentially, what you are seeing is a signal to ground at the rate of 5k times per minute. This is like a short. Your spark would likely not be getting to the pugs then. I would look very carefully at all of the wiring. Look for grease build up, arcing marks etc. I think this is your area of concern.

Someone please correct me if I am wrong, but this has to be the problem.
 

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Check your fuel pump wires by the driver's side manifold. Could be burnt and arching when you hit bumps. Common. Probably not, because it isn't blowing fuses, but it bears looking into.

Check the wires where they enter the distributor too...they often get crunchy with age.
 

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If your tach jumps to 5k but the motor does not, then I think you are seeing the problem. Something in the coil, distributor or spark control module is goofy. Essentially, what you are seeing is a signal to ground at the rate of 5k times per minute. This is like a short. Your spark would likely not be getting to the pugs then. I would look very carefully at all of the wiring. Look for grease build up, arcing marks etc. I think this is your area of concern.

Someone please correct me if I am wrong, but this has to be the problem.
I know absolutely nothing on how to check the spark control module, I'm glad you know what that spike means because I actually wasn't sure. That might help to narrow it down a good bit in terms of where my issue may be.

Check your fuel pump wires by the driver's side manifold. Could be burnt and arching when you hit bumps. Common. Probably not, because it isn't blowing fuses, but it bears looking into.

Check the wires where they enter the distributor too...they often get crunchy with age.
I'll definitely check that. I checked the wires on the distributor though, they were stiff but not crunch, insulation still looks to be in good shape (granted looks don't mean everything)
 

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I know absolutely nothing on how to check the spark control module, I'm glad you know what that spike means because I actually wasn't sure. That might help to narrow it down a good bit in terms of where my issue may be.



I'll definitely check that. I checked the wires on the distributor though, they were stiff but not crunch, insulation still looks to be in good shape (granted looks don't mean everything)
I do not know how to check the spark control module either, short of changing it. I do know that you have an electrical problem with what you stated about the 5k spike. This might be as simple as a bad ground somewhere in your system. I would check and clean all of he wires in/on/around your distributor.

I do not know S10's well. But I do build vacuum tube stereo amps, I have most of a degree in audio/video electronics repair, I worked on Mercedes and BMW's(electronic trouble shooting tech) for awhile. I'll see if I can dig up a schematic and give it a good once over.

once again, this is computer goofiness aside.............
 

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I just thought I would take a minute and explain what the spike on your tac means. I apologize if this seems condescending, it is not mean that way. I just am too new to the board to know who knows what yet.

Almost all tachometers in cars are powered by an electric impulse that comes from the ignition system. Each pulse that the ignition system generates also is equal to a spark on the plug. Most tach are made to be used on 4,6,8, etc cylinder cars. They are calibrated to he number of cylinders. So, if an ignition system generates 6k pulses in a minute for a 6 cylinder engine, that means each cylinder is getting 1k sparks per minute. This equates to 1k rpm for the motor.

So if a tach reads 5k rpm for the same 6 cylinder motor, the ignition system is now generating five times as many "sparks to the lugs" as it was before. That is fine if the motor is actually turning that much faster. If the motor is not turning that much faster, you are now getting multiple sparks in the cylinder without need. So you will get sparks in the exhaust stroke, in the intake stroke, on the combustions stroke. In other words the engine is no longer in time in any way. The sparks are going to be way out of time and your engine will not likely run. If it were to happen only for a slit second, your engine may continue to run but it is going to be an awfull mess. This sounds very much like what you are describing.

The questions is why is it doing this. Do you have a second tachometer. One that works on an induced voltage from the spark plug wire - similar to a timing light. If so, see if you can reproduce the situation and compare the two tachs. If they jive, and your motor is not actually increasing in rpm, it is likely in your ignition system. If they do not jive, then I would be looking for another source, like the ecm
 

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I did a little more research. According to the schematic I have your tachometer is controlled directly from your distributor. I did a little reading. I think your issues is in your distributor, either the electronic spark control module, or the sensor that triggers it.

http://www.wisegeek.com/what-is-an-ignition-control-module.htm

This is a real quick read, very general, but it explains my thinking.

good luck and keep us posted
 

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I did a little more research. According to the schematic I have your tachometer is controlled directly from your distributor. I did a little reading. I think your issues is in your distributor, either the electronic spark control module, or the sensor that triggers it.

http://www.wisegeek.com/what-is-an-ignition-control-module.htm

This is a real quick read, very general, but it explains my thinking.

good luck and keep us posted
My tach is aftermarket and run off the coil wires (Specifically the white wire that also runs into the computer) It has been on there for quite a few months now (I've ruled out the connection being bad, already checked it). Also, since my tach is run off the coil and it is possible that what you said earlier about the tach picking up a short to ground or something of that nature, would it make sense that the problem is before or in the distributor?
 

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My guess is it is in the distributor. specifically the electronic spark control or the sensor that triggers it. But wait, maybe we can get another person to chime in. I am not positive of this yet. I would hate to see you throwing money out the door and not get it fixed. I did read about an "easy" test for the spark control module. I'll see if I can dig it up and post it too.

mark
 
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