Well I'm back here posting again because I'm still dealing with a very frustrating problem that has been plaguing me for a while.

engine is a 360 TBI in a 90 pickup with a mild cam, very mild engine all around.

After starting my engine, and then accelerating, at 2,000RPM the engine pops or misses really badly, and the tach needle dives. I have tried different things, (checking the initial spark advance, etc. and I still haven't found the cause, or a solution to my problem). After it misses the first time, it WON'T do it again UNTIL I turn off the engine and restart it. I do not understand, and I'm absolutely stumped as to where to look?!?!

I have tried 3 ECM's, still having the problem. Ignition module replaced not too long ago, distributor about 1 year old. Could a sensor be causing this?? IAC??

I'm running the stock TBI fuel pump, which I know leaves a lot to be desired, but I don't think that would cause the miss I'm having would it??

Thanks

 

The popping and missing, especially at 2,000 rpm sounds like an ignition drop out. I would guess from your prior attempts at fixing it that youve alredy been down this road. Whats very odd is that it only does it once. I assume after the one episode it runs normally? What might cause this? I assume youve checked for codes. Has it ever thrown a code 42? Does the tach come back after this event and work normally thereafter?

The IAC can affect idle/off idle operation as it can affect idle fule mixture and throttle response. I dont see it causing the symptoms you describe though. Especially since your symptoms include the tach dropping off. The tach is driven straight off the coil primary. Either the engine is losing primary ignition pulse, causing th etach to drop. Or the tach is shorting out and pulling down the primary ignition pulse. One quick test might be to unplug the tach from coil harness and see of the problem doesnt subside. That would lead me to think that the tach is at fault.

If the problem is still there with the tach connected, I would look for an issue with the EST system. Either in the module, yes I know you just replaced it, the pick up coil , the EST circuit to the ECM, or more likely since the problem seems to cure itself after a single episode, the ECM itself. I wonder if after the episode the ECM isnt going into memcal fault mode. This would explain the single episode. If this is the case, I would expect to see a code 42. Just some food for thought.

 

Thanks for the informative reply ASE doc. Could you please elaborate on "ignition drop out" as I don't know alot about this really. It is extremely odd that this problem only happens once per run cycle and I still haven't figured it out. Yes you are correct is assuming that the engine runs normally after it hiccups. Yeah, I have checked for codes with a laptop and WinALDL, but I never find any codes. I think it has thrown a code 42 at sometime in the past, but as long as I've been dealing with trying to diagnose this particular problem there hasn't ever been a code thrown, which makes it all that much harder for me. Yes, the tach does come back and will act normally thereafter.

I did discover here recently that the plugs which plug into the ignition coil were broken and the engine vibration was throwing them off and I had to pull over and plug it back in. I got a plug that wasn't broken and soldered it on solidly and still have the problem. Is it possible that the ignition coil got fryed from that plug getting "tossed" a few times?? I have unplugged the tach before and it didn't seem to help, but I will try it again. Could you elaborate on the engine losing primary ignition pulse because I simply don't know about that. Electrical stuff is not my strong suit, but I'm trying...

How might I go about testing the EST system with a meter?? Again, if you could please tell me more about what you mean by the engine losing primary ignition pulse and what I can do to test for such a problem..

Thanks for all your help so far I really appreciate it.

Edit:
I unplugged the tach from the coil harness and went for a test drive, problem is still there.

 

Ignition drop out is loss of ignition signal. Basically, the ignition stops firing, or at least drops a portion of its firing events. The popping is unburned fuel igniting in the exhaust manifolds. The fact that this occurs only once per igntion cycle means that it is something that the ECM can eliminate by going into fault mode. I suspect the problem is in the EST system which includes the ign module, the EST circuit to the ECM and the ECM itself.

Remember too that your ECM only stores certain faults beyond the key cycle in which the fault occured. Once you shut the key off, the ECM may be dropping the code and shows no faults when you scan it. You may also have a faulty ECM which is not setting a fault code even though it goes into fault mode. That doesnt mean your ECM wont function okay otherwise and it may not be the cause of your problem.

Your problem is certainly going to be in your primary ignition system or EST circuit. The EST circuit includes the ECM. The primary ignition system is the portion of the ignition system which generates the pulses to fire the ignition coil. It includes the pick up coil, the ign module. The EST circuit works in conjunction with the primary ignition system so that certain failures in the EST system, such as a short to ground, will prevent the system from functioning.

Begin diagnosis by disconnecting the EST wire. Its the tan/black wire that you unplug to set timing. Unplugging this wire will put the ign module in bypass mode and it will function using a default timing map. This will also take the EST out of the equation. If the problem goes away with the EST disconnected, you have a fault in the EST circuit or a bad ECM. If the problem is still there, reconnect the EST, clear codes and move on.

Next, test the ignition coil itself. I doubt theres anything wrong with it but lets eliminate it as a suspect. Test both sides for resistance. Primary resistance should be .5-1 ohm. Secondary resistance should be 22-27K ohms. There should be no continuity between either coil and the iron core.

Once we verify that the coil is okay, lets check the pick up coil. Its a small coil above the ign module, inside the dist. It is the original source of the pulses which fire the coil and the injectors. So, its god. It connects to the ign module by way of a two pin connector at the back of the module. The wires to this connector are one white and one green. Test the coil for resistance. It should test at 800-1000 ohms. The test of the module itself is too complex to describe here. Autozone should be able to test it for you. I am especially suspicious of the ignition module and would probably replace it even if it tests okay. Be sure not to scrimp on this part. AC Delco is best.

If no problem is found up to this point then we will need to step up to drive testing which can get tricky.

Since this problem happens on the first acceleration event, be sure to perform all tests on cold parts. If all parts test okay cold, repeat test after warming engine. Careful not to injure yourself on hot parts. Let me know if you find anything.

 

First of all, thanks a ton for your help, it has been very helpful and informative. I really appreciate your willingness to explain this stuff to me.

I first began by trying the 3 total different ECM's I have just to see if I could rule that out, problem was still there regardless of the ECM.

I tried then disconnecting the EST wire to rule that out as well. Problem was still there.

About the time I was under the hood testing the ignition coil for resistance and lack of continuity was when I discovered my culprits!!

Turns out that I had a loose ground wire on the intake manifold and I also had a small vacuum leak. I tightened down the bolt for the ground and checked it for continuity at several places under the hood, seemed good to go. I also fixed the vacuum leak, the tube that connects the brake booster to the intake was loose so I tightened that up as well.

I either overlooked this stuff previously or it just happened recently. I'm just glad I got it fixed finally.

Thanks again for your help, I don't think I would have been poking around in the right areas to find my problem without your advice as to where to look! And I'm definitely keeping this thread at an arm's reach for future ignition related issues that I hopefully won't encounter.

Thanks

 

Awesome news! Happy motoring.