Diagnosing Slight Engine Miss

B

BASS

Member
Joined
May 25, 2001
Ok, for some reason, my engine seemed to have developed a slight miss, in which I can notice it at idle and can feel/hear puttt putt puttering through the exhaust while cruising. The car seems to boost fine.

I was thinking its either a spark plug wire, coil pack, or spark plug. Or maybe an injector?? Or bad gas?

The wires seem to be in good shape, and all the coil pack terminals are nice and clean. I want to run the motor and disconnect each wire one by one to see exactly which wire/cylinder has the miss.

Is it ok to disconect the spark plug wires from the coil pack ends while the motor is running? Or should I only disconnect them by the boots on the spark plugs?
 
You can disconnect them while it's running from either point, but it'll probably shock the $hit out of you. Disconnect one at a time while the engine's off, then start it and see if you can tell the difference. I can tell you from experience that it's easy to tell the difference between a motor that's running on 5 cylinders versus one that's purring on all six.
 
If you want to disconnect something while the motor's running, pull the injector plugs one by one. it will at least isolate your miss, then you can figure out whether it's a fueling thing or a spark thing.
 
I could be wrong, but i disagree. Why would pulling one wire kill the other plug on the same coil? I agree that if one of the coils was bad, it would kill two plugs. However if you pull a wire off of one coil tower, it should not kill the other.
 
It is the way a waste spark system works,the plug that is not under compression,is effectively the "ground" for the other cylinder.So removing one plug wire,kills both cylinders,on that coil.
 
I still think that's incorrect, but I would love to be proven wrong so I can understand. My understanding is that there are three coils on the coil pack. When one of these coils fires, spark goes to two plugs that are fired in tandem. One plug is on the cylinder on its compression stroke, one plug is on the cylinder on its exhaust stroke (this one is the "wasted" one). Whether one of these plugs is connected to its corresponding wire will have absolutely NO bearing on the other. The plug that IS connected to its wire will then act as the ground for BOTH. Let me give you a real world example.

Not too long ago, I was diagnosing a miss after buying a new chip - I pulled one wire at a time. The wire that was not connected to the plug still sparked. I felt it all the way down to my toes - I couldn't let go of the wire fast enough!!! Compression has nothing to do with it.
 
Turbotre said:
It is the way a waste spark system works,the plug that is not under compression,is effectively the "ground" for the other cylinder.So removing one plug wire,kills both cylinders,on that coil.
Click to expand...
Wrong! 87-we2 ur right if a coil is bad it will cut two cylinders pulling a plug wire will cut that one cylinder
 
if you pull a plug wire off of a coil terminal, it should only kill one cylinder. The reason is that the spark that should have gone to the plug you disconnected now arcs internally in the coilpack, or will jump externally to one of the mounting screws. So as far as the spark plug on the other end of that coil is concerned, the circuit is still 'complete', although it is a bit compromised.

Note that doing this is not good for the coil.

With these systems, replacing parts seems to be the only way to diagnose something like this. if it is missing at idle, it may be messing up your block learn, since the unburned O2 in the exhaust will make the O2 sensor read lean.

you could use an infrared temp gun to read the header pipes and see which cylinder looks "weak".

Bob
 
I have a friend that has that same issue. He had a slight skip since he got his car. He's replaced every ignition component, ecm, injectors, chip etc. He's had it with the orginal engine and with 2 rebuilt ones (bad builds, long story). I think he has an engine wiring harness or ground issue. That's pretty much all that's left.
 
I don't recommend pulling plug wire from coil, had a loose wire arc to bolt and kill ign module, i would up plug wire from spark plug, add a spare plug to the wire and ground plug, that way you can see if it has spark
 
what cam, and did this start before or after the cam was installed?

B
 
He first mentioned skipping several years ago when the engine was all stock. He had it rebuilt with a ductile roller 206/206. That engine was machined wrong and failed in short order. Next build with a different builder also failed do to builder error (I know, tough to find a local builder here). Final build done by Nick Micale. Runs fine except for a light miss at idle to about 1500 which it's had since I've known the car. I don't think anything electrical from the orginal engine is left. Probably wouldn't hurt to have the injectors cleaned. He had replaced the fuel pump and it has a hotwire kit too. Not sure if he has a translator and LT maf or not. Feel bad for the guy, he's got the fickle finger of fate following him...
 
I found this years ago from one of our members and save in my GN special info files. I have used this to also troubleshoot a miss that I had, and I wind up replacing the the coil and ignition module. Try it and see if you find your miss
 

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Pronto said:
He first mentioned skipping several years ago when the engine was all stock. He had it rebuilt with a ductile roller 206/206. That engine was machined wrong and failed in short order. Next build with a different builder also failed do to builder error (I know, tough to find a local builder here). Final build done by Nick Micale. Runs fine except for a light miss at idle to about 1500 which it's had since I've known the car. I don't think anything electrical from the orginal engine is left. Probably wouldn't hurt to have the injectors cleaned. He had replaced the fuel pump and it has a hotwire kit too. Not sure if he has a translator and LT maf or not. Feel bad for the guy, he's got the fickle finger of fate following him...
Click to expand...

That's too bad. If he was close to me I would help him out. I love to fix these things.

Most of the cars I see with this type of problem end up being cam and/or crank sensor. FFT

RL
 
I'm the guy with plagued Gn it has new crank sensor and recent wires also had problem with old cam sensor, coil pack and module. chips etc... I recently added power logger and have a log of car idled up at 1800 rpm popping in the scan tool tech area. Basically I have had the same issue since I bought the car and have replaced almost everything.
I think I need to get it on an old school ignition scope and see what it's doing, the car seems to drive fine but I'm not sure the car has ever been 100%
 
Is the popping violent or are we talking garble?

The log you posted does not say much. Get it on the road. Log the condition in high gear while cruising.

Rick
 
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