General mis-firing question regarding wires

safecracker · 04-26-2005, 05:22 PM

I have noticed on this forum that other people have had problems with rough running but not at high rpm. Some responses have been about a possible wire problem. This is my question, if there was a spark plug wire problem (loose or broken) why is it possible for a car to run good at high rpm, but not in the low range. Can anyone explain why high rpm would not be affected by this same problem? Thanks in advance.

Early_S_Man · 04-26-2005, 06:43 PM

If you have ever seen a defective set of spark plug wires in action at night or in a dark garage ... it is very much a case of random arcing , just like lightning bolts in the real world. At idle or part throttle ... the output power of an engine is significantly reduced by just a few lost sparks, and fouling of the plugs is a serious risk. The miss may be fully audible, or just be noticeable by a vibration or 'stutter' at certain rpms.

On a full-power run ... fouled plugs may clear up, or in a worst case scenario, the power lost to one or two partly-fouled plugs could still be made up [or covered up] by the output from the remaining good cylinders under full power ... and the 'miss' may seem to improve or disappear.

gsxrken · gsxrken's Garage

I'd hazard that it's not really better at high RPMs, but that it is more masked with all the rest of everything going on at high RPM. Changing my wires solved the reverse problem you describe... my car would occasionally break up and starting missing BADLY as RPMs increased, and then just as suddenly, clear up for awhile. Cap/rotor/plugs = no change, but after I installed Magnacore wires, it has never happened again. I suspect the stock 18 year old beru connectors were intermittant in terms of how much resistance or continuity they provided as they heated up.

strupgolf · 04-26-2005, 07:48 PM

My 86 3.2 has what appears to be very good looking wires. I cant see any breaks etc. If I run the engine in the dark and rev it up, if I dont see any arcing or sparks, does that mean the wires are ok? Thanks

911pcars · 04-26-2005, 08:55 PM

The voltage requirement to fire spark plugs increases when the engine is under a load, usually at lower rpm (as in acceleration). Thus, the spark is more apt to find a short cut under these conditions.

Sherwood

safecracker · 04-27-2005, 01:42 PM

Thank you for all of the responses.

patkeefe · 04-27-2005, 03:18 PM

Warren:
This was also my problem. After troubleshooting the ignition system and finding it OK, I replaced the plug wires. Car ran much better. Leaned out the mix...big improvement. I believe I had the cover-up scenario you aptly described above pre-existant to my repair. Thanks for your help.

Safecracker, assuming you're speaking of your turbo engine, introduces another set of variables into the idle problems, namely the turbo system. You could be running on three cylinders and still produce more power than my weakling SC.
Pat