Had a strange ignition issue over the weekend. NO SES codes. Cruising below 70 MPH, no issues. Above 70, the engine would stumble about every 20-30 seconds for maybe 0.2 second and would recover with no issues.

When I was coming down the mountain, the car would coast to 75-80. With no throttle input, the tach would drop anywhere from 200 rpm to 500 rpm for the split second, and would then go back to the correct RPM. Being that the converter was locked and no throttle input, the actual engine rpms should not have changed. What would cause the tach to drop?

I'm thinking the engine would stumble b/c if the PCM detected the RPMs dropped, it would pull injector timing as if the engine were closer to idle.

So where does the actual RPM measurement? The opti is only about 10K miles old, as are the wires and ignition module.

No issues at WOT, High RPM that I can find. Only cruising 70+.

 

Had a similar problem once. Just cruising along, everything was fine. Give it some gas at around 65mph or higher, and there was a slight miss. Turned out it was a bad plug wire. SES never came on, but when I plugged my scan tool in, I could see the misfire counts. It may be something that simple.

 

I would seriously doubt it's a plug wire. A bad wire shouldn't cause the tach to drop suddenly.

I'm wondering if something in the opti spark is going bad, but I would tend to think an opti issue would be more rpm based rather than cruising/non cruising