Slight occasional miss, first actual misfire today

[Doober]

That makes me feel better, I was thinking it was oil. I've had a few hits of detonation trying to dial in my timing with the computer, could that cause something like this? Within the distributor's limits I can program it to basically any amount of advance I want, referencing the MAP reading and RPM.

That's not the current table, just an example of the flexibility. Low KPa is high vacuum, high KPa is low vacuum.

 

[Doober]

So I checked both the plug and the wire, and their resistance is right around what I've read they should be (plug is about 4.5-5k ohms, wire is about 1.5k, checked both against another plug/wire). Funny thing is I picked up two plugs to replace the ones that had stuff on them, and one of them read about 45k... I double and triple checked to be sure I wasn't wrong on the reading.

What else may be causing this? Pickup coil? Module? Coil? I just started it and held it steady as I could near 1800rpm and it didn't miss, but I'd also just cleaned off the plug. 99% of the time it misses between 1700-2000rpm.

 

[95cardinal]

I had an HEI module that would occassionally misfire when it got really hot.
It would test "OK" on the tester, but after running several consecutive tests it checked "bad".

I thought they were either good or bad, but that one was flaky...

 

[t5montecarlo]

It may be a point to consider regarding the rpm that you are experiencing the miss. The surface chart you included begins a ramp up at 1800 rpm. What is the 3rd axis? I see rpm, pressure, and ??

 

[Doober]

The vertical axis is degrees advance. My table looks nothing like that now though. I suppose I could pull timing back and see if it makes a difference, but the fact is still that it was only on a plug or two. If the timing doesn't make a difference I may try a junkyard coil and see what that does. I don't think it makes a difference but the coil is mounted on the firewall sideways... it's a stock style one though, not oil filled.

 

[Doober]

I haven't really messed with this lately as I picked up the Fiero and have been driving that a lot more to help save on gas... I bought the distributor off eBay when I built the engine, but never disassembled it... overall it looked to be in good shape. Any chance the ICM getting hot could possibly cause this?

 

[ravenmetals]

I had a similar issue with my el camino. Would be driving and around 2000 rpm I would once in a blue moon get a little stumble in rpm. After maybe 6 months I took the car on a 5 hour trip to visit my dad and got a really good backfire(woke me). And after that the car would miss more often around the 2000 rpm mark. I tried to fix it before I drove back home from dad's and out of habit he told be to look at the ignition module. Sure enough no issues but I had a new one in the glove box so I just used it. Never had an issue again. I mean they are cheep and prone to failure.

 

[Doober]

I kinda figured that may be what was happening... I think it may be getting hot. The distributor is a unit I got from eBay, and I just dropped it in. I may try to find a used performance one or one from a junkyard... or just put some new heatsink compound on this one if I can find some.