1999 V10 cyclical misfire/rough running during long drives

[Shellkamp]

I'm not sure how else to describe this, but it runs and drives fine for about an hour and a half straight, then it'll start having what feel like misfires or loss of spark to the point it won't accelerate if your foot is on the gas, which is unlike a normal misfire that I'm used to. It almost feels like it's going to stall out but the RPMs generally don't get that low during the event.

The frequency increases as time goes on. So when I first notice it it might run poorly for 5 seconds, then be fine for a few minutes, then it does it every minute, then maybe 45 seconds before I get it parked.

If I let it sit for an hour or two, it runs fine again for the duration I need it to (usually only 20-30 minutes to get back home at that point.

Voltage at battery and engine grounds is consistent when it's running normally vs when it acts up. I'm starting to think the PCM might be getting "heat soaked"

Plugs were replaced 10k miles ago, coils and wires maybe 2 years ago. Truck has 419K miles and engine was rebuilt at about 350k. PCM and wiring harness are original.

Anybody have similar experiences?

 

[Kansas_Farmer]

I wonder if something 'electronic' is getting hot, then failing, when it cools, it works again. Like you said, heat soaked. If that's the case, something on a printed circuit board is expanding when hot, and that disconnects the trace you need to make whatever circuit effected work.
Only way I know of to fix that, replace the offending component.
Maybe a bad solder joint someplace that would do the same thing...

Hope this helps

Brendon~

 

[Rlaf75]

Hook a good scan tool up to it and see if there are any codes. Use the good scan tool to get live misfire data to see if it actually is a misfire.