There might be a bit to unpack for this issue. There was a small engine fire a few months back (squirrel or bird nest) that was centralized right where the wiring harness sits next to the vacuum electronic controls. After replacing the wiring harness with a donor from a 1990 (the only difference seemed to be the throttle body connections) and splicing and replacing a bunch of vacuum lines (this could easily be a potential culprit) it runs again. However, when the van is in drive or reverse, under load but not moving, the engine seems to sputter or misfire. Video attached. I've also replaced the spark plug wires (as a few were fried) from 7mm wires to 8mm wires. Not sure if that would have any affect on this issue.

Is there anything specific I should check for? Could a vacuum leak or mis-routing cause this issue? I really had trouble figuring out where the vacuum lines should go as most of the fittings were melted in that area. I used the following vacuum diagram, but it didn't seem entirely correct.

Thanks for the help.

https://photos.app.goo.gl/FDHJkwtA6uuVCQzC8

 

On my 94 318 5.2l, I had to replace my oxygen sensor high and in the pipe, the Throttle position sensor (TPS), EGR, PCV etc. Crankcase Breather. and my water pump clutch fan. Of course proverbial, spark plugs, cap ,. rotor, cables, ignition coil. Helped me a lot. Replaced the dash circuit boards for the console too.

 

Thanks for the insight. As an update, I went back and looked through the vacuum lines that were rerun looking for a leak. I couldn't visually find one, but that definitely doesn't mean there isn't one. I'm not sure if it's a coincidence, but when I replaced the spark plugs today I noticed that the two cylinders that are backfiring at idle, while in drive, are 8 and 6. Those are the two cylinders closest to where the fire was. I don't think that indicates anything but I thought I'd mention it here. Both of those plugs were badly fouled, as well.

I've mainly been concentrating on vacuum lines and things in the area of the fire to diagnose, but I'm wondering if I should be pulling apart the throttle body and cleaning it out as well? I suppose it couldn't hurt.