400E engine misfires
 

I was driving home from the gas station this morning when my solid Idle went flabby and it sounded like a bank of cylinders went out. No power whatsoever and a lawnmower like engine note. I've not torn into it yet. Any insights?

Has the wiring harness been replaced? That's my guess.

You essentially have two separate ignition systems post ignition module, each firing 4 cylinders. You can lose the coil, coil wire (unlikely to have complete failure), cap and /or rotor on one side and still have an engine that runs but with very low power like you describe. Additionally you can have a failure in the ignition module that can fail to fire one side.

I'd start by pulling either side coil wire from the distributor cap and checking for spark. Spark on both sides means check the caps and rotors. Spark missing from one side means check the coils. They are a PITA to get to but you could swap them and see if the problem follows the coil. If the same side fails to make spark with a known good coil then the module is suspect.

Wiring harness has been replaced, got the paperwork for it. So yes, Dux, that was the idea that I had... Now for the fun part.

M119 engine: Check for spark for each cylinder on both banks. Best to remove the plugs I think. If there are 4 cylinders without spark.... yes then you need to find the reason: coil or distributor.
There are 2 distributors, one at each cylinder bank bolted to the cam shaft ends- quite clever.
2 coils also... Each distributor provides spark for 4 cylinders according the firing order (each distributor provides spark for 4 cylinders but not for the bank but both sides.

Martin