I have a 1990 240 DL and it is not running right. When I check the temp of the exhaust manifolds cylinders 2 and 3 are half the temp of 1 and 4. I replaced the fuel injectors, plugs, wires and MAF. but no change. Checked all the vacuum lines as well. Get an error code of 3-2-2 (Mass Air Flow sensor burn-off signal missing or faulty) Checked the MAF wiring with a multimeter and everything checks out good from the harness. Check engine light is NOT on. Any ideas?

 

TA,
I just aquired a 1981 245 DL with a simular problem of running rough.
With the engine running one at a time I unplugged the spark plug wires at the spark plug, looked and listened for a change in idle, replugged and moved to the next. If there is no change in idle that cylinder isn't firing.
I found one that wasn't and replaced the plug wires. Smooth idle now.

I'm sure you've done this as I have.

(Edit) Re-reading your post you're further along than what I offered, post updates if you have them.