Volvo: Intermittent Engine Trouble, mass air meter, volvo experience

Question
I have a 1986 240DL wagon, and the engine has been failing intermittently since an ill-fated trip to the desert about two years ago. Since then I've been to two mechanics (neither of whom, it must be said, had much volvo experience) and lent the car for a while to a friend who's a 240 hobbyist, and nobody's been able to diagnose the problem. Most of the time the engine is fine, but seemingly at random, once it's been running for a while, it will suddenly start running very rough and will not go over 2krpm. Stopping and restarting the car will occasionally fix the problem, but not usually.

The fuel pump and O2 sensor have been replaced, the timing has been messed with (it's currently slightly advanced, which seems to keep the engine from stalling when this problem occurs), and tho the problem is not quite as frequent anymore, it still happens enough that I can't take the car on the freeway. Due to the apparent randomness of the failures, and the fact that the turn signals also fail intermittently, I suspect an electrical problem is at the root of this. However, my friend says he's been over the entire electrical system (and replaced much of it), so if there is indeed an electrical problem, it's apparently non-obvious.

Any ideas on how to begin tracking this down? Are you aware of any known issues with the 240s that might shorten my search, or am I just going to have to drive the car around 'til it fails, and then start checking circuits? Or is there some non-electrical skulduggery that could be happening here? Any thoughts are much appreciated.

Answer
The engine symptom you described sounds like it somehow starts running very rich at random (i.e., more fuel than air in the mix ratio).  Do you ever notice the smell of gas or fumes coming out the back when the car starts to run rough?

Assuming it does have a problem like that, I would start looking for problems to cause it.  The first place to look is the mass air meter.  If a random electrical issue affects this device (i.e., shorts it sometimes), it will default the emissions computer to a rich setting, which would cause the symtoms you mentioned.

Unfortunetly, because you have an 86, you could be the victim of engine wire harness decay, a problem until 87, where the wires under the hood would begin to deteriorate and short random things out.  I know in my 80, I had to buy a complete harness replacement because the decay was so bad, but it fixed all my problems which I would have never been able to track down.

So check the mass air meter, and that might start to point you in the right direction.  Other than that, short of replacement, going over all electrical wires under the hood with a fine toothed comb is what is required to weed out all possible shorts (beware, most wires are wrapped in protective sheeths, so its hard to get to everything).

As an additional resource, I suggest you post your question on www.brickboard.com, where more people who own and love 240s can give you some different feedback (they might think of something else easy that I left out).  Good luck!