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Engine won't run when warm

I have a 1990 Volvo 740 that is in great condition except for about one year now, the car will not start (engine won’t turn over) if I’ve driven the car around a bit within last 3-4 hours. Otherwise, when the engine is cool, it starts perfectly. I’ve taken it to the mechanic and everything checks out fine; to try to diagnose further, mechanic said he’d have to start taking apart the engine itself, many pieces, much time, lots of money …

You need to check and make sure that the ignition is working and fuel is getting to the engine when this trouble happens. Try spraying some starter fluid into the intake to see if the engine will run on it. If those things are good then you need to check compression and the timing. I would guess this trouble is due to an ignition problem.

I suggest a different mechanic, preferably one with Volvo experience. I seriously doubt if it is an internal engine problem.

BTW what do YOU mean when you say the engine will not turn over? For some it means the starter does not move the engine at all and for others it simply means the engine may or may not move but that it simply will not start.

This problem is intermittent, never happens when the car is “cold” - it starts right up every morning. When there is a problem this is what happens: I turn the key, you get all the normal lights and sounds but it won’t “catch” the combustion and resulting roar and running of the engine doesn’t occur. If I give it gas, that doesn’t make a difference. But sometimes, what has worked, is when I turn the key and if it doesn’t immediately catch, I let it go back, then turn the key again, maybe do this 4 or 5 times and each time it begins to catch a little more then finally the engine turns over (I don’t know if that is the right term) and everything is fine. If I try this trick (just turning key briefly, not holding it there) and it doesn’t start to work - if it doesn’t sound like it might almost make it on the first one or two tries - then it never reaches the point where it does "turn over."
By the way, I think by now you will understand that I am not a car person, but I do appreciate basic scientific principles, so if anyone can provide insight as to what might be happening, I would be quite thankful.
Also, my mechanic is a Volvo master mechanic.

Yeah, I’ve got the same deal with my 1985 760 GLE (barely 100,000 miles). If I turn it off while warm and then start it up again immediately or within the next 3-4 minutes, it starts fine. But if I try it after that short time has expired, it always has significant trouble. It takes 2 hours at least for it to start fine again. It will always start eventually when warm, but does take a little convincing. Did your condition worsen from what I’m describing, or did it just all of a sudden not start at all when warm?