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This may be a little long but info may be valuable to the diagnose.
Basically after running for a few hours on the highway I will get a random miss.
It may do it every 5 or 10 sec and it may go on for 2, 5, 10 minutes then go away.
I've had it come and go two or three times during a 19 hour trip from Pennsylvania to Florida. It usually takes two or more hours of running for the problem to arise.
Now this is not a new problem it has been going on for close to 8 months and I have almost replaced every electrical ignition part I can find.
The harness is about 6 years old. What I have replaced in the last year trying to find or should I say fix the problem.
NEW: Main Fuel pump, cap, rotor, wires, plugs, AMM, O2, air box thermostat, power stage, fuel relay, radio suppression relay, coolant sensor, new fuel injectors, coil was replaced with an old one I had laying around. There was no change, put newer one back in (50K). I also installed a new air filter and fuel filter with in the month.
Now ideas of what is left, Hall sensor? New distributor about 60K miles ago, FPR has about 100K miles on it. Lastly my Tech is leaning towards a weak valve spring? I replaced the motor 38K miles ago with a new re-manufactured Factory unit. It has been a wonderful engine with zero problems until this miss popped up about 7,500 miles ago.
Some more info that may help diagnose, it does have a temperature variable.
I replaced a bad coolant sensor back in January and had thought that solved the problem. Since replacing the sensor I never had the miss occur until this week when the temperature headed back into the 70's. Also if it is missing and I just nudge the throttle open just a little bit more the miss goes away. Just as if the miss is occurring at say 70 mph and I drop back to 65 mph it will go away for a few minutes. Then after a minute or two it will start up at 65 mph. Speed has no effect other than put off the miss for a minute or so. Again it comes and goes with no pattern or reason. I cleaned the throttle body last week and it had no effect.
I took it out on Tuesday night put it in 3rd gear and set the cruise on 70 mph (4200 rpms) to see if I could bring the miss on. Well it didn't take long for it to rear it's ugly head. About 10 miles in to the run was all it took for the miss to start. Usually if I had the car in OD at seventy miles per hour and I was turning 2500 rpms it would take a good two hours for it to start.
Once I shifted the car into 4th and engaged the OD, dropping my speed to 55 MPH at 2000 rpms the miss vanished for about a minute or two. Then it started back up again on the return trip back towards home. I got off the exit ramp and the miss stopped all together. Around town with stop and go traffic the miss never comes about. When it happens you can feel it and see it on the tachometer. The tach will drop about 200 rpms momentarily when you are at speed on the highway.

Thanks in advance
Al

1987 745 Turbo 321K miles with 38K miles on new motor run on Mobil 1 from first start.
 

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The 2 things Im am thinking of would be 1.The engine speed sensor. 2.The engine control module.
 

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Discussion Starter · #3 ·
Well after replacing every known electrical and fuel part that controls any part of the engine I finally found the culprit. I had a slowly dying ECM unit. Just on a hunch my Indy tech plugged in a borrowed known good ECM unit and I took the car out for a late night two hour drive last night. The miss never occurred last night or this morning. FYI it was getting worse over the last few months. It was starting up with as little of 30 minutes of driving.

I hit the internet and found a replacement 30 minute drive away for only $35.

Best part while over at my buddies last night a customer of his popped in that has a 1986 744 M46 with 25,000 miles on it and made a comment that his car has been doing the same thing for over a year and he can't find the problem. Well I guess this solution has solved two Volvo owners' problems.
 
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