Oldsmobile/Buick Repair: Misfire but maybe not, mass air sensor, olds cutlass ciera

Question
First of all I would like to thank you for your time, I own a 1985 Olds Cutlass Ciera 3.8L 2 door, with 85,000 miles and the 4 speed auto. I have a misfire, it happend as i was on the highway (in overdrive) a very bad vibration under engine load. So i started with the basics, checked for spark i have that, next fuel, i have great fuel pressure and idle and on the road. My brother is a GM tech. and he used a Tech 1 Scan tool to see if the TCC clutch was engaging at the improper time but that was not a problem. The Misfire only happens when the engine is warm and out of open loop. The car has overheated 2 months prior to this problem. The heater hose blew and ran out of water so it had to have gotten extreamly hot when it overheated amazingly still ran. I have replaced all hoses, radiator water pump due to the overheating. In the case of the misfire i have replace the pick up coil, Capasitor, plugs for the second time, TPS sensor, I work at an auto parts store so i replaced the Mass air Sensor and the problem was still there. But one odd thing happend, with the Tech 1 Scan Tool plugged it while driving the problem was extreamly unnoticeable until in overdrive, unplugged the problem occurs in all gears. Well thats about it, If you could advise me what to do, my GM brother is at a loss, i might think head gasket due the the fact that it leaks water but i beleve that the intake is to blame. could a head Gasket cause that, but go away mostly with a scan tool plugged in? i dont think so but anyway thank you for your time in reading my question. Forgot To say i have also Ohlm'ed the injectors and they are within specs.

 Billy Dethrow

Answer
Hello Billy,
Is that an 85 or a 95?
For the problem to go away with a tech 1 plugged in would, in my opinion, eliminate head gasket, vacuum leaks, and most mechanical problems that I can think of.
I believe you have a bad ground, probably on the ECM.

Now, I have a 95 Cutlass Ciera, and it had a terrible shake and shudder in overdrive, but almost went away in other gears, and it was a broken spring on the intake valve of one cylinder. It can be replaced without removing the head.
Found during a compression test.
That cylinder got up to the same compression as the others, but was somewhat eratic during the test....would drop pressure, then jump back up.

But at the GM Training center, several years ago, the instructor told us about the tool connected to the ALDL connector completing a bad ground, and I believe he said was the ECM. See if that rings a bell with the brother. Have him ask at the dealership.

Van