I've been working on figuring out what's wrong with my 86 2m4 for about a month now... It starts, idles fine, revs fine in park... but once i get going, it stops accelerating around 2K RPM, and won't go any higher no matter how hard I press the pedal.
I've got a whole 'nother thread going into details on this, but research has told me that a weak ignition coil CAN cause this sort of problem. Per service manual testing, the Ignition coil checks out. However, if the coil is internally shorted, it wouldn't necessarily fail the simple, and unspecific conditions the service manual suggests. The coil may still put out enough power to fire the plugs, and run the engine, but it may not have enough power to combust the air fuel mixture under load...
It was suggested to me, that placing a plug on the engine block, and listening for a loud "SNAP" would help identify if my coil was producing sufficient power. If it snaps loudly, it's good. If it's quite, or inaudible, it's bad. So, I used a pair of welding clamps and attached a plug to the block, pulled the fuel pump fuse, and cranked the engine. Can't hear it at all. I moved it over and clamped it onto the strut tower, so I could see/hear it better while cranking, and still no sound.
I'd just like some confirmation that this is a valid indicator. I couldn't find any other means of checking the output of the coil through a few hours of google-ing.
Also... I haven't seen enough spark plugs running to have developed much of an eye for it... but it LOOKS weak to me... for what that's worth.
[This message has been edited by Twilight Fenrir (edited 01-28-2015).]