So the 525 is in the garage and has been for the entire week. Something is SERIOUSLY wrong. Or I'm imagining it.

But here's the gist. There is a rumbling and shaking of the engine so bad that it shakes the ENTIRE Car at idle. This shaking continues up until 1000 rpm where you can barely feel it and above 2000 its smooth. I've gone through all the sensors and injection systems.

The only one that comes back out of test range is the Crank position sensor. Its 400 ohms and should be 1280. It sounds like I have a rod knock but its at the front of the engine. Its also causing the car to be a pain to start cold. I've got to pump the pedal. But starting Hot is a breeze. Both engine mounts are new. Crossmember mounts are new. Putting in the clutch makes no difference.

I'm leaving it at one of my garages and taking something else home. I changed all the Vacuum lines and cleaned the intake, idle air, and MAF. I also cleaned the CPS because it had a good layer of Gunk on it.

I'll guess some too....

Air intake boot, from air filter box to the throttle body: any hairline cracks between the bellows...? You can seal'em temporarily with silicone.... new one costs $25?

Coil, rotor, distributor cap? All easy to replace.... I'd do the rotor and cap first... cheaper.

Check the air mass meter potentiometer.... erratic change of resistance means getting a new one... $350 ouch! But this solved some really weird problems in my 89 525, like real lurching and dying out intermittently at stop lights.

That crank sensor seems to bear out investigation for sure.... I have never checked mine, but it was replaced by a shop about two years ago. The cable had come loose from its tiebacks and had been partially cut through by the fan pulley... result was a car that would not start.

As I said, Im just guessing... let us know what you find!

I have an M50 engine...no rotor or cap...coil on plugs.

Air boot is in good shape no cracks to be seen.

The mass air sensor checks out. The Resistance is within limits.

This might sound stupid but check your secondary fan. It sounds like the same thing mine is doing and my fan is trashed. when it is running I have no power at take off and it shakes the whole car. Scared the crap out of me the first time it happened, I had just finished my clutch change out. Anyway just a thought..

Check your compression. If a couple have no compression, will run real rough until rpm's up. Happened to me on a 4 banner Malibu I had for a week or two.