1983 300SD low compression #3? Next steps?
 

1983 300SD, All cylinders fine escept #3. Cold compression about 80. Warm compression about 200. Car only has 106k on it.

The motor shakes at idle, and nails loudly, but smooths right out at 1,200 rpm perfectly.

I switched out 3 known good injectors on that spot, and no change. (well, of course it wouldn't change as I show low compression).

Would a leakdown test show me what I got before I take the head off and look? How much for a shop to do that?

AGAIN, the craziest part is that it has all the power during normal driving, just that bucking at idle due to that compression. I'm NOT thinking this is fuel related due to the compression (but I WOULD if compression was ok).

Ideas?