This one has me puzzled:
My son has a '90 RS with the 3.1 V6 and it has a hard to pinpoint oil leak on the passenger side rear of the motor, actually seems to be coming down from above the starter. The obvious oil pan and rear main culprits have been examined and are not the culprits, nor does it appear to be a valve cover leak. There's so much ignition and emissions crap hanging on the right rear of the motor that its nearly impossible to see.
The only remaining things I could imagine it being are either the distributor (appears to be a major pain to get to) or the back of the intake manifold.
Are there any chronic leak places on these motors that might explain this one?
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I'm not very familiar with the 3.1, but AFAIK it's a cousin of the 3.4/2.8. I would check the manifold (its the two piece with the plenum, right?). I know LT1s love to leak from the back of the manifold. If you ruled out everything else back there, what else could it be?