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Hey ya'll, I just got my first truck a week ago. I have a 1991 GMC Sierra 2500 with a recently rebuilt 454 Big block, and I do need assistance. I just need some general knowledge on the specs, capabilities, good reference sites for parts and how to work on it.

There has been a couple issues though.
It was running great around town until I went on the highway then it would not go faster than 65, oil indicator went to 0 and started to overheat (although there was still oil), black gunk comes out of the exhaust when it starts, and lastly the biggest issue is the clonking sound. When I start it, there is a big clonking sound under the motor near my transmission I believe, but it quites down when I put it into gear. I believe it is the flywheel, however I don't wanna run it for too long, don't have the expertise to know for sure nor live near a auto shop to have them look without going to them.

any advise would be much appreciated. thank yall
 
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