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I am looking at the 87, 7.3 with 5-speed 2 wheel drive, guy wants $600 for it. it runs, but I am not real familiar with these engines. Right off of idle until about 1500 rpm the engine shakes the truck, not real bad but very noticeable. Smooths up by 2000 rpm and its making about 50 psi oil pressure. Doesn't smoke or make any other odd noises.
I have a 87 International medium duty with the non turbo IDI with low miles, my 6.9 seams a lot more responsive on the throttle compared to the 7.3
You may get more help in the correct forum, this is the DI powerstroke forum, you have an IDI. But, there may be others in this forum that can help you too.
I'll take a Wild Azz Guess and say it's probably the carrier bearing (center) in the drive shaft.
Here's the IDI forum: https://www.ford-trucks.com/forums/forum117/
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