Hey All,
We purchased a '91 F-250 that had been previously been used as a ranch truck. We got the truck running initially, but the alternator pulley backed out while driving and it overheated. The short of it, we had a long block rebuild done.

Finally got the motor back in, with much of the smog lines deleted. We just recently replaced all of the vacuum lines with new ones. The question here though, is the idle is super rough, almost like a misfire. I've cycled through each plug wire to see if I could find the culprit cylinder, but no Joy. It starts fine, but after a few seconds it starts rocking, this almost worse after revving up to about 2k and letting off. Seems like it was better before changing the vacuum line.

Any Ideas?

 

Is it fuel injected or carb? Is there still an EGR valve and if so try unplugging it and see what happens. Have you checked to see how much vacuum you have? Should be 20 at idle. If it has a carb and the EGR valve is on a plate under and behind the carb, check the under side of the plate, they were known for burning out between the intake passages causing a vac leak

 

Hi RP67F100, Yeah did most everything you mentioned here... EFI model. We dove back into over the weekend with an ODBI tester. When it prompted to bump the throttle, got blow out from the intake. After tearing it back down, I've got a ground lifter.
Got to say, the machine shop I took it to, 1.) put the wrong bolt in for the oil pickup tube and had to wait a week for them to find it, 2) left a broken intake manifold bolt in the head. Now this.

Thanks for your response. I have a determination to get this thing running, hopefully I can post some good news soon!

 

i would find out what cam the rebuilder used, esp since ford has 2 firing orders for a small block, the cam grinder should be able to give the specs .

 

very interesting