Hey guys!

I have a 1989 F-250 4x4 with a manual transmission (zf5) with about 92k miles on it currently. Really clean truck that I bought off of grandpa's kid a few months ago. I have put about 4000 miles on it without problems and now I am getting some interesting issues. Initially there was a loss of power I noticed when going up the mountain passes to ski. I threw some Lucas fuel additive in there a couple times to no avail. However, I improved this situation when I changed the fuel filter but I believe there is still some power and fuel delivery issues.

Upon some visual inspection, I noticed that there is hairline crack on my exhaust manifold on the drivers side of the engine. It is small and tight and I don't hear any hissing or terrible sounding exhaust leaks. I think this fracture has been there for a while and I didn't notice it when I bought the truck. I plan on fixing this but further down the line as I was told it is not urgent by a fellow truck guy? - Is this true? I plan on replacing the exhaust manifolds (probably both of them since they both look pretty old). Anything else I should watch out for or get done while I am down there?

The most recent and concerning issue is when I am driving - usually in 3rd gear - and I let off the throttle to "coast" but still IN gear, the truck does this jumping and jolting and doesn't drive smoothly. The tachometer is moving up and down around the 2k rpm mark and the problem will go away when I re-engage the throttle or depress the clutch so that the engine no longer has any load on it. This issue gets worse as the fuel levels decrease in the truck. For example - when I was running close to empty, the issue was much worse, but when I topped off with fuel, the jolting and jumping would not occur as frequently or as dramatically. No CEL is coming on.

I really care about this truck and I want to make it last as long as I can. It is in great condition for a 35 year old truck and I would like to keep it that way. This is also my first post on FTE and I am really stoked to be a part of the community. I would love to know if I am on the right page with my diagnostics and direction of thinking. I don't really have many personal resources to reach out to and have always tried to work on my own trucks and cars in the past. Thanks in advance for all your help!

P.S. - I have a new FPR, HP fuel pump, and fuel pressure gauge, OBD1 scanner and Haynes Service Manual on the way that should arrive soon to continue diagnostics.

 

First thing to do is to check for trouble codes. You said you have a scanner coming in the mail, but you can also do it with a paperclip. Here's how. https://www.ford-trucks.com/forums/1...aper-clip.html

Next thing I would do is check fuel pressure. I see that you already replaced some parts, but that doesn't mean they're within spec. The important test is to check the fuel pressure while the engine is making power, so either by power-braking or accelerating on the street. Here's how... https://www.ford-trucks.com/forums/1...rd-trucks.html