Im trying to diagnose why my 89 F350 5.8 is running poorly. If you drive it under 1/2 throttle and about 1500 rpms, it runs perfect, no missing or stumbling. however, if you are under a load (or somewhat lugging in a high gear) It loses all power and the motor shakes pretty bad. Unloaded, the truck actually accelerates faster at 1/2 throttle than at full throttle. I did plugs, wires, cap, and rotor and that helped, but didnt cure my problem. I pulled codes and the only codes I got were code 33 and 29. What Should I look at? Thanks and sorry for such a long post.

 

Check for any vacuum leaks,this would cause the engine to run lean under load.

 

Sounds to me like you may have the timing off.
Did you turn the distributor at all when you did the tune up?

And if you need me to I can look up what those codes are in my book, let me know.

And the best way to check for a vacuum leak if getting a can of brake cleaner and spraying it on vacuum lines. If any gets sucked in you will be able to tell, because it will run better momentarily.
-NashT

 

I never heard of using brake cleaner ?? Sure its not carb cleaner ?? Lew

 

carb cleaner works also check fuel pressure and fuel filter

 

mine is doing the same thing...did you ever get it fixed and running right...if so how...please help

 

My F350 7.5L is acting the same way. When timing it do you also disconnect the air pump control valve( the one that is connected to the Cat ) and pull the ecu plug?

 

My truck was running similiar. Here's what I did...

New distributor - didn't help the problem.

Checked the TPS - measured fine

Replaced coil - The truck seemed to start a little better, but this didn't resolve the problem.

Replaced plug wires - Got Napa wires which was a mistake. Get ford wires. The truck ran smoother after I replaced one of the napa wires which was grounding out.

Replaced plugs - The old ones were gapped incorrectly. Replace with copper core. The truck ran a bit smoother after new plugs

seafoam treatment - I'm not sure this helped too much, but I'd do it again.

check for vaccum leaks - I had one at the "tree" on top of the manifold. Engine ran a bit smoother after that was fixed.

Fixed intake leaks - I had several leaks at the upper to lower intake connection and at the lower to head connection. I stripped everything down and put on new gaskets. While I was there I cleaned out the intake which was caked in carbon and sludge

clean the TB - Mine was caked in sludge, even after the seafoam treatment. The truck ran much better after it was cleaned.

unhooked the EGR - Bingo, this was really it for my truck. The egr was malfunctioning. When the vaccum line was disconnected from it and plugged the truck runs like it should. Well like a 200k mile engine should at least.

 

finally.....slight kink in the fuel line.....engine is running 100 % now....just need to replace the fuel selector valve because I am losing fuel from the rear tank into the front tank

 

Truck was getting rough. Replaced plugs, wires, cap, rotor, battery, fuel filter, and starter. Ran good for a short time then started getting worse and worse. Replaced the egr valve, no change. Replaced the idle air control valve, no change. Replaced the fuel pressure regulator, checked plugs, driver side were scorched, and two wires were broke. Replaced all wires and plugs, ran fine for a day. Now chugging again after only one day. It's driving me crazy, please help. I have an 1988 f150, 2 wheel drive, with a 351 engine.

 

What kind of wires did you put in it??

 

This time around I put autolite wires in. Ive also ran two cans of seafoam thru it and had walmart do a complete fuel system clean.