I have a 1991 F250 4x4 with the 5.8L EFI and with the 5 speed Manual Trans.

I Was towing a boat home recently and notices while I’m in 5th gear cruising at 65 at 2000 RPM to 2500 RPM when I came to a hill and gave it throttle is seemed to stutter almost a miss or a hesitation so I down shift to 4th gear and gave it a bit more gas again it stutters and that is what made me concerned. When I got home and took the boat off the truck I took it down the street again and full throttle it seems the timing is off.

Also after warm up the truck idles at 1500 RPM until I rev it a few time then it slowly fall back to normal 800 RPM. (Throttle seems to get stuck but dose not)

I have replaced TPS (throttle position sensor), cleaned out throttle body, plates, Idle air control valve and Map sensor.

I borrowed my shops smoke machine and smoked the intake sealing up intake tubes and what seems to be charcoal canister. I have found leaks in EGR (not sure if that is normal) and the throttle plate’s shaft and where the TPS is.

My main concern is the lack of power under acceleration throughout the gears getting onto the highway.

Also when I first start up the truck and drive it down the road it acts fine and normal. But after a little bit, the truck dose its hesitation act and sounds like it is having an asthma attack struggling to get up to highway speeds.

At work I normally work and diagnose OBD 2 cars and alike and I bought this truck to learn about earlier stuff and to use it as a tow vehicle. So bare with me, I’m not used to working on 80s early 90s vehicles but I’m willing to learn!

So anything in response appreciated in advance!
Thanks!

 

clogged cat maybe?

 

My 88 F-250 had a hesitation problem with a non-Motorcraft ignition module. In fact, with several brands of ignition modules. Once I put a Motorcraft module on - no more problems.

 

so i was talkign to guy with a mustang (i know, not the same vehicle.) but he said he was having the same issue with his car. untill he replaced the Idle air control.(not jsut clean it liek i did.) That fixed his high idle at stop and it cleared up some studdering/misfiring while getting on the highway.

He has a standard 2003 mustang with the 6cyl in it. but has an idle air control.

im not too sure about it but shoudl i repalce the idle air control valve?

 

I have an 88 with a hesitation issue also, it runs and idles just fine but when driving and I get on the gas for more power it just stumbles, not bucking or anything like that, it just hesitates and misfires. I have Motorcraft plugs and wires and routed them per the ford tech bulletin.
Anyway of testing the ignition module? Or is it a shot in the dark? Are they expensive?

 

Check fuel psi. When you go from cruise or idle into putting your foot down hard on the gas, which equates into WOT, you remove vacuum from the FPR.Then the fuel psi is suppose to increase around 5-10 psi. If your don't have the extra psi, this can cause your bucking, missing, hesitation. Since the PCM knows from the TPS that all sensors should be seeing WOT from input, then using preset values, it expects higher fuel psi. The PCM uses open loop mode for the time it sees WOT.

Not saying your fuel psi isn't up to par, but as stated earlier, a clogged exhaust can create same problem and a bad PIP can give similar symptoms.

 

Thanks, I will have to check the fuel pressure. My exhaust is all new without cats so that's not the problem. Now that u mention wot, I ran a test with engine running with one of those code readers for the old fords (can't recall the name) anyway part of the test tells you to fully open the throttle, I got an error indicating operator error on that test. (I think I know how to floor it). That may be telling something about the fuel pressure. Hopefully Haynes manual explains how to check this. This weather is not helping, maybe I need to build barn to put this thing in. (Mirrors too wide and truck Is too long for garage)