Alright, i would appreciate any help anybody could give me. I am going to get very precise here. I just recently purchased a 1988 D-150. When i first got it it would not go over 55 mph. I replaced plugs, distributor, hall effect switch, air pump, diverter, hollowed out the cat converter, put a new o2 sensor in, new ignition coil, new cap, new rotor, new coolant temp switch, new air filter, tps, thermostat,Egr checks out good. map checks out good. I adjusted the timing to specs. now here we go.
The truck idles great. I put it into gear and it surges a few times when cold and then i step on the accelerator and it hesitates spits and sputters. If i floor it it will catch and take off like a raped ape. However a gradual increase in accelerator pressure and it spits and sputters UNTIL its over about 20 mph and then it runs fine. Oh yeah, i put a new TPS switch in as well. It still hesitates a bit and bogs a little bit, like its shifting into gears too early? Not sure. Anyway, the throttle body is sparkling clean, almost too clean. idling it reacts fine. no apparent vacuum leaks. The motor doesn't change rpms if i spray carb cleaner around to check for leaks. If the truck gets super warm, and i mean we are talking revving the **** out of it, etc, it runs great. even low end, everything runs fine. Its happened twice. We revved it while working on it numerous times to check stuff and clear it out and we thought the problem was fixed because it ran great. Then the next day everything is back to normal. No codes. The new o2 fixed the code that stated it was running too rich. No other codes. I'm at a loss here and am losing faith as i've already dumped more money than expected into this. Will dump more if somebody has any ideas. Thanks for your help. email is polishwarrior69@hotmail.com or just post to this thread. Thanks again.

 

Update for anyone trying to help me out. Also, the fuel pressure regulator tested out fine so far as i can tell. However, the vacuum line going from the throttle body or manifold into the regulator has no vacuum, and i don't know if its suppose to or not, or maybe only at a certain rpm. Hope this added information helps out some.

 

Fuel pressure regulator should have manifold vacuum full time. Unregulated (high) fuel pressure will cause an overrich A/F mixture, which would definitely explain your low RPM driveability issue.

 

CK THE VSS... CVEHICLE SPEED SENSOR AT THE TRANNY END OF YOUR SPEEDOMETER CABLE ....

 

With no vacuum at the port for the fuel pressure regulator would that indicate a problem with the TBI?