I have a 1988 F250 with the 351w EFI motor and automatic transmission. When I drive the truck it idles smooth and pulls very strong up to 45 miles per hour where it loses all power and appears to lean out, and backfire. This only occurs at 45 mph, once I make a complete stop it runs fine. I purchased the truck at auction and don't know anything about the history of it. I purchased it with both fuel pumps inoperable and replaced them in order to make it run. It idles fine and consistently pushes 37 psi at the rail after I replaced the fuel pumps. When stationary I can rev the engine from idle to redline with out the symptoms appearing. This problem is agravating me to no end any suggestions would be helpful.

 

Bad fuel filter?
Bad catalytic converter?

 

fuel filter was replaced at the same time as the fuel pumps. How can I check to see if the cat is bad or not?

 

drive it for a while and then get out and see if its glowing *works easiest at night*

 

ok, I will do it tonight. I just have a hard time seeing why it only does this at a certain speed, and not when idling or revved whilst stationary. is there any kind of speed sensor, or governor on these things?

 

still trying to figure this thing out, replaced a bunch of vacuum lines, checked the catalyzer, it is still duplicating the problem, it appears to be doing it hot or cold now and while stationary.

 

that does sound like a cat, you could disconect your exhoust pre cat and run it to see if it still acts up.

 

going to do that in the morning and will report back.

 

If your cat starts breaking apart inside the pieces can plug the muffler. A muffler shop can check this for you.

On edit: Did you verify fuel pressure while it was cutting out?

 

Doesnt sound like a Cat to me. Cats that are plugged or partialy plugged most of the time will give you a low power situation on acceleration all the time not just at a certian speed. I will wrap my brane around this one for a bit

 

I think it still could possibly be the Cat. I had one completely empty the fill all the way into the muffler (that I had just replaced). I only figured it out because I was trying to get to work and frustrated with the random bogging out, downshifted, floored it and actually blew the cat and rear muffler off. It was loud as hell, but it ran fine.

Try disconnecting the exhaust from the cat back as Mr Finch and 90 GT Dan suggested. If nothing else, it'll rule that out.

Did you check your O2 sensor? That's caused similar problems for me in the past ...

 

37 psi at power loss, rigged up the gauge so that I could see it while driving.

 

I was thinking that it maybe jumped time? Perhaps it is off by a tooth or something? The parts are only about $30, it may be worth doing just because.

 

My 89 F250 (351w) has a similar problem, I have found that the ignition timing isn't advancing. If I remove the timing plug and set the static timing to 30 degrees it is ok, I haven't had time to trace the reason for the lack of advance yet.