I am in need of some assistance here. I have a 1990 F-250 4x4 with the EFI 4.9L. I recently bought it, and replaced the fuel filter, O2 sensor. Ran it about two weeks, started having problems with cold starting (It's 0 pretty regularly here) so I pulled the codes, and replaced the IAT and ECT temp sensors, as indicated by the codes. Also replaced the MAP sensor, and did a complete tune-up (plugs, wires, cap, rotor, coil). Now, the truck has no power!!! I can get it to accelerate, but it just hits a wall at around 1800 rpm, and just is gutless above that. It will rev just fine at idle, and in the low gears (manual tranny) the problem is so bad, but trying to get it above 45 mph is almost impossible, it just bucks and hesitates, also under some light throttle conditions. I tested the fuel pressure at the rail and it is within specs on both tanks/fuel pumps and the regulator checks out. The only code I now have is 41, which correlates to "O2 sensor indicates lean" I am leaning towards replacing the fuel injectors, but am also concerned that I could be chasing a bad engine management computer or ignition module. Anyone have any experience with this sort of problem??

 

It sounds like a bad sensor common lead. With that many codes at once, it could only be that or a bad computer. Check the sensor common wiring according to a shop manual. It should be around 5v, I believe. Also, check the ECM ground. This is on the battery negative terminal, the small wire that comes off there. If this is unplugged, or has any corrosion, it could be your problem.

Jared

 

I had a similar problem with my truck after a tune up. Turned out to be a bad spark plug. It would sit and idle fine, and ran well at very light throttle, but under any load it wouldn't do anything. If you raise the hood in a dark place you should be able to see spark leaking out of a plug wire boot if this is the problem. Spraying water on the plugs, wires, and coil wire will help the spark escape. What brand of parts did you use anyway? Lots of parts store stuff is junk when it comes out of the box.

 

you could also try running some injector cleamer through the system, maybe its a bit pluged up. As i dont deal whith computer controlled cars ever listen to the other guys first though:-)