Running Out Of Ideas!

[Big "E"]

Hey fellas! I got a 99 Explorer with a 4.0 ohv, auto, 4wd. The old motor blew so i replaced it with a used one that has low miles and runs great. The only problem i'm having is under normal (slow) acceleration the engine will start bucking between 40 - 55 mph. Under harder acceleration or deceleration it will never do it. Runs smooth as can be. The code keeps showing the CMP and cylinder 1 misfire, so i changed the plugs and wires and the CMP. No difference. The engine runs too smooth in park and in gear to be out of time. The only thing i changed on the engine before i put it in was the CMP cause it was a 3 prong and my harness is a 2 prong. I know these engines used several different things for the same purpose that year. I guess cause they were phasing out the 4.0 ohv that year. I also thought if the CMP was the whole problem then i could splice the 3 prong fitting into the harness and put another 3 prong CMP back on the engine. I've always owned cars from the 50's, 60's and my current 74' cougar xr7 ( the exp is the wife's) and i'm REALLY getting tired of trying to figure out what's wrong with it. To much stuff to mess with! Any help would be GREATLY appreciated! Thanks!

 

[explorerguy89]

could be a coolant leak into the cylinder did the old plug have corrosion on it?

 

[Big "E"]

No, the old plugs didn't look bad. Just wanted to eliminate that possibility.

 

[jremington59]

Did the vehicle sit for a while between engines?

I know this is going to sound to easy but did you try putting a cn of good dry gas through it? I had a vehicle bucking once and it ended up a can of dry gas cured it.

 

[Big "E"]

Not really. We had the engines changed over in a couple days time. Never tried dry gas, have to give it a shot.

 

[410Fortune]

sounds like a fuel pump issue to me, that "bucking" or miss, is what a 4.0l likes to do when fuel pressure is low, pump getting weak. I would put a fuel pressure gage on it,
check the regulator and vacuum to it, replace the filter and listen to the pump itself

check the pressure at the rail and see if it drops at 3500+ rpm when hot, or the next time it does it

 

[budwich]

If I am not mistaken, the cmp causes "relationships" with "length of fueling" as injector timing and not firing (ie. engine timing). Your "conversion" from 2 to 3 wires may be an "issue" but you might need an engine analysis (ie. scoping) to see if this is a problem.