Missing engine what is the problem????

[smokeum]

I have a 93 explorer 4.0 with 95,000 on it. Would any one have any suggestions on this problem that I am having, I can’t seem to get to the bottom of it. The problem that I am having is when you run the X for a while it will start missing but not all of the time. I also noticed that when I get out of the X for a few minuets letting it run, after I get back in to take off it sometimes seems that it can’t get out of it’s own way. It doesn’t happen all the time though. It is not throwing any codes either.
Thanks


Here is a summary of what we did and found. We changed the MAF, ECT, O2 sensors, Thermostat and TPS. Also checked grounds and ran separate grounds. Monitored fuel pressure, no problem there. Changed lower intake manifold gasket to fix pinging problem.

We found that when we go up hills the O2's will function normally then go full rich and go into open loop with short-term fuel trim at 0. Then when we come down hills or come off gas and coast O2's will go full lean '0 volts' (this is not a gradual drop to lean, about 5 to 10 seconds of coasting then it just drops) and goes into open loop with fuel trims at 0.
If you just touch the gas pedal ever so slightly it immediately goes into closed loop and everything functions normally. (Reason we tried changing the TPS).

Someone also mentioned the O2's looking like a digital sign wave. Well ours do when you first start it up and will stay that way until you shut the engine off and restart, then they look normal. (Maybe a scanner issue?) Any other suggestions would be much appreciated, and I will definitely post a fix if we figure it out. Thanks again Guys and Gals

We are running out of ideas on what it could be..

 

[1IronMike]

When was the last time you changed the plugs and wires? That would have been my first thing.

 

[marragtop]

When it's running poorly, you need to try and isolate it to a specific cylinder if possible. Try disconnecting plug wires to assess the impact.