HELP!! '97 Explorer runs terrible after warmup!!!

[toddyg_23]

i have a '97 explorer sport 4.0 SOHC. this problem has me pulling my hair out! the truck runs and starts good when cold. after warmup, the engine starts missing and jerking at 2000 rpms and up. TPS is good. a friend said the alternator had a failing diode and was messing up the ECM. tested it and it was BAD!! replaced alternator, still same problem. could it be CPS? fuel pump maybe? i'm lost!!!!

 

[JohnJ]

If you had a bad alternator, you could have had some pretty wacky system voltage and possibly even AC going on there. It is possible this could have permanently damaged the PCM, but more likely it's just a bit confused right now. Try disconnecting the battery for ten minutes ago before you go nvts. Also, make sure the battery connections are nice and clean.

Also, a couple of questions about the missing and jerking:
Does it run smoothly at idle both in neutral and in gear?
In neutral at 2000 RPM how does it run?
Powerbraking at 1500 RPM how does it run?
Driving at WOT how does it run?
Do you have a check engine light on? If so, what codes?

 

[toddyg_23]

thanks for your reply john. it idles fine in gear and in neutral. at 2000 rpms and up in park AND in gear it misses. at WOT it's really missing. like ot's not getting enough spark, or fuel. mind you, it only does this when the truck's at operating temp. i've been told it could be a bad PCM, as you said, bad coil, fuel pump not having enough pressure, and/or a bad CPS throwing timing off. what do you think???

 

[JohnJ]

Based on that behavior I'd tend to suspect low fuel pressure more than anything.

If the CPS were going bad the computer should be able to pick up on that and throw a code for you. Warm only rough running could also indicate a dead O2 sensor, but that should throw a code as well. If the computer itself were going bad I'd expect your misfiring to happen at idle also.

If you can get your hands on a fuel pressure gauge it would be well forth checking fuel pressure. If no gauge is available, try changing the fuel filter and see if that gives you any slight improvement. If it does, that would suggest the pump may be dying.

Also, you don't by any chance have low coolant level do you?

 

[Mbrooks420]

Does the rough running happen gradually as it warms up, or all at once?

 

[marragtop]

I didn't see where you said you pulled codes. There may be a code stored even if the CEL is off. Have you tried pulling wires off one at a time to see if you can isolate the problem to a specific cylinder? That would help you determine if it's spark of fuel related.

 

[BrooklynBay]

Welcome to this forum! Clean the mass air flow sensor, and see if the engine coolant temperature sensor is out of spec.

 

[Toolbox57]

Any thing you find out I am interested also. I 'm working on the same problem.

 

[Toolbox57]

Look at the ground where it comes off the connection below the battery. There is a ground wire that feeds the computer that comes off the connection that if it is loose or corrosion it causes problems. I believe it is a black wire white stripe that goes on around behind the engine to the computer.