engine wont rev past 1500 warm, 2500 cold

[iand86]

So i got 93 ford explorer 4.0l, when i start the engine cold it will rev up to about 2500 rpm but wont go past. if i step on it then engine seems like it wants to go but it wont, it wont go past 2500 rpm. as the motor warms up it gets worse, ill be lucky to hit 1500 rpm. ive replaced the MAF and TPS sensor and air filter, to no avail. BUT when its warm and running like crap....if i pull over turn the engine off, pull off the maf and tps sensor, start the truck and let it run for 10-20 seconds or till when it dies, turn off the key, plug the MAF and TPS back in it will run better (better as in up to 2500 rpm still) for 2-3 minutes. if i run it for a while at 1500 rpm going up a hill the engine will begin to stutter and a check engine light will come on...then turn off (engine light) once its not underload. what in the hell is going on! i dont think its mechanical...sounds like a sensor...but which one??

Oh ya, replaced wires and plugs less then a year ago.

 

[iand86]

nothin eh? poopy

 

[pt_maes]

have you checked out your idle air control valve?

 

[Diff Whack Daddy]

I'd say fuel filter and/or fuel pump. Check your fuel pressure on and off idle. When the fuel pump goes bad, it can end up not being able to create enough flow and pressure to keep up with off idle situations and cause erroneous computer codes. I know this cause mine fooled the Ford techs at the dealer before and after replacing all the sensors and the catalytic converter, they finally listened to me and tested the fuel pressure.

 

[ranger7ltr]

I'd say fuel filter and/or fuel pump. Check your fuel pressure on and off idle. When the fuel pump goes bad, it can end up not being able to create enough flow and pressure to keep up with off idle situations and cause erroneous computer codes. I know this cause mine fooled the Ford techs at the dealer before and after replacing all the sensors and the catalytic converter, they finally listened to me and tested the fuel pressure.

That is messed up...Damn I hope that vehicle was under warranty so someone else paid for their throwing parts at the truck...

 

[iand86]

gonna change the fuel filter today and check fuel pressure...ill get back to you. whats the fuel pressure supposed to be at idle?

 

[ranger7ltr]

For a 1992 Explorer...

The fuel pressure should run between 30-45 psi...30 psi with high vacuum, 45 psi with wide open throttle...

 

[iand86]

thxs, attempting the fuel filter right now...but it hates me

 

[iand86]

well, changed the fuel filter...believe it or not...that seems to of been the problem. took it for a short drive and it'll rev out to 6000+. will have to see, when i brought it into craptire they cleaned the MAF and changed the air filter and it ran ok for about 5 minutes. when i take it out here for a long drive tonight ill let you guys know. If thats all it was...thxs maaaang.

 

[iand86]

Well, changed the furl filter checked fuel pressure...its ok. Car ran good for about 10 minutes after changing the fuel filter...but once again after about 10 minutes the rpms dropped to 2500 then 1500. Wtf...I'm lost...computer on the fritz? Ugh

 

[MrShorty]

1st step in diagnosing a "computer on the fritz" is to pull codes.

Did you still have the fuel pressure gauge hooked up when the engine acted up? What was the fuel pressure when it wouldn't rev past 1500 rpm?

 

[iand86]

No I didn't, it only acts up while I'm driving. 90% of the time when its in neutral or park it'll rev to the moon. I brought the truck in and they pulled codes while it was off...I think they got 121 and 122? its also having a really hard time starting....took about 20 seconds of turning over before the engine started.

 

[Diff Whack Daddy]

Sounds like a bad catalytic converter to me. In neutral or park, there is no load on the engine, therefore a lot less exhaust gasses to squeeze through the clogged converter. Have someone rev it while you have a hand near the exhaust pipe and compare that with a comparable vehicle. Getting worse after driving is explained by the exhaust system heating up and further blocking the catalytic converter due to heat expansion. You could also smack the catalytic converter with the engine off. If you hear a rattle coming from inside, not the heat shield, then that is another indication.

 

[MrShorty]

No I didn't, it only acts up while I'm driving. 90% of the time when its in neutral or park it'll rev to the moon. I brought the truck in and they pulled codes while it was off...I think they got 121 and 122? its also having a really hard time starting....took about 20 seconds of turning over before the engine started.

Both codes point to something with the TPS circuit. Were they KOEO or CM codes (see my notes on pulling EEC-IV codes thread to learn how to distinguish between KOEO and CM codes). Have you looked into the TPS circuit? I could see all of these symptoms (even the hard start) being caused by a TPS fault. TPS is a pretty basic potentiometer and shouldn't be too difficult to diagnose with a DVOM and a wiring diagram.

 

[iand86]

allright update....changed maf...tps...iac...fuel filter...air filter...still runs like ****. brought it to a shop they said the cats were clogged...had the cats removed...ran top notch for about 2 days. then one fine day going up a hill...it strarts running like a POS again. brought it back to the shop, they tried hooking there comp to it...said they couldnt even communicate with the ECM, so they said the computer was shot...got a new computer...still ####ED! oh man...ive dumped so much money into this thing im ready to torch it...who wants to have a bonfire?

 

[Diff Whack Daddy]

I think your back to the in tank fuel pump. Mine did the same thing and the pump wasn't able to keep up with the fuel requirement under load and after warmed up. Sometimes the fail completely and others only half way.

 

[muddypaint]

i had the exact same problem with my 94 xlt and i changed the fuel filter worked good then after a few days back to the same problem, changed o2 sensors crank cam, dealer said my pcm was bad, but i emtied the cats and it run great in the cold then when it was warm out it ran poor but i changed the fuel pump and it ran better and started too, but it is december.