Erratic RPM's

[mikewild]

I'm stumped, I just completed a 2.9 to 4.0L swap into my 91 5spd ranger. Everything seemed to work fine once I fired it up and did a test drive with it yesterday. Yesterday, I drove about 10 blocks to the store, everything was fine. Coming home, it all went south. Here's the symptoms:

Starts fine
Idles fine

As soon as I go to give it gas, the RPM's jump up and down like I'm pumping the throttle. I thought it might be the IAC valve, so I unplugged it, cleaned it and it made no difference. I thought maybe there was a ground wire issue as my negative cable looks like poo so I cleaned it all up. No effect. Checked the ground wires at the ECM, and they look fine. Today I went and played around a bit more, checked the vacuum lines, MAF, all look good. I unplugged the ox sensor, made no difference. Unplugged the TPS sensor, still no difference. I don't suspect a faulty fuel pump or filter as the truck drove fine with the 2.9 prior to pulling it. I also don't suspect the 4.0 ECM as I also drove it in the exploder without issue prior to yanking the 4.0. Out of 10 fires, it ran and revved perfectly as though nothing was wrong one time after completely shutting the truck off, then it went back to messing with me. There's no CEL or anything like that.

I noticed today when farting around with it, that as soon as I brought the RPM's up to around 2000 is when it started it's revving up and down routine, but if I kept it under 2000, I could shift through all the gears and get the truck somewhat up to speed.

Any help would be greatly appreciated

 

[cjones101812]

when connecting the vacuum lines up...did you connect a vacuum line to the BARO sensor?? if so, disconnect it the 4.0 uses a BARO sensor the 2.9 uses a MAP sensor. the MAP sensor has a vacuum line on it, the BARO does not, it reads atmospheric pressure.

i don't know it this would cause the problem but...

also, what is the fuel pressure??

 

[mikewild]

when connecting the vacuum lines up...did you connect a vacuum line to the BARO sensor?? if so, disconnect it the 4.0 uses a BARO sensor the 2.9 uses a MAP sensor. the MAP sensor has a vacuum line on it, the BARO does not, it reads atmospheric pressure.

i don't know it this would cause the problem but...

also, what is the fuel pressure??

sorry I'm not sure what you mean by BARO sensor. I did find one thing odd that the MAP sensor on the firewall below the winshield was different between the exploder and the Ranger, on the exploder there was the plug, but on the firewall there was a plate covering everything, ( no where to plug into) my Ranger had a female connection, but I still had the erractic rev's with that plugged in or not.

Thanks for the reply though, I appreciate it.

 

[mikewild]

Finally figured out the problem today. Turns out it was the CPS.