So I finally got my motor in, got everything together, & got it started. When I started it though, it revs up to ~2500 RPMs, runs there for a few seconds kinda roughly, backfiring, shaking etc. Then it will rev up to about 5 grand & hang there until I cut it off, & it seems to run smooth in that higher range, and idea what the cause could be? It also smokes, like it's burning oil, but I believe that is just the extra crap burning off.

 

You probly need to adjust the throttle positioning sensor or the air intake control motor. Those two control the idle.

On my 88 renix Jeep it had bad grounds going to those sensors.and was giving the computer weird readings and did the same thing, but renix is completly different.

 

I will see about adjusting those, and check my grounds, I hope it's something simple as that...

 

Lately I go to Advance auto and use their computer.
It shows a lot of info besides codes:
temperature
RPM
O2 sensor voltage
all sensors data
TPS %
Pretty easy to use
Last time I checked Jeep and rear O2 sensor had 1.2Volts.
Rear sensor Doesn't matter for 98, no codes. I am not going to deal with it.

 

The only problem is that mine is not street legal yet, I bought it out of VA not running, replaced the motor with one from a '99, & this...otherwise I'd be on my way to Advance right now

 

Upon further inspection it seems that I have a crack in my intake at the #1 cylinder



With this being the case, it makes sense, because the crack would cause a vacuum leak, thus pulling in more air, which would be like holding the pedal flat on the floor. It would run rough at first because it is receiving too much fuel and has to catch up with itself, once it does, it can just rev freely, would anybody else agree?

 

Absolutely. Vacuum leak was actually what I was going to suggest when I read your original post.

 

I will get on changing that & hope that's my only problem, good thing I have another intake!

 

Well I replaced my intake, & the same freaking thing is happening? Does anyone have any clue as to what is happening here? I need to try to have this thing going by Friday

 

Nobody?

 

Faulty tps or iac.

 

Alright I will clean my IAC tomorrow, is there any brand to go for or avoid if I have to replace them?

 

I bough the borge Warner brand from checkers and it's been fine. Has lifetime warranty on both. Each Ran me about $45/PC.

People say go to the dealer for Tps and I agree if you have the extra cash. If your on a pinch just go to any parts store and get a lifetime warranty one.

Make sure when cleaning the iac do not twist or push in the pintle needle.

 

Alright I'll do that & if it don't fix it, I'll guess I'll keep going through stuff until it's good

 

I don't know what year your xj is but have you tried a cumputer from a 99 xj that matches what signals should be going to everything