IAC valve

I recently swapped a junk yard 4.0 into my 94 xj and am having trouble getting it to idle. I removed the iac valve from the throttle body but left it hooked up and this increased the idle and it ran fine. When i rev it higher than idle it runs fine, it just wont idle for crap. Is there any way to test the idle air control valve ? In my repair manual it said the probe on this valve should never stick out more than 1/4 in and mine was about 3/4 in out and the book said this will ruin the valve.

Also would a bad tps sensor cause it not to idle and is there any way to test the tps sensor.

Try using carb and choke cleaner on it.

you can try and clean it, but really...you can get a new one from Autozone for like $45....

TPS shouldnt cause rough/bad idle...maybe high idle.....but your problem sounds just like IACV problems.

Heres some things I did that really helped with my low and unstable idle. I know the IAC may be your problem, but it could also just be a small part of the problem.

try cleaning the pintle on the IAC with sensor-safe cleaner (I used electronics cleaner on mine to be safe--carb cleaner can be harsh).

Also really make sure the recess in the TB for the IAC is spotless. The rest of my TB was shiny clean but the IAC recess had all kinds of crud built up. Took half an hour with a toothbrush and carb cleaner to get it really clean.

re-adjust the TPS to factory spec.

Check the contacts at the fuel ballast resistor. It's the white ceramic thing on the drivers side fenderwall with 2 wires coming out of it:

Clean the contacts with electrical cleaner then apply some electrical contact grease and make sure the contacts are good.

Clean every single damn electrical contact in sight with electrical cleaner and apply electrical contact grease(sorry, I just woke up so my brain isn't working well yet--can't remember the actual name of the stuff).

test the EGR solenoid, it has often been known to stick, causing rough idle when cold.

run a can of mopar combustion chamber cleaner through the TB, as per instructions.

A combination of all of the above completely cured my rough idle. I used to idle at around 450-600 HOT, around 800 COLD. The idle was never very smooth and was constantly "hunting" and running like crap. I did all these things I mentioned above and now have a solid hot idle of 760RPM, with no variance. Cold idle in park is slightly high at 1k RPM, but that's because the TPS is set a little higher than factory spec, basically so I have the same as an "extended idle"