Was pulling a tree out of my back yard with my 99 XJ a couple months ago. Check engine light came on. Code was for the IAC. Replaced the IAC, held battery cables together for a bit, hooked back up and still had the code. Reset CEL with a reader but code came back on after about 10 minutes. Drove the Jeep for about 20 minutes on the freeway. At idle I was pushing almost 2500 rpm under load, over 3 grand in park. Checked codes again and it had both IAC and TPS low voltage codes. This was a few weeks ago. Today installed new IAC and TPS, held cables together, hit the freeway. Idle is down to around 1100 at a stop under load but about 1500 - 1600 in park. Cleared the codes before driving and CEL came back on and both codes are back.

Could I have screwed something up in the computer? I am at my wits end. Can't afford to take it to a shop right now. I had a 96 XJ prior to this that needed the computer replaced and cost me $500 bucks....

 

I think you want to check both the IAC and TPS with a multimeter at this point. The TPS has three wires - One is for a ground. One is for a power supply to it. This wire should test having 5 VOLTS on it. And the third wire is for the signal it produces coming from the TPS to the PCM. The signal wire should have about 0.2 VOLTS on it at idle and rises along with throttle input up to near 5 VOLTS at maximum throttle. The voltage should sweep cleanly with no lags or erratic spots from zero throttle starting at 0.2 VOLTS to 5 VOLTS at full throttle. You can back probe the TPS at its connection with the ignition key turned to the run position (engine off) and see what it does.

Next, you can test the IAC with a multimeter and see what you get for this sensor too.

Check the connectors and wires of both of these sensors. There may be nothing wrong with the sensors themselves. There could be a problem of resistance or a ground wire connection at the sensors or along the wiring harness to trace if things do not test properly at the sensors when you test them.

Pull the Throttle Body and clean the inside of the IAC really well with a soft bristle brush and parts or brake cleaner. Just remove all of the rubber o-rings and gaskets to not ruin them with the chemical cleaner. Be careful of the pintle inside of the IAC to not damage it while scrubbing.

Clean the whole Throttle Body in the same way..

If these parts have not been cleaned recently there is most likely a lot of gunk and carbon buildup inside of them that can cause engine running issues similar to what you have happening.

The code being thrown makes me think it is a little more than just dirty Throttle Body and IAC having carbon build up. But, it should be done for what your Jeep has going on with it. The codes make me think it is likely a problem with the sensors, at one of their connections, or with their wiring.

 

Thanks for the reply. I will test these today. The throttle body and the orifice for the IAC have both been cleaned thoroughly.

 

When I cleaned up the IACs on my Jeeps I took the housing for the IAC off the throttle body and cleaned all of the way up inside of there really good too. Mine both had a lot of carbon inside of this spot. There was probably more carbon inside if there than on the rest of the parts. It seems like an important space to clean to me because it is such a small space and is where the pintle end mates up. It would be difficult to clean well without removing the IAC housing from off the throttle body. You would not be able to see and tell if you got it all either. I think because it takes a special hollow star-point bit to remove the four screws of the IAC housing is why it does not ussually get done? It is probably not detrimental if it does not get done or anything. It might not effect things to do what your Jeep is doing. I still think it is worth it to get it clean inside of there. I think it could cause somewhat better idle performance on an otherwise normal running Jeep.



This is when I did mine.

It is not easy to get any of this area cleaned without removing it. It seems to me the pintle could 'stick' inside of there if you only tried cleaning from the hole where the stepper motor pintle slides into without removing the IAC housing.


You can apply battery power to the IAC while it is on the bench if you wanted.. If you were to remove it and keep it inside of its housing you will be able to see it slide into and out of its location here inside of the housing assembly by doing this. Maybe you would see it getting stuck up somewhere along the way during its movement or something?

 

I went back and re-read your initial post. I am a little stumped on the issue. At first your idle was way up there near 3000 RPMs. This sounds classic as a TPS issue having that high of an idle. You got an IAC code at the time then though. You replaced the IAC then right? And it did not fix the very high idle right? Just a little while later you had a TPS code get thrown and replaced it. This brought the idle down considerably much closer to 1000 - 1500 RPMs after putting in a new TPS sensor. Maybe it was the TPS sensor all along and initially the code for IAC was sort of in a way coincidental...

Try to hear me out on what I am thinking. I think it is very possibly true what I am thought up? I hope so for you because I know you say you are at witts end and cannot afford expensive mechanics who will probably charge you too much wether they fully fix it or not.

So, right now you have two codes one for TPS low voltage and one for IAC low voltage right.. even after having replaced both these sensors already. This may be an issue and I am going to come back to it. I cannot see both of these being low like that without some sort of global type of reason. There may be a reason for these two codes. But, maybe these are not what is causing your problems? Just to be sure is it P0122 and P0508 for the two codes? There can be different codes for these sensors depending what is happening with them...

So, I think your main high 3000 RPM idle issue was indeed TPS related. It is just one of the classical signs of a faulty TPS when idle speed gets above 2000 RPMs... You replaced it and got your idle more reasonable at 1000 - 1500 RPMs. This fixed part of the problem for you.

What if you had two totally different problems happening and you have only fixed one of them.

One of the other main reasons for high idle is having a vacuum leak. A vacuum leak should ussually always be leaking about the same and consistantly like your is doing steady 1000 - 1500 RPMs. IAC may be up and down. Replacing the IAC did not do anything to help anyways. Vacuum hoses and such should not raise the idle speed as high as 1500 RPMs though either. Except for at one location. And this sounds like it may actually be somewhat common a thing too.

I think possibly you now still have a vacuum leak remaining located at the intake manifold. The intake manifold leak is a larger leak than any old vacuum line. There are bolts in the intake manifold and you can either pull out a torque wrench and check them out like that. Or, just get a wrench out and make sure all of the intake manifold bolts are all snugged down real good and tight. You could run and point an unlit propane torch at and around your intake manifold prior to tightening the bolts listening for a change in engine sound if you wanted.

Now that the TPS is fixed.. the remaining low voltage codes on both the IAC and TPS may be sort of a red herring to the issue of high idle. There may be a problem with that and all but, maybe not to cause the symptom of a high idle RPMs. A low voltage in both of these sensors at the same time could be that your battery is not the best anymore, the connections/pin-connectors all along the wiring harness everywhere are needing cleaned, battery terminal connection, and main grounds. I would clean all of the grounds every last one in the engine compartment and pull and clean every single wiring harness connector to include the three at the PCM.... No matter if having an issue or not. If this has not been done in some time. It is excellent preventative maintenance in and of itself. Since you are getting low voltage codes at multiple sensors. It only makes all that much more sense to just do it!

I apologize if this was long or if there are errors and such in this post. I was trying to type it up quickly and have to run so no proofreading this one.

Good luck

(edited to add):
One other location to check for a leak is around the fuel injectors where they mate up. Pouring ether or something on all around them may lead to finding a leak this way too. Or, possibly the injector o-ring seals too. Just an afterthought..

 

check all vaccume lines. high idle means air is entering the intake, whether it is thru the throttle body or any vaccume line will cause high idle. keep in mind the gasket under the throttle body also, the intake gaskets and the vaccume hoses connedted to the intake. make sure when you put in the new IAC, that you didn't pinch the "O" ring, it can also cause this. good luck.

 

I know this reply comes way late but.....

I will be testing the sensors and connectors this weekend. I know part of the high idle is from the IAC. Neither of the two new ones match the old one exactly but all P/N match. The old one seems to be a little skinnier and sticks out farther into the TB. BUT...... none of the IACs actually move. I had them each removed from the throttle body and keyed the ignition and nothing happened. You can also see through the throttle body that the IAC is allowing air around the throttle plate the entire time.

Also should mention 2 other issues that arose..... First, when I was trying to pull the stump out, at the same time the CEL came on for the IAC, the fuel gauge was going from full to empty a bunch of times, stopping after a couple days. Then most recently, the Jeep sat a while and the battery died. I jumped it and let it run, even though it was running about 1400 rpm, to charge the battery. The check gauges light came on and my volts were spiked. After about 10 minutes it showed as normal.