Renix idle goes up and down at stop

1990 Wagoneer Limited, when it's in idle the RPMs go up and down (every couple of seconds) and then it eventually dies out. Seems worse if I have to come to a hard stop. Just started doing this today. Been searching and found some stuff on TPS but it sounds like if that is bad (or out of adjustment) you would just get either too high or too low of an RPM, not the cycling I'm seeing. Does this sound like a bad EGR valve? If so, should I just replace?

Works fine if I'm at highway speed, etc. - drove it about 200 miles today, only had problems in local traffic.

Are you running the A\C or defrost? That would cause the compressor to cycle. If not, check for vacuum leaks.

Either a bad TPS or dirty IAC are best bets. If some carbon has built up and come partly loose in the throttle body at the IAC (Idle Air Control ...stepper motor) then it can interfer with the IACs ability to tune into a perfect idle causing it to hunt. Just clean it with some carburator cleaner in the spray can with engine at high idle (hold the throttle at about 2000 rpm so the engine does not die). OR pull the IAC and clean it and lube it but do not turn or pull on the front part on the threaded shaft!

IF the TPS has developed a bad, noisy worn spot at the idle possition on the TPS internally it needs to be replaced. Either, or both could be the problem. The TPS can be tested with an old style analog meter for noisy signals, versus a smooth output from idle to WOT.

While you are at it, check the engine and jeep body ground versus the battery negative post. Should be less than one ohm.

88 Wagonman said:

Are you running the A\C or defrost? That would cause the compressor to cycle. If not, check for vacuum leaks.

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It does it whether I'm running the A/C or not. I definitely have a vacuum leak somewhere, because it does the whole "defrost" only thing unless I am off the gas. I'm pretty sure it's in the dash somewhere and I've just been too lazy to take the whole thing apart. Been doing that for a while, but the idle fluctuation is a new symptom though.

Double check the ground as was stated. Clean the IAC. Check for vacuum leaks.

If the HVAC has a leak bad enough to default to defrost only, the vacuum leak is probably driving the IAC nuts. Take off the vacuum hoes going into the cabin, block it at its source, and see if the problem goes away.

Take a quick look around the battery for your vacuum leak, they tend to happen in that area.

I took out the IACV and it was filthy. Cleaned it out good with some carb/choke cleaner and stuck it back in, now it won't start at all. If I take it out, it starts right up... so I'm gonna replace it now and see what happens. Still no luck on finding the vacuum leak though.

The start / no start issue after cleaning the IAC tells me you might not have a vacuum leak, just an IAC problem. Did you clean the throttle body too?

The start / no start issue after cleaning the IAC tells me you might not have a vacuum leak, just an IAC problem. Did you clean the throttle body too?

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Cleaning the whole thing is important. If the venturi is dirty and the throttle plate doesn't close properly, there is a contributing problem. The IAC is intended to monitor the air around a closed throttle plate that seals properly. If the computer is spending all it's effort trying to control the idle, as it is told to by the TPS, it's doing so by leaning out the injectors, possibly opening up another can of worms. This causes the system to try to operate outside of its pre-determined parameters, and there's only so much the computer can compensate for. Make sure the whole throttle body in clean and operating properly, if only to check off one thing that can't be giving you grief.

heyhar said:

Cleaning the whole thing is important. If the venturi is dirty and the throttle plate doesn't close properly, there is a contributing problem. The IAC is intended to monitor the air around a closed throttle plate that seals properly. If the computer is spending all it's effort trying to control the idle, as it is told to by the TPS, it's doing so by leaning out the injectors, possibly opening up another can of worms. This causes the system to try to operate outside of its pre-determined parameters, and there's only so much the computer can compensate for. Make sure the whole throttle body in clean and operating properly, if only to check off one thing that can't be giving you grief.

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No, I didn't clean the whole thing, I will do that this weekend. Judging from just sticking my finger in the hole while I had the IACV out, it is probably filthy. My finger was jet black with oil/sludge. The TPS tested well within range so I don't think that is the problem.

Good news is I was able to get it working, for now. First, I had to plug the IAC in (not installed) and start the car. This got the IAC to push the "valve" all the way out. Then I was able to lube it with some WD40 and screw the "valve" (sorry not sure of the actual term) back in by hand until it was almost closed. BTW, a new IACV from NAPA is $116.00 (which was a deciding factor in further troubleshooting!) Started up, but still had the rough idle.

I still had the EGR system in the back off my head as a culprit so I turned my attention to the EGR solenoid. Checked for vacuum at the EGR solenoid, and it was good. Plugged this line and suddenly, the idle dropped to a nice steady 800rpm or so. Plugged the vacuum line back into the EGR solenoid and the car died. I took the line off and plugged it with a screw, and it's now running like a top. This leads me to believe the problem is within the EGR. Took her for a test drive and the vacuum problem is still there, but better now - I assume this is because I plugged that line I have more "overall" vacuum now. I'm going to replace the EGR, solenoid, IACV, and the TPS, but will pull from a junkyard or look on eBay for a permanent fix to save some dough. Vacuum line leak is still bothering the crap out of me though.

Sounds like the EGR vacuum diaphram has a hole in it. Just out of curiosity, to you RENIX gurus, how important is the EGR valve to the operation of your XJs? I remember back in the '70s and '80s, the EGR, in addition to burning (supposedly) the unburned hydrocarbons lurking in the exhaust, I found it useful, in my Pintos (stop laughing) to control pinging, as the exhaust gas would displace the hotter burning oxygen in the intake charge, thus lowering cumbustion chamber temps. The only problem was that they were constantly coking up, requiring frequent cleaning. I had installed a new one on my '78 Firebird, 3.8 V6, but had to leave it disconnected because the tranny's vacuum modulator vacuum port was right next to where the EGR was on the intake, and when it operated, it fooled the modulator, and the tranny would shift funky. Great engineering, GM. Now with my '92 XJ being the oldest, no more EGRs!

I am sure that 5-90 will step in if he becomes aware of it, but his thought on the EGR valve is that it is actually counter productive on the 4.0. My personal experience is that he is correct. My suggestion is that you take the valve off, cut out a piece of aluminum or steel the same size as the mounting surface, and remount it with an extra gasket. This will in effect stop the valve from operating. I usually don't suggest removing an emmisions component, but in this case it actually runs CLEANER without it!. Just my .02

Clean the pintle on the IAC. Get some Qtips and carb cleaner and use them to clean the TB where the IAC is inserted. The pintle interfaces with an orifice. If they are both not clean, you will have issues.

Did you take the pintle off? If so, the IAC is most likely toast. The same goes for if you forced the pintle to change position. Try cycling the key on and off for 20 seconds each position for about a dozen times to give the computer the chance to adjust the IAC.

A hunting idle could also have to do with the vacuum at/between the Map sensor (mid bulk-head, black/green connector, i think 3 wire). And the small vac line runs to the side of the TB.

old_man said:

Clean the pintle on the IAC. Get some Qtips and carb cleaner and use them to clean the TB where the IAC is inserted. The pintle interfaces with an orifice. If they are both not clean, you will have issues.

Did you take the pintle off? If so, the IAC is most likely toast. The same goes for if you forced the pintle to change position. Try cycling the key on and off for 20 seconds each position for about a dozen times to give the computer the chance to adjust the IAC.

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When I took the IAC out for the initial cleaning, it was already in 3 pieces. I screwed the threaded part back in to it by hand so it was almost all the way closed, sprayed it some WD40 and stuck it back in. Works fine. I'll triple-check the vac lines to the MAP sensor today and clean out the entire throttle body.

overflow said:

When I took the IAC out for the initial cleaning, it was already in 3 pieces. I screwed the threaded part back in to it by hand so it was almost all the way closed, sprayed it some WD40 and stuck it back in. Works fine.

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It is still defective, if only because you found it in pieces. IT is not built to be dis-assembled. It is not normal for it to separate into parts. You can expect it to give you continued problems. Autozone sells them for about $60.