89 XJ RENIX low rough idle

My 89 XJ started having a low idle a couple weeks ago. It starts up fine, to 1200-1100 rpm drops down to 500 rpm, idles rough. In drive slows down more, turn AC on goes down to 3/400 and wants to die. Tried cleaning IAC, and cleaned TB, helped a little, replaced with 2 other IAC's no help. Pull a vacuum line instantly gains couple hundred RPM. Replaced TPS and adjusted, no change. Pull vacuum line to MAP dies instantly. Off idle runs great w AC. Platinum plugs, new sp wires in last year. Definitely NOT EGR. - don't ask, Free flow 2 1/2 in exhaust, cone air cleaner, mopar cam. I've been driving my MB diesel the last month and a half as I just bought and it gets a lot higher MPG and burns Veg oil! Searched and read 9 pages of low idle, but...

I Also have the engine and chassis grounded VERY well, please don't comment about any "ground loop".

You likely know it should idle up with the A/C on. Try spraying a little penetrating oil behind the piston on the IAC and then let a a couple of drops of synthetic oil flow behind the piston.
Test your battery and charging system. My IAC seems to act up more when my battery is getting tired.
May be some sort of resistance in the ECU power in circuit. I've found low voltage to various components over the years.
Check your grounds, sorry I couldn't resist.
I've had crappy idle turn out to be the distributor cap on numerous occasions.
If you have the auto, you may want to check the ECU half of the TPS for in and out values and see if it's radically out of spec.
If all else fails you can open up the idle air adjustment in the drivers side of the TB, behind the slug cover. Clean out the air passage and open it a quarter of a turn at a time, until you get a hundred or so more idle RPM. Don't try to adjust it perfectly. All I've even been able to accomplish is to influence the minimum IAC position a little. Any adjustment you try to make, the ECU is going to try and correct for. Sometimes the idle can be influenced a little hot, before it gets to way too much during warm up.

Which side of the TPS did you adjust ( I am assuming it is an automatic tranny), and what idle voltage did you set it to?

Did you verify that the TPS ground wire itself is reading less than 1 ohm to the battery ground post?

Set tps to .80 (and is consistent) Checked O2, MAT MAP CTS CPS all withing specs. 3 different AIC - NO DIFFERENCE when unplugged, guessing wiring to ECU is problem. Turned up idle by increasing air bypass - under aluminum plug on drivers side of tb as temporay fix (doesn't change TPS setting, if you change idle stop screw you will change TPS).

I also had a nasty noise which ended up being loose tc bolts, probably caused by low idle speed engine vibration - On anther post. Going to install Mercedes turbo diesel see still another post.

Check the EGR valve...it may be leaking.

CPS can check "within specs" and still not output a reliable signal.

EGR not the problem, 1-blocked off, 2-isn't leaking to intake as idle speed would increase. Not CPS as increasing air bypass setting solved low idle speed.

Replaced IAC latch relay also.

Try setting the TPS idle (ECU side) to 0.83 volts. There is differing data out there on the proper setting, one of them indicates 0.83 volts for the idle setting. Then disconect the battery for 5 minutes, reconnect, start and run. If idle is too high, restart it three times to clear the Renix Memory, and see what the final base idle is.

Also make sure the TCU/ECU ground wire has less than 1 ohm between it and ground with ignition off. Lastly check the WOT TPS/ECU side voltage, and input voltage (TPS/ECU wire / Pin C to the battery ground with power on , engine off and post the results here. If they are off could be ECU, ECU to TPS wire, (pin C) or the TPS itself.

There is lot of new, recent data on thouroughly testing the TPS posted at the end of the "ReniX Files" thread here. Search "ReniX Files" under title in the OEM forum.

Don't know if it would be a problem to idle that low. My 89 XJ has idled that low since the day I bought it (and longer, according to PO). I've gotten 12k miles on it, and she's still running strong........

I'm slightly curious....could this be some sort of timing issue?? I've done the check/replace ground routine.....I would think that bad timing would cause misfires and the like......basically, not let this thing run as strong as she does.

About the only thing that will cause a low idle that isn't caused by a severe miss (engine shakes), is the IAC. If the throttle body where the IAC fits is clean, then it pretty much says it is an IAC problem. The computer outputs a sequence of voltage pulses on the different windings of the IAC. This controls the direction and amount of movement of the stepper motor inside. What are the things that can keep the stepper from working correctly other than a defective IAC stepper motor??.....Well if the computer is not outputting the correct sequence due to failure, or the pulses are not making it to the IAC. Unplug and replug the computer connectors. On my XJ, there was a connector that many people have found to be flakey. It was called C101. Jeep even published a TSB about it. They recommended cutting the connector out and wiring the two sides directly together. This is a fairly large multi-pin connector that resides in a bracket above the vacuum booster. If you cut it out, use crimp lugs, not wire nuts. I recommend soldering the lugs after you crimp them and cover them with heat shrink tubing. It will take a while since there are about 20 or so wires in the connector. Beware, some wires have the same color code and some wires only show up on one side of the connector.

When I rebuilt my rig, and was swapping over a new harness, I cut the connector out as a preventative measure. Now if you have two people and don't mind standing on your head, you can pull the computer connector and do a continuity test on the wires from the computer all the way to the IAC. If you need, I can send you a schematic that shows the pinouts in question.

I cleaned the 101 connector last year, when I installed a nice used engine harness -old Injector wiring was getting bad. Everything is grounded very well, I made 3 new additional grounds between under dash, firewall, engine and battery. CPS is directly wired to ECU. IAC to ECU is next. Went Offroad a couple miles today ran just fine, 10 puddles under a foot deep. All connectors getting waterproofed again.

Drove over 50 miles now, 5 city, 5 offroad, 40 highway, idle speed increases when hot, Still can't use AC when idling. Runs fine.

Checked C101 and cleaned (Was very clean still) and sprayed liquid wrench2 on both sides. Checked IAC to ECU wiring, under 2 ohms resistence all 4 wires. Checked latch relay for 12+ and ground, OK. Installed another CPS runs smoothly - with bypass opened up. Turn on AC and/or shift to drive always drops RPM.

what's the input side of the TPS read?
setting it to a fixed voltage is not the proper adjustment procedure, it is supposed to be a percentage of what the input is.

87manche said:

what's the input side of the TPS read?
setting it to a fixed voltage is not the proper adjustment procedure, it is supposed to be a percentage of what the input is.

Click to expand...

Is the "percentage of what the input is" TPS calibration procedure in any FSMs that you have?

I am quite familiar with it, but it is not in my 87 FSM, and it's not in his 89-90(?)FSM. I am just curious where the "percentage of what the input is" TPS procedure came from. Perhaps it started with the 91 OBD-1 models?

Do you know its origin or have an FSM with it, if so what year did it show up?

that comes from the renix fuel injection manual, it's on billavista tech archive on pirate.
I'm not going to link it.

anyway, I don't have it right in front of me, so I can't tell you the percentage.

but, the adjustment procedure goes something like this:

Find the ground with your multimeter, then measure the input voltage coming to the TPS. It should be somewhere around 5 volts, mine measures out about 4.7. This is the reference voltage from the ECU.
Then you divide that to get the percentage the manual says you're supposed to have, and that's what you set the TPS to.
Mine is about .78. I know that it's different for manuals and autos, mine's a 5 spd.
Let me dig up the page in the PDF and get it up.

straight from the renix manual, step 4, page 32:

Divide the output reading by the input voltage.
the desired value is 4.15V.
Example: If the input voltage is 5V and the output voltage is 4.15; divide 4.15 by 5=.83=83%.

So, you need to set the TPS for an auto at 83% of the input voltage on terminal A.