The RenX Files: High idle problems

OK I don't think I am hasta, though it may sound like it at first. If you have any Renix experience please read this, as I think it lends a new perspective to an old, preplexing, frequent problem with high idles.

My old high idle Nemisis has returned uninvited. :bawl:

I have discovered that when I start the Jeep (87 Renix, 4.0), and if it idles to fast (recently started idling at 1000 to 2000 rpm again) and if it refuses to drop back down to normal, then if I disconnect the TPS sensor, the idle IAC control will finally operate ( there is about a 3 to 4 second delay) and lower the idle speed to a normal idle.

Then if I plug the TPS back in (re-connect the harness) the idle stays normal, unless I press the throttle again, (which I do not do because it eventually screws up the idle speed again if I do, not always right away, sometimes right away, sometimes it is a slow creep over minutes).

Then once I have gotten the IAC idle speed adjusted to the normal idle as I just described, I then unplug the IAC and drive the jeep. One of the interesting things is that once I have done this procedure the jeep runs great, including a roughly 30 to 50% increase in apparent horspower :cheers: when in gear that it has been lacking since I bought it 2.5 years ago.

I have tested the above problem, resolution procedure at least 8 times over the last week, and it is quite (100%) repeatable! Also if I disconnect the TPS and then press the throttle the engine dies right away.

Now it occures to me that the TPS must be working properly based on its flawless performance when the IAC is disconnected, and that it is still actually doing something control wise (data input to the ECU). It also must still be doing something, feeding good data to the ECU since it runs great when connected and dies when I press the accelerator if it is disconected!!!!! Right?????

Also, since the IAC works correctly when the TPS is disconnected, I must conclude that the IAC is also working when given the correct ECU instructions.

So what the hell could be causing this?????

To the best of my knowledge there are no battery, alternator, ground wire, or ignition problems (all of the ignition parts are new), and all the sensors have checked out recently and are fairly new.

I am looking for the non-obvious solutions.

I am thinking that either the ECU is getting faulty RPM data at some point, which makes no sense at all based on the way the engine runs, or it is giving faulty information to the IAC, but only when the TPS is connected and the throttle is pressed, or when the engine is first started. But why would it do this?

Why would it correct the idle speed, operating the IAC only after temporarily disconnecting the TPS????? Wiggling the connections at the IAC and the TPS has no effect on the system.

I eventually have to temporarily plug the IAC back in for real cold morning start ups, otherwise it tries to run and idle at about 250 rpm taking 20 minutes to warm up as a result on real cold morning first starts.

I frequeantly ( Couple times a year ) have to sray w-D in the connections on the Intake and snap them back togther. Works everytime. If you wash down your engine or get into Hi H20 areas they seem to start that crap.

boise49ers said:

I frequeantly ( Couple times a year ) have to sray w-D in the connections on the Intake and snap them back togther. Works everytime. If you wash down your engine or get into Hi H20 areas they seem to start that crap.

Click to expand...

Which connections?

If you are talking electrical connections, that is not the problem here. They are good and solid. Also it is not a sticking IAC in this case, as it works if I disconnect the TPS.

OK it is "what if" time!

What if the voltage from the ECU to the TPS (circuit) temporarily dropped a little due to a poor connection somewhere else (other than the tps itself as I am sure it is good and tight) like the firewall bulk connector (I forget its name, C-102?), or the return volatge line from the TPS on its way back to the ECU?

Would that tell the ECU that the TPS was asking for more speed?????

Or tell it to slow down the idle even lower than normal????

What about the TPS's B wire ground that runs back to the ECU as a common sensor (C-4) ground for all the ECU sensors? If that ground was weakening occasionally what effect, if any, would that have on the idle speed?

Ecomike said:

Which connections?

If you are talking electrical connections, that is not the problem here. They are good and solid. Also it is not a sticking IAC in this case, as it works if I disconnect the TPS.

Click to expand...

After reading deeper I figured that out. Sorry !

I have a 1990 XJ with the RenX motor and im haveing a Low(350-400rpm) idel problem any possibity it could be the same problem?
-Keep Jeepin

help same problem with 87 4.0! ! ! its driving me nuts. been trying to sort this for 3 days now. tried changing sensors, removed and cleaned tb . cleaned connections. when it happens it gets really slow and sluggish. idles 1500-2000rpm. it pushes and doesnt want to stop. jamming the throttle doesnt stop it. help! has anyone had any luck fix fixing this?

I am having the same prob, but i am new to the jeep lifestyle so i really havnt done much to figure it out yet. just a lack of the necessary knowledge. sometimes its fine sometimes its really high, and sometimes its really low. no pattern and never under any certain conditions. i could use some of that troubleshooting help too.

tighten the intake manifold and exhaust manifold. this is most likely. . .this fixed everything. runs great now. maybe your tps is intermittent or you have a failing iac. almost all of the upper and lower bolts were loose on mine. one bolt underneath was half way off. i just had a vacuum leak from the intake manifold. good luck!

blibra said:

tighten the intake manifold and exhaust manifold. this is most likely. . .this fixed everything. runs great now. maybe your tps is intermittent or you have a failing iac. almost all of the upper and lower bolts were loose on mine. one bolt underneath was half way off. i just had a vacuum leak from the intake manifold. good luck!

Click to expand...

Bet it runs and sounds a lot better. Those 2 bottom ones closest to the firewall suck to tighten.

ya i feel stupid now being just some loose bolts. . . now it runs perfect besides the noisy power steering pump.

good to know i will try thta

blibra said:

help same problem with 87 4.0! ! ! its driving me nuts. been trying to sort this for 3 days now. tried changing sensors, removed and cleaned tb . cleaned connections. when it happens it gets really slow and sluggish. idles 1500-2000rpm. it pushes and doesnt want to stop. jamming the throttle doesnt stop it. help! has anyone had any luck fix fixing this?

Click to expand...

I don't feel the list bit sorry for your terribly long drawn out three days of exaperation chasing an idle speed problem, I have been chasing the ellusive multiple idle speed gremlins in the same jeep for 1.5 years now!
!!!1. So I hope that makes you feel better (or did it make it worse?) I have thought I had fixed mine a dozen or more times only to have it come back a month or two later. Each time something else seemed to fix it briefly.

OK, I see now reading your last post that you fixed yours! :clap: congratulations!!!!

Update by Ecomike:

On sunday, Feb. 4th, I worked on my oil filter adapter (another thread) While I had it out I was finally able to get to the ground wire mess on the passenger side of the engine block. What a mess, I ahd read about it and been told about it (5-90) but I had depended on the driver's side wire from the block to the firewall to fix the problem.


Bad idea. :lecture:

Turns out there about 4-5 other smaller wires that all attach to the same ground connection, including the battery. So even though I had a good ground from the engine to the firewall and to the battery negative post via the drivers side cable and the battery to the firwall run, all the electronic guys (ECU, ICM, TCM, and who knows whate else) grounds were loose, oily, dirty, oxidized....and they all are grounded at the rear passenger side of the engine block near the dipstick (not easy to get to, I had to remove the oil filter first).

So I cleaned up all the mini ground connections and reattached them. I also cleaned up the +12 Volt connections at the main start/run relay next to the battery + post.

So when I finished with that and the oil filter adapter project I reattached the IAC connector and the jeep ran great (idled perfectly, ran great, shifted great, even the 1 quart oil leak per 100 miles was gone:clap for 100 miles until tuesday evening when the fast idle gremlin (FIG, go figure right?) resurfaced again all of sudden after 5 to 6 hours of driving and multiple trips.

I am going to recheck the TPS voltage next to see if the improved controls ground affected the idle possition voltage.

For now I am going back to disconnecting the IAC controler connection with it set for an average idle of about 450 rpm cold, 600 rpm warm and 750 rpm hot in park.

To all those who claim to be a Renix expert here (and to those who just are Renix experts, even if they do not claim to be one),
I have not received a single answer to my oringinal question in this post!!!!!!!

Have I stumped the experts??????? :rattle:

Please help!

Did anyone replace the IAC relay?

I would just say your TPS is wonky. Didn't you post about a shifting problem a while back? It seems the TPS would cause the same idle problems you are having now, as well as the shfiting problem.
I wouldn't be surprised if, when you cleaned the grounds next to the oil dipstick that you got a higher voltage on the TPS, and this threw off your adjustments. Go back, test the voltage and see if it changed anything. If it did, make the necessary adjustments. I've been meaning to get around to cleaning those ground wires but they are a bit of a pain to get to so I have been putting it off.

I'm a fan of the Renix but have just began to learn about then,...........is there any grounds for the computer that attach to the body(under the dash?:dunno: )

What is it, that's the same every time it acts up,............just after a rain, after a long drive, only when ya baby it, or when you drive it like ya stoled it, full tank of gas.... next to empty, when the defrost is on, when the a.c. ins one EXC.:eyes:

ask your self these question and more when it acts up. If we can find something that is there every time it act's up then we have a path or a direction to go.


Flash.

langer1 said:

Did anyone replace the IAC relay?

Click to expand...

NO! Is there a relay between it and the IAC that I am not aware of???????????? Or are you asking about the IAC itself. I have tried, replaced the IAC with new ones twice.

If it was a relay why would the IAC operate 3 seconds later and reset the idle speed to the proper idle speed for the current operating conditions (such as air and coolant temp, etc) when I disconnect the TPS while the engine is running?

Then when I reconnect the TPS it stays at that correct idle speed until sometime later (totally variable, no pattern*) when I start driving it (operating the accelator*) again. Sometimes the idle speed stays normal for a while after the TPS disconnect fix, other times it acts right up after operating the TPS-throttle again.

The TPS is also new (less than 12 months old, with less than 1500 miles on it), and keep in mind that I recently had one of the longest stretches of proper idle operation (500 miles, 4 weeks) since I started having idle problems 1.5 years ago.

But back to your question, I thoroughly cleaned all the under the hood relay contacts and harness connector contacts as best as I could. The harness (female) connectors are hard to get at. I may try wiggling them the next time the idle wanders off again!!!!!

langer1 said:

Did anyone replace the IAC relay?

Click to expand...

Were you referring to the power latch relay that sets the IAC position using a timed power down delay after the engine is powered down?

If yes, I do not know for sure, I have replaced on or two of the small square plug-in relays, and swapped some around to test them so I am not sure. I have cleaned the male pins on all the under the hood relays including the power latch relay if it is under the hood (?).