The RenX Files: High idle problems

[Ecomike]

@Ecomike I haven't gotten a chance to do the running sensor tests yet because I had to get my butt to work and I was late... Now as for the torx idle screw... I've cranked it in all the way, closing the port. This brought the idle down at the time that I did it, but it seems to have compensated for it and is back up at the high idle. I figure it has to be either a vac leak somewhere or a sensor that is telling the computer to open up the idle air motor for some reason... I wonder if my new O2 sensor is faulty... any way to test it without expensive equipment?

Easy way to test O2 sensor for that problem is to just disconnect it. If it still idles high, it is not the O2 sensor. If the high idle stops, it may be a good working O2 sensor adjusting AF ratio and raising the idle to compensate for the vacuum leak.

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=oxygen+sensor+test

http://www.naxja.org/forum/search.php?searchid=4018847

You can take some pliers and bend the ear that rests on the idle stop screw on the throttle body, or use the idle stop screw that the ear rests on to adjust it. The idle stop screw on mine was seized, so I just bent the ear. I would open the torx screw back to where it was, then try adjusting the idle stop ear or screw, maybe adjust both, but you want some air going through the torx screw hole on the throttle body, you do not want it closed all the way.

Do you have the proper size orifice restrictor in the small tube line that goes to the rear of the valve cover and to the intake manifold. If it is too large, it will act like a vacuum leak and cause a high idle.

An idle that only rises with increasing engine temp is usually a stuck IAC, a manifold gasket leak, or a bad temp sensor.

 

[joe_peters]

Just for a recheck--didn't see where you posted up the year/trans? Auto? If so, are you checking idle in DRIVE, after a good 20 minutes of operation when both the transmission and engine are fully warmed up?

Pull the intake tube, and as Ecomike mentioned adjust the throttle plate stop screw--3/32 allen--by backing it out until it no longer has any effect on the throttle plate, then turn it in until the first perceptible movement of the throttle plate and then STOP. Its only function is to keep the steel throttle plate from slamming fully closed and damaging the alloy throttle body.

Agree with Ecomike on putting the torx screw back to where it was.

Now, you will probably need to re-adjust the TPS.

The IAC is a stepper motor. There is only one way to test it--with a tool the dealership has or used to have (not the DRB scanner). If it is energized to extend when it is not mounted on the throttle body it will extend too far and break. IACs don't fail that often, but they do become "lazy". If you have a few Renix in your local salvage yards grab a couple, odds are one will be good.

Anyway, post up what you end up doing. Has the 3000 rpm idle happened again?

Good luck.

 

[Ecomike]

Just for a recheck--didn't see where you posted up the year/trans? Auto? If so, are you checking idle in DRIVE, after a good 20 minutes of operation when both the transmission and engine are fully warmed up?

Pull the intake tube, and as Ecomike mentioned adjust the throttle plate stop screw--3/32 allen--by backing it out until it no longer has any effect on the throttle plate, then turn it in until the first perceptible movement of the throttle plate and then STOP. Its only function is to keep the steel throttle plate from slamming fully closed and damaging the alloy throttle body.

Agree with Ecomike on putting the torx screw back to where it was.

Now, you will probably need to re-adjust the TPS.

The IAC is a stepper motor. There is only one way to test it--with a tool the dealership has or used to have (not the DRB scanner). If it is energized to extend when it is not mounted on the throttle body it will extend too far and break. IACs don't fail that often, but they do become "lazy". If you have a few Renix in your local salvage yards grab a couple, odds are one will be good.

Anyway, post up what you end up doing. Has the 3000 rpm idle happened again?

Good luck.

All good points Joe. Interesting comment on the throttle stop purpose. I did not know that. He mentioned 89, Manual below recently, IIRC. I forgot the TPS might move and need re-adjusting after resetting the throttle stop. Good catch. I have succeeded in testing spare unmounted IACs by momentarily connecting loose ones to the IAC connector on a running engine.

 

[joe_peters]

Good tip on testing the IAC, I will have to try that. I was going to make an IAC test jig to allow mounting and testing of travel, but its kind of low on a long list of Jeep-Do's.

 

[Ecomike]

Jeep-Do's????

Second cousin to Jeep-RTIs?


LOL, Jeep Round To Its!:wave1:

 

[joe_peters]

Jeep-Do's????

Second cousin to Jeep-RTIs?


LOL, Jeep Round To Its!:wave1:

LOL! Yeah, took off the 2" receiver to sandblast and paint it 6 months ago--hoping to put it back on this weekend--if I can get round to it!

 

[xytrix01]

Yay for lunchtime wrenching! I unplugged the O2 sensor and the RPMs went up. Not by much, but I could hear a change. Not that then I suspect, so i plugged it back in. Next up I reset the torx screw back to its original position. I did this with the engine running and it didn't change the idle at all... I suspect that as I opened it more, the IAC closed to compensate.

Next, I backed off the throttle stop screw until the idle came down to around 700rpm. I let it run and it stayed there for a good 10 min. This made me happy.

Then i took it for a drive around the block.

Idle shot up to 1500rpm and stayed there.

This made me sad.

I turned off the truck and immediately restarted it. Idle dropped back to 700rpm.

It has got to be a sensor at this point... that isn't vacuum leak behavior at all. But which one???

 

[joe_peters]

Ok, look, if you are using the throttle plate stop screw to set your idle then you are screwing up other things. Period.

Again, that 3/32 allen screw should not be having an effect on your idle speed, if it is it is screwed in WAY TO FAR. Set it as I explained in the earlier post. If it is set correctly, and your XJ won't idle correctly, FIX WHAT IS WRONG and causing the problem instead of trying to work around things and screwing up the engine management system.

Good luck.

 

[xytrix01]

Ok, look, if you are using the throttle plate stop screw to set your idle then you are screwing up other things. Period.

Again, that 3/32 allen screw should not be having an effect on your idle speed, if it is it is screwed in WAY TO FAR. Set it as I explained in the earlier post. If it is set correctly, and your XJ won't idle correctly, FIX WHAT IS WRONG and causing the problem instead of trying to work around things and screwing up the engine management system.

Good luck.

That exactly it! Perhaps i worded it screwy..

Pull the intake tube, and as Ecomike mentioned adjust the throttle plate stop screw--3/32 allen--by backing it out until it no longer has any effect on the throttle plate, then turn it in until the first perceptible movement of the throttle plate and then STOP. Its only function is to keep the steel throttle plate from slamming fully closed and damaging the alloy throttle body.

I did exactly this and the idle ended up at around 700RPM. It was screwed in like 4-5 turns past that point. Not sure when/how this got set this way, I have never touched any of the idle controls on this car before.

 

[joe_peters]

That exactly it! Perhaps i worded it screwy..



I did exactly this and the idle ended up at around 700RPM. It was screwed in like 4-5 turns past that point. Not sure when/how this got set this way, I have never touched any of the idle controls on this car before.

Ok, got you. Some PO or monkey mech messed with it along the way--both youth and beauty are fleeting, while stupid is forever.

Did you reset the TPS? What reference voltage and output voltage numbers did you end up with?

If you are still getting the intermittent high idle I would go back to the EGR and consider a temp block off--just to prove out it is the source, then replace it if necessary or put it back into service.

Good luck.

 

[Ecomike]

You are doing fine. Good results all. O2 is not the problem! You are right, a rapid change of 700 to 1500 rpm, from start to stop is not likely to be a vacuum leak, BUT, the system starts open loop, no O2 sensor data, and tries to run at 750 rpm based on the CPS sensor, but once the engine goes closed loop (it uses O2 sensor data), it will sense the vacuum leak's excess air, and Renix ECU will increase fuel flow to match stoich ratio up, and that results in the later increase in rpms, due to the vacuum leak!!!!! Also vacuum leaks can come and go, or change with engine temp.

Now because you fixed the PO screw up, by setting the idle stop properly, you now need to adjust the TPS idle voltage, as the TPS moved with the adjustment of the idle stop. Once you have reset the TPS, start and drive the Jeep, shut it off, then restart, shut off, cycle that 3 times. Ignor any screwy idle speeds, until AFTER the third restart!!! That seems to clear out the old TPS idle stop data!!!!!

Also your test data seems to show that the IAC is working.


One last note, make sure the floor mat is not hanging up on the throttle cable under the gas peddle!!! ASK ME WHY!!!!:laugh3:

 

[xytrix01]

One last note, make sure the floor mat is not hanging up on the throttle cable under the gas peddle!!! ASK ME WHY!!!!:laugh3:

Valid point! I'm sure that isn't it though because I don't have floor mats... or carpet for that matter!

I changed out the TPS for my junkyard spare and set it to .814VDC. That was a long drawn out process as I broke off one of the aluminum screws holding the TPS in place and ended up having to drill and tap the throttle body. Thank goodness I have a full shop at work!

Anyway the problem didn't leave, but has at least developed something of a pattern.

From a cold start the engine revs up to ~2000rpm for just a second right after it catches and then settles in to idle at what looks to be 5-600 RPM if I don't touch the throttle at all the idle will stay this way without any major changes until the engine warms up to operating temperature. Revving the engine up by hand still allows the idle to drop after the throttle is returned to the closed position. Now if I drive the jeep at all, even just a block, the idle comes to rest at ~1100. Without touching the throttle I shut off the engine and imediately restart it and the idle drops back down and stays there. It seems to be somehow dependent on engine loading... This is repeatable behavior, and i must have shut the jeep down and restarted it 10 times on my commute home tonight with the same result every time.

Here is a video I shot showing the idle. I also noticed that it seems to have trouble stabilizing, and its jumping around a little. I never really noticed this before, but then again I wasn't really paying attention.

http://www.youtube.com/watch?v=vocYY0pP8lw

The sudden Revs are me hitting the throttle btw... it isn't doing that all by itself... thank goodness!

 

[joe_peters]

Are your RPM figures with the dash tach or an external one? The low cold idle is a symptom--did you test the CTS numbers? Do you have this site bookmarked: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Also, just for grins and giggles, smear some heavy grease on the throttle body where the ends of the throttle plate shaft stick out--want to check for air leaks there.

 

[Ecomike]

Sounds to me like it is way past due for brand new TPS.

Have you read this entire thread? :read: LOL

The TPS has about a 1/2 dozen quirks to its partial failure modes, and some of them are near impossible to test for, short of installing a new one. They get noisy internally with age due to physical wear. Try a new TPS before you waist anymore time and money.

 

[Ecomike]

You might try testing the TPS idle voltage cold, then hot, to see if it is changing during warm up as the idle rises. Keep in mind that the TPS idle voltage is not an absolute number to the ECU (As many probably think it is), it is a reference low that is checked by the ECU at start up, and then averaged with 2-3 prior lows from memory, as best as I can tell. So if it moves a little during warm up, then resets when you restart the jeep, it can be a noisy (due to temperature changes) TPS idle range on the TPS. The TPS does wear out, it uses, is a carbon based potentiometer.
Otherwise, it may be the intake/exhaust gasket leak I previously suggested, but the TPS is easy to change!!!!!, and very suspect if it is used, from a JY!!!!!

 

[xytrix01]

I'm waiting for some more parts to arrive... didn't really want to pay the prices @ the local kragens... $180 for the new TPS? I think not! So I ordered them from rockauto.com I'll update more once the bits arrive.

 

[Super mud]

Hi I have an 87 4.0 and experiencing high idle problems almost word for word to what you suffered. Im not new to the renix system and believe I know a good bit about it but it's been kicking my ass on this one. I had got it running awesome for at least a couple months after the gremlins vanished but just yesterday while my jeep is torn apart in the driveway cause the d35 bearing failed, got so hot that when I down shiffted it twisted the axle end off and left me stranded 100 miles from home, It came back at the best possible time to make me more mad then i've ever been.

Now it's a bit different: Will idle consistently at high rpms, never returns to normal unless I unplug one of the 2 TPS connectors. The IAC moves and closes to normal. Now what I figured out is after testing the TPS (brand new replaced cause of tranny problems) With one of the connectors unpluged, voltage is .80 idle and around ~4.5 at WOT. Now with everything plugged in voltage is 1.5 idle and about 2.8-3 WOT.
So heres the bastard but now i need to figure out what is causing this. Checking all grounds which have previously been gone through the couple months ago.

One more concern I want to check out is it seems I do not have the large number of grounds I've been hearing about checking. (i am very familair with my wiring harness since I'm constantly testing wires and recently have been rewiring and rerouting the harness for a better and cleaner layout but the problem was not post the wiring work) i have the battery to bellhousing/back of engine/flex plate ground, intake manifold to top of firewall ground strap, 4 smaller ground wires on 2 ring connectors which I had found disconnected and reconnected to the valve cover bolt and i have a small battery to chassis ground which I put on. i noticed where there were like 2 ring connectors with broken stands of wire attached to the block right behind the dipstick tube (I hear alot of the dipstick tube ground) but I removed them and didnt think much of them at the time since I could not find a broken wire source. Even if this was a problem and somewhere I had a broken ground I could not find...theres something else that caused the recent electrical problems.

I've been reading this thread thoroughly but will try to finish it at a later time since staring at the computer for a length of time gives me a massive headache. I have sent this message to ecomike for help and posted it to here for added info and consideration.

 

[joe_peters]

Super mud:

Sounds like someone tore the dipstick grounds and relocated them to the valve cover.

There is a stud on the passenger's side of the block, close to the distributor. That is where the main ground cable from the battery should be attached. That stud is also where I relocated my dipstick tube grounds to.

The firewall ground should be running from the rear stud of the head to the firewall. Sounds like someone relocated that too.

Not quite sure what you are doing with your TPS. Unplug the 4-wire connector, key ON, measure between "A" positive and "D" ground--mark down that voltage, that is your "reference" or "supply" voltage. Hook the 4-wire connector back up. Back probe the connector and measure "B" positive to "D" ground--that is output and MUST be set to 83 percent of the reference voltage. If you can't get the TPS to adjust to that amount it likely needs replacement.

Post up what you find.

 

[Ecomike]

Good advise Joe on putting the the grounds back where they belong. Intake manifold and valve cover have gaskets and are not a great place for the grounds. The four small wires with loop ends belong near the dipstick grounded to the block, along with the large HD ground from the block to the battery. Especially important while cranking the engine.

On his TPS it sounds like the TPS wiring harness wires got damaged somewhere. He mentioned a drive shaft failure, so I would look for nearby wires that got hit. He should also check the O2 sensor wiring as it likes to get tangled in the front driveshaft area too (ask me how know? LOL).

Either that or the brand new TPS is bad? No, wait, how about transmission wires, could they get tangled in the drive shaft failure?

This is an AW4?

We need a more precise description as to which TPS wire connector is disconnected, and reconnected that changes the voltage readings!!! The AW4 version has separate ground wires on each TPS connector. The square one, 4 pin is for the AW4 computer, 3 pin is for the ECU.

This one indeed sounds odd, so far.

 

[Super mud]

Yes this is an AW4 and now I understand why the TPS has 2 connectors of the same wires for the tranny and ecu. Though whenever I unplugged either connector (for tranny or ecu) the idle jumps back down and has a .80 output reading. Now does the tranny connections run through the ecu or does the TCU run off a completley seperate harness by itself (been meaning to find this out) I'll be messing with the grounds today since i just got home from school.
Also it wan't a drive shaft that broke but a rear axle and i havent been messing with any thing near the electronics lately so i doubt I caused a break somewhere and since it happened out of the blue.