Very High Idle

[AeroNautical]

Before you say it, I've searched the forums for answers, and found 3 possibilities. First was the TPS. So I adjusted it accordingly, no go. Next was thoroughly cleaning the IAC, which I did and though it did help my engine start up alot faster with fewer revolutions, still didn't fix the high idle. I also cleaned my throttle body intake manifold. Next was the throttle linkage was getting stuck. That's not the case because it moves freely and smoothly through its entire motion. So, ideas? It's always idled a bit high, but now it's like I have my foot on the gas and won't let go. It occasionally likes to idle a bit lower, but then it realizes it isn't supposed to do what I want it to and jumps back up to a higher rpm. 

[ComancheKid45]

Check your manifold bolts,...very likely a few have worked themselves loose

[onlyinajeep726]

http://www.jpmagazin...es/viewall.html

 

Scroll down to the part titled, "Run, Run, Rudolph"...

 

Was my problem on my '88 XJ until I tightened the bolts.

 

Run, Run, Rudolph
Maybe it is age, maybe it is mileage, but we've seen a lot of Renix-era ('87-'90) Cherokees where the bolts holding the intake and exhaust manifold to the six-cylinder engine work themselves loose and cause the engine to race around 2,000 rpm.

Prevention: Tighten the bolts, dummy!

 

Cure: Tighten the bolts, dummy!
Seriously though, if your Jeep isn't doing this yet, just check the bolts. If it is idling erratically, grab your 9/16-inch wrench, get the Jeep to operating temperature and tighten the bolts while the Jeep is hot. Be wary of over tightening, though-they like to snap. If you let the leak go for too long you might need to just bite the bullet and put in a new gasket, but often tightening is all that is needed.

[AeroNautical]

Alright, I'll give it a shot. From the looks of it, it's a project in itself trying to get to the lower manifold bolts. From what I can gather it requires removing the fuel injectors and bar, and that metal heat guard on top of the manifold, is that right?

[cruiser54]

Alright, I'll give it a shot. From the looks of it, it's a project in itself trying to get to the lower manifold bolts. From what I can gather it requires removing the fuel injectors and bar, and that metal heat guard on top of the manifold, is that right?

Remove the air cleaner assembly.

 

What instructions did you use for the TPS adjustment? If you used mine, you would have done a sensor ground test and tested/adjusted the TPS on the "engine" side.

 

 

RENIX TPS ADJUSTMENT

Before attempting to adjust your TPS be sure the throttle body has been recently cleaned. It's especially important that the edges of the throttle butterfly are free of any carbon build-up.

IMPORTANT NOTE: With the Key OFF, and using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS. The letters are embossed on the connector itself. Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it parallels the valve cover and also over near the MAP sensor on the firewall. If you see more than 1 ohm of resistance, or fluctuation in your ohms reading, some modifications to the sensor ground harness will be necessary. The harness repair must be performed before proceeding. I can provide an instruction sheet for that if needed.

MANUAL TRANSMISSION:

RENIX manual transmission equipped XJs have only a flat three-wire TPS mounted on the throttle body and it provides data input to the ECU. It has three wires in the connector and they're clearly embossed with the letters A, B, and C. Wire "A" is positive. Wire "B" is ground. Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors. Note the voltage reading--this is your REFERENCE voltage. Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For example: 4.82 volts X .17=.82 volts. Loosen both T-20 Torx screws attaching the TPS to the throttle body and rotate the TPS until you

have achieved your desired output voltage. Tighten the screws carefully while watching to see that your output voltage remains where it is supposed to be. If you can't achieve the correct output voltage, replace the TPS and start over.

Sometimes, after adjusting your TPS the way outlined above, you may experience a high idle upon starting. If that happens, shut the engine off and reconnect your probes to B and C. Start the engine and while watching your meter, turn the TPS clockwise until the idle drops to normal and then rotate it back counterclockwise to your desired output voltage.

AUTOMATIC TRANSMISSION: RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-wire connector, same as the manual transmission vehicles have, and it is tested the same as the manual transmission equipped vehicles, as outlined above—FOR ALL ENGINE MANAGEMENT RELATED ISSUES.

However, the automatic TPS also has a square four-wire connector, clearly embossed with the letters A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module. THIS SQUARE FOUR WIRE CONNECTOR IS USED FOR TRANSMISSION/SHIFTING RELATED ISSUES ONLY. Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your REFERENCE voltage. Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage. For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage. If you can't, replace the TPS and start over. So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and the other side feeds the TCU.

FOR AUTOMATIC TRANSMISSION RELATED ISSUES: Check the square four-wire connector side of the TPS.

If you have ENGINE ISSUES check the flat three-wire connector side of the TPS.

For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.

Revised 09-22-2012

[AeroNautical]

These are the steps I took: http://comancheclub.com/topic/11634-renix-throttle-position-sensor-tps-testing-adjustment/

I honestly don't think that's the problem, however. It really started to idle higher when I cleaned the IAC. It was covered in carbon crap, and looks brand new now. I reinstalled it, started it and it idled nicely for a few seconds, then just kept getting higher.

[MJ_Milam]

When it high idles, try tapping on your MAP sensor. mine used to do the high idle on start up. tapped it one day and it dropped it back down. before I replaced it I waited a few more times for it to do it and eveytime I tapped the MAP, it worked. long story short, bad MAP for me.

[cruiser54]

Try adjusting the TPS this way:

 

And, did you pull, twist or in any other way get rough with the IAC?

 

RENIX TPS ADJUSTMENT

Before attempting to adjust your TPS be sure the throttle body has been recently cleaned. It's especially important that the edges of the throttle butterfly are free of any carbon build-up.

IMPORTANT NOTE: With the Key OFF, and using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS. The letters are embossed on the connector itself. Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it parallels the valve cover and also over near the MAP sensor on the firewall. If you see more than 1 ohm of resistance, or fluctuation in your ohms reading, some modifications to the sensor ground harness will be necessary. The harness repair must be performed before proceeding. I can provide an instruction sheet for that if needed.

MANUAL TRANSMISSION:
RENIX manual transmission equipped XJs have only a flat three-wire TPS mounted on the throttle body and it provides data input to the ECU. It has three wires in the connector and they're clearly embossed with the letters A, B, and C. Wire "A" is positive. Wire "B" is ground. Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors. Note the voltage reading--this is your REFERENCE voltage. Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For example: 4.82 volts X .17=.82 volts. Loosen both T-20 Torx screws attaching the TPS to the throttle body and rotate the TPS until you
have achieved your desired output voltage. Tighten the screws carefully while watching to see that your output voltage remains where it is supposed to be. If you can't achieve the correct output voltage, replace the TPS and start over.

Sometimes, after adjusting your TPS the way outlined above, you may experience a high idle upon starting. If that happens, shut the engine off and reconnect your probes to B and C. Start the engine and while watching your meter, turn the TPS clockwise until the idle drops to normal and then rotate it back counterclockwise to your desired output voltage.

AUTOMATIC TRANSMISSION: RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-wire connector, same as the manual transmission vehicles have, and it is tested the same as the manual transmission equipped vehicles, as outlined above—FOR ALL ENGINE MANAGEMENT RELATED ISSUES.

However, the automatic TPS also has a square four-wire connector, clearly embossed with the letters A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module. THIS SQUARE FOUR WIRE CONNECTOR IS USED FOR TRANSMISSION/SHIFTING RELATED ISSUES ONLY. Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your REFERENCE voltage. Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage. For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage. If you can't, replace the TPS and start over. So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and the other side feeds the TCU.
FOR AUTOMATIC TRANSMISSION RELATED ISSUES: Check the square four-wire connector side of the TPS.
If you have ENGINE ISSUES check the flat three-wire connector side of the TPS.

For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.
Revised 09-22-2012

[AeroNautical]

I was gentle with the IAC, but I did pull the sheath back and pray some carb cleaner in there, as well as spin the sheath and what felt like a spring on the inside a little. I didn't think much of it, but could that have effected it?

[RockMJ]

Based on your posts I am sure you have done this but have you checked for vacuum leaks in and around the intake manifold? Generally with the IAC you are fine cleaning it in the manner you did. If it is shot to begin with it really would not matter. Did you torque the manifold bolts down to spec? It sounds like you reworked your TPS to spec as well. If you have done all this minus the vacuum check, go over the lines and make sure nothing is unplugged, broken, or missing. A buddies 90 XJ was running away all of the sudden and upon inspection a tap on the manifold went bad and gave up the ghost. Quick replacement and re-routing of the vacuum lines fixed his problem. Good luck

 

RockMJ

[AeroNautical]

Checked all my vacuum lines, there was one loose, but I believe it went to the charcoal canister or whatever that emissions thing is in the back of the engine. Plugged it back up and didn't fix the problem. Also check all my manifold bolts, they were all torqued down. I'll try messing with the maps sensor, and hope that's the problem, because I'm running out of solutions fast, haha. 

[AeroNautical]

Update: I tapped the map sensor, as that's all I knew how to do, but it doesn't effect it. I went to a jy and swapped the IAC valve with one off of a grand cherokee (4.0) and it still gives the same problems, so it's not that. When I tested the TPS, it gave me 4.88 volts, and 17% of that is .83 for idle, which is what I have it set at. Does that seem right? Like I said, I checked all my vacuum hoses with the old carb cleaner method, and no spikes in the idle. I did purposely disconnect my vacuum hose from the back of the valve cover leading into the exhaust manifold, and plugged the nipple with my finger, and that lowered the idle quite a bit, but it obviously couldn't breath, so that's not a solution. It only idles higher when it's warm now, and it does idle lower briefly, as if the computer is compensating before it goes back up. 

[RockMJ]

When you adjusted the TPS was it a new unit or the same old one? The part is not all that expensive and if everything else checks out replace the sensor, adjust it accordingly, and go from there. Just trying to think without having the Jeep in front of me, when you pulled the IAC motor out did you make sure the gasket was intact when it went back in? If I was in your shoes and after going through everything I would just replace the TPS sensor setup and adjust it and go from there. Good luck

 

RockMJ

[Exgrayxj]

In another post you replaced the valve cover.

I'm not sure, but it looks like the rear hose is connected to ...what?

On a stocker, that rear vacuum hose is also a restriction, taking the place of a pcv valve,not connected straight to manifold vacuum.

If it idles down by plugging that, Viola, there is the problem.

 

On an old xj I had,(the exgrayxj) I used a Chrysler pcv valve and got a vented oil fill cap/breather.

In other words, straight vacuum will cause a high idle.

 

Maybe I missed something.

[AeroNautical]

I seem to have fixed the problem. How you ask? I have no idea. I did several things at once, without testing the idle between adjustments. Replaced some vacuum lines, map sensor, recleaned iac, double checked and readjusted tps. Turned it on, it sounded fine. Let it warm up for several minutes and its sounded better than it ever has before, a nice, low, strong idle. I appreciate everyones help, though I don't know who to credit for the method that fixed it, haha. 

[MJ_Milam]

I seem to have fixed the problem. How you ask? I have no idea. I did several things at once, without testing the idle between adjustments. Replaced some vacuum lines, map sensor, recleaned iac, double checked and readjusted tps. Turned it on, it sounded fine. Let it warm up for several minutes and its sounded better than it ever has before, a nice, low, strong idle. I appreciate everyones help, though I don't know who to credit for the method that fixed it, haha. 

All that matters is you fixed it. Congrats :cheers:

[AeroNautical]

Well, It idles high again, about 2500 RPM. Let me tell you the scenario leading up to it:

On the highway going about 70, fifth gear at about 2000 rpm. I jumped out of my seat to stretch really quick and accidentally floored the pedal. It didn't race, stutter, nothing bad happened, just gained a bit more speed. I run into some traffic and slow to a stop. I hear the engine racing a bit, look down and it's stuck at 2000 rpm. Normal for me is about 800 to 1000 idle. I checked all my vacuum lines with carb cleaner, but couldn't test much more. Did I knock something lose or what? Doesn't make much sense to me. The throttle body seats correctly at idle, also, so that rules out something happening to the throttle linkage. 

[1987Comanche]

Replace the TPS and readjust.  Maura (my old '90) would eat a TPS about 1X/yr but the initial diagnosis was difficult as the TPS voltage was always correct.  A manual trans TPS is tough to find in the jy but the auto trans TPS will work as well.  I always kept at least 1 spare on hand...

[jeephack]

I had a problem with a very high idle a couple of years back. Jeep would start and idle at around 3000/3500 rpm. After going through all the aforementioned procedures to no avail, upon closer inspection I found that one of the wiring harnesses had come dislodged in the dash and fallen across the throttle cable next to the firewall preventing the cable from retracting fully. A quick repositioning and securing of said harness remedied the high idle. Probably not your problem but I know firsthand that it can happen. Only takes a minute to check.

[AeroNautical]

Replace the TPS and readjust.  Maura (my old '90) would eat a TPS about 1X/yr but the initial diagnosis was difficult as the TPS voltage was always correct.  A manual trans TPS is tough to find in the jy but the auto trans TPS will work as well.  I always kept at least 1 spare on hand...

Turns out your very right. I tested my throttle by "flooring" it, and when I let it reset the TPS stuck, got a screw driver and unjammed it. Took it off, cleaned it throughly and it doesn't stick quite as bad, but at least it's a temporary fix for my idle. Thanks for that heads up!

 

You say the auto tps will work also? Awesome, rock auto has the manual tps for almost $100, but the auto for $20. I went to a junkyard today that had atleast 10 renix era cherokees and one comanche. ALL of them were scrapped, and I was left with a 98, 99 and 00 XJ. I couldn't believe they cleaned house like that. 

[cruiser54]

Use a good quality "auto" TPS.

[1987Comanche]

Glad my input led you to the problem :thumbsup: .  The output from the auto TPS and manual TPS are the same and it will bolt right up to the throttle body.  I had to remove the air intake tube (rubber piece from airbox) to install & properly set the TPS (didn't need to do this with the manual).  I'd also cap the TCU output plug (square 4 wire) so it's waterproof. But, as you mentioned, the auto TPS is a he&& of a lot cheaper...

[conker]

Guess I oughta do this to mine too, changed my MAP sensor hoping to get rid of my intermittent 2000rpm idle/2000rpm jump on startup and it didn't go away.

[cruiser54]

Guess I oughta do this to mine too, changed my MAP sensor hoping to get rid of my intermittent 2000rpm idle/2000rpm jump on startup and it didn't go away.

 

It's usually bad sensor grounds or bad TPS.

[AeroNautical]

Glad my input led you to the problem :thumbsup: .  The output from the auto TPS and manual TPS are the same and it will bolt right up to the throttle body.  I had to remove the air intake tube (rubber piece from airbox) to install & properly set the TPS (didn't need to do this with the manual).  I'd also cap the TCU output plug (square 4 wire) so it's waterproof. But, as you mentioned, the auto TPS is a he&& of a lot cheaper...

I've actually got that square plug on the harness, could it just be plugged in even with a manual tranny? Figure I could get 2 birds with one stone and water proof both connectors. 

 

 

 

Use a good quality "auto" TPS.

Have any brands you'd recommend?