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Different solution for high idle.....

89jeeplaredo

NAXJA Forum User
Location
joliet
After doing much research on here for my high idle situation on my 89 Cherokee w/ 4.0L, my high idle was resolved by something else other than the common things such as the IAC, EGR, vacuum leak, TPS....etc.

I adjusted the little set screw on the throttle body a little bit and it brought my idle down to right where it should be.... down from 1300 to 800 rpm's in park w/ about 17 inches of vacuum.

At first I tested every other component electrically and mechanically (which were within spec) and tried to find a bad vauuum leak to no avail. Since I have only owned the vehicle for a few weeks now, I was not sure if the previous owners tampered with the set screw.

So I adjusted the screw and presto, problem solved so far! Just thought I'd share.:)
 
Glad that solved your problem. However, that screw is adjusted at the factory to correspond with the signals sent by the SBEC. So, you haven't actually solved the problem, but altered the idle to work with the incorrect signals the SBEC is sending based on an unknown condition..

The point being is you have solved the sypmtom and not the cause.

There is nothing wrong with this approach, it's just not ideal.

With the above said, I'll be the first to admit I have used the same approach on a 1990 XJ and drove me nuts trying to find the actual problem. A few months after altering the idle set screw I discovered the real problem. Thankfully I had logged the change I made to the idle set screw so I could undo the change I made.

Kent :)
 
Yes, that's what I actually thought too, that it's not really the proper corection, but it did work.

On an optimistic note, maybe a former owner screwed around with the set screw because something else was causing the enigne to idle to low. Maybe I just brought it back to it's correct setting. hahahaha, yeah right, time too go drink a beer!:yap:
 
Anytime you change the throttle plate stop screw (the idle screw). The values change on the TPS. You might want to go back and double check the TPS.
On mine, I noticed somebody had bent the stop to adjust the idle. I guess they were to lazy to find an allen wrench that fit.
It took me some trial and error and a few TPS adjustments to get it all corrected.
 
I just started having the same problem,idles @ 1700rpm,sometimes I will let it idle for about 10mins & it drops down slowly & might go back up when you drive it, some times so high you have to shift into neut'l @ stops. So do adjust the screw or bend the linkage or drink a beer?? I checked the TPS & the ohm's were right on, what sensor feeds the PCM for idle? just like my throttle, I'm stuck! HEH! 90 XJ. 4.0L,auto
 
Check and see if your IAC is sticking. Spray some (light) oil down the IAC port, while the motor is running, to help wash much of junk out and lub the IAC some. You may have to remove the IAC and scrape off the crust on the piston and the seat. Let a drop or two of oil flow behind the piston. Don't try to move the piston or remove it.
I occasionaly spray a little oil down the IAC port (a few times a year), just to prevent trouble.
If the IAC service doesn't solve the problem, plug off all the vacuum lines except the MAP and hook them up one at a time and see what happens. The vacuum line to the vacuum canister where it runs under the battery or through the front braces are likely spots for trouble. Check your intake manifold bolts (carefull of the studs, they break easy). Tighten down the TB, the gasket can shrink. Listen to your vacuum brake booster for a hiss, when applying the brakes, diaphram may be leaking vacuum.
A typical small vacuum line leak will idle at or slightly above 1000-1100 RPM, a major leak near 2000.
 
When spraying around the injectors to find a vacuum leak, listen for the engine to speed up slightly. If it does, check the manifold bolts for correct torque. They tend to loosen over a few years. This was mentioned - don't pass it up.

Renixs don't get rebooted. They just are.
 
papacan said:
I just started having the same problem,idles @ 1700rpm,sometimes I will let it idle for about 10mins & it drops down slowly & might go back up when you drive it, some times so high you have to shift into neut'l @ stops. So do adjust the screw or bend the linkage or drink a beer?? I checked the TPS & the ohm's were right on, what sensor feeds the PCM for idle? just like my throttle, I'm stuck! HEH! 90 XJ. 4.0L,auto

I have had and fixed this problem several times on my jeep, each time it had a different casue. The IAC was bad once, the Air Intake manifold Temperature Sensor another, and finally having the TPS sensor connected to the harness connection backwards.
 
TiRod said:
When spraying around the injectors to find a vacuum leak, listen for the engine to speed up slightly. If it does, check the manifold bolts for correct torque. They tend to loosen over a few years. This was mentioned - don't pass it up.

Renixs don't get rebooted. They just are.

Near as I can tell the only way to boot (reboot) a Renix is to drive it until it replaces the old bad sensor information with good new sensor information. The FSM's I have (1987 Renix) indicate they (Renix ECUs) do retain some old sensor data info, but that it can only be updated and replaced with new sensor data by driving it for a while.

Mike McGinness
South Houston, TX
 
8Mud said:
Check and see if your IAC is sticking. Spray some (light) oil down the IAC port, while the motor is running, to help wash much of junk out and lub the IAC some. You may have to remove the IAC and scrape off the crust on the piston and the seat. Let a drop or two of oil flow behind the piston. Don't try to move the piston or remove it.
I occasionaly spray a little oil down the IAC port (a few times a year), just to prevent trouble.
If the IAC service doesn't solve the problem, plug off all the vacuum lines except the MAP and hook them up one at a time and see what happens. The vacuum line to the vacuum canister where it runs under the battery or through the front braces are likely spots for trouble. Check your intake manifold bolts (carefull of the studs, they break easy). Tighten down the TB, the gasket can shrink. Listen to your vacuum brake booster for a hiss, when applying the brakes, diaphram may be leaking vacuum.
A typical small vacuum line leak will idle at or slightly above 1000-1100 RPM, a major leak near 2000.
I found a maj vacuum leak,fixed it, & it seemed alright for awhile and it started going back to norm to high idle & back,so sprayed the TB & cleaned the black sooty stuff,making sure to spray the IAC port. Started it this AM for moca run,idle fine for 5-6mins & then went to 2000rpm & stayed there for the round trip of 6miles untill I shut it off,about 1hr ago?
 
You only need to disconnect the battery (Key Off) for about 10 seconds to clear the ECU memory.
 
Saudade said:
You only need to disconnect the battery (Key Off) for about 10 seconds to clear the ECU memory.
Saudade,

The Renix ECUs do not have any volatile memory that can be cleared by disconnecting the battery. 89jeeplaredo listed an 89 (so he is dealing with a Renix ECU) model in his first post.
 
Ecomike said:
I have had and fixed this problem several times on my jeep, each time it had a different casue. The IAC was bad once, the Air Intake manifold Temperature Sensor another, and finally having the TPS sensor connected to the harness connection backwards.
I pulled the IAC,looked like a good seal contact 360 on the pintle,cleaned soot,nothing serious,light oil,checked TPS volts in spec,& found that the connector from the ECU had the letters ABC up & the conn.from the TPS had the letters PED up, so I reversed the TPS to mate ABC TO ABC but it did not want to go together easy, so I left it the way it was,so do I force it together or not? which is right? This is HELL on ice!
 
dealing with similar issues myself...as to the connector I can verify it should hookup ped-abc...abc-abc actually gives ya cba-abc if that makes sense.
 
UncleBud said:
dealing with similar issues myself...as to the connector I can verify it should hookup ped-abc...abc-abc actually gives ya cba-abc if that makes sense.
Okay,thats the way it was abc-ped,so the only thing I haven't checked is the IAT sensor,does that have a big part in the idle? & is the resitience check{ohms} that accurate?
 
You may have a short in the TPS, remember the side closest to the TB is for the ECU motor controls. Hook up an ohm meter, perferably and old needle type and open and close the throttle. It works just like a volume control, variable resisitor. The needle on the meter should move smoothly with no jerks or dead spots. My TPS had a dead (open) spot, the motor would crap out at a certain spot on the throttle, almost the opposite of what you have.
I have no personal experience, but I doubt the IAC or the motor temp sensor (lower left of block), would make the idle go up to 2000. Never seen it.
The temerature resisitanc etables for the test have been posted. All you need is a thermometer and and ohm meter.
The IAC will, the TPS will and a major vacuum leak will cause a really high idle.
You can do a quick check on the IAC without removing it. In the morning before you start up, do an ohm check on the IAC and on the motor temp. sensor (block). The temperature should be close to the same, the resistance should be close to the same.
With a flashlight, you can look down the IAC opening and when you work the throttle a little can see the plunger move. When the throttle opens the plunger closes and/or makes some sort of adjustment.
 
8Mud said:
but I doubt the IAC or the motor temp sensor (lower left of block), would make the idle go up to 2000. Never seen it.
The temerature resistance tables for the test have been posted. All you need is a thermometer and an ohm meter.
The IAC will, the TPS will and a major vacuum leak will cause a really high idle.
You can do a quick check on the IAC without removing it. In the morning before you start up, do an ohm check on the IAC and on the motor temp. sensor (block). The temperature should be close to the same, the resistance should be close to the same.
With a flashlight, you can look down the IAC opening and when you work the throttle a little can see the plunger move. When the throttle opens the plunger closes and/or makes some sort of adjustment.

8Mud,

I have a few questions, and comments reagrding your reply. First for clarification, did you mean to tell him to test the (IMATS) Intake Manifold Air Temperature Sensor (Renix calls it the MAT sensor) or the IAC motor with the ohm meter? I think you meant the MAT Sensor.

It has been my experience that the two temperatures (the MAT and the CTS) do vary widely during warm up for the first 10 to 15 minutes on my 87 Renix, 4.0.

Also, I had a bad MAT sensor that was causing the idle speed to increase during warm up, and was causing hard starts when the morning temperature had changed a lot since the last start. If the ECU gets bad air temperature data it will adjust the IAC motor based on bad data and speed up or slow down the idle incorrectly. Also, If the ECU can not get data from the MAT sensor it must guess at the air temperature, causing erratic idle speeds.

Papacan said he had already fixed a vacuum leak, and tested the TPS and cleaned the IAC.

Papacan,

Yes the ohm test on the "IAS", properly called the MAT (Renix name) is accurate and reliable.
 
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