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Idle problem while in drive

searchingforXJ

NAXJA Forum User
Location
Canada
So a problem developed last night with the idle of my 99 XJ, 4.0L, AW4 with ~106k miles. While in drive, when I take my foot off the gas at any speed it idles at around 1100-1250RPM. Normally it would go down to 750ish RPM. It won't drop down to the normal 700-750RPM until I come to a complete stop. If I shift into N while coasting it will drop down to this RPM as well. It's annoying because I have to hit the brakes harder to stop and it's not the way it's supposed to work. It's almost like a cruise control because it will maintain the speed I'm at if I'm on a level road.

It started last night after I had a high temp condition on the engine. I was stuck in the drive-through line for 15 mins. and the temp went up to 250ish F before I shut it off. The check engine light never came on but the check instrument light did. I was able to get back onto a road and get her up to speed to get the temperature back down to normal. I haven't had a chance yet to check if the electric fan is not working. If it is broken, could that have cause the high temp condition?

Could an overheat like this cause the weird idle condition or is it just one of those goofy coincidences?

I checked for codes by holding in the trip button and turning the ignition on. It cycled through all the #s on digital display and then showed the mileage. I didn't show the serial # as I read somewhere it should.

Does anyone know what controls the idle speed while it's gear and your foot is off the gas (and off the brakes)?
 
I'm having a little trouble with the overheat causing an erratic idle. Could be coincidental.

You really need to find the root cause of your overheating though as that can toast a head gasket or a head. If you are experiencing "temp creep" only when idling and it runs at the proper temp when on the road when you have good air flow, I'd suspect the fan clutch first. They are easy to replace. SEARCH for more information on them.

BTW, you can't check for codes on a 99 the way you are describing. It doesn't work. You will need a code reader or a scan tool. A check engine light sets a code, a check gauge light does not.

If never before cleaned or replaced, I would clean the idle air control. It has the most effect on your idle speed. It's easy to do. I've included a procedure.

The IAC is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform.

CLEANING THE IAC

1. Remove the IAC with a torx 15 driver (2 bolts)
2. Gently wiggle out the IAC from the throttle body
3. Clean the IAC with throttle body cleaner (not carburetor cleaner ) Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle as it is fragile and you can damage it pretty easily
4. Also clean where the IAC seats in the throttle body with the same throttle body cleaner
5. Consider cleaning the entire throttle body itself as long as you’re at it
6. Reinstall IAC and check idle quality
 
I'm having a little trouble with the overheat causing an erratic idle. Could be coincidental.

You really need to find the root cause of your overheating though as that can toast a head gasket or a head. If you are experiencing "temp creep" only when idling and it runs at the proper temp when on the road when you have good air flow, I'd suspect the fan clutch first. They are easy to replace. SEARCH for more information on them.

BTW, you can't check for codes on a 99 the way you are describing. It doesn't work. You will need a code reader or a scan tool. Did the Check ENGINE light come on or the check GAUGE? A check engine light sets a code, a check gauge does not.

If never before cleaned or replaced, I would clean the idle air control. It has the most effect on your idle speed. It's easy to do. I've included a procedure.

The IAC is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform.

CLEANING THE IAC

1. Remove the IAC with a torx 15 driver (2 bolts)
2. Gently wiggle out the IAC from the throttle body
3. Clean the IAC with throttle body cleaner (not carburetor cleaner ) Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle as it is fragile and you can damage it pretty easily
4. Also clean where the IAC seats in the throttle body with the same throttle body cleaner
5. Consider cleaning the entire throttle body itself as long as you’re at it
6. Reinstall IAC and check idle quality

Thanks for the info. I picked up some throttle body cleaner on the way home last night to clean the IAC and wouldn't you know it, the idle problem went away on the way home.

Regarding the overheat, that was the first time it's ever happened. I checked and the mechanical fan is working, but electric one never came on. I read somewhere that it should automatically come on when the A/C is turned on, and I turned it on and waited a few mins and it never started spinning. Would that cause an overheat? The A/C had been running at first, but when I saw the guage strat to go up I turned it off. I should have turned the heater on full but didn't think of it.
 
The self healing Jeep hasn't been invented yet so I'm guessing your idle problem will return....eventually. Cleaning the idle air control makes sense even as a preventative measure, so I'd still consider doing so. It's too easy not to and especially since you already have the cleaner you'll need.

How did you check your mechanical fan clutch? A lot of techniques of testing them are not 100% conclusive. It could still be that. Temp creep at idle always bring the fan clutch into play as a suspect.
 
The self healing Jeep hasn't been invented yet so I'm guessing your idle problem will return....eventually. Cleaning the idle air control makes sense even as a preventative measure, so I'd still consider doing so. It's too easy not to and especially since you already have the cleaner you'll need.

How did you check your mechanical fan clutch? A lot of techniques of testing them are not 100% conclusive. It could still be that. Temp creep at idle always bring the fan clutch into play as a suspect.


I am still planning on cleaning the IAC.

Once it was up to operating temperature I opened the hood and looked at the fan. It was spinning. Are there times when it could be warm and the fan doesn't spin? I thought the clutch was just to allow the fan to spin freely while at speed so it doesn't create a drag on the engine.

School me on exactly how the fan/clutch is supposed to work if you don't mind.
 
I'd use the SEARCH function on this website, and also Google mechanical fan clutch operation for a more detailed explanation and testing procedures.
 
I'd use the SEARCH function on this website, and also Google mechanical fan clutch operation for a more detailed explanation and testing procedures.

Thanks I'll do that. You seemed to be well versed on the subject so I thought you might be willing to share some of your knowledge.
 
Fan Clutch 101:

When the temperature of the air flowing through the radiator and hitting the bi-metal spring on the front of the fan clutch reaches 170 degrees it will start to engage. At that time your coolant temperature is about 30 degrees higher than the air temp at the spring.

To check: Warm the engine to normal operating temperature. Shut off the engine. The fan clutch should be fully engaged, if not, replace the fan clutch.

Idle or slow driving in town/traffic temperature creep No. 1 suspect is the fan clutch.

Your electric fan usually won't engage until 230 degrees, which is way too late if the fan clutch isn't working properly.

I recommend replacing the stock fan clutch with the WJ HD fan clutch, NAPA #272310, direct fit, $44.
 
Fan Clutch 101:

When the temperature of the air flowing through the radiator and hitting the bi-metal spring on the front of the fan clutch reaches 170 degrees it will start to engage. At that time your coolant temperature is about 30 degrees higher than the air temp at the spring.

To check: Warm the engine to normal operating temperature. Shut off the engine. The fan clutch should be fully engaged, if not, replace the fan clutch.

Idle or slow driving in town/traffic temperature creep No. 1 suspect is the fan clutch.

Your electric fan usually won't engage until 230 degrees, which is way too late if the fan clutch isn't working properly.

I recommend replacing the stock fan clutch with the WJ HD fan clutch, NAPA #272310, direct fit, $44.

Thanks. I also did the google search as suggested and found that if you disconnect the bi-metal strip the fan will then act as a normal one. and if it no longer overheats while idling I'll have the culprit.
 
Fan Clutch 101:

When the temperature of the air flowing through the radiator and hitting the bi-metal spring on the front of the fan clutch reaches 170 degrees it will start to engage. At that time your coolant temperature is about 30 degrees higher than the air temp at the spring.

To check: Warm the engine to normal operating temperature. Shut off the engine. The fan clutch should be fully engaged, if not, replace the fan clutch.

Idle or slow driving in town/traffic temperature creep No. 1 suspect is the fan clutch.

Your electric fan usually won't engage until 230 degrees, which is way too late if the fan clutch isn't working properly.

I recommend replacing the stock fan clutch with the WJ HD fan clutch, NAPA #272310, direct fit, $44.

I thought I read somewhere that the electric can comes on with the A/C, I guess that's not true?
 
I thought I read somewhere that the electric can comes on with the A/C, I guess that's not true?

Yes, if working properly the efan will come on either with the a/c or when the coolant temp reaches approx. 230 degrees.
 
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