97 5-speed XJ Fast Idle when warm

I can almost set my watch to it - right at the end of my 6 mile stop-and-go trip home from work, as I navigate the last few turns and speedbumps at around 10 mph while leaving it in first gear and frequently depressing the clutch - the idle soars to 1500 rpm. Engine/coolant temp is normal.

I have a new OE TPS in my toolbox :guitar: I figure I'll clean the IAC while I'm out there.

1. Sounds like a logical solution, right?

2. Anything I need to know about setting/resetting either component?

3. Any air/manifold temp sensors at fault?

TPS and IAC are logical, my bet on the IAC. Use a TB safe aerosol cleaner (not ordinary carb cleaner), and while you have the TB open/off, inspect the throttle plate to make sure it isn't loose.

On the post-Renix XJs the TPS is not adjustable, and the IAC never was.

I can't think of any air/manifold sensors that would cause your problem.

Generally, an intake leak would increase the idle, but I would expect it to be more prevalent.

Could your motor mounts be loose and allowing things to move around too much?

If you have a/c is it off?

What is your idle speed when normal?

I think you're on the right track by starting with a thorough cleaning of the idle air control. They get carboned up over miles and time.....here's more on it.

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The Idle Air Control (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 and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

1. Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one cam be kind of hard to get to)
2. "Gently" wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged
3. Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.
4. Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner
5. It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled
6. Reinstall IAC and check idle quality

Thanks for the quick replies. Motor mounts are good (replaced last year - still tight) and A/C is off. Normal idle while warm [closed-loop] around 750rpm. Vacuum hoses are all intact, as is the air cleaner/intake.

Starts fine, except for "Jeep thing" fuel check-valve long-cranking startup.... idles smooth where it's supposed to... just races between pumping the clutch over the series of speedbumps right before I get home - and continues when I finally park.

I'll replace the TPS and clean the IAC and TB. Thanks again for the quick thorough replies.

Be sure an give us an update.

Removed the IAC and WOW! there was a lot of carbon caked onto it. Spent 20 minutes shaving it off with my knife. Polished it clean with TB cleaner and used cleaner-soaked Q-tips inside the IAC housing and the TB. I had a ridge of carbon around the butterfly.

Held off on replacing the TPS. Didn't want to violate the repair Commandment of Thou Shalt Not Shotgun and lose my base. :lecture:

Idles smooth now, no warm fast-idle, and a happy side effect is noticably better running in all gears.

Mission accomplished. :smoker:

Very logical use of rifle instead of shotgun.

Any thoughts on this exact same problem on a '90 MJ 4.0 Renix, 5 speed, 4WD Command Trac? I thoroughly cleaned the throttle body and have tried 3 different IACs (all used, all do the same thing). I checked the resistance of both CTS and Air Intake Temp Sensor and both are within spec (cold and hot, though I had to guesstimate the coolant and intake manifold temp). It will idle around 1,000RPM when you first start her up.

1987Comanche said:

Any thoughts on this exact same problem on a '90 MJ 4.0 Renix, 5 speed, 4WD Command Trac? I thoroughly cleaned the throttle body and have tried 3 different IACs (all used, all do the same thing). I checked the resistance of both CTS and Air Intake Temp Sensor and both are within spec (cold and hot, though I had to guesstimate the coolant and intake manifold temp). It will idle around 1,000RPM when you first start her up.

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Three different IAC units safely eliminates that as a cause! High idle can often be caused by a vacuum leak. I would perform a comprehensive search to ensure you don't have one. I like to use a fluid like spray carb cleaner or even a spray bottle of water. Spray around all suspect components/hoses and if the idle changes while you're spraying, you've found your problem.

I think your vintage still has an EGR valve. Keep in mind that vacuum is a key component of activating/operating your EGR valve. I'd start in that area.

Maybe some RENIX guys will chime in with more specific recommendations as the RENIX years have their own quirks.

OP states "It will idle around 1000 rpm when you first (cold?) start her up."

Only on COLD starts? What is the idle speed after 5-10 minutes of engine operation?

Yes, 1,000 RPM after a cold start. After 5-10 mins the idle is usually up to 1500 RPM although sometimes it's only up to 1250. Give it 20 minutes or so and it will always be at 1500RPM.
I can check the intake manifold bolts and EGR vacuum lines next.

Did you clean the TB where the IAC mounts. Carbon inside the pocket behind the IAC is the most common problem. I spray in there and use a small cleaning brush the finish off with a Qtip until it comes back clean.

The idle should drop as it warms up. Loose bolts on the TB to the manifold or actual intake manifold bolts can also cause it.

Check the Air Charge Sensor with an ohmmeter. I can't remember the numbers but you should be able to find them with a search, if not PM me and I will look them up.

The entire throttle body (and butterfly) was filthy. I actually pulled the IAC and threaded paper towels in the IAC opening and out the top until they came out relatively clean. It took 2+ cans of throttle body cleaner.
Do you mean the air intake temp sensor (on the intake manifold)? I probed it and referenced it to the chart in the 1990 XJ FSM and it seemed ok. In fact the air intake sensor and coolant temp sensor had great correlation until the engine warmed up. Once the engine was hot the coolant temp sensor dropped a lot further which makes sense as the intake air should be colder than the coolant.

So how is your idle now?

Is it still higher when warmed up?

Thinking of things that could raise the idle when hot: An intake leak that when hot increases, such as a system with vacuum that is at least partially temperature controlled, such as the EGR system, or possibly a loose intake that when cool is held tighter by pressure of mounts or something and when hot and things expand it opens more?

test/adjust the tps.

The idle is still 1500 RPM when warmed up but the truck will now idle on it's own when cold. It was so filthy when I bought her that she'd only run with a foot feathering the throttle. Regarding vacuum leaks what does the canister behind the bumper run? The vacuum line running to that has to be 10' long.
I thought about disconnecting the EGR and seeing what happens. My understanding is that most of the 4.0 EGR valves no longer work anyway.
Also are there any grounds in particular I should clean before I adjust the TPS? As far as I can tell it's never been touched.

The vacuum reservoir behind the bumper is just that--it maintains a reserve of vacuum so that when you are on the throttle or the engine is under a heavy load your vacuum operated accessories will still function. If there is a loss of vacuum, your heater/defrost will default to defrost, if you have a D30 disco axle it won't function, the coolant control valve won't function and if you have cruise forget about it. I relocated my vacuum reservoir into the engine bay, under the coolant pressure bottle. Got rid of about 7' of vacuum line. My 1990 EGR system is fully functional, and I intend to keep it that way. That said, general consensus is that the EGR system on the 4.0 was never needed in the first place, was eliminated on the HO engine, and is no big thing. However, WHEN obamagore gets done areas currently without anal-retentive smog inspectors will be a thing of the past. Numerous LSD Zombie controlled state governments are considering pass-or-crush legislation. Disable systems at your own peril. Biggie grounds: Battery - main to engine block; ICM, ECU, o2 sensors grounded to the dipstick tube; and one lousy single ground strap from the rear of the head, driver's side, to the firewall. There are numerous other grounds, big one at the rear of the vehicle for lights, etc. I always recommend adding an additional battery ground to the passenger side fenderwell. Makes no sense to have a single body ground to the entire unibody.

I spent some time working on the MJ today and, while the truck runs better, I still have no change in idle speed. I swapped the MAP sensor, replaced the vacuum quick disconnect with individual vacuum lines, checked the bolts on the intake, pinched off the vacuum hose running to the vacuum reservoir, adjusted the TPS to 0.84V, put in a new O2 sensor (it was the original) replaced my valve cover with one from a '96 XJ (CCV issues), and replaced the distributor (had a problem starting when hot). None of these dropped the idle speed at all. So I guess the question for the '90 is: what's next?

Well, could go really old school. Have you checked to see if the base idle screw has been adjusted in too far? 3/32 Allen, pull the tube and then adjust the screw out until it no longer makes contact--throttle body totally closed--then adjust in until you can just perceive the slightest movement of the throttle plate. Bolt the tube back up and give it another try.

joe_peters said:

Well, could go really old school. Have you checked to see if the base idle screw has been adjusted in too far? 3/32 Allen, pull the tube and then adjust the screw out until it no longer makes contact--throttle body totally closed--then adjust in until you can just perceive the slightest movement of the throttle plate. Bolt the tube back up and give it another try.

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Still chasing my idle problem as in my other post have mentioned.
Question concerning doing this adjustment.......
If it idles at 750ish while in gear (mines a 88 I6 automatic),
Will doing this adjustment affect just the idle speed while in N or P.... or both in gear and in P or N?
On start up mine starts out at around 750 rpms then as it warms up goes to 1250-1500rpms.Still chasing my idle gremlins.