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  #1  
Old December 17th, 2010, 20:07
dieselenthusiast dieselenthusiast is offline
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what causes high idle?

What are all the possibilities that could cause high idle. It doesnít happen all the time, but sometimes the XJ has a high idle. While idling high, I typically pop the hood and manually adjust the throttle to bring the idle back down. However, sometimes it tries to kick back up. Thoughts?
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  #2  
Old December 17th, 2010, 20:26
joe_peters joe_peters is offline
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Re: what causes high idle?

Dude--year, engine?
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  #3  
Old December 17th, 2010, 21:05
dieselenthusiast dieselenthusiast is offline
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Re: what causes high idle?

Quote:
Originally Posted by joe_peters View Post
Dude--year, engine?
With all the questions that you've answered in my posts, I thought you had that memorized by now, LOL.......... 2000 XJ, 4.0L
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  #4  
Old December 18th, 2010, 00:07
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Re: what causes high idle?

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Originally Posted by dieselenthusiast View Post
What are all the possibilities that could cause high idle. ?

A vacuum leak, a dirty throttle body, or a dirty Idle Air Controller.
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I spent a few hours researching how to jack up an XJ, but even this is more complex than a normal car.
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Last edited by Tim_MN; December 18th, 2010 at 00:16.
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  #5  
Old December 18th, 2010, 03:41
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birchlakeXJ birchlakeXJ is offline
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Re: what causes high idle?

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

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 can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

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.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

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

Reinstall IAC and check idle quality
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  #6  
Old December 18th, 2010, 07:47
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Re: what causes high idle?

Not as common as an air leak, but a leaking fuel injector would do it too.

Most common is an air leak, in any vaccum line, intake manifold leak, or sticking IAC. Exhaust leaks near the O2 sensor have also been reported to cause high idles. The TPS can also cause it if it is on its last legs, and it is giving inconsistent idle position resistance. Lastly bad wiring, battery, positive or grounds can add to the engine speed issues.
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  #7  
Old December 18th, 2010, 08:06
dieselenthusiast dieselenthusiast is offline
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Re: what causes high idle?

Thanks everyone for all your responses. It’s obvious that you guys have done a lot of research and have the passion to understand how your XJ works. My last UOA showed fuel and traces of coolant in the oil, so I’m pretty much milking this thing along. Overall, the motor seems to run good with no overheating problems. I’m at 175,000 miles now, which isn’t really high mileage for the 4.0L, so I’m torn between putting money into this motor, or just saving up for a new motor.
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  #8  
Old December 18th, 2010, 09:01
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winterbeater winterbeater is offline
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Re: what causes high idle?

I would guess that the IAC is sticking. On my 2000, I can create a huge vacuum leak by disconnecting the power brake vacuum line, and the system is smart enough to bring the idle right back down in a few seconds.
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  #9  
Old December 18th, 2010, 09:22
dieselenthusiast dieselenthusiast is offline
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Re: what causes high idle?

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Originally Posted by winterbeater View Post
I would guess that the IAC is sticking. On my 2000, I can create a huge vacuum leak by disconnecting the power brake vacuum line, and the system is smart enough to bring the idle right back down in a few seconds.
Thatís good to know, Iíll probably start with the IAC first.
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  #10  
Old December 18th, 2010, 16:58
joe_peters joe_peters is offline
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Re: what causes high idle?

Really, other than the 0331 head issue the 4.0 is a tough cookie and can go 300+ with regular maintenance.

Depending on your needs I would suggest you save up for a new head and drive it until the problem really brews up.
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  #11  
Old December 18th, 2010, 17:54
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Re: what causes high idle?

Since the oil analysis showed fuel in the oil, that says it is highly probably you have a leaking or sticking fuel injector. If it gets bad enough it will ruin the compression quickly on that cylinder!!!!! Or you have a bad plug(s) or wire(s) that are not getting a hot enough spark on one or more cylinders.

I would pull the injectors and clean and test them. Same for the spark plugs, wires, and the cap and rotor!!!!!

Small coolant in the oil leaks may be reduced by adding bars leaks brown fiber pellets to the coolant. They are dry, you crumble them and add them to the radiator.

If too much unburned fuel is getting past the combustion chamber a good, working O2 sensor might also see too little O2, and compensate by adding O2, thus resulting in an increased idle.
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  #12  
Old December 18th, 2010, 20:35
dieselenthusiast dieselenthusiast is offline
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Re: what causes high idle?

Quote:
Originally Posted by joe_peters View Post
Depending on your needs I would suggest you save up for a new head and drive it until the problem really brews up.
Iím saving $300 a month for a new engine, new injectors, new water pump, new alternator, new battery, new drive belt, new hoses, new aluminum 3 core radiator, new upper and lower hoses, new thermostat, new heater core, new engine mounts, new starter, new coil packs, new plugs, new injectors, new header, new front pipe, new converter, new catback, and new sensors. IĎm budgeting $5,000 for parts and labor. So Iíve got a long time (17 months) to go before I can fully fund and execute this project. This gives me plenty of time to buy parts when I can find a good buy.
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  #13  
Old December 18th, 2010, 20:43
dieselenthusiast dieselenthusiast is offline
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Re: what causes high idle?

Quote:
Originally Posted by Ecomike View Post
Since the oil analysis showed fuel in the oil, that says it is highly probably you have a leaking or sticking fuel injector. If it gets bad enough it will ruin the compression quickly on that cylinder!!!!! Or you have a bad plug(s) or wire(s) that are not getting a hot enough spark on one or more cylinders.

I would pull the injectors and clean and test them. Same for the spark plugs, wires, and the cap and rotor!!!!!

Small coolant in the oil leaks may be reduced by adding bars leaks brown fiber pellets to the coolant. They are dry, you crumble them and add them to the radiator.

If too much unburned fuel is getting past the combustion chamber a good, working O2 sensor might also see too little O2, and compensate by adding O2, thus resulting in an increased idle.
Here's my first report:

Unit ID: 2000 Jeep Cherokee
Second ID: Inline 6
Unit Type: Unleaded Gasoline Engine
Application: Transportation
Unit Mileage: 171,673
Lubricant Mileage: 2,113
Lubricant: Amsoil XLT SAE Synthetic 10W30


OVERALL SEVERITY OF REPORT: (High end of ABNORMAL)

SUGGEST performing cooling system pressure check. Lubricant and filter change is suggested if not done at sampling time. Coolant indicators (Sodium, Potassium) are at a significant level. Flagged additive levels are different than what should be present for the lubricant that is identified for this unit. Please provide unit model number so we may compare data to the proper standards for this unit; Replace oil filter and top off system with fresh make-up oil if not done when oil sample was taken. Re-sample in 3,500 miles or 65 hours. Resample in 30 days.

Areas that are flagged:

CONTAMINANT METALS:
Sodium: 41 PPM (High end of NORMAL)
Potassium: 197 PPM (High end of ABNORMAL)

MULTI-SOURCE METALS:
Molybdenum: 8 PPM (Low end of ABNORMAL)
Manganese: 27 PPM (High end of NORMAL)
Boron: 41 PPM (Low end of ABNORMAL)

ADDITIVE METALS:
Magnesium: 584 PPM (Low end of ABNORMAL)
Zinc: 1063 PPM (Low end of ABNORMAL)

Here is my second report:

Unit Mileage: 174,257
Lubricant Mileage: 2,358
Lubricant: Amsoil XLT SAE Synthetic 10W30

OVERALL SEVERITY OF REPORT: (Critical)

Check for source of FUEL LEAK (injectors, lines, etc.). Fuel is at a SIGNIFICANT LEVEL; Lubricant and filter change is suggested if not done at sampling time; FUEL DILUTION has caused viscosity to decrease SEVERELY; MODERATE POTASSIUM level could be a coolant indicator; Coolant leaks at this level will most likely not be detectable through normal diagnostics; Suggest MONITORING COOLANT LEVEL closely between samples; Magnesium is slightly high for this lubricant; Please provide until model number so we can compare data to the proper standards for this unit; Resample at half intervals.

CONTAMINANT METALS:
Sodium: 14 PPM (NORMAL)
Potassium: 45 PPM (Low end of ABNORMAL)

MULTI-SOURCE METALS:
Molybdenum: 36 PPM (NORMAL)
Manganese: 6 PPM (NORMAL)
Boron: 198 PPM (NORMAL)

ADDITIVE METALS:
Magnesium: 92 PPM (Low end of ABNORMAL)
Zinc: 757 PPM (NORMAL)

Fuel: 3.6% (High end of ABNORMAL)
Viscosity 9.0 (Critical)
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  #14  
Old December 19th, 2010, 08:08
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Re: what causes high idle?

I would pull the injectors and clean and test them, or have some one else clean and test them. I have found that a 9 volt battery and a simple air test (blow through the injector like you are blowing up a balloon works for large leaks and simple injector tests). Or just put in new injectors (buy them now, and use them for testing) now while you still have an engine!!!!!

Change the oil, then run the engine a few minutes. Shut down and then run compression tests on each cylinder to see how much damage there is compression wise, if any.

Is it using oil yet? If so how fast? Did oil consumption change between the two tests?

I would not be too worried about a coolant issue reading all that data. I would just flush the coolant system, and change the coolant, and add maybe one brown pellet (Bar's Leaks http://www.barsproducts.com/HDC.htm ) the one I described below to the coolant. If you caught it soon enough and change the injectors immediately you may be able to save the engine.

I am not sure Amsoil isn't using some potassium in the oil? Not sure it is a sign of coolant all the time? I think I saw where some oils were using potassium, and some coolants use only sodium, not potassium. Are you using any oil additives, or gas additives at all?

All that gas in the oil may have broken loose and dissolved some trash, old engine deposits, and artificially changed some of those metals numbers by dissolving and grinding up old loosened deposits. May not be all just metal wear if you caught it early enough. I would be worried about the oil pump and oil pump screen if a lot of trash came loose!

Fact that sodium and potassium dropped is encouraging on the coolant leak issue side of the equation!

Just curious, why don't they test for other metals? Like iron, chrome, nickel, etc?
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  #15  
Old December 19th, 2010, 08:28
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winterbeater winterbeater is offline
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Re: what causes high idle?

As Ecomike has brought out, the main problem seems to be fuel contamination of your oil. Have you studied your spark plugs? They should be mostly whitish with possible light tan color. If one or a couple of plugs are darker than the others, suspect that cylinder (injector, spark plug, compression, etc.).

While you have the plugs out, run a compression test. That is the best indicator of engine condition (notwithstanding bearing wear).

One other point that I don't believe has been touched on is operating temperature. Are you running at least 195 temperature? Not doing so can result in your oil contamination also.
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