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  #1  
Old February 12th, 2018, 09:24
Patriot_XJ Patriot_XJ is offline
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low idle bad mpg...

so ive seen similar issues discussed a thousand time on here but none seem to be like mine... If you know of a thread link it and i will be happy to check it out

Ok so my 2001 XJ has a low rough idle (600-750 range), its a brand new jasper 4.0. After i get on the gas it runs like a dream. So far i have a new IAC, cleaned the throttle body, checked for vacuum leaks, replace pcv ccv whatever you want to call them valves.
Only code being thrown right now is an evap code
new plugs with new engine
Im not sure where or what to start checking next???
also have pretty bad mpgs right now too, dont know if its related or not?
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Old February 12th, 2018, 11:25
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Re: low idle bad mpg...

How many miles, is the engine broken in yet ? Did you install a genuine Jeep IAC or a cheap crappy Chinese clone part ?

About 750 rpm is about normal idle speed for a Jeep 4.0L, what are you defining as rough idle ? If the Check engine light isn't ON, there should be no misfires.

What data do you have for "bad mpg's" ?
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Last edited by Tim_MN; February 12th, 2018 at 11:30.
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Old February 12th, 2018, 11:26
RCP Phx RCP Phx is offline
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Re: low idle bad mpg...

I would start by checking the fuel pressure and look into some new injectors.
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Old February 12th, 2018, 11:40
Patriot_XJ Patriot_XJ is offline
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Re: low idle bad mpg...

replaced with a good IAC, by rough idle i mean a surging idle, tach is constantly jumping 200 rpm or so. and if i am say backing out of my driveway (so just barely applying throttle) and let off the jeep almost stalls but picks right back up. does not always do that though
Jeep just has around a 1000 miles on the new engine maybe im just being paranoid???
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Old February 12th, 2018, 15:50
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Re: low idle bad mpg...

Fix the "Only code being thrown right now is an evap code" code first. Sounds like a vac line to carbon canister or solenoid valve on the Evap system/Carb canister. Check the Gas tank vent hoses and Gas cap.

Sounds like a problem with the O2 sensor as well (Bad MPGs), check the O2 sensor wiring, and the 12-14 volt feed from the O2 heater relay to the sensor. You can test the O2 output with a DC volt meter 0-1 volts, google my old thread "ecomike naxja.org Testing Oxygen Sensors " here for instructions.

Wandering idle is not a rough idle, it is wandering. Rough is a missfire and shake. 600 is not really that low.

If those two fail to solve the problem clean the Intake temp sensor, air temp sensor and test it. But a wandering, idle is usually a bad heater in the O2 sensor or damaged O2 sensor wiring on the pre-cat O2 sensor, or a variable vac line leak.

Check the spark plugs for fouling and proper color since it is a new engine!!!

There a thousand places you could have a vac leak. Intake manifold gasket, injectors orings, hoses, throttle body gasket...... HVAC Vac line.....

Might try a Vac gauge to make sure the valves are sealing and that is not the engine!!!
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Old February 12th, 2018, 15:52
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Re: low idle bad mpg...

Also since it is cycling, that probably means the IAC is actually working!!! It is blamed for just every idle issue there is that is not caused by it. In 12+ years I have never seen a single post here where an idle speed issue problem post here was actually caused by (and fixed) a bad or stuck IAC.
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34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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Old February 12th, 2018, 16:02
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Re: low idle bad mpg...

As suggested, fix any known issues. If the gas mpg's are low by 30-40% or more, suspect O2 sensors issues. Test the O2 sensors for proper function, inspect the O2 wire plugs and wire harnesses for corrosion or damage, inspect/test the O2 fuses.

I have had 2 IAC fail, the symptoms were a random failure to maintain a steady idle down to 500 rpms or less when decelerating and sometimes when not moving at all, so it could happen.

There would be no reason to randomly install new fuel injectors, they should be tested.
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Quote:
Originally Posted by bimmerjeeper

Blanket caught in the rear liftgate latch. Now I can't open.
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Old February 12th, 2018, 16:59
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Re: low idle bad mpg...

The only bad IAC I ever had on 5 Renix jeeps was the one I forked up the first time I cleaned, or tried to clean one, LOL. I wonder if the HO IAC is the same as the Renix IAC?
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34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #9  
Old February 13th, 2018, 04:15
Patriot_XJ Patriot_XJ is offline
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Re: low idle bad mpg...

thanks guy i appreciate the help, I will take a look into your suggestions and post back when the issue is resolved
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Old February 15th, 2018, 17:31
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Re: low idle bad mpg...

Do a vacuum check. A clogged cat can cause what you describe.
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