RPM fluctuates/rough idle when fans turned on??? - Jeep Commander Forums: Jeep Commander Forum
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post #1 of 27 (permalink) Old 01-20-2011, 10:22 PM Thread Starter
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RPM fluctuates/rough idle when fans turned on???

Before anyone crucifies me (Jeep5253) I did search around the forums a good bit typing in "idle surge" "rough idle" "rpm fluctuate" etc!

Here's my problem, it only occurs (from what I've noticed) when the fans are on regardless of heat or a/c.

When idling at a complete stop or when in park the rpm's set at about 500-600. Every minute or so the Jeep will get a light shake from the engine and the RPM's will rise and fall between 700-400 and then back to the normal idle.

It sounds like it has to do something with a sensor but I am unsure. PCV? IAT sensor?

I appreciate the help!

Current: 2013 Dodge Challenger R/T 6 Speed

Traded: 2006 Jeep Commander Limited Hemi 4x4
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post #2 of 27 (permalink) Old 01-20-2011, 10:31 PM
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2006 Steel Blue 5.7L HEMI Limited 4x4 Build Date: June 9, 2006 Hour 14
Interior: Dark Slate/Light Graystone with Saddle Brown Leather Seats
Quadra-Drive II, NAV, DVD, Trailer Tow Group IV, Off-Road Group II, Remote Start, UConnect, Rear Backup Camera, Chrome Wheels,
Magnaflow cat-back exhaust, Daytime Running Lights enabled, 2" Rough Country Lift with 265/70R17 Goodyear Wrangler AT/S
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post #3 of 27 (permalink) Old 01-20-2011, 10:33 PM
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Well....for whatever it is worth, and I assume you mean the cabin heater fans, my 07 4.7 displays the same behavior.....actually, the cabin fans don't have to be on....it still displays that behavior.

Sort of when the better half is in the store and I'm in the Jeep, in Park, listening to radio/waiting situation.

Finding this occured only idling while parked.....since new and now at 37k with no changes, I decided it is the nature of the beast and I decided the odds of my determining the cause/correction was nill.

That being said, when in gear, say, at a light, it is stable.
I suspect, that when the engine is unloaded, it hunts for a ideal fuel mixture....this is seen as a idle fluctuation of 2 hundred RPM or so.

Rob
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post #4 of 27 (permalink) Old 01-20-2011, 10:44 PM Thread Starter
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The only reason I am wondering is because I've never noticed it before. It occurs both in park and in gear at a light. Is there anything I could clean for the heck of it? I'm just about to turn 50k so I'm probably due a good cleaning of the engine.

Current: 2013 Dodge Challenger R/T 6 Speed

Traded: 2006 Jeep Commander Limited Hemi 4x4
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post #5 of 27 (permalink) Old 01-21-2011, 04:06 PM
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My 4.7 does the exact same thing.
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post #6 of 27 (permalink) Old 01-21-2011, 05:19 PM Thread Starter
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Sounds like it's normal then, that works for me! haha

On a side note I did wash the entire engine bay/radiator during my tow hook install (PITA btw!) and I think a couple pounds of mud washed out. My driveway looks like a trail.

Current: 2013 Dodge Challenger R/T 6 Speed

Traded: 2006 Jeep Commander Limited Hemi 4x4
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post #7 of 27 (permalink) Old 01-21-2011, 06:25 PM
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Well, I am telling you what my 4.7 does.......I think it would be more helpful if some 5.7 owners report in.

Since it occurs in gear, it may be looking for some attention.....possibly throttle body cleaning or IAC concern.
Could even be a coolant temp sensor reading out of range.
The best approach in my opinion is to get a good diagnostic, done in real time, with a Starscan or a independent shop using the All-Data programs.

At this time, nothing is far enouph out of range to set a light or code, but a good diagnostician might see something on the screen.

If, however, you start hearing from HEMI owners with the same concern, I would avoid getting to concerned unless someone found a fix.

Rob
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post #8 of 27 (permalink) Old 01-21-2011, 06:47 PM
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Similar Problem

I would look into a throttle position sensor (TPS). It's $25ish at the local parts store. Bolts with a torx screw/screws to the driver side just below the air intake. 10-15 minute change. Something you could do yourself and it will probably cure your problem.

My 06' 4.7 would ramp up and down while in Park in the morning when I first started it. Not under load (Drive), but in park. Mine started out doing the same thing and then it went over the 1000 RPM range.

I changed mine with about 38,000 (45,000 now) and no problems since.

Good luck and keep us posted.

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post #9 of 27 (permalink) Old 01-21-2011, 06:53 PM Thread Starter
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I'll take a look at the throttle body tomorrow to see if it could stand to be cleaned.

Edit: After reading some more threads it sounds like it could be the IAC going bad. I noticed today when coming to a stop the slight hesitation and rpm stumble as some described.

Current: 2013 Dodge Challenger R/T 6 Speed

Traded: 2006 Jeep Commander Limited Hemi 4x4

Last edited by Chapman89; 01-21-2011 at 06:55 PM.
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post #10 of 27 (permalink) Old 01-21-2011, 07:00 PM
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Like any problem, the first thing to do is check for stored DTC's.

Have you replaced the spark plugs when the maintenance schedule says too?

Eroded spark plugs will cause rough idle, loss of power and even stalling.

Ignition cables will degrade with use and time as well, I think they are called for replacement at 60k miles/5 years, if you've got more than 5 years on ignition cables with less than 60k, I'd consider replacing them.

Check your owners manual, Chrysler has been equipping some of their vehicle with suppression ignition cables instead of the more common resistive cables, the suppressive cables often can last the life of the vehicle.

Cleaning the Throttle Body, especially the IAC circuit, can help.

The Hemi has an Hydraulic Fan doesn't it? Something I was shocked, I guess its something they've doing on trucks for years, I've never seen it on cars. Could the fan be causing the load that drags down the idle and makes it rough? Its supplied by the PS pump, maybe there is something with the PS and fan that needs correcting.

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