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Old 09-07-2012, 11:34 AM   #1
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Why am I idling at 1250RPM?

2004 TJ 4.0 with 96,097 miles on it. I usually idle right around 750 RPM, but occasionally when I'm stopped (obviously!) my engine starts idling just above 1k and surging to 1250. Air filter, throttle body and IAC are clean, plugs are good, TPS is fairly new and I run fuel system cleaner with every oil change.

Any ideas?
Thanks, gents!

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Old 09-07-2012, 12:20 PM   #2
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Since you assure us your IAC is clean, I'd suspect you have a minor vacuum leak somewhere. Cracked vacuum hose perhaps. Spraying something like carb cleaner or WD-40 at your various vacuum hose connections while the engine is idling may help find a leak if one exists. When a leak exists and you spray it with carb cleaner etc., the leak will suck the cleaner into the engine and you'll hear an RPM change.

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Old 09-07-2012, 12:23 PM   #3
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Mine does it when it's really hot out and I'm in traffic with the AC on.
I just thought it was trying to cool its self off
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Old 09-07-2012, 12:40 PM   #4
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Quote:
Originally Posted by tailhole
Mine does it when it's really hot out and I'm in traffic with the AC on.
I just thought it was trying to cool its self off
Yeah, that's the most common situation in which it happens to me too. But the engine raising its RPM isn't going to cool the engine at all.
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Old 09-07-2012, 12:42 PM   #5
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Fan turns faster. More air.
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Old 09-07-2012, 12:42 PM   #6
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More water moving.
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Old 09-07-2012, 03:15 PM   #7
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Quote:
Originally Posted by Jerry Bransford View Post
Since you assure us your IAC is clean, I'd suspect you have a minor vacuum leak somewhere. Cracked vacuum hose perhaps. Spraying something like carb cleaner or WD-40 at your various vacuum hose connections while the engine is idling may help find a leak if one exists. When a leak exists and you spray it with carb cleaner etc., the leak will suck the cleaner into the engine and you'll hear an RPM change.
So what is a "good" idle? Mine hangs around 500-600 and to me it seems too low......
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Old 09-07-2012, 03:20 PM   #8
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Mine idles at 450 so don't ask me
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Old 09-07-2012, 05:58 PM   #9
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Follow up: turning the AC off fixes the problem instantly. Turning it back on bring the idle right back up... Hmm...
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Old 09-07-2012, 06:23 PM   #10
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Food for thought, just because and iac is clean doesn't mean it's not haging up.
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Old 09-08-2012, 12:41 AM   #11
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With A/C off, engine up to temp, and the jeep not moving what does it idle at? It should be right around 750/800.
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Old 09-08-2012, 08:26 AM   #12
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Originally Posted by James249G View Post
Follow up: turning the AC off fixes the problem instantly. Turning it back on bring the idle right back up... Hmm...
Every vehicle that I've ever owned with AC kicks up the idle if the AC compressor is engaged. I assume its to prevent a stall due to the increased load, but that's just an assumption. What you are describing sounds normal to me.
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Old 09-08-2012, 10:32 AM   #13
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The engines have the smarts to keep the engine idle at an appropriate RPM when the a/c is turned on but it won't raise the RPM above the normal RPM just because the a/c is turned on.

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