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GGINN
10-22-2009, 07:56 PM
My daughter has a 98 Grand. 4.0 auto. When it idles it revs about every 20 seconds. Not bad but it is not normal. So about 600 rpms to 900 rpms, about every 20 seconds. What could this be? Any input would be helpful.

Greg
Lukesdp
10-22-2009, 09:03 PM
My 4.0 likes to REV up sometimes as well, i always figured it had something to do with power steering. Its very annoying when i change to reverse and it wants to jump. Maybe my leaking steering lines have something to do with it...
Ggg
10-22-2009, 11:28 PM
Are any accessories turned on like the defroster or heater?
GGINN
10-23-2009, 03:29 AM
I believe the heat was on. Does that make a difference?
RangerPaul
10-23-2009, 08:09 AM
I don't know about heat, but today's defrosters use the AC to dry the air. The compressor cycles on and off, and with it the idle.
SWMBO
10-23-2009, 08:21 AM
my cherokee will rev up and down in idle if i have anything related to the defrosters on. does not do it with heat only tho.
Ggg
10-23-2009, 09:25 AM
What Paul and SWMBO said is what I was thinking. On gas engines if you listen with the hood open and at idle, defroster on, you will hear the click of the magnetic clutch on the AC compressor immediately before the idle increases. On diesels there is just too much noise to notice it.
RangerPaul
10-30-2009, 08:18 AM
GGINN, any update on this? I have a friend at work who has the same thing as what Lukesdp described.
GGINN
10-30-2009, 02:21 PM
No, sorry. Will have to ask the daughter tonight. I will post my findings later this evening.

Greg
RangerPaul
11-05-2009, 09:00 AM
Anything?
GGINN
11-05-2009, 11:23 AM
Well you guys really keep me on my toes:) I just texted her and asked her to let it run without the defrost on today, and let me know how it went. I will have results sometime today, hopefully:slaphead:
GGINN
11-05-2009, 01:30 PM
Well I just spoke with her. It is happening still.Shes gunna bring it over and I'm going to have a look. Again:mad2:
Swap
11-05-2009, 07:00 PM
Try to disconnect the AC connection under the hood! :shhh:
GGINN
11-05-2009, 07:07 PM
Ok, I'll try anything at this point. :chin:

Thank you...
Ggg
11-05-2009, 09:40 PM
Another bad battery? Maybe the ECM is trying to compensate for low voltage. I have a battery load tester.
Swap
11-06-2009, 05:39 AM
Yep, check your battery, the ECM will act stupid if that's the case.
GGINN
11-06-2009, 03:54 PM
The only way I have to test it is to take it out and run it down to batteries plus. My wifes 06 GC had a bad cell, and it reaked all kinds of havoc.
RangerPaul
11-25-2009, 07:54 AM
My friend here at work finally took hers in, and the shop replaced the throttle position sensor, the throttle body gasket and the AIC or IAC or some three letter thing that I'm not familiar with. It's all good now.