RPM Fluctuation

[Frederick]

2000 Ford Expedition 4.6. RPM fluctuates about 300 RPM’S when idling in gear at red light only when a/c is on

Topic

[Frederick]

2000 Ford Expedition 4.6. RPM fluctuates about 300 RPM’S when idling in gear at red light only when a/c is on

[Lorrin Barth]

Does the rpm decrease when the A/C kicks on?

[joey]

is your MIL on and have you tried pulling the codes. any recent repair history how long has this been going on.

[Frederick]

This has been goin on for about a yr.The RPM’S are at about 600-700 when a/c is off and stays level when in gear at red light.If I turn a/c on it increases about 300 rpm and goes up and down every couple seconds. Had alternator replaced about 9 months ago and it stopped for awhile and started again. Just had the fuel pump replaced and it was ok for about 30 mins.When this happened a yr ago it did the same thing but fluctuated twice as much.

[Bryan Carter]

Sounds like either an IAC problem, or your AC system is low on refrigerant.

[joey]

go get the codes pulled before you replace any more parts in it. have you checked your belt tension and when was your last tune up.

[Frederick]

This has been goin on for about a yr.The RPM’S are at about 600-700 when a/c is off and stays level when in gear at red light.If I turn a/c on it increases about 300 rpm and goes up and down every couple seconds. Had alternator replaced about 9 months ago and it stopped for awhile and started again. Just had the fuel pump replaced and it was ok for about 30 mins.When this happened a yr ago it did the same thing but fluctuated twice as much.

[Frederick]

What does pulling codes mean and how do I do that

[joey]

go to your local auto parts store call fist see if they offer this service it is usually free and can show you any diagnostic trouble codes that have been set which is many cases it the ticket to starting an accurate diagnosis

[Ingvar]

1. loose engine belt
2. air leak somewhere
3. ac comp is causing too much resistance to engine spin
4. BAD alternator. Yes, I saw you replaced it. Means nothing. I had 3 bad one night outta da box.
5. how is your air filter?

[Lorrin Barth]

In the engine computer there’s a setting for engine idle rpm and a setting for engine idle rpm with the A/C compressor engaged. Anyway, it would be interesting to see if the change in rpm coincides with the activity of the A/C compressor clutch.

[EricTheCarGuy]

As suggested the best place to start is by fixing any problems you have associated with engine codes. They are your best clue as to what the issue is. Additional information on solving idle issues can be found here.

http://www.ericthecarguy.com/faq/solving-automotive-idle-problems

Good luck and keep us posted on your progress.

[Dan]

FWIW, I had this (perhaps) exact problem with the ’01 Taurus that I recently purchased. When the blower was turned on, for either hot or cold air, while idling in gear, the RPM would fluctuate. However not 300rpm, more like 100 rpm. But the cause seems to have been the outrageously heavily corroded positive battery terminal and a weak battery. I put in a new battery, cleaned up the terminal and the problem appears to be solved.

[Bill]

Sounds to me like the A/C compressor is short cycling. This could be caused by a low refrigerant charge or a faulty low pressure cut out switch.

[Author]

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