fluctuating RPM's with AC on

[the98stang]

Hey guys the other day I noticed my cars idle fluctuating with the AC on. It drops down to about 500 rpm's, then moves back up to about 800. It doesn't do it ALL the time though. Could my AC need more freon or something? Thanks for any help!

Tanner

 

[Kilgore Trout]

This is usually caused by low freon.

 

[the98stang]

This is usually caused by low freon.

What other symptoms will appear if I have low freon?

 

[Dragstr05]

Does your A/C blow cold? Have you had your A/C charged lately? The fluctuating in rpm is likely your AC compressor cycling on/off too frequently. I know for sure an overcharged system can cause this, because its happened to me.

 

[the98stang]

Does your A/C blow cold? Have you had your A/C charged lately? The fluctuating in rpm is likely your AC compressor cycling on/off too frequently. I know for sure an overcharged system can cause this, because its happened to me.

Yes it blows cold. No it wasn't charged recently. Car has 65k .

 

[Pennywise2]

Do you have underdrive pulleys?

 

[the98stang]

Do you have underdrive pulleys?

no I don't

 

[the98stang]

Do you have underdrive pulleys?

no I don't

 

[Kilgore Trout]

One thing you could try that does not cost anything is to reset the PCM and then have the car re-learn the fuel trims using the procedure outlined in your owner's manual.

If that does not work have a shop check your freon level or check it yourself.

 

[sgarlic]

Have you cleaned the IAC valve lately?

 

[Stang|ess]

How do you check the freon level?

My car's RPM jumps from 700 to 1100 every few seconds when I'm at a stop. I can hear the clutch engaging and disengaging and it's really irritating because people thing I'm revving at them.

 

[mity2]

u take it to shop, and they'll hook up manihold gauge to the shreader valve, and check your freon. In past, some of my car(my Geo) was doing samething with A/C and RPM. But A/C was working good.
Turned out that freon was low, which made compresser to go on more freaquent then it was designed. But because compressor was working overtime, it would still blow cold air. Just I was putting unneeded stress on my compressor.

I think they added something like half of can(7Oz) or so R123, and it was fine after that

 

[the98stang]

Well I guess I could have a shop check the freon level. But there is another problem that might be related. If I'm cruising at any given speed, put her in neutral and let my foot off the brake to coast, the RPM's drop down to 500, then back to 800-900. It will do that a few times before staying at correct idle. I have a new PCV valve and grommet. The IAC has never been cleaned or replaced. Do you guys think the IAC is causing this to happen?

 

[mity2]

Is PCV Grommet seated right?
Mine did up'n'down idle thing. Turned out that my grommet wasn't seated right, and causing leak.

Check that

 

[Pennywise2]

I think the freon is low in your car.You should get
the freon level checked to see because my rpm's
would fluctuate when my freon level was low and
now it doesn't do it any more.

 

[the98stang]

Ok I'll get the freon level checked. I'll check that PCV grommet too..

 

[mr-mike]

vacum leak

99% of the time when the idle acts up with the AC on its a vacum leak the loss is probally one of the doors inside the dash or vacum pods is leaking its a hard fix as there way under the dash but if you can gain access you may find the leak , vacum leaks are late model fuel injected cars is quite common , hope this helps

 

[the98stang]

Guys I got the IAC out but I just realized I don't have any damn carb cleaner! Can I use wd-40 or is that not a good idea? Also, just curious, but what is that black cap-looking thing do? And is it safe to spray the cleaner directly into the IAC holes? Or should I spray it onto a rag first? Should the gasket be replaced too (mine looks fine, no cracks)? Sorry for all the questions, I just don't want to F up the IAC.