Help, Car Idles Out of Control when Heat or AC on

[MainstreamStang]

Hey guys, well basically what the title says. Whenever I turn on the heat or the AC my car idles High then Low then high again and doesn't stop. Does anyone know what this might be?

Thanks

 

[sgarlic]

You sure it does it with the heater on? I would say clean or replace the IAC valve, but there isn't much increased load on the engine itself (except maybe the alternator,) with the heater that the IAC should be compensating for. Did you mean it does this on defrost?

 

[MainstreamStang]

I'll double check when I leave work, maybe it is with the defroster on. I think this might be the case. Is the IAC valve hard to replace?

 

[Kilgore Trout]

Nah, the problem is low freon.

 

[MainstreamStang]

Nah, the problem is low freon.

Wouldn't the AC blow hot or warm then?

 

[Kilgore Trout]

No, when the freon gets low it causes the compressor to cycle on and off and this screws up the idle real bad. It will still function and blow cool air however...

 

[MainstreamStang]

Would I get the same issue with the AC OFF and the HEAT ON? Sorry for all the questions but curious.

 

[Kilgore Trout]

Well, a lot of people misunderstand which settings the compressor actually runs in.

Straight up is OFF, the settings to the immediate left and right of OFF do not run the compressor. ALL the other settings DO in fact run the compressor.

Your idle should not be affected when the compressor is actually off if low freon is the cause.

 

[MainstreamStang]

You sure it does it with the heater on? I would say clean or replace the IAC valve, but there isn't much increased load on the engine itself (except maybe the alternator,) with the heater that the IAC should be compensating for. Did you mean it does this on defrost?

OK, when the heater is on, it DOES NOT happen, only when the AC is on or the Defrost is on (either hot or cold). Still Freon or IAC?

Edit: I just remembered I replacced the IAC last year, I will try the FREON, thanks!

 

[zratedmustang]

any updates on this, I'm having the same problem...

 

[Kilgore Trout]

Check freon...

 

[Mike97gt]

OK, when the heater is on, it DOES NOT happen, only when the AC is on or the Defrost is on (either hot or cold). Still Freon or IAC?

Edit: I just remembered I replacced the IAC last year, I will try the FREON, thanks!

Killgore is right about this.

Your low on freeon, when the A/C compressor is running the PCM raises the idle, normally the PCM can adjust and keep the idle smooth, but when the A/C is short cycling due to a low freeon condition it cycles so rapidly that the PCM can not adjust the idle fast enough and you end up with the idle goign up and down.

If you watched the A/C compressor when it does this you will see that the idle jumps high everytime the A/C copressor clutch shuts off.

So what you need to do is have the leak in your A/C fixed and the system refilled with freeon

 

[mr-mike]

its a vacum leak more than likely probally one of the hose lines that open the doors to the heater or ac .... they are tough to find as there under the dash , take off the glove box & check by pinching off the lines if the idle changes you have found it , will take some time