Climate control increasing idle

[bnqdiy]

I have a E220 avantgarde w211 facelift model 170bhp, I don't know if this is a problem or not but seems to be very random. When the climate control is on without air conditioning on the idle increases to about 850 rpm then sometimes drops back to 750 rpm but mainly stays at about 850 rpm. If I turn the climate control off the engine drops back to 750 rpm and stays there but as soon as the climate control is turned back on it's back up to 850 rpm again with the air con off. Also if you turn the temperature down to 18 or below it drops the rpm back to 750 but as soon as you go above 18 it's back up to 850 rpm. If I turn off the climate control it drops back to 750 rpm and then turn on all the electrics, rear heater etc it still stays at 750 rpm which to me suggests it's definately something to do with the climate control.
I have had it around to my local motor engineers and even they are stumped by this and haven't got a clue why it is doing it.
Any help or suggestions would be appreciated.

 

[Sp!ke]

I think its normal for revs to fluctuate slightly as the aircon compressor kicks in and out.

 

[sinewave]

Turning on a lot of electrical items will make no difference to idle speed as the alternator and battery output is enough at idle for most things.

Even though the A/C is on does not mean the compressor is engaged as the cabin may be at set-point so no cooling is required.

When the Stat calls for cooling load the compressors electronic clutch engages the compressor and a mechanical load is then applied to the engine via a belt etc

It's this load (drag on the engine) that causes the engine management to 'pick up the idle speed' slightly to compensate otherwise the idle would be dragged down to potential stall levels if un-compensated.

The fluctuations are just the ECU not being able to maintain a steady idle against a constantly changing A/C load.

 

[bnqdiy]

So does the air con compressor still kick in even if the air con is off because this is happening with the air con switched off.

 

[whitenemesis]

Later model's a/c compressors are constantly engaged but have a bypass valve to modulate pressure. This is, AFAIK, to maintain a flow of refrigerant in the system and keep the seals healthy.

Suggest you get the car on STAR and see what's going on.

 

[bnqdiy]

Thanks for all the fast replies, I was just making sure there wasn't a serious problem that needed looking at.

 

[bnqdiy]

Had a bit think of the replies and am still a bit none the wiser, does anyone else have this problem or is it just my car. It really sounds like the engine idle speed is running fast. Also I can understand that the air con compressor could kick in even though the air con is off but this would not explain why the idle would go back to what it should be if the temperature is set below 18oC. Another thing I have found is that when the car is ticking over at 850 rpm if you turn the climate control off and then back on it goes down to 750rpm but if you switch from drive to park it goes back up to 850rpm. I have had a few days where the engine idles permanantly at 750rpm for no apparent reason and mpg seems better but then for no reason goes back to 850rpm.
Would really like to know if anyone else is experiencing this.

 

[Frostytel]

Had a bit think of the replies and am still a bit none the wiser, does anyone else have this problem or is it just my car. It really sounds like the engine idle speed is running fast. Also I can understand that the air con compressor could kick in even though the air con is off but this would not explain why the idle would go back to what it should be if the temperature is set below 18oC. Another thing I have found is that when the car is ticking over at 850 rpm if you turn the climate control off and then back on it goes down to 750rpm but if you switch from drive to park it goes back up to 850rpm. I have had a few days where the engine idles permanantly at 750rpm for no apparent reason and mpg seems better but then for no reason goes back to 850rpm.
Would really like to know if anyone else is experiencing this.

Sounds to me as if you may possibly have a split vacuum pipe somewhere

 

[bnqdiy]

Will get them checked. Has anyone else had this problem?

 

[bnqdiy]

vacuum hoses ok, car went to normal 750 rpm the other day for no reason lasted a few days then for no apparent reason went back up to 850 rpm, this even has my mechanic stumped as he has checked everything it could possibly be with nothing obvious at fault.

 

[bnqdiy]

Ok this just gets wierder, I have noticed if my car is on an incline when I start it the revs are normal at 750rpm but if I start it on the flat they are at 850 rpm unless I turn the climate control off which takes it down to 750 rpm. Really need some advice with this one so any help would be greatly appreciated. It has to be something to do with the climate control or air con even though I have that switched off.

 

[grober]

Ptc heater

You must understand that climate control means just that- the maintenance of a fixed temperature. This sometimes means heating as well as cooling the circulating air. IRRC on diesels there is a PTC heater element -bit like a mini radiator in front of the heat exchanger radiator in the aircon unit--- its rated at about 1000watts I believe. This is a considerable load which would be reflected in a need for an increased alternator output= greater engine load. My guess is this element is kicking in and out putting an extra load on the engine leading an an increase in engine revs to compensate?

 

[joecash]

its all to do with the set up. This is the equivalent of putting pressure on the throttle pedal when going up hill to maintain the same speed. The revs will not increase but the fuel demand will be more to maintain the same speed. Likewise when an a/c compressor kicks in there will be an increase in power demand from the engine. If the set up is correct there should not be an increase in indicated revs. If there is an increase in revs either the set up is incorrect or there is a problem-just my view

 

[flango]

Its perfectly normal when under high load and staionary engine revs can increase slightly. In my W208 when at idle turn heating on to max and revs increase by about 100rpm it mentions it in either the manual or tech bulletins. Both our volvos do exactly the same and its mentioned in the manual to compensate for increased electrical load engine revs may vary at idle. Thats why there is a specific list of what must be switched off when setting the base idle setting on these cars

 

[Satch]

MB, Toyota and Ford vehicles in our household at present and every single one does it.

Thinking back I do not think I have ever had or driven a vehicle with aircon that did not do it to some extent.

Worst was a Honda with a small engine where you could feel road speed drop when the A/C cut in!

 

[BrabusD6]

I have the same problem in my 2007 E320 CDI. Can't get any answers from anyone. It happens off and on and the only thing I notice fixes it at a stop light is to turn the system off completely. Then the idle goes to 750. Even if you turn the system back on, it will stay at 750 until you take off and stop at the next light where idle will be back at 850. This issue started at about 120K miles. Before then it never did this. Did you ever get your issue resolved?

 

[bnqdiy]

Nope mine is still the same and no further forward with any answers. I have also now noticed if I start my car on a hill the revs are normal at 750rpm but on the flat they go back to 850rpm, total mystery to me. Needs to go on the star machine I think to check for any faults. If you get any further info let me know.

 

[Charlie87]

Is this happening when the engine is cold, or not reached operating temperature ?

 

[bnqdiy]

This is when the engine is at normal running temp.

 

[sspeed]

I cant see what you are worrying about!
You have been answered several times now and yet still you are looking for a problem that doesn't exist!
Enjoy the car, drive it and stop worrying over a normal process !