Rough Idle when Air Con is On
 

As title says my zt cdti will idle normally if the aircon is off however if the car is left idle and aircon is on the idle is rough with revs increase slightly and then drop off. Turn off the aircon and it will idle prefectly smooth again. I presume the aircon is loading the engine somewhat. Is this normal? Its rather annoying if your stuck in traffic around town.

Sorry missed cdti bit

Aircon compressor does put a slight load on engine and drop in revs slightly but shouldn't be massive.

Thought you might have done Click. :D

Paul,

Check the crankshaft pulley. On the diesel the multi-layer construction is prone to weaken and eventually fail. It could be that the extra load of the air conditioning compressor is putting additional stress on the pulley and affecting the idle.

Simon

The ECU should detect the extra load when the a/c is turned on and adjust the idle speed accordingly but I am not sure what sensors it uses to do this.

Cheers guys thanks for help. I've been offline for last few days bank holiday weekend over here so only following up posts now.

I'm getting exactly the same.
Is there anyone on here that has resolved this?

As said above David, crankshaft pulley breakdown is the prime suspect. I'd suggest removing the wheelarch liner and carrying out an inspection.

Simon

I'm unsure what I am looking for.

The pulley isn't noisy and I'm not sure I'll see much without taking the belts off.
Any ideas?

There is plenty of information about this on the forum David. The diesel crankshaft pulley isn't solid metal; it is constructed partly using a flexible rubber-like material. It is this which fails. If you use the Google search facility at the top of this page you will find out all you need to know.

Simon