Cannot idle with airco on

[mgwanderer]

Hi guys,
a friends Tam appears to have problems at idle. Car idles a bit lumpy at 1000/1100 revs but has plenty of oomph when driven. If the airco cuts in when idling the engine dies.
On checking the exhaust temperatures per cylinder number 6 is far colder (90 degs) than the other five and as it is nearer the bulkhead I would have thought it would be warmer. the spark plug seems to be the same colour as the others
Any ideas

Many thanks in advance

[sixor8]

If the engine is stalling when the air-con is turned on, it is not responding to a change in engine loading. Could be as basic as running lean (poor fuel supply / pump), but could also be an ECU issue.

[mgwanderer]

I agree but why is number 6 so cool when in my mind it should be the warmest of all the exhaust tubes.

It has a new ecu fitted which has solved a lots of the old problems and the car is very quick but it is just this idle problem.

[astonman]

Lumpy idle and no.6 cold?
Have you checked the throttle volumes using syncrometer?

[spitfire4v8]

Could be no6 running high airflow, or low ..

but ..

whenever i see lumpy running issues and something amiss with no6 cylinder the first thing that goes through my mind is valve clearances.

No6 is hard to get to and you'd be surprised how many service agents just don't bother to adjust the clearances on no6 ...

[astonman]

Good point,I hope you're wrong.
I had similar running on an engine with a stretched valve ( overheated on weak methanol fuel).
The exhaust tappet clearance was zero.
Could get same issue with valve recession over time and no maintenance.

[non_linear]

Worn butterfly spindle bearings?

[mk1fan]

Easy checks;

Balance of flow through the throttle bodies with a syncronometer. If out, re-balance, set idle speed and reset adaptives (simple but need the IT equipment).

Harder checks;

If no different then when is the next '12k' service? If soon, mention cool exhaust temp of no6 to the service agent and ensure they check and set valve clearance at the back.

If no different then into testing the wiring you venture ...