Hi all,

I have owned my car for almost 1 year and I love it. It is summer time these days so I always turn on the AC in the car when I am travelling with my family. What I can notice is that when AC is on, I stop at a red light and leave the gear stick in D position, the idle could drop to below 600 rpm for around half a second and i can obviously feel engine shaking and then rpm goes back up again above 600 rpm. This symptom can be magnified if my car is stopped, the stick is in D position and I steer. But if I put the stick into N then the rev drops less.
This problem disappears if no AC is turned on. Could that be the AC pump is seizing and the engine is struggling to keep the idle rev when AC is on? Or could that be ignition coil issue?
In addition, in case I need to replace it with a new AC compressor, any one knows the Denso part number of the AC compressor for XJ8 L with rear climate control?
Thank you everyone in advance.

Car: 05 XJ8 L with rear climate control.

 

The system is designed as a climate control system with the expectation that the air con part is on all the time to dehumidify the air. In winter, this air is then heated to warm the car. In summer, the heating matrix circuit is shut-off by flaps and the car cooled.
You are not saving any money turning it off; these systems need to be kept in use. As you are finding, lack of use is now causing problems. Why not have an air con service and get the refrigerant and lubricant replaced and see how it goes.You definitely don't want to be whacking-out on a compressor before cheaper alternatives have been exhausted. The compressors on these cars run all the time and control is via a swash-plate that varies the pumping stroke. If the air-con is off the swash plate is normalised so no pumping occurs.

 

Noted with thanks! however, the AC is very cold and I am very happy with it, just that it drags the engine quite a lot, expecially when idling, causing the drop in revs and vibrations. I just wonder if it is normal?

 

The system is meant to compensate for loads on the engine, so there may be something not quite right. On my car I never notice any problem when idling and this summer we've had 30C + temperatures.

 

I would suggest renaming the title of this thread to something like "idle speed drop" or "erratic idle" instead of pointing toward the a/c. My car has a similar issue but it has not been enough of a problem for me to dig into (mine dips RPM for a second sometimes at idle, but comes right back). The fact that it happens more with the a/c on could indicate a compressor issue, yes, or possibly an electrical issue as the cooling fan will be running at speeds less than approx 35 mph. An aging cooling fan motor could be putting excessive load on your alternator. But that's just a guess. Hopefully others can reply with some specifics for you to check.

 

 

Hi all, thanks for all the replies on this thread. I have finally fixes the problem, by replacing the brake booster. It was certified that when I step on the brake pedal when slowing down the car, the idle will drop to a point where I can feel the engine hesitate and then back to 6xx rpm. The symptoms became more and more serious for the past few weeks so I believed that it is the brake booster causing vacuum leak when brake was applied.
i replaced my brake booster, bled the brakes and tested the car, no more engine rpm drops.
Just sharing the info in this forum so other members can check their brake booster if having the same symptoms.