Hi Bruce

There can be a number of things going on.

I’ve done well adjusting my idle up a bit. Some makes of cars have controls that speed up the engine a bit when you turn on the compressor to compensate for loading by the compressor. Ours do not.

If your engine is working properly (compression, valves, injectors, filters, timing and throttle linkage) are all working as they should you may want to consider a Sanden AC compressor conversion. I’ve found the OE systems quite marginal when converted to r134. They run higher pressures and the condenser is under capacity. The Sanden seems to put a lot less load on the engine in the installs I’ve seen. MPG and power goes up noticeably. My understanding is the efficiency is better than the old R4 units. Around here the hot setup is Sanden + parallel flow condenser.

The easy path is check the idle. Then check for basic tuning stuff I mentioned above.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles