I have had some not great vehicles in the past. So, I used to have a 72 Chevy Impala, with a 350 engine. It had been a good car, but, I was in high school when I had it, so, I didn’t take any care of it. Who knows what important features I neglected, oil, water, etc… in it’s upkeep.
So, towards the end, I would run the air conditioner. Of course, it put a strain on the engine, and the car would run a bit more weak. Understandable; but, the strange thing about it was, that while I was driving, and turned off the A/C, the car would continue to run like crap for about 30 secs, MOL, and then go back to normal.
This happened in another vehicle or two that I had owned. I can’t really remember what the others were.
So, to be more precise, I am wondering why the engine didn’t go back to normal immediately, rather than the 30 second delay?
One of our resident auto experts can probably give you a better answer, but without knowing the design of your AC system, my guess would be that it had a relay controlling the compressor clutch and that relay was worn out and starting to stick a bit when it turned off.
When I turn off my AC in my apartment, by flipping the thermostat switch to OFF, the fan continues to run for about 30 seconds. When I turn it on, the fan doesn’t start for about 30 seconds. I think the reason for this is so various components will have time to return to normal before being left unventilated, or before being stressed by the ventilation of the fan.
Maybe there are some component in car ACs that need to equalize for all components to go on or shut off, and are designed with lag times. In a good-working car, one does not notice this, but when your old Chevy does it, you can still detect the difference during the interval.
An analogy woud be a modern car with an electric cooling fan, that can continue to run after everything is shut off.
Carbureted car (ugh!) carb was no doubt out of adjustment, idle up circuit not or barely functional, and car way out of tune.
Turn in the AC and engine struggles mixture is wrong idle is low one cylinder might be loading up.
Turn the AC off it takes a bit for everything to clean up again.
Turning on the AC might have bumped up the idle a bit. eg to 900 rpm if its running right.
Turning off the AC also turns down idle …to eg 600 …
basically the rpm was low and then fuel going unburnt and its missing and carrying on, and attempting to run it at the low RPM doesn’t clear up the result of rough running.
Apropos of nothing. I had a '72 Impala at one time. Had the 400 small-block in it. You could practically stand in the engine compartment to work on it. One neat little feature it had was a small fan in the back deck to use as a rear window defogger. IIRC, the AC compressor in it was HUGE and was bolted on with a clamp in back and one in front. It was built by Frigidaire which GM owned at one time. Another feature I liked was the climate controls were to the left of the steering wheel so your passengers couldn’t mess with them.
The way it’s supposed to work is there’s a little solenoid that is energized when the AC turns on that bumps open the throttle linkage a little. Most likely what happened is that either the solenoid broke or was out of adjustment, but someone either couldn’t or didn’t want to fix it and decided instead to do away with this high idle/low idle nonsense and set it to a happy medium idle: fast enough that it wouldn’t quite stall the car with the AC compressor on, but slow enough that the car wouldn’t drive off on its own too quickly with it off.