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1999 Ford Escort Zx2 2.0 DOHC Rough Idle drop in RPM when heat on

So I turn the heat on, and my engine feels like it’s going to stall out, the RPM’s drop by 200. What could this be?

Also, It does this if the car is in neutral, or any gear, stationary or moving.

Also, I did get a new water pump, and my check engine light is flashing a code for P420. Haven’t asked mechanic yet. It’s been doing this for two cold seasons now.

Your car is fairly sick and you need to just take it to your mechanic.

In one of your follow ups you said that “my check engine light is flashing a code for P420” - clarify that since your check engine light won’t flash codes. It will flash, however, and if you are driving with a flashing check engine light then you risk turning what may be a simple, repairable problem into a much more expensive, perhaps car-killing problem.

As for the rpm drop, there are a couple of things. One is that the fan does put some load on the electrical system. If your battery/charging system are not healthy power draws can lead to engine stumbles. Its also likely, however, that your vent selector is set to defrost which will activate the A/C compressor. The A/C compressor puts a major load on that tiny engine.

Anyway, it doesn’t sound like you are going to be dealing with this so you just need a good mechanic. Your battery & charging system should be checked out. You should also make sure that all maintenance is current - spark plugs, wires, filters…

.< I shouldn’t have said the light was flashing, it’s steady. Catalytic function below threshold. I just changed the battery and the alternator. Starter is new as well.

The ZX2 has only a couple of vent position selections where the A/C compressor does NOT run. If you use the bi-level position, the A/C runs even though you haven’t turned it on. The compressor loads the engine and drops the rpm. Read your owner’s manual.