Car is a 90 GXL, emissions intact (minus cat).

The car started acting very strangely. It was on the edge of being dangerous to drive. I first noticed about a week ago that throttle response was twitchy and would randomly buck on deceleration. When the AC kicked on it would shoot to a 4000 RPM lopey idle. When it idled at a stop it loped at 1600-2000 (very quickly as if it was a bad port job). About 70% of the time when I would come to a stop the brakes would stiffen up like there was no vacuum boost and make it very difficult to stop from 70 MPH. A little dangerous... The idle was a little embarrassing at a stop so I shut the car off. When I restart the car it idles at 5000RPM! I tap the throttle like its the AWS jamming, but it only stops when the car starts to move.

I though, OK... the TPS is bad I'll replace it no problem I have an extra. Replaced it..

It threw codes 12 and 18 referring to the TPS full and narrow range being fixed in limp mode. That explained the idle, but it didn't explain the jump in RPM when the AC turned on (I know the BAC would do that, but why so much. It should be based on vacuum not TPS readings right?). It also does not explain the brakes freezing up. It seems like a massive vacuum drop or leak, but why would code 12 and 18 cause this? Surely Mazda would not program their ECM's limp mode to do this...

Anyway, I did change the TPS and reset the battery to clear the codes and it seems fine now. I'm just stumped why a good car would suddenly act like this all at once.

Can anyone tell me? I replaced the alternator 2 weeks ago, but prior to that I have had to do zero work no this car in about a year.

 

Well, when the engine is revved really high, it probably won't have vacuum any more, explaining why you lost the brake boost.

Now, the real question is why is it revving so high? Try unplugging the BAC at idle with the AC on and see what it does. I'm wondering if there's something else idling it up when the AC comes on. I didn't think the BAC could idle it up to 5000 rpm by itself

Did you see what the TPS was set to?

 

you should always have a full brake booster reservoir unless the check value has failed. i would check that for your brake issue.

as for the high lopey idle, can't say for sure what would have caused that. sounds more like a throttle linkage issue than anything else, or the tps itself was holding open the throttle. if your brake booster is on it's way out some of those symptoms sound related.