I have been fighting the 221 / 231 Check Engine codes on my 95 945. Multiple possibilities are cited as causes for these codes, including O2 sensor, AMM, and air intake not going through the AMM. I have also read in the FAQs that the ECU can be involved.

I am trying to make sure that it is not an air leak before I start putting parts on it, and have fixed several already. I had noticed that I could get an idle surge if my foot was planted firmly on the brake, though not if only planted lightly. However I had always been in traffic when I noticed this and did not think of trying it in the quiet of my driveway until today.

So, today I noticed that I could hear an air movement sound when my foot is planted firmly on the brake. I don’t know enough about the inner workings of the booster to know for sure that this is abnormal, but I suspect that it is not supposed to do that.

In the interest of “full disclosure” I replaced the check valve with one from NAPA that fit the booster and hose, but which was actually a replacement for a Ford Bronco. The surging existed before I replaced it, but I had not listened for the air sound before replacing. The old one did act like a one-way valve when I tried it off of the car, as did the new one.

So, is it true that there should be no sound of air movement associated with steady foot pressure on the brake pedal?

Thanks,
Charley