So I was testing my newly rebuilt brakes, pumping them repeatedly (testing pedal travel), and noticed that the RPMs took a brief hop everytime I applied the brakes and then released them. Then as the engine warmed up, no more hop. Of course when the engine is cold, it runs rich, so a tiny bit more air through the brake booster line helps burn the excess fuel, raising the idle briefly.

When the engine warms up, the ratio leans out to the correct mix, and a bit more air really has no effect on idle speed, so the idle doesn't hop when the brake is applied and then released.

If my ratio were too lean, I imagine the extra air would momentarily reduce combustion efficiency, and the RPMs would drop.

The lesson: If you want to know how your mixture is doing while you're driving, just pump the brake. If the idle hops up, you're running rich. If it stays the same, you're running about right. If it briefly sags, you're running lean. Of course that's just the idle, but it's useful info, right?

This little trick should work with any car that has power brakes.

That sure beats texting while driving.

I think you are reading a bit much into that. Vacuume drop perhaps?

Horsemann, this isn't really much different from pulling a small vacuum hose to see whether the idle picks up or drops. The only difference is that you're introducing unmetered air via the power booster, rather than through a hose fitting. The attractive thing about this test is that it's so easy and so broadly applicable. It's also fairly subtle. However, the cold idle enrichment caused about a 200 rpm blip in idle when I pumped the brakes, which is readable enough.

@siteunseen: Er... I guess I should have said "while sitting in the driver's seat with the engine running, PARKED," rather than "while driving!!!"

That's funny! I need to learn those little smiley face things.