check engine when braking

[tmac5]

I have an '85 Buick Electra. The "check engine soon" light comes on when I break, stays on for awhile after I stop braking, and then goes on. There have been a couple of times when it just comes on but the majority of the time, it's when I brake. Any thoughts on why it's doing this?

[Blue Bowtie]

Does this also occur if you depress the brake pedal when the vehicle is not moving, or only when braking while moving?

If it happens while sitting in PARK and braking there may be a problem with the power brake booster dropping vacuum to the point where the MAP sensor faults. It might be very helpful to retrieve the stored error codes from the ECM. All that is required for this is the ignition key and a short jumper wire (or paper clip) and the instructions.

[tmac5]

Yes, it comes one when I first get in the car and depress the brake pedal to go into reverse out of the driveway.

[Blue Bowtie]

I'm suspecting there may be a problem with the brake power booster allowing too much leakage. This would be a vacuum leak that the engine (and MAP sensor) cannot tolerate at idle speeds. However, there is a remote possibility that the problem is electrical.

One potential method to determine this is to disconnect the vacuum hose to the brake booster, then start the engine and depress the brake. There is no need to move the vehicle and you may not want to anyway since there will be no power assist for the brakes with the vacuum hose disconnected. If the problem does not occur with the vacuum line capped, start looking more closely at the brake booster.

[Tech II]

Or the check valve going into the booster....

[Blue Bowtie]

Originally Posted by Tech II

Or the check valve going into the booster....

Correct.

That would be part of the possibly "looking more closely at the brake booster."