Engine revs to 3000rpm when brake pedal pushed

[cloaked_chaos]

I had a very weird experience last night. I stopped at a gas station to fill up my tank. When I finished pumping my explorer started normally, but when I pushed the brake pedal it would rev up to 3,000 rpm and stay there until I let go of the brakes. I put it in drive, same thing. I revved the engine with the gas pedal a few times, still revved when pushing the brake pedal. I shut the engine off and restarted it, same problem. I checked the carpet and floor mat, neither were interfering. I looked under the hood. The throttle cable was clear of any obstructions. All my vacuum lines were fully attached (they have a tendency to pop off if I don't fully start the engine and it backfires a little). I pulled the brake booster vacuum line and there was only a slight change in engine rpm, which is normal when pulling a vacuum line.
After a couple minutes of trying to troubleshoot this, I noticed that the engine rpm was dropping while pushing the brake pedal. After I pushed the brake pedal a few more times, it only revved a slight amount. Then it went away altogether.

This is the first time I have ever experienced this, and I have not experienced it since then.
The only thing I had done differently than normal was I filled it with 91 octane instead of the normal 87 (already had half a tank of 87) and I put a fresh quart of oil in the engine about 10 miles back when I stopped at Autozone.

Does anyone have any ideas as to why this happened so I can prevent it from reoccurring in the future?

 

[mehedgec]

Maybe a bad check valve on the vaccum line to the brake booster?

 

[R&T Babich]

... All my vacuum lines were fully attached (they have a tendency to pop off if I don't fully start the engine and it backfires a little)....

How many miles on the engine? Our old '92 started doing the backfire/blowing off vacuum lines thing around 140k. It died at 219k with a blown head gasket, but looking back on it I think the timing chain was probably causing the backfires. Our new '92 and '93 have around 100k on them and don't have the backfire issue when it doesn't fully start. A Ford van 351W I had with 120k was backfiring, idling weird at times (pre-computer) and dieseled. Checked the timing chain slack and it was almost 15 deg. Replaced the chain and it ran great until it died at 240k.