Could I have messed something up while changing the battery?

Dear Car Talk

Dear Car Talk | May 01, 1996

Dear Tom and Ray:

The local mechanics think I'm crazy. My 1989 Chevy Beretta with a V-6
engine and automatic transmission doesn't idle correctly, and I believe it
is an electrical problem. Here's why. Until one day last week, the car was
perfect in every way. It started, idled, ran fine and got good gas mileage.

Then the battery died. I replaced it myself, and the first time I started
it with the new battery, it was a different car. Now it idles erratically,
hesitates, sometimes revs itself to 2,000 rpm, sometimes barely idles at
750 rpm, and occasionally even stalls at traffic lights. None of this
happened before the new battery.

A couple of mechanics say it needs a complete tuneup, but I believe it's
much simpler. Since the symptoms came on so suddenly (literally, it ran
perfectly when I shut off the switch to change the battery, and it was
messed up when I turned it back on) I believe I must have inadvertently
damaged some electrical component while replacing the battery, although I
didn't touch anything except the battery terminals and cables. What's my
car's problem? -- Bubba

TOM: I can think of two possibilities, Bubba. The most likely is that you
accidentally knocked off or broke a vacuum hose while you were changing the
battery. A vacuum leak would explain all your symptoms.

RAY: If it's not a vacuum leak, then it could be electrical. GM computers
DO lose all of their engine management settings when the battery is

TOM: But the computer is supposed to automatically re-learn all of the
correct settings from the engine sensors once the battery is reconnected,
and you've taken a sustained, moderate-speed drive.

RAY: So I suppose it's possible that you just haven't had the car over 30
or 40 miles an hour for long enough allow the computer to reset itself --
at least when you wrote us this letter. And if that's the case, by the time
you read our answer, your car will probably have miraculously healed

TOM: If not, then the computer may not have reset itself as it's supposed
to. And in that case, I'd suspect a bad PROM.

TOM: PROM stands for Programmable Read Only Memory. It's a replaceable chip
on the car's computer that tells the computer which GM engine it's

RAY: Ask your mechanic to put his scan tool on your engine and check for a
bad PROM, Bubba.

TOM: I had a bad PROM once. It was 11th grade. I asked Alice
Winkerschnitzel to go with me. She said sure, but she asked me if she could
bring a date.

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