response 23 of 40:

Apr 1 17:05 UTC 2003

I understand the "secret", but I found that the saleman at the dealership
I dealt with was the same person for new and used cars.  That is, they
sold both, and all the salespersons had full information about both.
In this situation the "secret" is irrelevant.

response 25 of 40:

Apr 1 23:04 UTC 2003

I've used a version of the method toasty describes with good luck.

response 27 of 40:

Apr 2 14:09 UTC 2003

Ah, all right.  I was thinking of their early ones, about which no one
seems to have anything good to say.

I found my biggest problem at car dealerships was convincing them I
didn't want to buy an SUV, and no, I didn't want to test-drive one just
to be sure.

response 29 of 40:

Apr 3 17:51 UTC 2003

Wow.  Your dad's luck has been a lot worse than my dad's luck.  I think
part of it is my dad found a car dealer early on that was honest.  He
never bought a bad car from this guy.  In fact, there were a couple of
occasions where he told him, "you don't want to buy this car."  You're
more likely to find this kind of honesty in a small town, since word
gets around fast and you don't have a constant supply of victims like in
a larger city.  A dishonest dealer in a small town is not likely to stay
in business long.

response 31 of 40:

Apr 4 02:31 UTC 2003

My dad bought a number of used cars from service stations & repair
shops that sold 'em on the side (for customers unloading old cars).  
The places were locally owned & knew dad as a long-term customer,
so they gave him good cars too keep him that way. 

response 33 of 40:

Apr 6 10:55 UTC 2003

Older cars with open loop control systems & carburetors had to do weird
things with A/C & automatic transmission for the idle speed.  So, yes,
on those cars, engaging either would make a difference (of course,
engaging an automatic is going to affect engine speed anyways).  I have
never heard of a car where engaging electrical accessories will increase
idle.  Most newer cars have closed loop control systems and "constant
idle"; there's no reason why idle would increase for any accessory on
such a car.  On my car (1988 volvo, efi) when the a/c cycles on or off,
the engine idle speed glitches slightly.

On *very* old cars, at idle, the battery would not charge.  On those
cars, the headlights would get noticeably brighter when engine RPMs were
increased.  This is still true on many modern motorcycles.

Nearly all modern spark ignition engines depend on a functional
battery/electrical system.  The only exception I can think of is some
lawnmowers which use a magneto instead.  EFI certainly depends
electricity; E = "electronic".  If the battery fails "in operation", the
car may continue to operate until the electrical load exceeds the
charging sytem's supply -- ie, high load, low supply - headlights & all
accessories on and engine idling "in gear" stopped with an automatic.  I
was once in a car where the battery post had rotted through (battery
acid) and snapped.  Car worked fine until a stop light, then it was a
total systems failure.

Older diesel engines did not need electricity - so on such vehicles
(probably mostly trucks) a battery failure won't kill the engine.
Starting it might be a problem (although there are diesels that can be
started using compressed air or other means).  I believe modern diesels
are switching to the use of electronic closed loop systems (pollution,
economy, efficiency), so a functional electrical system will be required
there.

The april issue of CR has a much more complete list of things to look
for in a used car.  I don't think they mention anything about idle speed
(which on most cars is hard to judge anyways) but they do mention hard
braking, also having a friend watch to see if the wheels line up with
the car's motion (if not, could be frame or wheel alignment issues),
also looking for signs of car repair work, and other stuff.  A good
mechanic should be able to tell you if there's anything "off" about the
car's idling or other characteristics, and can look for a lot of other
things (wear in suspension components, brake pad life, battery
condition) that you could not necessarily tell just by driving.

You should still budget money for things the mechanic might not catch.
The older the car, the more likely it is that really strange stuff could
fail.  Not all of these are practical to find, so any older car *is* a
gamble.  This is why they're worth less money.

response 35 of 40:

Apr 8 17:18 UTC 2003

the "12V" automotive "standard" is actually 13.8VDC, so a gauge indicating
14VDC isn't a reason for concern.

response 37 of 40:

Apr 9 02:52 UTC 2003

True, the variation is interesting.  #35 was directed more at people who
didn't realize 12V isn't always 12V, not so much at you.

response 39 of 40:

Apr 11 17:35 UTC 2003

In English please?