heater problem on 2001 Honda Civic LX

I purchased this vehicle from a (Toyota) dealer
a couple of weeks ago. There is an odd heating
problem.

Sometimes it behaves, and I have full control of
good hot heat inside the cabin.

Other times (especially when the ambient temperature
goes down to the 30s or below) I can't get any heat
at all from the vents, or hardly any at all. The
temperature gage was warming up to just below half.

But sometimes, it goes higher. I was afraid it was
really overheating, so once I had it towed back to
the dealer. They said it was a bubble in the cooling
cycle, and gave me the car back after backflushing
it. They assured me that it had coolant.

So it behaved in the middle (warmer part) of two
days again. But in the evenings (colder) it won't
give me heat.

And tonight the temperature gage went high again.
I don't believe it is actually overheating based
on the diagnosis so far. So I continued to drive
it this time. It went up, down to 3/4, once to
half, back up, up to red, up to pegged, and I
kept driving. With no heat in any vents no matter
what I did with the controls. And it did not boil
over.

Is there a T sensor on/in the engine that may be
defective and bads signals pegging the gage and
causing the inside heat not to get any hot water?
(I have no idea how much "smarts" they have in the
heating system of this car).

Any other ideas? I've reported it to the dealer again
and will be going back, but they had it two days and
it came back in the same condition, so I'm hoping
for a real diagnosis here.

Thanks, John

 

At fist glance, it sounds like you have air in the cooling system.
some cars are particularly difficult to purge air from, and about the
only way is to disconnect a hose from the heater system and (using a
hose) force water through the system until there are no air bubbles at
all. If you have an "automatic climate control system" this can be more
difficult, as the valve to the heater core may not be open under many
circumstances... Sometimes, low coolant level in the main loop (engine
block and radiator) will result in the guage excursions you describe,
and can essentially kill your engine if you ignore them. A blown head
gasket can either be the cause or result of running under this condition...

Once the heater core and plumbing is purged of air, you can proceed to
the main cooling loop (engine block and radiator). I'm assuming your
dealer just purged this loop and didn't pay any attention to the heater
loop.... (not an unusual situation with many of the dealers today...)
Most of the time, the block/radiator cooling loop is below the level of
the valves and/or piping that serves the heater core, so purging the
block look won't necessarily purge the heater core loop. Once upon a
time, the heater core would eventually get purged and the excess air
would show up as a drop in the "expansion tank" but this is not
necessarily the case with many vehicles today.

Just about any time you see the temperature guage moving around,
particularly if you see it into the red, you have a problem with air in
the system. This can also be exacerbated by a malfunctioning thermostat.

Other issues that could be part of your problem:
If the guage never went beyond mid position (ever) and you have problems
with cold heater output when the guage is in the lower half of the
normal area, and typically this only happens for the first 20 to 50
miles of driving, I'd suspect a thermostat stuck open (or missing). If
you've hand the vehicle only a short while, and it operated relatively
normally during the summer, and as you approach winter the temperature
takes longer or forever to reach normal, I'd think someone took out the
thermostat.... (typically, this is done if a car overheats in the
summertime, instead of fixing the root cause which is usually a plugged
radiator core).

 

condition...

Update: They are replacing the head gasket tomorrow.

Thanks guys! (at least one person replied privately and nailed
it too!) I never would have thought that would have been it.
I told the service writer this morning, and they eventually
concurred. Fortunately, since I just bought it, they are
covering the cost! Phew! Let's hope that ends it for good!

Thanks, John