No heat at low rpms

Car is a 1990 Integra. I live in a cold climate (eastern Canada). After
a cold start, engine warms up normally, just idling (a bit slow IMO but
it's always been like that). Driving around with the engine warmed, up
the heater puts out lots of heat (normal).

Problem is after the engine is warmed up, if I let the car idle for a
couple minutes the heater air gradually starts to get cooler and cooler,
and after a couple minutes the air is almost cold. Engine temperature
doesn't change (engines stays warm), the temp needle always sits at the
same exact spot in the temperature gauge (about in the middle). Now if I
rev the engine a bit, like 2-3000 rpms (or if I start driving), heater
air temperature rises instantly back to normal (hot). The air temp stays
hot as long as the engine is revving, it seems like there is no coolant
flow to the heater core when the engine is just idling, and cold outside
air eventually cools down the heater core and the fan then blows cold
air. When the engine revs faster the heater core gets hot fluid and the
heater puts out warm air.


What could be causing this?
That's pretty annoying as I can't get much warm air in the car when city
driving (not much heat comes out waiting at traffic lights) and I can
barely get the windows defrosted. Driving at highway speed is not a
problem, lots of heat.

Coolant level is normal, All coolant hoses and thermostat have been
replaced a year and a half ago. When the engine warms up the temp needle
always rises and then stabilizes to the same spot, no matter what the
outside temperature is. Heater blower works normally, normal air flow
all the time.

Any ideas?


Carl
1990 Integra

 

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The rad is full to the very top, and the reservoir is half-full?

'Curly'

 

has it always been like this or is it a recent phenomenon?

basically, if the weather is extremely cold, and the engine's not
working, it's not generating as much heat as it could. similarly, if
the engine's not revving, it's not ciculating coolant as fast as it
otherwise would. the car will never be able to produce as much heat in
these conditions as it could when working.

that said, /if/ this is a recent phenomenon, it could be that the heater
core has become partially clogged. if you had the coolant changed a
while ago, & the techs didn't use deionized water, there could be
corrosion buildup within the heater core sufficient to take the edge off
the thermal transfer. you may be able to use a chemical flush to clean
the system, but be very careful to use the neutralizing solution after
the main cleaning, then flush a couple more times with deionized water,
before refilling with good quality coolant. there is a small risk with
this procedure if the rad/heater cores are too corroded, but it usually
works fine.

 

Seems like rpm related, possibly a flow problem. Either clogged system or bad
water pump impeller...

 

Jim,

Is it corrosion that makes tap water unsuitable for coolant? I thought
it would be buildup from alkaline constituents, like calcium and
magnesium carbonate. BTW where I live the water is very hard, so I try
not to let people I know 'just use tap water' because 'it will be all
right'.

Mark

 

Well, I added some today, it took about a cup in the rad. The overflow
reservoir was a bit below the min line, so I filled it up the the MAX
line.
I felt like it wasn't as bad as yesterday, the heat output seemed much
constant. But I didn't drive much and the weather was much warmer today
(yesterday when I first noticed the behavior the temp was around -15C
(5F).
I'll keep an eye on it over the next few days.

Thanks

Carl

 

I first noticed it yesterday. I don't do much city driving with this
car, sitting at traffic lights waiting, engine idling. At highway
speeds, I usually get plenty of heat.
So it may not be a recent phenomena.
The temperature was around -15C (5F) yesterday, we usually get much
colder weather later in the winter. -25C (-15F) is not uncommon, and the
last time we had that (last winter) I didn't notice any heat problem. I
use the car almost exclusively to go to work, most of it at highway
speeds.

Carl

 

=====================

If there was air in your rad, but the reservior was fairly close to
normal, you've got a problem. The cap and hoses should be checked for
leaks. You're sucking in air, probably while cooling down. (or a leaking
head gasket) There shouldn't be any air in the rad.

Don't use _any_ tap water in your rad. Honda premix is safest.

'Curly'

 

can be both. limescale is common, but aluminum is highly reactive once
its passivated layer is disrupted too. modern good quality antifreeze
formulations take care of it normally, but adding rogue minerals to the
system in the form of tap water makes its job much harder.

 

Just a follow up, looks like the coolant I added did the trick. Today's
weather was pretty cold (-27 deg C) and I had plenty of heat coming out
the heater with the engine idling. I still notice a slight increase in
heater output temperature when I race the engine to 2-3000 rpms, but
nowhere as dramatic as before, where the temperature was going from cold
to very hot in a couple seconds as I revved the engine. Looks like the
coolant level was a bit low, now when the temps get a little warmer I'll
just open the bleed screw on top of the engine to make sure all the air
is gone.

Thanks for your help

Carl