No heat at idle - heats up with revs

[bean]

Hi guys,

New problem surfaced with the 90' Legacy. Starts great and warms up fast, but when idling at a light the heat slowly gets cooler and cooler. (Engine temp stays the same - dead center) As soon as revs come up it's instant heat again.

Doesn't seem to be the thermostat as the engine temp is fine, and it warms up fast. Water pump isn't very old (30K kms), and everything else is the same age. (OEM thermostat, all hoses, etc etc.)

A friend is having the very same problem with his newer legacy wagon, tried a new rad cap and found the heat to be hotter, but it still cools at idle.

Has me stumped - but it's currently -35C (-44C with the wind chill) here so not sure if the cold is actually just out powering the heater core at idle.

Any thoughts?

[dscoobydoo]

I once had a thermostat stuck open so that at the temps you speak about, the car would not put out anything more than a slightly warm fart at idle. On the road, I was forced to wear two coats.

At those temps, people often put cardboard in front of the radiator to help block some of the air flow. You might try it.

[bean]

Thanks for the input. Cardboard in the front helps the car warm up faster, and helps keep it at operating temps on the highway in the cold - no effect at idle.

With the thermostat - engine temp should then fluctuate with the heat temp - but mine isn't. It only takes a few hundred RPM to get the heat back.

More I think about it - I think it just might be too cold!

Rabin

[juice91si]

Thanks for the input. Cardboard in the front helps the car warm up faster, and helps keep it at operating temps on the highway in the cold - no effect at idle.

With the thermostat - engine temp should then fluctuate with the heat temp - but mine isn't. It only takes a few hundred RPM to get the heat back.

More I think about it - I think it just might be too cold!

Rabin

someone i know is having the same problem as you on his honda, if he figures it our ill let you know, the solution may be similar.

are you in canada? weve been seeing -40s with the WC for the last week

[bean]

I'm you're neighbour, and arch nemisis in football. (not a big football fan - but the rivalry is fun) I updated my profile - thought it had my location already.

Does your Subaru have full heat at idle in this cold? Needs to idle a while - but it definitely gets cooler and cooler till the revs come up a couple hundred rpm.

Has me baffled - but it just could be an inefficent heater core after 370k kms. Hopefully the temps will get into the teens and I'll be able to see if the problem goes away with the cold.

Rabin

[Legacy777]

The symptoms highly point to a lazy thermostat or a non-OEM thermostat.

If you haven't replaced the thermostat recently, that's what I would suggest doing. If you have a garage, it shouldn't be too bad. Just remove the two bolts holding on the housing to the water pump. That will drain the radiator and engine once the thermostat is removed. Then just put the new one in along with a new rubber gasket, and fill with coolant.

There is a bleeder/vent screw on the passenger side of the radiator. Make sure you leave this off as your filling, and initially when you start up the engine to allow all the air to escape.

[bean]

Thanks Josh - and it is baffling - but I don't think it's the thermostat. It's OEM, and it only has maybe 20K miles max since it was done. Engine temps are rock solid.

If it was the thermostat stuck open, the car would not warm up at all in these temps. It's really fricken cold - most people can't even comprehend how cold it is. Currently it's -24C (-12F), but the last few days were -37C (-35F)

In the mornings it started fine, and with the high idle is blowing warm air by the time I get in it. (remote starters rule) It was only if waiting in traffic the temp falls at the heater. Engine temp stays fine.

If it continues after this cold snap - then I'll definitely do the rad cap and thermostat (both OEM) again. Still bothers me that engine temp is so solid, and that it only takes 2-300 rpm to regain good heat.

To me it really points to the flow at idle through the core not being sufficient to heat the cold air. Curious about the guy in Winnipeg - if his car is fine then my core might be plugging up and just not as efficient as it should be. (Buddy's 98' Leg wagon is doing the same thing too)

Rabin

[Legacy777]

Could be flow as your thinking.

When's the last time the pump was replaced?

Does putting the trans in neutral & revving the engine bring the heat back?

[bean]

Pump was done at the same time - OEM as well. Not sure if you remember - but I've had the car a while. Total basket case when I bought it. $5k+ in parts alone later and it's my daily driver beater.

Engine was pulled, resealed, water pump, head gaskets done, all new hoses, etc etc. TONS of work on this car - and it's in great shape mechanically - quite rough shape cosmetically.

The moment it starts revving at ~1200 or more - heat's fine. Engine temp never changes and is rock solid. Temp gauge comes up fairly quick so I think the thermostat is working fine.

Rabin

[93forestpearl]

I've noticed this in both of my Legacys in cold temps. When I pull away from a light and load/rev the motor, the temp from the heat noticably increases. I've just always blown it off as not enough coolant cycling through the heater core to maintain.

[juice91si]

definitely weird. if its any reassurance, i believe you do have a problem. in the same temps i have hot heat even at idle.

[ericem]

I had the same problem. Never really cared TO much. When my rad went, did my radiator cap, all hoses, and the thermostat along with a gasket, and now I have no issues. Generates heat fast and holds heat, I usually always need to turn down the heat once the engine temp is up. I also used Subaru coolant. Be sure there is no air in the system either, but that would lead me thinking the engine would overheat.

[wtdash]

Before my turbo swap, I had the same issue - w/new coolant and Tstat - little heat @ idle; fine @ cruising speed. And this was @ 0 F/-18 C, so i'd just be glad it's running @ -35 C - Holy Puck!

[douglas vincent]

Same issue here. Never figured it out. Was frustrating, not to mention cold....

Maybe check the direction of the heater hoses? try swaping which hose goes where? I "think" thats what I did when i replaced the last motor, and now everything is dandy.

[93forestpearl]

Top hose goes on top?

[bean]

Hmmm - I never thought of swapping the hoses at the heater core.

Did some more playing last night and the car idles at the second notch on the tach (In gear) which I assume is 750 rpm (first notch is just off the bottom, second is halfway to 1000 rpm). Raise it up to just 900 - 1000 rpm and heat is fine.

Juice - can you confirm which heater core hose goes where? It's still bloody cold here so this can wait till it warms up a bit. I have a garage with a furnace, but it's not insulated - so it's useless in these temps. Once it's -15C it should be fine.

Car is unbelievable in these temps. Starts great, great heat (when not idling) and the AWD with 4 winter tires makes for some awesome winter driving fun. Have an event Jan 1st and will try to post pics of me running.

Thanks for the input guys - helps a lot to bounce around ideas.

Rabin

[mexicanzero]

personally i'd think its the heater core
if all the passages are fairly blocked up at idle with low coolant flow it might be going through only 1 or 2 "lines" when you rev up a bit and flow increases it might start squeezing its way through a few more lines heating up more of the heater core.
when you get a chance unhook the hoses to the heater core and stick some clr in both hoses let it sit for a while then flush it out with water and then blow out the water and refill the cooling system.

just an idea.
-Alex

[Freerange]

if it's below 15 F, I have the same problem, no or little heat at idle, good heat above 1000rpm's in my 90 and 91 Lego's, and in my 85 toy p/u (well the toy just plain has weak heat) but I've narrowed it to tired heater core in the toy and I'm reasonably sure it's the same problem in the Lego's. At some point I'm going to remove the t-stat and run it open cycle just to see what happens. None of the cars overheat, all have great coolant depth and oem t-stats, rad caps, water pumps, raiators. I don't really want to swap heater cores(pita factor) so I'll stick it out, just thougt I'd chime in. My 92 t-leg has broken fan controls, so I'll report on that later.

[evolutionmovement]

Keep an eye on coolant appearance and level (though they generally don't reveal their problems). A bad HG could be putting air into the system, which collects by the water pump impeller under idle, reducing circulation. When you hit the gas, the surge pushes heated coolant through the system, including the heater. If that is the cause, the issue will continue for a little while as it is before becoming bad very quickly as the volume of air increases and collects by the water pump. A trick that might get you home in that case is to pop off the bottom hose at the water pump quickly before reattaching it and adding coolant for what spilled. If that is impractical, squeezing the two radiator houses might be able to dislodge some of the air from the pump cavity to similar effect. What that should do is clear the air out of the way so the pump can circulate before the air builds up again, but some air may end up in the gooseneck, giving a high heat reading that may or may not be accurate (but always assume the worst) as the temp gauge only shows what the temp is at the sensor, itself. Not saying that's it, but making you aware so it doesn't catch you by surprise.

[jefferson]

Another thing to check into is whether or not the ac evaporator is plugged by debris that won't let air flow over it very well. Pull your heater fan and take a look at things. It sounds more like a lack of flow of coolant through the heater core to me, but still would be worth checking the air flow too.

Jeff

[bean]

Car has had the head gaskets done, and had an OEM water pump at the same time.

The issue is new this winter, didn't do it last winter that I can remember... Still thinking that it's the heater core itself and will likely try flushing just the core when the temps are less severe.

Air flow through the core is fine, and when rev'd the heat gets pretty damn hot - faster you rev it - more heat. Screams coolant flow to me... (Engine temp dead in the middle the whole time.)

Still - got a new rad cap on order, and will likely put another OEM t-stat in and change the coolant while I'm in there to flush the core out. Won't isolate the cause if it's resolved unless lots of crap comes out of the core - but it certainly won't hurt doing some preventative maintenance at the same time.

As an aside: Tomorrow is our first rally-x - and it's currently blizzarding out... I'm nut running studs - so lots of snow is a blessing. Can't wait to rip around the lot in it...


Rabin

[japanesecowboy14]

Easy one.... had this problem a couple times.

Burp your radiator... open the cap & let the car warm up... keep pouring coolant in as it "burps" air out, and once the coolant hits top level and stops, (40 or so minutes worth of doing this PITA) you should be good. VERY TINY air bubbles will affect your heat when idle.

[bean]

I think you're right...

After the rally-x the car had super hot heat at idle. I was like WTF? Now it's an intermittent problem.

Resevoir levels were fine, rad level was right near the top so I kinda ruled it out.

I think a new cap and bleeding the cooling system with the front end up nice and high should fix it for sure.

Rabin

[japanesecowboy14]

Yeah usually they will show right at top, even when car is warmed up, but if you leave it for a couple minutes, it will eventually work it's way down...The only peeve i have about the old legacys is the PITA it is to put in coolant BECAUSE you can't just fill to the line, you have to sit & babysit it until the air gets out.

[japanesecowboy14]

Yeah usually they will show right at top, even when car is warmed up, but if you leave it for a couple minutes, it will eventually work it's way down...The only peeve i have about the old legacys is the PITA it is to put in coolant BECAUSE you can't just fill to the line, you have to sit & babysit it until the air gets out.