My problem is that I'm not getting much heat out of the vents. It's only lukewarm, and the airflow is restricted like the car isn't warm yet. The air used to get hot enough it could almost burn you if you left your hand right in front of the vent.

I just completed a bunch of tuneup stuff on the car, though none of it dealing with the cooling system (just cleaning the intake). Since then the Temp gauge points to ~185° [1/4 up from cold] for quite some time before reaching up to ~210° [1/2 way], which is where it used to point most of the time. I've felt the radiator and hoses and it seems that the gauge is correct. It could be that it was always this way and I never really noticed.

I can feel a change in air flow, so I know the door in the airbox is doing something when I turn from cold to hot. Airflow is normal and good on cold or other vent settings, it only suffers on hot. I plan to open up the airbox in the passenger footwell this weekend to look around. My thoughts are:

1. There may be something jamming inside the airbox. (Easiest to check, so this is first.)
2. The heater core spontaneously got a fat clog in it and needs flushing. (I know this is a common problem with Dexcool, but I always imagined it would be more gradual of a problem.)
3. The thermostat is broken and coolant isn't flowing where it is supposed to. If it's stuck open, then maybe the coolant won't reach temp because it's circulating through the radiator at all times.

I checked coolant levels in the radiator and overflow tank and both are good and clean as well. I had the water pump replaced ~7 years ago, so I would think that's still good, but I can do the rev test to be sure. I repaired the vacuum line to the vacuum canister from the HVAC spiltter about a year ago, so it shouldn't be that either (plus the top vents work on cold, and all vents are restricted on hot). Any thoughts?

 

Is the airflow lower on hot, or just the temperature of the air not hot? If the air is cold, or not all that hot I would suspect the thermostat is stuck open. If the airflow is lower on the hot settings only, I have no ideas.

 

I would suspect # 3 is your problem. Replace it. Is that the ls1 in your avatar/ car in question?

I had the same exact issue when my thermostat went and was stuck open. Honestly that's best case I would be hoping for that, it's an easy fix. The car should take longer for the temp to go up and feel the luke warm air you do feel? And is reflected by the temp gauge changing and even reading lower while driving vs idling. This is everything my car did. The Tstat is a cheap easy swap to see if that's your problem.

If your heater core is clogged that shouldn't effect your temp gauge or you have major issues.

 

Thanks for the quick replies guys. To answer your questions: The flow of hot air was always less until the car got warm. Not the temperature, but the force of the air (on hot that is, on cold it was never temp-dependent). I assumed this was normal so that it wouldn't blow cold air on you while the coolant was warming. It still behaves this way, just now it never gets warm, so the air flow is always weak. It gets warmer and the air flow gets a little stronger, but it's only lukewarm, even driven on a warm day.

Yes, that is my Trans Am in the avatar and listed in "my garage," and it is the one I am asking about. I'm not certain about the temp gauge being lower than normal, I could just be noticing it more now. It does seem to hang out at 185 for a while, is that normal? The only thing that makes me think it might not be the thermostat is that it never really gets hot on my 30 minute drive to/from work. When I park it, it will get up to 210 sometimes, but even then, it's not very warm.

I wonder if all the deposits and the car running rich with a high idle before had it running a bit hot. Maybe now that I've fixed all that, it doesn't warm up as quickly? Whatever it is, the change was pretty drastic versus just a couple of weeks ago (used to be, by the time I got home I'd turn down the heat as it was so warm it was drying out my face and hands).

 

I have a similar problem, though my heater hoses are not as hot as as the radiator hoses. Changed thermostat (with a advanced auto 86*C, 186*F, that acts like about a 200*-210*F according to HP tuners, and the stock gauge) and it made no difference. I cooked my old one in front of a space heater to test it, it opened and closed but I don't know at what temp, but still needed a new one because of burning it. You could boil it if you wanted.

My next step is a good flush, and I'm thinking about using prestone yellow mix with anything coolant instead of dexcool. I'm sure it needs flushed anyway.

If that doesn't do anything I guess the next step is changing the heater core.

 

It sounds like it might be trail and error. I still think if it's not heating up when you drive it (say if the tstat is stuck open) because you're constantly getting air flow and over cooling. There's like 2 things possible to cause over cooling and 90% of the time it's the tstat. My ls1 stock always ran at the 200 mark on the gauge, no hotter or less until my tstat went.

As said you can do the simplest test and boil your tstat and see if it opens and closes once cooled. And just be stuck buying a new one regardless. If it's another issues someone else is going to have to chime in.

 

I would check the plastic "T" valve...it might look good but it can be cracked, leaking vacuum air and maybe the door in the dash isn't opening all the way when HEAT is selected. Also, check the vacuum port on the back of the intake to make sure the hose is firmly attached and has a tight seal.

Get the engine up to operating temp and touch both heater hoses where they attach to the water pump. If they are not BOTH equally hot....you have a clog in the heater core. Hot coolant always travels through the heater core, non-stop, no matter what the t-stat is doing.

Clogs are soooo friggin rare....Dexcool on its worst day cannot clog a heater core or anything else in a cooling system........unless its like mud for some reason. But that would be a slow process you would have seen over time a long time ago.

You said the coolant is clean and full. Its not the Dexcool.

Also, the water pump is working....if it wasn't you would bury the needle into the red in 15 minutes. Water pumps never stop pumping coolant, they fail because they spring leaks only, they always move water the same.
(and yes, a water pump shaft can shear, its 1 in a million)

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(Note, if you don't want to read all the updates, just skip to the last paragraph.)

So I opened up the lower dash today and cleaned around in there. I tested the blower motor, and that seems good. I didn't get to the hot/cold flap, but I'll have to find that one of these days. Is it inside the box with the heater core? I'm assuming it's either in that large black box to the left behind the glove box, or above the blower motor somewhere.

Funny that as you were typing this, I was inspecting these very things. I had the vac line from the T to the canister break off below the headlight a couple of years ago, so I know without that vacuum, the upper vents won't work. They are still functioning well, so that should not be a problem. I checked the T and used my vacuum gauge, which all showed good results.

You were right though. There was a vacuum leak, only much farther upstream. No, it was not the fitting at the back of the intake either. I even double checked that by hand even after the vac gauge showed the line to have good vacuum. Here is the culprit: I had just replaced the oily PCV valve.

The old PCV was partially clogged, so it probably wasn't losing much vacuum. The new one, however, was sucking on the end of a broken straw. While reaching around the back of the intake to check the vacuum line, I found the rubber junction where both PCV breather tubes from the heads join and lead to the PCV valve and the intake was rotted away from the oil inside it. Split into two pieces. The system is basically a network of 4 plastic tubes protected with foam sleeves and joined with rubber connectors at the junctions. The driver's side L fitting on the head was rotted, the 2-heads-into-1-PCV-fitting behind the intake was rotted, and the tube in which the PCV resides was rotted. I was already aware of that last one, and had taped it up until I could fashion a replacement, but the others were wide open.

I purchased a rubber L-fitting, some rubber tubing and a high temp plastic T-fitting. I built a new PCV "collar" that the PCV valve lives in with 3 sizes of rubber tubing. The outside one was 3/4" and I shoved the PCV valve inside of it. I even pried the edge of the hose up, so that the flange at the widest point of the PCV valve would fit inside for a tight seal. Then, I shoved a smaller piece of tubing into that and finally an even smaller piece of tubing into the intermediary piece using some dish soap as lubricant. In this way I built a very short adapter and wrapped it in electrical tape, though it's so tight I doubt it could leak even without the tape. I replaced the rubber fitting shaped like =- (kind of a "Y" with the top ends parallel) with the plastic T-fitting and a bit of rubber hose.

I don't know if any of this has to do with the heat, but it needed fixing and should prevent a loss of vacuum when it is needed most (under load for example).

After doing the above repairs, I ran the car at idle in park and let it get a little warm. The vents seemed to be warmer, but that could also have been from sitting still. I noticed that when I revved the engine, it would get warmer and then when I let off, the air would instantly cool back down. In other words, the heat was RPM dependent. I hadn't noticed this before, but it told me there may be something obstructing the flow of coolant and it needs more force from the water pump.

I took that as a sign and did the garden hose flush of the heater core using the hoses at the water pump. It didn't flow much at first (although no debris came out) but after a short stint, it seemed clear with good volume. It's hard to get a good seal with the end of the hose, though, so I could be imagining the improvement. I refilled with coolant after, and at least idling in park, it feels plenty warm now. I'll have to see how it behaves late at night when I'm at work on Monday.

It IS Dexcool. No rust, no contaminants, almost no brown fuzz, nice and fluorescent orange. I had to refill a lot of it after the heater core flush, so now it's even cleaner.

Good to note on the water pump. I hadn't thought of it that way.

The only issue I have remaining is that I suspect the door which switches between hot/cold in the vents may be remaining slightly open at either extreme or even have an obstruction. When the car is hot, vent set to full cold (no AC, just vent), the air seems just a tad warm to me. When set to heater, it seems the air flow is somewhat restricted compared to the cold vent setting. This may just be due to the heater core being included in the loop, but I'd like to check. Any guidance from someone who's done this already? How do I access the hot/cold flap?

 

Have you taken the 2 minutes to just swap in a new tstat? That would rule out it being a simple issue vs something else and odd one you have going on.


How you described your flush sounds normal and unfortunately unless you're lucky the fix wont last long, they're usually to far gone when you notice a problem in my experience. Did you see if both hoses are equally hot? As stated before i think that means you have a flow issue, how ever don't think that's causing your problem, just the lack of heat.

 

I drove the car from cold in the rain and wind today (not warm out) and the heat was almost instantaneous. Before I even got to the corner the vents were warming up with the temp needle still at cold. This is actually better than I remember it being when I bought the car 8.5 years ago (but it's been a while). The airflow is still a bit weak, so I think the flap needs to be looked at, but the heat part is solved for now.

Now that I think of it, there have been maybe 20 times since I've been driving this car that I heard a gurgling in the passenger foot well and wondered what was up in the heater core. Let's hope that it stays clear and doesn't need to be re-flushed or replaced anytime soon. New ones are cheap online, but I hear the install is a PITA. Still, if the problem returns soon, I’ll do the replacement. I did not get to feel the hoses (or get your response) until after I flushed the core, so I don’t know how they felt before.

I didn't do the thermostat mostly because I was impatient to do the work right away and don't feel like waiting for one to ship or paying $20-$25 for one at the parts store if I don't even need one. I also didn't feel like dumping the coolant, but I wound up doing that anyway to clean out the heater core+pipes. After seeing the circulation begin inside the radiator cap with sufficient warm up during fill up/air extraction, I really don't think it's the thermostat. The temp does seem to hang around 185° for a bit but it does get to 210°, I suspect this is what it used to do, I just hadn’t paid as close attention—plus it’s cold out, so I’m sure it will hit 210° quicker in the summer.

Thanks again for all your help and suggestions. I’d still like some info on how to access the hot/cold flap if anyone has any.