Intermittent heat from recently-installed heater core

[Chilezen]

I replaced a leaking heater core with a used core from a junkyard car. It doesn't leak, so it does the job...

Did several bleedings. First on flat ground, I didn't know it had to be raised. So I assumed air was trapped. Bled the system about 3 more times, progressively with the front of the car higher and higher, following the various guides to properly bleed it.

As I was bleeding it, the air out of the vents blew pretty warm with my hand right by the vent, so I figured, yeah that's good enough. Air bubbles don't appear to be coming out anymore from the screw.

But on the freeway, the air blows cold.

On the freeway, I would turn the temperature dial from hot to cold, wait a few seconds, and turn it back to hot... and I would momentarily feel warm-ish air come out.

At red lights, while the car idles, I get warm, near-hot air! When I turn the temp dial to cold, it gets cold. Excellent! Fan speed hardly affects air temp either way.

So if I'm getting hot air during idle when the car isn't moving, but literally cold air on the highway... would you suspect I have a stuck-open thermostat? I wish I could tell you what my instrument cluster temp gauge says, but it doesn't work (dead si board, yet another thing to fix). So I can't say what the water temp is. Would you suspect I still have air in the heater core?

1987 325is, nearly everything is stock, but don't assume everything works. Thanks in advance!

[superj]

sounds like some kind of door issue. like the one that opens to get air from the outside is not staying closed when driving

[sileighty]

If your water gauge doesn't work I would definitely sort that out first. My car cools down a lot on the freeway (needle starts going into the blue zone) but can get to the halfway point when idling. My symptoms are very similar to yours but I don't remember my air being necessarily COLD on the freeway, so perhaps your thermostat is stuck open more than mine is.

[Chilezen]

I have confirmed that the outside-air flap on the blower motor does open and close when I push the "recycle air" button on the console.


Yes, I'm going to fix the temp gauge soon. You're saying you also get warm air in your vents at idle? I hope I find the solution too so we can both have hot air on the freeway!


I'm trying to reason whether or not an air bubble is still trapped in the core. For what reason would enough hot coolant be able to pass through the core at idle, but hot coolant isn't flowing in when cruising?

The water pump pushes more water at cruising speed through the system. So you would agree that more hot water getting pushed into the core, and the fan has more heat to push through the air into the vents. But that's not the case; it's cold. So would an air bubble limit the amount of hot water being pushed into the core? Yes. Would an air bubble limit the flow at cruising speed but not idle speed? It shouldn't.

Would an open thermostat allow cooled air to flow into the heater core? It shouldn't, because the water pump should be pushing cooled air into the block, and out the back of the head into the heater core. Unless I put the hoses on backwards? The heater inlet/outlet is switched? Possible. Would an open thermostat be a problem whether or not the hoses are switched?

[superj]

I think maybe your hoses being backward would be more of a problem than the thermostat.

[efficient]

I think maybe your hoses being backward would be more of a problem than the thermostat.

I was going to say the same thing. If i remember correctly the hose from back of the head goes on top.
If you switch them prime the heater core before puttimg the hoses back on.

[Chilezen]

The top hose is connected to the head.

Further, it doesn't matter if I am on the freeway or not. I can rev the engine to 3K while stationary, and the air temp will drop to cold. Then when I let it idle, the air warms up again. I don't understand how an air bubble can cause this, but I don't see any other factor being possible.

[earthwormjim]

The top hose is connected to the head.

Further, it doesn't matter if I am on the freeway or not. I can rev the engine to 3K while stationary, and the air temp will drop to cold. Then when I let it idle, the air warms up again. I don't understand how an air bubble can cause this, but I don't see any other factor being possible.

That sounds like air pockets to me. At idle, water pressure is low, thus allowing air pockets to form and develop, blocking off portions of your water loop.


When you rev the engine, you're increasing the water pump speed, and pushing those air pockets around or compressing them, temporarily allowing trapped water to now flow through the system, which is replaced by fresh cool water from your radiator.

The water pump pushes more water at cruising speed through the system. So you would agree that more hot water getting pushed into the core, and the fan has more heat to push through the air into the vents. But that's not the case; it's cold. So would an air bubble limit the amount of hot water being pushed into the core? Yes. Would an air bubble limit the flow at cruising speed but not idle speed? It shouldn't.

Would an open thermostat allow cooled air to flow into the heater core? It shouldn't, because the water pump should be pushing cooled air into the block, and out the back of the head into the heater core. Unless I put the hoses on backwards? The heater inlet/outlet is switched? Possible. Would an open thermostat be a problem whether or not the hoses are switched?

You've got your theory backwards by the way, reving the engine doesn't bring more hot water to the heater core, it brings more cold water to the engine block, and thus more cold water to the heater core. In a closed loop system, inlet and outlet water temperatures are hardly any different, provided flow rates are high enough.

[Chilezen]

This is the answer I was looking for, thank you! So there is an air pocket trapped in there still. Would you recommend taking off the hoses and pouring water in? I've already spent an hour with the front of the car jacked way up, trying to bleed it.

Would you also say the thermostat is fine? Or should I look for a separate test to condemn it?

[earthwormjim]

This is the answer I was looking for, thank you! So there is an air pocket trapped in there still. Would you recommend taking off the hoses and pouring water in? I've already spent an hour with the front of the car jacked way up, trying to bleed it.

Would you also say the thermostat is fine? Or should I look for a separate test to condemn it?

Thermostat stuck open can also cause the problems you are having. I thought you had recently replaced it, which I why I mentioned air pockets, but I don't see you mentioning it in this thread really.

If you have no idea when the thermostat was last replaced, just replace it. Make sure with the right temperature part, someone put a 70C thermostat in my car, so it was really not reaching properly regulated temperatures. Had to use an 80C part.

If you are really curious about your thermostat, you can put it in a pot of hot water with a thermometer.

Bleeding wasn't very hard on my car, I had the front on ramps, and I did not need to pour water into the hoses. I just filled the expansion tank, and massaged the hoses as I poured more water in. I left the bleed screw open while filling with coolant. Then let the car get up to temperature while still on ramps, and opened the bleed screw regularly till I had a smooth stream of water coming out. I think the key for me was massaging the hoses quite vigorously, that pushed any air bubbles around into the expansion tank.

I recommend driving around with a socket or screw driver for a while afterwards, that way you can quickly open the bleed screw if you see temperatures start to climb.

[AndrewBird]

Don't jack the entire front of the car up in the air, just the side with the coolant expansion tank. Lift it from the side until the opposite side tire is just coming off the ground. Set the theater to max heat and fan speed 1. Then leave the fill cap off the expansion tank and run the engine until fully warm, reving it up occasionally to get coolant moving. Leave the bleed screw alone. A lot of the time it can actually let air in.

You may not be waiting long enough for it to get hot. It will take 30 minutes or longer just sitting idle. Fix your temp gauge and try again. The SI board has no effect on the temp gauge.

[uturn]

I think maybe your hoses being backward would be more of a problem than the thermostat.

Superj got it right! The inlet of the heater core is the bottom connection. The outlet is the one connected to the solenoid valve. That valve also acts as a check valve so if you connect the wrong hose you get little or no flow.

The hose from the back of the head attaches to the bottom connection of the heater core.

[Chilezen]

I asked a tech-buddy of mine, and he asked about the condition of the fan clutch. I didn't know exactly, just that it is fairly stiff. So he suggested that it may be too stiff, and pulling too much air, which would make sense why I feel cold air when the car revs for a while. The fan is over-cooling the water...

and to add to this, a couple months ago, my fan spontaneously exploded while cruising on the freeway. I couldn't understand why suddenly I heard a loud crash from my engine bay, but noting seemed wrong, until I popped the hood. The fan must've hit something. Nope! My buddy said it died from fatigue of a stiff clutch. So with his suggestion, I took the fan off the engine and drove around.... and I noticed that the heat didn't get as cold as before. So, that's good! The clutch may actually be the problem! It's not getting as hot as I'd like, but this discovery is good.

Anyway. back to the heater.

The thermostat is about 4 years old, at the time when I did a full engine refresh.

Tonight, I removed it, saw that it had a stamp that reads 176F/80C, and then boiled it in a pot with a thermometer poked in. I watched it slowly open right at 176 :)

Then after reinstalling that, I switched the heater lines. I raised the front as I poured water in, massaged the hoses, over-filled the tank, got the engine warmed up, didn't really do anything with the bleeder, as it seemed to be bled on its own. So I took it on the road.

Superj and uturn are correct! I now have exceptionally hot air in my vents. The problem was a simple matter of backwards coolant lines. The hose from the back of the head attaches to the bottom connection of the heater core. Thank you everyone!

[superj]

Badass!! Glad you are back to normal

[earthwormjim]

That's the best solution to problems like these, user error. At my work whenever I encounter problems in our products, I'm always relieved when it is user error, and not a defect or design flaw, or real failure.