Heater blows cooler when idling. I'm getting ready to replace the water pump this weekend and I was just wondering if the water pump was on it's way out would that cause the heater to blow cooler when the motor is idling?

What range T-stat?

If the t-stat is good and the truck gets up to temp than I would suggest back flushing the heater core, that's what fixed mine, I have heard it is not an uncommon problem.

What range T-stat?

I'm not sure. Stock as far as I know. Gauge always seems to read about 195.

Are you doing a complete dual thermostat & hi volume 130GPM water pump change over ? It is all stock parts from the 1996 1/2 & later trucks.

hows the radiator cap? If the pressure is too low, or the cap is going bad, it wont pressurize the system enough which could cause sluggish coolant movement in the smaller passage ways (heater core). Cap should be a 16 PSI cap. If your gonna fix up the coolant system, get a new cap anyway.

I'm replacing with a stock pump and replacing the heater hose. I'm going to modify that quick-connect on the heater hose.

The overflow cap is new (16lb) and is 1lb more than the original that I had on there.

I do believe that it has been the decision of the powers that be that you can blow a heater coil if you allow full pressure into the line, so you will want to use the right part in the replacement of the quick connect.

Might search for the correct nipple to use.

I do believe that it has been the decision of the powers that be that you can blow a heater coil if you allow full pressure into the line, so you will want to use the right part in the replacement of the quick connect.

Might search for the correct nipple to use.

Thats a possiblity. The truck was worked on seven or eight years ago when the original heater hose got a hole in it. The shop replaced with a regular piece of heater hose and screwed in a new end where that quick-connect was. I noticed a few months ago when replacing the hose again(just temporary until I got around to doing the pump and new stock heater hose) that the nipple that was screwed into the manifold was not reduced at all and the whole setup allowed alot more flow than the original setup.

Would the heater work at all with a blown heater coil?

What is the correct nipple size?

My owners manuel says it's perfectly normal for the heater to blow room temperature or even cool air at idle.

My owners manuel says it's perfectly normal for the heater to blow room temperature or even cool air at idle.

That would be one less thing to worry about then. I was just curious for the most part. As far as I remember my trucks heater has always acted that way.

If the heater coil goes you will notice a smell of coolant in the heater air, fogged up windshield, loss of coolant, and it would probably be leaking to the inside on the floorboard.

If the heater coil goes you will notice a smell of coolant in the heater air, fogged up windshield, loss of coolant, and it would probably be leaking to the inside on the floorboard.

I am loosing some coolant but I'm pretty sure that it is on the outside firewall where the heater hose connects.

When the heater coil goes what exactly happens? What is the heater coil made of? I don't even know what it looks like.

I do believe that it has been the decision of the powers that be that you can blow a heater coil if you allow full pressure into the line, so you will want to use the right part in the replacement of the quick connect.

Might search for the correct nipple to use.

My truck and the other 6.5's at work don't have the quick connect anymore. Instead we have straight nipples with the heater hose attached. One truck has been that way for 8 or 9 years now w/o a problem. I'm not saying its ok like this, as I am in no way an expert on these things, but it is the way we run!:)

I'm not sure what pressure it's rated for. Just saw it posted here about the pressure causing damage.

I think it's basically an aluminum core right. Just like a small radiator.

It will handle what ever the rad or cooling system is rated for, the orfice in the heater line WILL not reduce the pressure in the heater core, it only affects the rate of flow thru the core
the heater core is in effect a mini rad., if you have coolant leaking out of the AC drain (heater air box) then you could have a heater core or line leak inside the box.

You would think in a regulated pressure system it would be the same pressure.
However when there is flow involved the pressure changes relative to the restriction of flow.
Only thing I can think of is the flow rate if inlet tubes are large enough to allow high flow and then maybe inside the core the pressure is increased by restriction of the flow.

Something I saw recently is a small water line from the blocked off port in the back of the head piped into the heater hose. This would give you instant heat in the cab.

From what your saying you have a leak now. It could either get inside or outside depends on where it's leaking.

It would seem that if you use something smaller than the nipple on the radiator where the return heater hose goes you should be fine.

If the pressure is too low, or the cap is going bad, it wont pressurize the system enough which could cause sluggish coolant movement in the smaller passage ways (heater core). ...the orfice in the heater line WILL not reduce the pressure in the heater core, it only affects the rate of flow thru the coreYou would think in a regulated pressure system it would be the same pressure. However when there is flow involved the pressure changes relative to the restriction of flow.
Wow, three conflicting statements! First, my expertise is in compressible flow (air) and not non-compressible flow (water) so I'm not as sure about my take and terminology on this as I would be for an air-cooled engine, but I think I have it right.

First, although scrufdog's comment seems intuitive I think it is incorrect. Pressure in the cooling system doesn't improve flow to its various parts. Cooling system pressure is present only to allow the coolant to get hotter than 212F without boiling so it can carry more heat to the radiator. Pressure does not drive coolant flow in the system.

DavidPhillips, in a system open to air a restrictive orifice would allow higher pressure upstream of the orifice (from a spigot to an open garden hose, for example). However, in a closed system it moderates only flow. I'm pretty sure that water pumps in a closed system do not pressurize anything - they just push water (remember, water is not compressible). So they are really circulator pumps, not pressure pumps. A good example of this is that there is no pressure in the radiator when you start the engine and water pump - it has to get hot to pressurize. If the water pump pressurized the system there would be pressure immediately. Because cooling system pressure is clearly created by the expansion of hot air [in the expansion tank?] and not by the water pump the entire system is pressurized equally.

So nicg, I think you are correct - an orifice in a closed system would only regulate flow, not pressure.

Feel free to disagree - I've been known to be wrong:) , but I think this is right.

Wow, three conflicting statements! First, my expertise is in compressible flow (air) and not non-compressible flow (water) so I'm not as sure about my take and terminology on this as I would be for an air-cooled engine, but I think I have it right.

First, although scrufdog's comment seems intuitive I think it is incorrect. Pressure in the cooling system doesn't improve flow to its various parts. Cooling system pressure is present only to allow the coolant to get hotter than 212F without boiling so it can carry more heat to the radiator. Pressure does not drive coolant flow in the system.

DavidPhillips, in a system open to air a restrictive orifice would allow higher pressure upstream of the orifice (from a spigot to an open garden hose, for example). However, in a closed system it moderates only flow. I'm pretty sure that water pumps in a closed system do not pressurize anything - they just push water (remember, water is not compressible). So they are really circulator pumps, not pressure pumps. A good example of this is that there is no pressure in the radiator when you start the engine and water pump - it has to get hot to pressurize. If the water pump pressurized the system there would be pressure immediately. Because cooling system pressure is clearly created by the expansion of hot air [in the expansion tank?] and not by the water pump the entire system is pressurized equally.

So nicg, I think you are correct - an orifice in a closed system would only regulate flow, not pressure.

Feel free to disagree - I've been known to be wrong:) , but I think this is right.

So...would an oversized hole be a problem.....for the heater core?

i recently replaced the heater core in my jimmy. its just like a mini coper radiator with 2 coper pipes coming off of it. for a few months before it blew i had slight fogging on my windows, but i thought nothing of it because it went away after having the defrost on for a while. i was also losing coolant slowly. low and behold a few months later i was driving down the road and smoke started filling the cab, the next day there was a puddle of coolant on the pass. floor. it was the heater core. i replaced it but the biggest problem was (and every so often still is) trying to find something to get the coolant off the windows. i tried everything from water to windex to brake parts cleaner. the thing that i found that works the best is this rain ex anti fog(for interior glass) it comes in a black box and works well for a while. all the windows still fog up every so often but it wipes off easily and its not hard to reaply the rain ex if i really feel like it. good luck with the heater core.

Heater blows cooler when idling. I'm getting ready to replace the water pump this weekend and I was just wondering if the water pump was on it's way out would that cause the heater to blow cooler when the motor is idling?

It's normal for it to blow cooler when idling this time of year, mine did this morning, being -28C, my temp gauge showed low till I started moving, where is your temp gauge setting at when it's blowing cool air? When mine gets up to engine temp, I get more then enough hot air at idle. A cold front helps, if you do install a cold front, keep a eye on your temp gauge when towing.

Wow, three conflicting statements! First, my expertise is in compressible flow (air) and not non-compressible flow (water) so I'm not as sure about my take and terminology on this as I would be for an air-cooled engine, but I think I have it right.

First, although scrufdog's comment seems intuitive I think it is incorrect. Pressure in the cooling system doesn't improve flow to its various parts. Cooling system pressure is present only to allow the coolant to get hotter than 212F without boiling so it can carry more heat to the radiator. Pressure does not drive coolant flow in the system.


Well yeah, I realize that the pressure is to allow coolant to get hotter. I'm going off experiance of selling auto parts for 6 years, 4 of which I was an ASE Certified Parts Technician. I can remember a few occasions that bad heat was fixed by a new pressure cap. I know there is no scientific basis for this, you may know more about flow dynamics, but I 'm just going off what I have seen.

Well yeah, I realize that the pressure is to allow coolant to get hotter. I'm going off experiance of selling auto parts for 6 years, 4 of which I was an ASE Certified Parts Technician. I can remember a few occasions that bad heat was fixed by a new pressure cap. I know there is no scientific basis for this, you may know more about flow dynamics, but I 'm just going off what I have seen.

Sorry scrufdog! I didn't mean to imply that the pressure cap solution didn't work - I just had a different idea about WHY it worked. I suspect it works because a working cap allows the water to get hotter and therefore the water passing through the heater is hotter. On my van the heater is supplied by coolant that has just left the engine and is on its way to the radiator, and its pretty hot. Under pressure probably hotter than 212. That's my theory anyway, and I'm stickin' to it!:)

Sorry - I should have spent more time thinking about what my comment sounded like.

My truck has the original setup and parts on it.

The heater works. I noticed that there is a small pipe where the coolant goes out of the crossover and back to the heater core, and then back to the radiator. I also have a bypass on my water pump.

I have also noticed that for some crazy reason the top radiator hose is smaller than the bottom. Could be coincidence, or there may be a reason for it.

Maybe there is no good reason why the line is smaller or maybe there is. However if I had a large nipple and hose that someone put on there and I was changing my heater core because it just started leaking for some unknown reason, and since I saw it mentioned here by someone that knows more about it than me, I would put it back like it was designed to work.

And that's not conflicting with needing a radiator cap the way I see it.

Sorry scrufdog! I didn't mean to imply that the pressure cap solution didn't work - I just had a different idea about WHY it worked. I suspect it works because a working cap allows the water to get hotter and therefore the water passing through the heater is hotter. On my van the heater is supplied by coolant that has just left the engine and is on its way to the radiator, and its pretty hot. Under pressure probably hotter than 212. That's my theory anyway, and I'm stickin' to it!:)

Sorry - I should have spent more time thinking about what my comment sounded like.

No need to apologize. I didnt think you were 'cuttin' down on me. I was explaining how I got to my possible solution.

:Handshake

Is there a suction port on our water pumps, if so a guy could return the heater core there, for faster warm ups, like the old northern chevy gassers, enstead of returning to the radiator and constantly recirculating cool water through the block to the heater.

Regards to replacing the quick disconect with a straight fitting, I wont do it, had to many problems with it(mainly crappy cores nowdays), they make replacement quick disconects out of steel or install an orfice. I run about an 1/8" orfice in one of mine, works fine here in texas(rarely below 20 degrees here), guys up north will just have to see what you need, also will make your a/c work better in the summer by not have as much heat circulated in the cab.

I know it doesn't make good sense but there is pressure there to be limited. Think turbo on an engine running, it pumps air, atmospherical pressure is on the inlet and outlet of the compressor(outlet not hooked to engine), put a pressure guage on it and it reads zero, now hose hooked up intake and heads create a restriction and you now have pressure. Your water pump is pumping water through the heater core and if there is a restriction in the core or after it, it will build pressure in the core.

When my 92's quick disconect broke, I used a regular hose barb, well the heater core started leaking, thought well its eight years old, replaced it, new one lasted a few months, thought defect, carried it back, got a new one, same thing, tried one from my radiator dealer, same thing, tried two from chevrolet. Last core has lasted 4 years, since I put my orfice in, needless to say I can put one of those cores in in ten minutes flat.