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Unread 01-02-2014, 11:42 PM   #1
tcanthonyii
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94 4.0 with almost no heat

This is ticking me off. Been fighting the issue for a couple of years. 2.5 years ago I got this thing. It's a 94 4.0 limited. 2 years ago I put in a new radiator as the old one was leaking. At that time I put in a new thermostat that I tested in boiling water on the stove. I had better heat but it's still just warm air. Then last year the thermostat housing was leaking so I fixed that again replaced the thermostat with one I tested on the stove. Still same deal on the heat. I flushed the heater core out this summer. This winter, still just luke warm air. I popped the hood last night and the system wasn't pressurized and I could hold onto the hoses. It was cold last night, 12 degrees, but the thing was running for nearly an hour running around town an sitting around idling.

I had an idler pulley fail on me on Christmas day. After I got done fixing it, just for giggles I pulled the cap off, with engine cold and just started up and coolant immediately spilled out of the radiator. On every other vehicle I've ever worked on it didn't do this.

I'm at a total loss as to why this is happening. I can live with it but I got married this fall and now have a wife and 2 kids. While we have 2 other vehicles we do take the jeep a lot and it's easy for us. The kids get cold in it and start complaining.

Any ideas?

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Unread 01-03-2014, 12:09 AM   #2
JasonStebbins
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Radiator cap controls the pressure, 18 pounds is stock. If the hoses aren't getting hot that means there is no flow to/from the heater core. That would explain no heat as well. A bad water pump could also cause similar symptoms.
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Unread 01-03-2014, 01:05 AM   #3
jnicewan
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Were you feeling the temp of the heater hoses? They should be getting hot when the engine is at operating temp, hot enough that you might only be able to hold for a few seconds, check a couple places aling the hoses, not just real close to the engine connections. Bad water pump is possible but you would probably see an overheating problem but maybe not. I would lean toward a clogged heater core. If you dont mind making a mess and losing some coolant then disconnect both heater hoses. Run water from a garden hose through the hoses and core from both directions to attempt to flush out the crud. This is not a guaranteed fix but it might work, a stopped up core may not flush, a weak core might burst if you let the water pressure build up.

If the heater is operating good and blower is turned up you might notice the return heater hose being cooler than the supply but at least one of them should be hot if coolant is flowing good through the core. If the heater hoses are hot but still no heat you might start looking at the temp blend door, actuator motor or some type of airflow restriction.
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Unread 01-03-2014, 12:51 PM   #4
tcanthonyii
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I did flush the heater core out last spring or summer, I can't remember which. It had great flow. We had the garden hose running through it in both directions and the water was clear coming through it. I've seen nasty heater cores and this wasn't one of them.

On the feeling of the hoses, the upper radiator hose I could wrap my hand around all day the night before last. It wasn't even hot. So in turn the heater hoses were only warm as well. Obviously the return was cooler the other.

I forgot to mention that the temp is only 150 degrees or so on the temp gauge so It's like the whole engine isn't heating up.
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Unread 01-03-2014, 01:17 PM   #5
PolkaPower
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A clogged heater core means nothing to whether the motor itself is warming up. The motor has it's own loop with a bypass and can operate fine regardless of the heater core. Some folks put a shut off valve in the heater core line to turn off circulation during hot summer months so the A/C gets cooler.

The problem is with the motor not heating up. When my crappy failsafe Tstat stuck open this was the same scenario. Winter time it never fully warmed up and I barely had heat. On the highway the motor ran at 150* or less. I put a stock Mopat Tstat in there and everything runs great now. Warms up fast and super hot heat.
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Unread 01-03-2014, 03:21 PM   #6
LouC
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If your engine is running at 150* is WAY too cold you will never get good heat that way. Normal operating temp is 210, I have owned our 98 4.0 since new and it always ran at that temp. If the engine runs at 210 and the heater core is not clogged and has no leaks and no air in the system the temp of the air on full heat should be about 140 degrees or so. I had to get the core replaced last winter and the heat has worked great since then. Sure you don't have a small leak in the core? if so you will get air in the system (you will hear a water rushing sound from behind the glove box when you first drive it in the morning) and it will never get hot.
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Unread 01-03-2014, 03:48 PM   #7
zjosh93
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Quote:
Originally Posted by PolkaPower View Post
A clogged heater core means nothing to whether the motor itself is warming up. The motor has it's own loop with a bypass and can operate fine regardless of the heater core. Some folks put a shut off valve in the heater core line to turn off circulation during hot summer months so the A/C gets cooler.
On the 4.0 the heater core is the bypass. There is no separate engine bypass.
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Unread 01-03-2014, 03:50 PM   #8
PolkaPower
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So if the heater core plugs up then the motor overheats? That is a terrible design. The motor is designed to operate normally regardless of the heater core right? It would be ridiculous for that to depend on if the heater core is functioning.
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Unread 01-03-2014, 04:34 PM   #9
zjosh93
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I've never pinched off the heater hose to find out but there are only two inlets to the water pump: the lower rad hose and the heater return hose. The thermostat housing has only two outlets: the upper rad hose and the heater outlet. There is no separation in the head between the thermostat and the heater outlet but there is on the thermostat housing and the gasket so any water making past the closed thermostat to the radiator is not by design. With the thermostat closed the only water circulation is from the head out to the heater core and back to the water pump. There may be some random motion around in the block but I doubt it's much. If you look through the hole where the water pump mounts the water pump blades just kind of swirl around in a big cavity around the number one cylinder.

You would hope that you could run the motor with no heater core but from what I've seen I wouldn't try it.
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Unread 01-03-2014, 04:54 PM   #10
PolkaPower
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All I am saying it that from an engineering standpoint it would be a very terrible design to make a motor that couldn't run if the heater was broke.
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Unread 01-03-2014, 07:41 PM   #11
tcanthonyii
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Quote:
Originally Posted by LouC View Post
If your engine is running at 150* is WAY too cold you will never get good heat that way. Normal operating temp is 210, I have owned our 98 4.0 since new and it always ran at that temp. If the engine runs at 210 and the heater core is not clogged and has no leaks and no air in the system the temp of the air on full heat should be about 140 degrees or so. I had to get the core replaced last winter and the heat has worked great since then. Sure you don't have a small leak in the core? if so you will get air in the system (you will hear a water rushing sound from behind the glove box when you first drive it in the morning) and it will never get hot.
I may have a leak in the core. I'm not ruling that out. I don't smell anything or have wetness that I've ever noticed like is typical. Even if I did you would think it's more apt to over heat than run cold. I do hear the nise behind the glove box from time to time but I'm never loosing coolant either. That's why I say I've got a weird one. I'm sure I'm missing something, but I have no idea what.
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Unread 01-03-2014, 08:03 PM   #12
LouC
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I had exactly that for about 2 winters, not enough of a leak to really notice but the heater did not work like it used to. Then last winter it started fogging the windows and then, saw a/f coming out the rear heater ducts. That was it, time for a new core.
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Unread 01-03-2014, 08:04 PM   #13
tcanthonyii
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When that happened was your temp gauge also running cold?
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Unread 01-03-2014, 10:42 PM   #14
jeepyard
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What brand of thermostat are you using? I bought a stat from the o'reillys one time and it did the same thing. I did all the thing's you did and finally got me a gates thermostat from rock auto and put it in and now the gauge goes up to 210 and my heat is working like it's supposed to. I have bought thermostat's from o'reillys since and haven't had this issue since. It may have been just a bad one but its fine now!!
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Unread 01-04-2014, 12:23 AM   #15
LouC
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Quote:
Originally Posted by tcanthonyii View Post
When that happened was your temp gauge also running cold?
Nope the engine temp was always normal.
The heater hoses were also pretty hot, but the air getting in lowered the heat in the core.
Back then it was just lukewarm. With the new core it is blazing hot. Even in 12* weather we had today.
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