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View Full Version : 99 cherokee laredo heating problem



Z28Envy
12-16-2010, 08:54 AM
A friend I work with told me her cherokee wasn't blowing any heat. I asked her if the temp gauge was real low when driving and she said it was just a bit over the 100 mark when driving in this cold weather. I told her it needed a thermostat more than likely so I told her I would do it for her being she doesn't have much money and was moving into a new apartment. I put the new thermostat in and started it up topped off the coolant and let the thermostat open. The engine got up to operating temp and the heat was good. I took it for a drive and didn't even make it a few blocks down the street when the heat started to go down. It got pretty cold just going around the block but the temp stayed just below the 200 degree line. I figured there was air in the system but I don't see any bleeders any where so I decided to put it up on ramps hoping that would help get the air out. The radiator cap was cold when I got back from driving it too.

I let it cool down then took the cap off being there was still pressure in the system. I started it up without the cap on and the fluid started to slowly rise until it started to overflow slowly. Never got any air bubbles just a steady overflow like the water pump is pushing it out. Maybe this is normal though I haven't seen this before on other cars that I have bleed air out of. Once the thermostat opened the coolant went down but would come back up everytime. Must be the thermostat opening and closing? I put my scan gauge on and the temp will move up and down from about 190 to 210 even while driving but doesn't go below. I tried another t-stat but the same thing happens. I have read about blend doors being a common problem on these but don't think this is what my problem is. I have also read the heater cores get plugged on these a lot too and the hose going in is hot and coming out is cold after I drive it and the heat goes away but if you let it idle like in the warm building I change the t-stat in the hoses are both hot so I don't think it's the heater core?

Could it be a bad head gasket or exhaust gasket and that is what's pushing the fluid out at idle with the cap off?

UnderPSI
12-16-2010, 03:50 PM
My guess would be blend doors. Check at the firewall if the return side of the heater core is getting hot. If one side is hot and the other isn't, it's the core. If not My guess is blend door. Means pulling the whole dash out.

SLOWC5
12-16-2010, 04:11 PM
we can do the blend door if you need it

Jeff

spooln30
12-16-2010, 08:16 PM
Do they have a heater control valve. Cuz the one that's hot is probably the in and the cold is the out. If it does have a heater control valve it may work off vac. So check that as well. I don't think the blend door would have anything to do with one hose to the heater core being hot and the other cold. It's gotta have a valve otherwise both hoses would be hot unless the core is blocked.

Z28Envy
12-16-2010, 09:16 PM
My guess would be blend doors. Check at the firewall if the return side of the heater core is getting hot. If one side is hot and the other isn't, it's the core. If not My guess is blend door. Means pulling the whole dash out.

When the engine gets up to operating temp both hoses are hot. The inflow tube is a little bit hotter than the outflow tube but both are hot. When I go drive it and the heat goes away, the hose going in will still be hot but the outflow will be only warm or even cool. The radiator cap will get hot if it's just idling but if I go drive it the cap get real cold. When it sits idling the heater will get hot but as soon as I drive it the heat will go down fast.

I can turn the heat dial from warm to cold and you can feel a difference in the temp.

Z28Envy
12-16-2010, 09:16 PM
Do they have a heater control valve. Cuz the one that's hot is probably the in and the cold is the out. If it does have a heater control valve it may work off vac. So check that as well. I don't think the blend door would have anything to do with one hose to the heater core being hot and the other cold. It's gotta have a valve otherwise both hoses would be hot unless the core is blocked.

I'm not sure if there is a valve I know I haven't seen one anywhere unless it's somewhere I can't see it.

spooln30
12-16-2010, 09:25 PM
Usually they are somewhere on the firewall where the heater core hoses go into the passenger compartment. But I also have seen them being apart of the core or inside the HVAC box. I have ALL DATA at home I will check when I get home for ya in about 25-30 min. I'll also check TSB's and I-ATN. If the jeep doesn't have the valve.

Keith

spooln30
12-16-2010, 09:32 PM
Is it a Grand Cherokee or just a standard? Also which motor if a Grand?

spooln30
12-16-2010, 09:41 PM
Ok, I just checked and they don't have a valve so the coolant will pass through the core at all times, both hoses should be hot,same temp. So what that tells me is either the heater core is plugged or maybe the water pump is bad. Check to see if both of the radiator hoses are the same temp. I have ran into the same issues where it would get up to the temp but you don't have heat. With the weather being cold it more than likely wont over heat. Now summer time way different story.

So check that both of the radiator hoses are the same temps and if they are disconnect both of the heater core hoses and get a foot long piece of hose that fits the core and flush it with a garden hose. Remember to flush it both ways and also in between flushing back and forth blow air from a compressor. That will unplug it for sure if it's clogged.

Z28Envy
12-16-2010, 09:49 PM
It's a 99 grand cherokee laredo 4.0 v6. I will have to check the radiator hoses. I know the top one gets hot but I didn't feel the bottom. I will try flushing it hopefully that will do it.

Thanks for the help!!

spooln30
12-16-2010, 09:57 PM
Not a problem. And the only function of the blend door is to allow air to pass thru the heater core thus giving the hot air into the passenger compartment. Same when you want AC.

So I will say it will boil down to the water pump or a plugged heater core. Hopefully it's just the plugged heater core. The water pump on the 4.0 I-6 are pretty east to do.

Keith