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  #1  
Old November 19th, 2007, 10:38
tstaples tstaples is offline
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engine's hot, but no heat, then heat, then no heat

So.. this seems really odd to me.. and it doesn't really make sense that it would be the usual thermostat issue.

I start my 99 XJ and let it run, go out and the temperature is to the middle of the guage where it's supposed to be for it to blow hot air... so then i turn on the heat... cool air comes out. So i get on the road to go to work... a while down the road when i'm stepping on the gas all of a sudden hot air will come out... then... it'll go back to cool air when i slow down a bit.. then hot air.. then cool air... seems like if i stomp on the gas it'll blow hot air... maybe i'm insane..

anyone ever seen this before and know what it could be? I was thinking about changing out my thermostat just because it's probably a good thing to do before the winter.. and doing a radiator flush... but i'm not so sure that's what it is

any ideas? i'm baffled..
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  #2  
Old November 19th, 2007, 10:49
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AZRockRunner AZRockRunner is offline
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Re: engine's hot, but no heat, then heat, then no heat

Sounds like its the Heater core. Do a good chemical flush, change therm and see if that clears it.
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  #3  
Old November 19th, 2007, 11:01
JoshRountree JoshRountree is offline
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Re: engine's hot, but no heat, then heat, then no heat

Blend door cable might not be working?
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  #4  
Old November 19th, 2007, 11:06
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Re: engine's hot, but no heat, then heat, then no heat

Vac problem sounds like. I'd do a good flush of the system, either professionally or use the prestone 7 hour flush, change the Tstat and go from there. Use a garden hose to push water thru the two heater hoses, don't over do it, most house water pressure is around 50PSI the heater and cooling system work around 16PSI so DON'T BLOW UP THE CORE...

I just replaced the Tstat in the ZJ, wow, once it gets warmed up that thing is toasty.... the old Tstat came out in 4 pieces so it was for sure stuck open and never got above 110F.
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  #5  
Old November 19th, 2007, 11:07
tstaples tstaples is offline
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Re: engine's hot, but no heat, then heat, then no heat

DAMN.. i was hoping not to hear that...
so is it better to flush the radiator.. or is there a way to flush just the heater core itself?
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Old November 19th, 2007, 11:12
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RichP RichP is offline
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Re: engine's hot, but no heat, then heat, then no heat

Quote:
Originally Posted by tstaples
DAMN.. i was hoping not to hear that...
so is it better to flush the radiator.. or is there a way to flush just the heater core itself?
Best home method I've heard is drain the system and refill with a gallon of vinegar and top off with water then go drive it some, do a search on the time involved in driving it. I have not used the vinegar solution.
To me the most success I have had is with the Prestone 7 hour flush, drain, dump in the flush and refill with water then drive for 7 hours, drain and flush the system then refill with 50/50 mix of coolant and distilled water. While I have the system empty I replace the hoses, Tstat, belt and use a garden hose to GENTLY flush the heater core, make sure the heat is turned on before you shut the engine off or the core will be closed by the system. Then use the hose to alternate between the top and bottom hoses so you get flow pushing and pulling, DON'T try to push the houses water pressure thru it either or your next question will be how to replace the heater core and you don't even want to go there. Don't let it sit overnite with only water in there or you may end up doing freeze plugs too if it gets too cold

I also usually install one of the prestone back flush kits too when I do this.
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  #7  
Old November 19th, 2007, 11:13
89xj 89xj is offline
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Re: engine's hot, but no heat, then heat, then no heat

the heater core is very simple to flush.

there are only 2 hoses. in and out.
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  #8  
Old November 19th, 2007, 11:23
tstaples tstaples is offline
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Re: engine's hot, but no heat, then heat, then no heat

my house water pressure is very low.. so it'll probably be fine.. just gotta wait for a day when it's not below freezing so my garden hose wont be frozen.. maybe some day this week i'll stay home from work and do a flush. I'll go get the prestone 7 hour flush.

i DEFINITELY dont want to deal with replacing the heater core... i'm praying that's not it.

now, what's this blend door? could that be giving an issue?
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Old November 19th, 2007, 16:06
Hawk Hawk is offline
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Re: engine's hot, but no heat, then heat, then no heat

Make sure you're not leaking any coolant from the water pump. My dakota was doing exactly what you're describing when my water pump started leaking. Definitely do the flushes first, but keep an eye on that water pump.
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  #10  
Old November 19th, 2007, 17:13
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90Blue_XJ 90Blue_XJ is offline
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Re: engine's hot, but no heat, then heat, then no heat

Blend door is a vacuum operated door in the ducting to your interior heating. It opens the air flow between the heater core and the outside air vent. If the vacuum line is leaking or off then the blender door will either be open or closed usually. Sometimes it is just flopping back and forth in older XJs. Yours sounds like there is a vacuum leak since it opens and closes with engine speed. May be hard to find the leak.
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  #11  
Old November 19th, 2007, 17:21
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techno1154 techno1154 is offline
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Re: engine's hot, but no heat, then heat, then no heat

Also the valve that allows the (hot) water to go through the heater core is also vacumn operated. Well I guess this do not apply in his case since a lack of vacumn opens the valve toallow the hot water to pass. I would say there is a problen with one or more of the vacumn operated doors. Need a FSM to trouble shoot.
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  #12  
Old November 19th, 2007, 19:31
Jess Jess is offline
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Re: engine's hot, but no heat, then heat, then no heat

I have used vinegar to flush the coolant system and it does work well. It seems every time I use a chemical flush, within 2 months the bearings in my water pump go bad. I have been told there is no link, but have my doubts.
Whatever flush you decide to use, make sure to get all of it out of the system.....just pulling the lower hose won't get it all out. It has to be flushed out.
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  #13  
Old November 19th, 2007, 20:06
98XJeep 98XJeep is offline
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Re: engine's hot, but no heat, then heat, then no heat

if it was a vac problem it would have heat when sitting idle(high vac) vs. under acceleration (low vac), it might be a plugged or restricted heater core, or bad t-stat, they can have similar symptoms, but have you checked the coolant level? low coolant can also do exactly what your taliking about.
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  #14  
Old November 19th, 2007, 20:26
xalexjx xalexjx is offline
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Re: engine's hot, but no heat, then heat, then no heat

could be a sticking thermostat too
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  #15  
Old November 20th, 2007, 08:12
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1985xjlaredo 1985xjlaredo is offline
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Re: engine's hot, but no heat, then heat, then no heat

It sounds like the heater control valve could be messed up.
It could be the heater hose is being pinched just enough to not let coolent flow through and then the core cools down. I have had the same problem and the latter was my issue.
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