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Thread: Poor heater output at idle, 2005 Grand Cherokee. I'm stumped!

  1. #1
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    Poor heater output at idle, 2005 Grand Cherokee. I'm stumped!

    This one has me stumped. Vehicle is an '05 Grand Cherokee, 3.7L with the manual HVAC. The head gaskets and timing components were replaced along with the heads resurfaced about 1 year ago due one HG leaking after overheating.

    When the outside air temp is 25*, and engine temp is a steady 203*, the vents will blow only 75* when the engine is at idle yet when the engine rpm is raised to 1500 rpm the outlet temp increased to 135-140*. The engine warms up quickly, temp on the scanner stays a constant 203-204* once warmed up and the system pressurizes quickly. The temp difference between the heater hoses is only 10* at idle with the heater on high.

    First thought was the heater core. It was only 4 years old, but figured that was the most likely culprit given the symptoms. Replaced that along with the soft heater hoses & thermostat for good measure. I also cleaned the hard lines from the firewall to the core by snaking a piece of wire through them with a small piece of rag attached to the end. That resulted in no change. None whatsoever.

    After that I redid the HVAC relearn/calibrate procedure. It did it's thing, with no codes, and still no change in heater performance.

    Thinking it was a flow issue at this point, I replaced the water pump as a last ditch effort. Wouldn't you know, no change. In fact, the temps from the vents lowered a little bit, now I'm lucky to get 130* out of the vents going down the road. I wasn't a big fan of the new pump's impeller design, lots of sharp edges; where the OEM unit appears more aerodynamic if you will.

    One thing I noticed while burping the system after the pump change is that the temp out of the vents was 125* at idle. This is while the coolant level was an inch or so low in the radiator and the cap removed. It was about 40* that afternoon. After it cooled down and I ran some errands, the vent temp was till low at idle.

    Anyone have any idea on what might be going on or ideas to help diagnose this?
    “If the people of the nation understood our banking and monetary system, I believe there would be a revolution before tomorrow morning.” -Henry Ford

  2. #2
    Hybrid booster
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    Re: Poor heater output at idle, 2005 Grand Cherokee. I'm stumped!

    not sure how the coolant routes on that system.
    does it have a divertor valve or heater valve?.
    sounds like maybee you arent getting hot coolant to the core.

  3. #3
    Supporting Member II Turbo Mopar Contributor rgl10's Avatar
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    Re: Poor heater output at idle, 2005 Grand Cherokee. I'm stumped!

    You said both hoses are 10 degrees apart. Do they run 190 or so when the engine shows 200? If both are that hot then you have an air flow problem. If they both are lower like 120 or 130 at idle then its going to be a coolant flow problem. I worked on an import car a couple months ago that we had changed the water pump four times on with different brands of water pumps till we found one that would produce enough flow at idle to keep the heater working in cold weather. Even with the same brand of pump we found that the impeller was pressed on at different heights so maybe you have a pump problem like that.

  4. #4
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    Re: Poor heater output at idle, 2005 Grand Cherokee. I'm stumped!

    There is no diverter valve. I measured the temp of the metal tubes that run from the firewall to the heater core to get a better reading than through the hoses. With the engine temp at 203, the tubes were 160* and 165*, engine at idle, heater fan on high. This was after driving across town, figuring that 30 minutes of driving would get them plenty hot. I'm thinking that this really is a flow issue, probably the distance between the timing cover and the impeller is too great like you ran into.

    I'll use the old pump I have as a test to see if it's possible to pull the impeller away from the housing a smidge.
    “If the people of the nation understood our banking and monetary system, I believe there would be a revolution before tomorrow morning.” -Henry Ford

  5. #5
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    Re: Poor heater output at idle, 2005 Grand Cherokee. I'm stumped!

    Do you have a cabin air filter? Could it be plugged?

  6. #6
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    Re: Poor heater output at idle, 2005 Grand Cherokee. I'm stumped!

    No cabin air filter. I am surprised they don't have one. There is plenty of airflow.
    “If the people of the nation understood our banking and monetary system, I believe there would be a revolution before tomorrow morning.” -Henry Ford

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