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Water Heater Control Valve?

This is a discussion on Water Heater Control Valve? within the Technical / Maintenance forums, part of the General Discussion category; 88 K1500 5.7 is not getting heat to the heater core. I flushed the core, water came out clean. I ...

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    Question Water Heater Control Valve?


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    88 K1500 5.7 is not getting heat to the heater core. I flushed the core, water came out clean. I don't think there is any water getting to it from the engine in that the hoses don't get got. According to my best guess, there is no heater core valve per se on the hose. The blending door under the dash directs heat from the core to the cab, correct? If this is the case, its working. I can see it move as I adjust the cabin temp on the dash.

    This old truck has chronic temperature guage voodoo that my mechanic couldn't even figure out. A few minutes after startup, it will spike and the red light will come on, but a thermometer on the radiator shows the temperature to be okay. It will then roll back down to normal operating temperature and stay there for the most part. We've burped the radiator many times and let the truck run for a long time without any more spikes.

    I'm may be having two different issues with the cooling system, but I wanted to include the guage issue just in case someone could connect the dots to a common cause.
    Last edited by TRSFA; 12-09-2009 at 06:43 PM.
    88 K1500 Siverado 330,000 miles and counting.

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    Re: Water Heater Control Valve?

    Make certain that your heater hoses are attached to the appropriate connections on the engine. Generally, the supply to the heater core is the larger of the two and comes off either the intake manifold (next to the thermostat housing/water outlet) or directly off the water pump. The return (smaller one) dumps back into the system at the top of the radiator.

    They can't both be on the engine (i.e. one on the water pump and the other on the intake manifold.) if they are there will be no flow. If you are piped up like that, it could also explain your temp gauge spike.

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    Re: Water Heater Control Valve?

    If the trucks up to temp, feel both of the heater lines.(use caution, theyre gonna be hot) If both are warm, your obviously getting water through the core.
    1998 2dr 4wd Z71 Tahoe Sport - Daily driver - just a K&N for now
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    Re: Water Heater Control Valve?

    Thanks for the responses! The hoses are connected properly, the larger supply hose coming off the intake manifold and the smaller return going to the top of the radiator by the filler cap. I'm going to pull the supply hose off the intake. It may be clogged. My former mechanic put a junk yard engine in the truck that had leaking freeze plugs with allot of crud behind them, which may have made its way to the hose connection at the intake. I'm assuming that water should constantly circulate through the heater core and is directed to the interior of the cab by the blending door as controlled by the temperature adjustment on the dash. Does anyone know of another setup on this model that would have a heater control valve somewhere inline that could be the culprit? Thanks again.
    88 K1500 Siverado 330,000 miles and counting.

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    Re: Water Heater Control Valve?

    Quote Originally Posted by ZZ4Blazer View Post
    If the trucks up to temp, feel both of the heater lines.(use caution, theyre gonna be hot) If both are warm, your obviously getting water through the core.
    After the engine warms up, the hoses are luke warm, as is the air coming off the heater core. Not hot like it should be. My guess it restricted flow of water making it to the core.
    88 K1500 Siverado 330,000 miles and counting.

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    Re: Water Heater Control Valve?

    I'm having the same exact problem, subscribed!
    1983 Chevrolet K20
    355, holley 670, intake, headers
    700r4 / np208
    10 bolt corp from
    14 bolt sf rear
    4" suspension lift
    3" body lift
    16.5 black wagon wheels
    36x12.5x16.5 military oz

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    Re: Water Heater Control Valve?

    I'll try to pull the intake hose later today and report back. If you figure it out, let us know. This old truck still runs like a champ, but know one locally seems to be able to troubleshoot this issue. It's been driving me nuts.
    88 K1500 Siverado 330,000 miles and counting.

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    Re: Water Heater Control Valve?

    To check for flow - lower the level in the radiator an inch or two - you should be able to see the return from the heater core dumping into the upper section of the right hand radiator tank. Should be about like a garden hose with the valve open 1/2 turn.

    If you have good flow (and you are SURE that the engine coolant is at the proper temperature) check for heat loss through the core. On a cool morning, with the cab temp control all the way over to hot and the fan on fast, you should be able to feel a difference in the return line (cooler). If not then either your temp control flap is not seating properly or the heater core is scaled up on the ID of the copper tubes. This scale can be enough to interfere with heat exchange and still allow plenty of water to pass through.

    You could try clean it with a heavy concentration of Prestone coolant flush or some CLR/ZUD type descaling compound and a little circulating pump.
    Last edited by chengny; 12-10-2009 at 07:14 AM.

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    Registered User 88K2500's Avatar
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    Re: Water Heater Control Valve?

    Quote Originally Posted by TRSFA View Post
    I'll try to pull the intake hose later today and report back. If you figure it out, let us know. This old truck still runs like a champ, but know one locally seems to be able to troubleshoot this issue. It's been driving me nuts.
    I know, I just fixed my off-idle stumble problem this morning. Coolant Temprature Sensor.
    It runs like new now. But I'd be more happy if the heater blew hot.
    1983 Chevrolet K20
    355, holley 670, intake, headers
    700r4 / np208
    10 bolt corp from
    14 bolt sf rear
    4" suspension lift
    3" body lift
    16.5 black wagon wheels
    36x12.5x16.5 military oz

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    Re: Water Heater Control Valve?

    A radiator shop may be able to do a chemical flush on the heater without removing it from the truck. Some vehicles seem to be built around the heater core and getting them out is a bitch. Wife's Taurus was like that.
    '04 Silverado 1/2 ton 2WD ext. cab dk. grey. 5.3 liter, locking diff. XLerator exhaust - offbrand but throaty. Borgeson intermediate shaft, Bilstein HD shocks.

    KJ6LFD

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    Re: Water Heater Control Valve?

    I checked the flow from the heater core into the radiator, it appears to be flowing well. Pulled the case off the heater core so I was able to touch it, and it never really got uniformly hot, just near the intake side. This is a relatively new core, by the way. Even after running for about thirty minutes, none of the hoses on the radiator or the heater core got too hot to handle. I'm beginning to think someone put a bad or low opening thermostat. The dash gauge stayed around 130 at idle and maybe up to 200 when I raced the engine for a bit. I'm getting a bad squeaking that is hard to pinpoint, that I thought my be the water pump. Oh well....
    88 K1500 Siverado 330,000 miles and counting.

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    Re: Water Heater Control Valve?

    pick up a thermometer at a parts store, one you can put in your radiator and see what temp the coolant is at. if it is not at 180-195* check your t-stat and water pump.if both test good i would first change both heater hoses you could have a collapse in side. if still no heat have your heater core flushed or replaced.

    good luck

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    Re: Water Heater Control Valve?

    Update: As far as the heater not working, the seals on the casing for the blending door above the heater core have deteriorated, which keeps the door from sealing out all the cold air. That's gonna be a hard fix.

    The temperature gauge spiking appears to be the gauge itself. Local cluster repair shop cannot get parts for an 88.

    Any suggestions?
    88 K1500 Siverado 330,000 miles and counting.

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    Registered User 88K2500's Avatar
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    Re: Water Heater Control Valve?

    I fixed my heat problem. It was the heater core.

    Removed it one evening, installed the next evening. About 8 hours of work. It was not an easy job.
    1983 Chevrolet K20
    355, holley 670, intake, headers
    700r4 / np208
    10 bolt corp from
    14 bolt sf rear
    4" suspension lift
    3" body lift
    16.5 black wagon wheels
    36x12.5x16.5 military oz

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