Heater Help

[jsett83]

Searched "heater" in this forum and could not come up with anything relating to my specific issue. Any help would be greatly appreciated.

1987 V10 with 5.3/4L60E

new thermostat, coolant level is good, heater hoses hot

blower motor has all four speeds, truck will heat with fan on low but not when fan speed is on high. Any suggestions?

Thanks in advance

[v30crewcab]

sounds like the heater hoses aren't hooked up correctly. not getting flow thru them. one has to be near the lower water hose for suction, and the other on top of the motor for return. unless the heater core is plugged.

[Dead Parrot]

What temp thermostat? Mine had a 160 when I first got it and the heat output was poor. Changed to 195, heater will run you out of the cab on high.

It is also possible the new thermostat has failed mostly open.

Any chance the blend gate is stuck mostly in the cold position?

If your core return goes to the water pump, and nothing else works, may be worth the hassle of temporarily plugging the pump return and sticking the return hose into the radiator to make sure you have good flow through the heater core.

[tucsonjwt]

I will throw my guess in here - clogged heater core or leaking heater core.

[jsett83]

Thermostat is 195, new as of 2 months ago...noticed this problem once before LS swap. Thanks for the suggestions, will check blend gate and try to flush heater core.

[tucsonjwt]

While you are investigating, look for signs of green streaks from copper oxidation indicating leaking heater core, hoses, heater core nipples, etc. If you had an old engine which did not have regular coolant changes/flushing you could have junk precipitate out of the coolant and settle in the heater core over time. Rusty looking coolant can contain some big flakes of iron from the engine and fittings which can settle out in places like the heater core which don't get used for months at a time in the summer.

[jsett83]

*Update* Flushed the heater core and it blows hot heat for about 10 mins then cooler air returns....any suggestions? I was thinking it was the heater core for sure

[chengny]

If the truck is AC equipped, make sure the compressor is not kicking in. Just pull the connector off the low side control switch (on the filter drier shell) - the compressor clutch is fed through that switch.

[jsett83]

Right now I have a sanden compressor, but i have not charged system/hooked up wires to it. Waiting until next spring. When I hooked up heatr core lines i hooked the bigger line coming of the LS water pump to the bigger opening of the heater core. I assume this is correct??

[donut]

I believe the supply is off the manifold with the return to the radiator or water pump.
I may be wrong or the LS engines may be different.

I think your supply would be from below the thermostat (intake manifold on the old style engines) and return after it.