This has been a problem for a while, but with winter coming, I'd like to get this figured out. I have no heat in my 92 F150 with a 300 inline 6. I replaced the heater core-no difference. I replaced the thermostat with a 195F one, then a 205F one-no difference. I took off the heater hoses to look for blockage-no difference. I put the thermostat in water and boiled it to check for performance-it worked fine. The temp gauge warms up to the "N" in Normal, then drops slightly below that. On hard highway driving, I can get the truck to run at or above the N in normal. I blocked off the rad completely last winter, and although the engine got up to the "M" in normal, there still was no heat. Half blocking the rad did nothing either. I did a compression test on the engine-I got 160 psi on all cylinders when the engine was cold. I did this thinking head gasket, but the engine runs fine.

I find that the heater hoses don't get very hot except down near the engine itself. I even put insulation around the heater hoses-no difference.

Can a water pump malfunction and cause this? This truck won't run hot no matter what I do, and I can't get any heat.

The air conditioning works fine. I took apart the dash, checked the temp **** and cable, the plenum door-all seem OK.

HELP!!!!

 

There has to a valve somewhere that allows the water to pass thru the heater core.

 

I know the valve you mean, but this truck doesn't have any valves in the heater hoses. I guess in the 92's, the coolant circulates thru the heater core all the time.

 

Did you check the temperatures of the core inlet and outlet hoses? If hot coolant isn't flowing, you won't have any heat. If there is normal coolant flow, you have a temperature blending/control problem.

 

Like stated above, check the heater hoses inlet and outlet temps, both should be hot.
If so and still no heat, the blend air door is stuck to cold. Check its operation, cabling etc.

 

OK, an update. I installed a new water pump, although the old one seemed fine. That didn't help.

Then I took a light duty radiator out of a 94 and installed it, still no heat. Truck runs the same temperature.

The cooling system doesn't seem to develop pressure. There is no circulation of hot water up the heater hoses.

So now I've done rad, thermostat, water pump, heater core-what's left????

 

It sounds to me like you might have a plugged up Heater Core, even tho it is new......you could try taking both heater hoses off it and using a piece of copper pipe or something to connect them together and run the engine to see if the hoses heat up to the touch. If so, you have a plugged heater core.......if not, maybe still a bad new water pump.

hth
Vaya con Dios,
Chuck, CABGx3

 

Another thing I remember from a LONG time ago, was that I replaced a Water pump, and the new gasket was one that was used for multiple applications and it had different die-cut places where I had to punch out to open the heater water passage port.........naturally, I didn't know that at the time and had to learn the hard way......but that gasket effectively cut off the flow of water out that port.........something to consider if the "bypass" I mentioned previously does not allow the hoses to heat up.

Vaya con Dios,
Chuck, CABGx3

 

Thanks Chuck. All good thoughts. It is actually my third heater core. The first one wa only a year old, the second was one out of a wreck at my place, then a third new one. I also checked that gasket you talked about.

It's strange that the whole thing won't seem to pressurize. I'm wondering about head gasket or crack, but all cylinders checked out at 160lbs.



John

 

Even a brand new part can be defective, such as a plugged brand new heater core, or a bad brand new water pump. You will not know where you are until you either bypass the heater core or open it up and blow through it.......the heater circulation circuit/plumbing is of course fairly simple, but you must isolate which part of it is blocked to know where to go further. Then you can crack open the bypass to see it the Waterpump is pumping water.

Vaya con Dios,
Chuck, CABGx3

 

Chuck-I can yank the heater hose right off and there is no circulation, with the old or the new pump.

I just got looking on ebay and found a 12 volt bosch water pump that looks like it could be installed inline in the heater hose. Has anyone ever resorted to using one of these? I figured I could install it low down so gravity would give it antifreeze to fire up to the heater core.

John

 

OK cool.......now either the hose is plugged inside or the water pump is not pumping.........no chance that the pulley is slipping on the water pump shaft is there?.....pull the hose off the water pump and see if you have any flow coming out of there.

Vaya con Dios,
Chuck, CABGx3

 

If no water coming out of the Water pump outlet port to the heater, maybe you can hit the outlet port with some compresed air to maybe dislodge a blockage?

Vaya con Dios,
Chuck, CABGx3

 

Still getting nowhere.

Does anyone know if the heater pipe that leaves the waterpump PULLS coolant back to the engine, or does it PUSH coolant up to the heater core?

John

 

I would not imagine that any Engineer would design the system to PULL water through the Heater Core, but hey, in this day and age, anything is possible. I figure that you've either got a blocked hose, or a bad waterpump, or one of the ports is plugged so that the waterpump cannot pump (like the gasket thing I described earlier)......since you said that the waterpump is turning and there is no flow out the heater hoses at the Heater core.

Vaya con Dios,
Chuck, CABGx3