No Heat

Its possible that its the thermostat, but i doubt it. The thermostat would cause the car to overheat, or it would stay cold and take a long time to heat up if it does at all. You also might have a broken heater core. Is there any Antifreze coming out into the car floormats around the passanger side?

If it was the t-stat you'd have an underheating or overheating engine, too much coolant or too much water same thing.

Sounds like you've got yourself a plugged heater core, or the blend door in the HVAC system is stuck.

Find the two heater hoses running thru the firewall on the passenger side. The line that runs from the back of the intake to the firewall is the feed line, the line that runs from the firewall to the radiator is the return line. After the truck heats up, the lines should be the SAME temperature. If the feed is hot and the return is cold, you get yerself' a plugged core.

Doc

Low coolant if it's anything like the older trucks. The coolant isn't high enough in the radiator to get into the heater core.

GMC Gremlin

I'd agre with Gremlin, likely cause is just low coolant, when it gets low enough engine temp stays normal, but you lose heat.......if it gets lower than that, you'lll begin overheating.

Its not because you have low coolent that would just cause it to overheat.

No it wouldn't "kubby" It can cause loss of heat in the cab, and then further then that it can cause over heating issues. And usually low coolant is indicated by LOW ENGINE TEMP, because there is no coolant heating up the temp sensor. The engine is very hot, but not according to the sensor.....

GMC Gremlin A man dealing with these issues as we speak... heater core next. Hopefully after that no more leaks, no more top offs.

Ok if it is at that point where theres not enough for heat but wont overheat, depending on where you live dont put in too much water with the antifreze.

You could use straight water or straight coolant and get the same symptoms. It is solely based on the amount of coolant in the engine. That IS the way it is in my 1990 and my 1985 K5. It may not be like that in his engine.

GMC Gremlin

What is involved to up plug/clean out a heater core??

You're both wrong. Coolant flow to the heater core originates from the intake manifold. The line running to the rad is the return line. If his coolant was low enough to NOT make it into the HC, his engine would be severely overheating.

Doc

Bwahahahahaha!

Doc

Remove both hoses (carefully, you can rip the nipple off) and put a garden hose up to the nipple that was attached to the radiator.

GMC Gremlin

Replacing it is the best way, but a powerful garden hose and a brass spray nozzle can do wonders.

Doc

Guess I run my truck continuously over heating, consistently, daily, as do others.... from other boards... and other walks of life.... different years.... :thinking:

GMC Gremlin

That's a good one. Is this a joke? You do know there is a Water Pump, and it forcing water not only threw the engine, but the Heater Core also! The Engine would have to be so low of coolant that the water pump was pumping AIR. In which case the Engine would be over heating. The likely problem is flow of Hot coolant threw the heater core. Didn't mention any leaking. So it's either plugged, or a Valve to control the flow is stuck closed. If it's the Core, it's better to replace then to flush out. Heater Cores are pretty cheap. It's just changing them sometimes that can take a while. It's better then trying to get it flushed out, only to get the crap into some other part of your system, and or have the core plug up again a short time later, or start leaking all over the inside of your car. This would also be a good time to get your hoses all replaced also.

How about how does the coolant LOOK? When was the last time the cooling system was flushed out? Usually lack of Maintenance is what cause this type of problem. People do the Oil Changes, but really how many out there flush the other fluids? Maybe the transmission. But Brake fluid, and Coolant, even Differential oil, ect.. All are Important, but most never thought about until there's a problem and your stuck someplace.

It's no joke - My 93 4.3 just had a leaking heater core. When the coolant got down to about 1/2 capacity the heater would blow only cold air, but there was still sufficient coolant to keep the engine in the normal operating range ( a little high, but still in normal range ). Add coolant and the heat returns to the cab.

BTW - this is exactly the same thing that happened to my 76 Chevy.

I agree, I have seen coolant levels low enough to cause no heat yet not overheat the engine. Only in cooler temperatures though (winter)

Larry

M Rothwell wrote:

Is there any coolent leaking at all? on your driveway? Because im having the same problem now and its just a broken water pump. I lost heat, added more coolent and got heat. But now its starting to find a home all over the driveway.

Well if your Water Pump is leaking enough that the coolant is low, the Pump would be pumping Air, and that being the case, no coolant would be getting pumped to the Heater Core. Fix the pump, fill it back up and all should be just fine. While your there take a GOOD LOOK at the Hoses and Belts. If not, and you have that nice new Water Pump on there you may end up with a blowout from a weak, Mushy old hose.