Hello,

I've noticed on my 89 Monty SWB 3.0 V6 auto, that the heat only gets hot when I'm on the gas. If I'm braking coming up to a stop light the air goes cold. I can hear some sort of relay, or motorized valve switching once I get on the gas again, then the heat is hot again. At the same time the temperature gauge floats closer and closer to the redline when the heater air is cold, but as soon as the "valve" or relay or whatever switches again, the temp comes back down to normal. This sounds like a combination of my truck not cooling very well, and some hvac related mechanism not getting the correct information. Does this sound familiar to anyone? Sorry if my explanation is lacking, it's a hard symptom to put into words. <img src="/forums/images/graemlins/smile.gif" alt="" />

Have you checked obvious stuff like is your coolant topped up? An air bubble in the system can do weird things with your heater. Are you sure it's not your waterpump?

The gauge moving up to red would worry me.

Low coolant level in the radiator will do that. The water doesn't get high enough to reach the heater core unless the motor is revved up. You have to pull the radiator cap to see this (COLD engine). If you have a wrong or clogged radiator cap, or empty coolant recovery reservoir, or plugged reservoir hose, you can get this pretty easy. OR, the head gasket could be blown due to the overheats and the water is disappearing into the combustion chambers. Check for 1-2 VERY clean sparkplugs, or run a compression test. Or a combustion products test on the coolant.

If the head gasket is not already gone, it would be very smart to retorque the head bolts. Most any overheat will start them loosening up. One pretty sure indicator of a blown gasket is loose head bolts.

There are a couple of other causes for your problem, but these are the ones to check NOW. Or you'll be looking up the "replace the head gasket" post...

I haven't let it get to the red, its just gone higher than normal, so hopefully the HG is fine. I topped off fluid about two weeks ago, ill give it another look when i get home to the truck. thanks for the responses guys.

describe the sound
Knocking, buzzing, what?

I remeber having a knocking and it was something connected to the heater/ air controls. I thought it was a vacuum control of some sort. Above to the right of the drivers gas foot. The fix was ignoring it...


I didnt have the other symtoms, though.

+1. Air bubble.

Note well. If you have ever lost the ground on the voltage limiter on the back of the gauge pc board, the most obvious sign of which is a fuel gauge that doesn't peg past F when you fill it up, both the fuel gauge and temp gauge will always read low.

There should be no relay switching or motorized valve stuff when you get on the gas from throttle closed decel. The heater water valve is above/right of the go foot, covered by a cone topped plastic cover held on with one screw. I'd make sure it's opening up all the way.

"Burp" the cooling system when you fill it up. Crank it and let it run up to op temp with the cap off (observe flow in the radiator top tank). I like to point the nose as uphill as I can. Raise the idle up by hand several times and see if it passes some gas. You should see the level drop as the idle goes up. I like to hold the idle high and cram in the last glug of coolant, and keep it high until I can get the cap on. Unless you have some kind of flow restriction, any air bubble should be purged over time as it gets to the radiator and is burped out into the coolant reservoir, and is then replaced by new coolant on the suck back.

Thanks for the replies everyone. The coolant was a *little* low. It didn't seem to change much in terms of the issue I was running into. But upon further inspection the thermostat seems to be defective. It's being replaced along with the water pump, timing belt, and the other engine and accessory belts that consequently needed attention anyhow.

I love how cheap the parts for these trucks are <img src="/forums/images/graemlins/kewl.gif" alt="" />

Thermostat fixed the issue. Hot heat, and a consistent engine temperature. <img src="/forums/images/graemlins/kewl.gif" alt="" />