Heater issues

[Chadly]

Heater has always worked great, now when its the coldest its been in the last 90 years, it acts up.

Will blow good hot air, then will blow cold air, then hot again, etc.

When its blowing cold the engine temp rises, when blowing hot the engine temp goes back to normal, and it cycles through this over and over.

Any ideas? I dont even know where to start looking.

 

[Eveready]

Heater has always worked great, now when its the coldest its been in the last 90 years, it acts up.

Will blow good hot air, then will blow cold air, then hot again, etc.

When its blowing cold the engine temp rises, when blowing hot the engine temp goes back to normal, and it cycles through this over and over.

Any ideas? I dont even know where to start looking.

That has to be a flaky engine thermostat. When it is cold the stat is open and the water going through the heater reflects the very cold air outside. When the stat closes the water jacket in the engine heats up and the water circulating through the heater is hot. Several have reported similar problems and replacing as many as three different thermostats to find one that works. I would use the redneck fix and tape cardboard over about 2/3 of the radiator... just remember to remove it when the weather warms up .

 

[Urambo Tauro]

I too want to say thermostat, but then again, doesn't the t-stat only affect coolant flow to the radiator...?

What truck/engine are we talking about here? Is this one of the later ones that had heater valves?

 

[Eveready]

Unless more has changed than I think it has the heater hoses come off the engine behind the thermostat but usually near the water pump. If the stat is open and the weather is extremely cold the water going to the heater core is going to drop like a rock. According to the shop manual the heater hose comes from the back of the intake manifold (definitely behind the thermostat) so the stat would definitely be controlling the water headed toward the heater. If the stat is flaky the heater would be also.

 

[Urambo Tauro]

On my truck, the only thing downstream of the thermostat is the upper radiator hose. The heater core is fed directly from the rear of the manifold, near the distributor. I don't know much about the engine's internal flow pathways, but any time the engine is running (regardless of t-stat temp), coolant is flowing out of the heater core into the radiator, to be picked up by the water pump. My truck has no bypass hoses or heater control valve.

But some other models do...

 

[redfishsc]

Yeah this has to be a lousy thermostat.

If the thermostat is staying open too long, the coolant temperature during super cold weather is really going to drop a lot. It sounds to me like your thermostat it's closing way too early.

 

[Tomcat58]

That has to be a flaky engine thermostat. When it is cold the stat is open and the water going through the heater reflects the very cold air outside. When the stat closes the water jacket in the engine heats up and the water circulating through the heater is hot. Several have reported similar problems and replacing as many as three different thermostats to find one that works. I would use the redneck fix and tape cardboard over about 2/3 of the radiator... just remember to remove it when the weather warms up .

I had a head gasket fail between 3 and 5 cylinders and decided to fix my engine temperature being 140 degrees I replaced both heads with rebuilds that were completely gone through. Shaved heads was a must !

I went to Napa and bought a new thermostat and installed it on my reassembly and when I fire it up the temperature climbed up to 220 before I freaked out and shut it down. I couldn't figure out the problem and was stuck on the NEW stat was defective. I ask Napa if this happens much and they said it does happen enough and recommend a stat called a safety stat.

Well it was a bad new stat and the safety stat fixed the problem by bring the water temp to 195 degrees. A must for the computer to run correctly ! Heater works great now

This was on a 1991 5.7 350 CI C2500 Silverado. I just wanted to share how even a NEW thermastat can fail too.