Hi there,

My 1995 5.8L has an issue with intermittently blowing heat. The blower works consistently, but will randomly switch between hot and cold. From what I have read so far, I understand this might be an issue with my blend door actuator or heater core, but before I go down that process I am wondering if something else could be at play.

I am about 99% certain my temperature gauge is bad. For instance, right now it has been parked with the key off for about 3 hours on a 50 degree night and the needle is still resting on the "L" of the word "normal." I've had the truck for about a year. When I first got it, it seemed to always run cold, with the needle never getting higher than the letter "N." About 3 months ago, it started reading hot instead. The change happened abruptly, but when I check the temperature on the block near the sending unit with a laser light thermometer, it reads right around 200 degrees. I already replaced the sending unit and the temperature sensors, but this had no impact on the wonky gauge reading. The truck seems to be running just fine, I'm not getting the sense that anything else is wrong.

This brings me to my question...Could a faulty gauge reading "Hot" cause the heater to stop functioning? I know this would be expected behavior if it truly is overheating, I don't know if a hot gauge but normal engine temp would cause this issue though. I don't think it should matter, but thought it was worth asking before I start running down other possibilities.

Thanks for any insight-
Brett

 

No not on these trucks. The temp gauge and the heater are not related.

 

Are you sure the vents are actually switching between heat and defrost? With low/no vacuum the HVAC actuators will default to defrost mode.

 

It could be your thermostat sticking. It gets hot, then all that cold (cooler) coolant flows through, then its cold, repeat.
You could have a vacuum leak, causing the blend door to actually open/close.

If its like my truck, its low on coolant, has two core super cooling rad, never heats up, until it over heats.

Keep the coolant level topped up. Hot air through the heater core will blow cold air.

 

Sorry, I'm new here. The forum layout is a little different from what I'm used to.

Thanks to all of you for the help! I think "wwhite" nailed it, but I'm still digging. Yes, it was low on coolant. I blamed the gauge for being faulty, but that was my mistake. I really feel like I dodged a bullet on the overheating. I hear so much about how fickle the gauges are, I made a bad assumption.

I added a half gallon of coolant directly to the radiator and now it's reading a comfortable coolant temp and the heater is blowing hot. I still have some issues to sort though.

I assumed it wasn't low because the overflow reservoir is plenty full. However, it seems that level never changes, no matter what the engine temp or how long it's been running. Now I'm suspecting that possibly there is a clog in the line between the radiator and over flow res? It may be totally off base, but I'm wondering if one day when it was running warm the expanded coolant overflowed into the res, but for some reason it wasn't allowed back into the radiator? I haven't noticed any leaks, so I'm questioning if the radiator was low because coolant was trapped in the reservoir.

Also, I noticed that when the engine was cold started, the top radiator hose was flattened, but it rounded out again after the engine got up to temp. I think that means air in the system, so I ordered a burping funnel to try to get the air out.

 

If this is true, you might want to change your radiator cap and also make sure the hose to the coolant reservoir is not plugged up.

 

This was definitely true. I think this is where it all went sideways. The hose going to the reservoir was completely air-tight clogged, and the coolant in the reservoir was full of sludge. I drained the reservoir and washed it, cleared the blockage, and refilled with clean coolant. Currently idling and waiting to purge air from the system. Seeing how bad the reservoir was, I should probably be planning a full system flush. I dod get a replacement cap while I was at it.

 

My cooling system was very poorly maintained when I bought the truck, so I extensively flushed it. Expect to have to flush the heater core every now and again, as mud and such that broke free will accumulate and plug it.

Other than that, it's 100% worth it.