1995 GMC K1500 4x4 P/U - Heat

[tken35]

I just checked the temperature coming out of the vent after running the truck for a good 45 minutes on the road and the hottest it got was about 82 degrees. Am I correct that this seems kind of low? Also, I noticed that the compressor seems to cycle on and off intermittently, is this normal? The truck has 100,000+ and is not used very frequently, but I did change the coolant this past summer.

Thank you in advance!

[777stickman]

Check both heater hoses going to the firewall. If they're both hot then the core is ok. If one's cooler then the core is plugged.

Next is the temp control door. Not sure if your '95 has vac controls or electric.

A/C should only cycle when an A/C selection is made or DeFrost is selected.

[tken35]

Well I just ran the truck for about half 1/2 hour and the upper radiator hose is slightly warm, the lower radiator hose is ice cold, and both heater hoses going to the firewall feel almost as warm as the upper radiator hose (but that isn't saying much). With regards to the temp control door, we did replace the actuator a couple of weeks ago and the defrost, vent, and floor are now working; however, I do notice that the air blowing out of the defrost is not even.

What should be my next step?

[corning_d3]

Might be time to replace the thermostat

[tken35]

Can anyone explain the differences between thermostats? Some are 160 degrees fahrenheit, some are 180 degrees fahrenheit, while others are 195 degrees fahrenheit, which is best? The vehicle is 5.0L TBI 8cyl.

[777stickman]

For your truck, the 195 is needed for the Electronic Engine Controls.

[j cAT]

Originally Posted by tken35

I just checked the temperature coming out of the vent after running the truck for a good 45 minutes on the road and the hottest it got was about 82 degrees. Am I correct that this seems kind of low? Also, I noticed that the compressor seems to cycle on and off intermittently, is this normal? The truck has 100,000+ and is not used very frequently, but I did change the coolant this past summer.

Thank you in advance!

you can check the thermostat operation out of the vehicle. see if it works correctly a number of times in and out of a pot of boiling/near boiling water. use a thermometer. test new thermostat as well.

remove the heater hoses at the engine , both then flush out / and back flush out the heater core. not too much pressure. see what is drained out in the bucket.

If you do not flush the heater core at the coolant change, low heater temp is common...for some reason all the coolant debris collects there.

at about 30 deg f you should have 130-145 deg f air coming out the vents.. blower on mid range.

3 years ago I had a lifetime warrantee thermostat new start to fail after one year. the temp on the dash in warm weather would lockin at 190 deg f. when the temps dropped to 30deg f or lower it would take a long time to rise from 175 deg f to the 190 deg f..replacing the thermostat no delay and rises steady to 190 .

since you replaced air duct components most likely this is not the problem. I would however make sure you have no vacuum leaks . I believe much of your air duct controls is powered by vacuum. a leak will cause doors not to move ..

[Schurkey]

Originally Posted by tken35

Well I just ran the truck for about half 1/2 hour and the upper radiator hose is slightly warm, the lower radiator hose is ice cold, and both heater hoses going to the firewall feel almost as warm as the upper radiator hose (but that isn't saying much).

Almost certainly the thermostat.

Originally Posted by tken35

With regards to the temp control door, we did replace the actuator a couple of weeks ago and the defrost, vent, and floor are now working;

The temp control actuator won't change the functioning of the defrost/vent/floor air distribution. That's a different actuator, if my '88 is any guide.

Originally Posted by tken35

however, I do notice that the air blowing out of the defrost is not even.

Huh? I don't understand. (Unless you mean that there's somewhat more air flow directed to the driver's side than to the passenger side, which is normal)

Originally Posted by j cAT

at about 30 deg f you should have 130-145 deg f air coming out the vents.. blower on mid range.

I can't vouch for 30 degree F ambient air intake, but if you have ~70F air intake, the heater output temp should be within 25 degrees of the thermostat rating. My '88 is throwing 170+ degree output temps, 195 degree thermostat.

Originally Posted by j cAT

I would however make sure you have no vacuum leaks . I believe much of your air duct controls is powered by vacuum. a leak will cause doors not to move ..

The 88--and I believe this applies to that whole generation of full-size trucks--uses three electric actuators (they look like a tiny Grand Piano) to move the doors. No vacuum.




When I had heater problems with the '88, it turned out to be a rotted foam gasket between heater inlet formed into the sheetmetal of the cab, and the plastic heater inlet/recirculation/fan motor box at the extreme right side of the cab interior. I knew that the engine was properly warming, though. Had a 205F thermostat in it for a couple of years trying to warm the cab before I figured out that foam gasket was degraded. (Looked like foam, until I touched it--and it turned to jelly.) Didn't seal worth beans, created a gigantic cold-air leak under the dash. Every similar foam gasket on the heater system was similarly degraded. I replaced with generic adhesive foam strip rolls sold in the local home-improvement store.

[tken35]

Well, it turns out it was the thermostat afterall! I finally got around to installing it and I just measured the vent temp and it was around 142 degrees .

I hope my picture clarifies my previous comment about the air flow not being even coming out of the defrost: Is this normal?

[j cAT]

Originally Posted by tken35

Well, it turns out it was the thermostat afterall! I finally got around to installing it and I just measured the vent temp and it was around 142 degrees .

I hope my picture clarifies my previous comment about the air flow not being even coming out of the defrost: Is this normal?

thanks for posting the fix. 140 deg f sounds good.

[j cAT]

Originally Posted by tken35

Well, it turns out it was the thermostat afterall! I finally got around to installing it and I just measured the vent temp and it was around 142 degrees .

I hope my picture clarifies my previous comment about the air flow not being even coming out of the defrost: Is this normal?

I believe it is normal that the middle provides little flow on the dash def vent.