Luke Warm Heat

[Jerrys1990]

Couldn't find a clear cut answer during search. My '90 has weak heat now that it's getting cold out. I warm it up 10-15 mins before I leave and the output is luke warm at best. Even driving freeway to work, it doesn't really get "hot." Assuming the thermostat is the correct 195 degree piece, do these trucks just have crappy heat? The temp gauge, which I don't really trust anyway, doesn't read where I would think it should for 180-195 degrees. Blend door is electronic and appears to be opening all the way. Though, the HVAC controls don't like to work when it's this cold....took a few minutes for it to respond to button pressing. The low speed fan relay chatters too but that'll be a separate post. Anything I should check before I replace the thermostat and coolant?

 

[RichLo]

When was the last time you flushed the cooling system?

I would start with unhooking the heater core hoses and back-flushing it multiple times then run some radiator flushing additive as per their recommendations and then back flush the heater core again before refilling the system with fresh coolant.

I just did this in my winter beater blazer and it went from not getting the inside of the truck over 50 degrees to face burning heat within a couple miles of my house.

 

[L31MaxExpress]

When was the last time you flushed the cooling system?

I would start with unhooking the heater core hoses and back-flushing it multiple times then run some radiator flushing additive as per their recommendations and then back flush the heater core again before refilling the system with fresh coolant.

I just did this in my winter beater blazer and it went from not getting the inside of the truck over 50 degrees to face burning heat within a couple miles of my house.

The 87 G20 van had the same issue when I got it, no heat at all. Zero flow through the heater core. I gently backflushed it and it will cook you out of it with a 170F thermostat.

 

[454cid]

The 87 G20 van had the same issue when I got it, no heat at all. Zero flow through the heater core. I gently backflushed it and it will cook you out of it with a 170F thermostat.

I think it's worth noting that the OP is in MI, and you're in TX. Unless he's a penguin, I'd recommend the 195

Jerrys1990 minimal heat, first thing I'd do is the thermostat. Last three vehicles I've gotten, I've had to do the thermostat. Two of them, and maybe the third as well, were OEM. (I don't know why this is bold and I get get rid of it)​

 

[L31MaxExpress]

I think it's worth noting that the OP is in MI, and you're in TX. Unless he's a penguin, I'd recommend the 195

Jerrys1990 minimal heat, first thing I'd do is the thermostat. Last three vehicles I've gotten, I've had to do the thermostat. Two of them, and maybe the third as well, were OEM. (I don't know why this is bold and I get get rid of it)​

There are enough 0°F days here and we have had some -15°F mornings in the past few years. -23°F is the coldest temperature on record in Texas. 170F will cook you out even in -15°F. My temperature knob is very rarely more than 1/4 off the coldest setting after the engine warms up on either the Van or Tahoe. When we had Snowmagedden come through about 3 years ago it was colder here than in Chicago because of the weird jetstream behavior.

 

[Schurkey]

My '90 has weak heat now that it's getting cold out. I warm it up 10-15 mins before I leave and the output is luke warm at best. Even driving freeway to work, it doesn't really get "hot." Assuming the thermostat is the correct 195 degree piece, do these trucks just have crappy heat?

Absolutely not...when everything is working as it's supposed to.

The temp gauge, which I don't really trust anyway, doesn't read where I would think it should for 180-195 degrees.

Yeah, I'd start with thermostat and flush. Before--and after--the flush, be sure to pull the block drains to get rid of the flush water trapped in the block. Failure to do this leads to NOT removing some sediment, and for many folks, weak freeze/boil protection due to the extra dilution of the anti-freeze.

A 350 will have a block drain plug (steel/iron, 9/16 hex) in the left bank, the knock sensor also serves as the drain plug on the right bank.

Though, the HVAC controls don't like to work when it's this cold....took a few minutes for it to respond to button pressing. The low speed fan relay chatters too but that'll be a separate post.

I posted about my non-responsive '88 HVAC control head, you ought to review that thread even though mine wasn't related to "cold".
www.gmt400.com/threads/hvac-control-head-in-dash-multiple-issues-mostly-fixed.63927/

HVAC Control Head in dash--multiple issues (Mostly fixed)

1988 K1500, AC + heat, (Option C60) the usual push-button control head with the "blue" vacuum-fluorescent electronic display panel. Apparently the '88 “black” control head is a one-year deal, the '89 and '90 have grey plastic faces but function the same. This control head design has a long...

www.gmt400.com

 

[GoToGuy]

Like they said, the heater water circuit has a flow restrictor built in , so the flow is slower through the in cab heat exchanger. To use the heat. Therefore any dirt, rust , debris in water flow as it reaches the heater core will slow and settle in the heater core passenges. ( like a settling pond ). No or small flows = no or low heat. Gently back flush, not full blast. Full city water pressure can be up to 50 psi or more, don't explode the heater core. Makes sense now?

 

[Jerrys1990]

Excellent info. Thanks, all.............I have not done the coolant flush since I got it. As soon as the budget allows, I'll be working on this.

 

[Jerrys1990]

Absolutely not...when everything is working as it's supposed to.


Yeah, I'd start with thermostat and flush. Before--and after--the flush, be sure to pull the block drains to get rid of the flush water trapped in the block. Failure to do this leads to NOT removing some sediment, and for many folks, weak freeze/boil protection due to the extra dilution of the anti-freeze.

A 350 will have a block drain plug (steel/iron, 9/16 hex) in the left bank, the knock sensor also serves as the drain plug on the right bank.


I posted about my non-responsive '88 HVAC control head, you ought to review that thread even though mine wasn't related to "cold".
www.gmt400.com/threads/hvac-control-head-in-dash-multiple-issues-mostly-fixed.63927/

HVAC Control Head in dash--multiple issues (Mostly fixed)

1988 K1500, AC + heat, (Option C60) the usual push-button control head with the "blue" vacuum-fluorescent electronic display panel. Apparently the '88 “black” control head is a one-year deal, the '89 and '90 have grey plastic faces but function the same. This control head design has a long...

www.gmt400.com

Any recommendations for a quality thermostat while I start shopping for parts? Started reading your HVAC panel thread too....interesting stuff. Mine locked up again yesterday when it was 29 degrees and wouldn't operate until I turned the parking lights on. Sure was strange.........

 

[Schurkey]

I put a CarQuest (Advance Auto) (Motorad--made in Israel) thermostat in my '88. This is a 205 degree unit.

I've been dissatisfied with it ever since. Slow to open--temp is way past the "210" marked on the dash gauge--must be around 225 or more before it opens. Temp then drops way down. Back up...way down. Takes forever for it to stabilize--20 miles or more.

I'm likely to replace it someday, but I don't know what to use either. I've had good--and faulty--thermostats of pretty-much every brand.

There was a time that I really liked the concept of the Robertshaw 'stats. I have a shitload of them as part of an eBay purchase--started with an entire case, but I've resold about half of 'em. Problem with that case, is that they're 160-degree units. Nearly worthless in most of the country.