03 Safari overheating but no heat

[axisg]

2003 Astro/Safari, 4.3 AWD Van w/ rear heat. Its been -20c / -4f for a couple days now

Driving home last night ( just a couple kms / miles really ) and I had no heat whatsoever from the vents ( both front and rear ) however Engine temps were fine.

Once home I opened the hood to check. Heater core lines are cold, rad hoses are warm but not hot. I popped the rad cap and it was lukewarm. Overflow bottle and the rad were both full. Tested the antifreeze a couple weeks ago to -40/-45

Went in for supper ( 30 mins ), came out to inspect further. Started the van to run and I had heat. Def head scratcher but OK. Let it run for 20 + mins. Got up to operating temp and the heat worked fine. Rad and hoses still never got "hot " but it was def warmer than before. Obviously the heater core lines were warm too ( front and rear )

So I started and let the van warm up for less than 10 mins this morning and went to work. Engine temp was fine but no heat again . Got about 2 blocks and temp started rising enough to turn around for home and it was overheating by the time I got it back to the house ( still no heat )

So I parked the van and walked down to the uhaul and grabbed a half-tonne for the day to get me around. Ordered up a new thermostat, and a water pump but still scratching my head.

My thinking is that if the thermostat was faulty ( stuck closed ) then I should have interior heat. If the waterpump was bad it should be making some noise, or leaking from somewhere. But its dry all around from what I can see. Cant really see much flow thru a full rad when its at idle but when you rev the engine it comes out the top of the rad so its producing some flow. If the diverter valve was faulty ( blocks the coolant flow when the ac is on ) it should not cause the van to overheat

Anybody have some other useful suggestions ? Dexcool was all flushed out last summer over the course of a week and switched over to reg green antifreeze. I am leaning towards a weak waterpump or sticky thermostat. The van is 10 years old, 200k kms on the clock and never been opened up. Its been solid since day #1

 

[Brother Al]

I had a similar problem in my old '03 Blazer.. (safari/astro is based on the "S" chassis). At about 80K, I started having heat issues.. or the lack there of... thing baked my skin off when I first got it.. gradually the heat wouldn't seem to rise enough to defrost the inside of the windshield...
The 100K mile life Dex-Cool Antifreeze is to blame.. from what I understand, GM came up with it to help reduce leaks caused by their cheesy plastic intake manifolds...I also understand that the formulation used was changed by GM at some point because of premature deterioration... As it breaks down, the rubberized anti-leak stuff that is in it starts to congeal on stuff, reducing cooling/heat transfer effectiveness. My heater core was starting to plug with it, as was my radiator. Tried home flushing it.. to no avail, had my mechanic power flush it.. nada. When I removed the heater core, there was a thick layer of the rubbery crud inside.. I imagine that the passageways were pretty plugged up in both the heater core and radiator. The thermostat was changed by the dealership at about 56K because it was in for warranty service repairs and I had them do it, (it had the extended-GM warranty). I typically change them every 50K or whenever I buy any vehicle over 50K. I did not change the radiator because I did not have any temperature issues at the time, outside of having enough heat to defrost the windshield from the inside.

 

[Bonnewagon]

Thermostats stick both open AND closed. When you change it, test both the new and old in a pot of boiling water using a candy thermometer. Many fail this test.

 

[pencero]

Not sure if this will help at all but when I had this problem on another car it came down to the thermostat gasket. Apparently it had eroded which was causing my computer to read the water temp incorrectly and throwing off the whole game

 

[85GPLef41]

Thermostats stick both open AND closed. When you change it, test both the new and old in a pot of boiling water using a candy thermometer. Many fail this test.

x2 I agree or air in the system could be a culprit.. I recently experienced this on a 01 safari.. I let it idle and burped air out of the radiator & heater and it worked good! also have you used any "sealers" ? They could clog the heater cores quickly....

 

[wonderboy24]

try having the system flushed

 

[pontiacgp]

I use one of these to fill and bleed the system. Napa has it for $28 I think, I got one one sale for $22 a few weeks ago

 

[woody31]

the only correct solution is to replace the radiator and the heater core,,,anything else and you are wasting time and money,,,,same issue on my suburban,,,,replaced both and now it will run you out of there,,,,

 

[axisg]

Got it sorted out. The no-cabin heat appears to be a rotten vacuum line to the 6 way diverter valve. It was fine up top but it had rotted out under the heater core and split open where it plugs into the vacuum ball ( no vacuum = no heat which is opposite from my earlier assumption of the valve operation )

I pulled the thermostat and checked it against a new one 3x and they both opened at appx 200 deg. I put the new one in and swapped the waterpump ( and a broken 6 way valve I broke trying to replace the vacuum line behind it in -20 cold 😳 ).

Drove it last night and again this morning. I will need to bump a little air out of the system but I got heat again and even in this cold the van is warmer than in recent memory !!

pic of the offending vacuum line. These Astro Safari vans are notorious for all the vacuum connections and I ended up swapping out over 10' of line which I had actually already done 7 years ago when I first bought the van !

 

[Brother Al]

LOL... we were all coming up with all kinds of potental possibilities... confounded by the most simple of parts.