I have a '97 Jimmy 4x4, I have had this truck for a few months..bought it for this years winter months while while summer car is set in storage for the Canadian winter.

For the whole time I've owned this truck, this thing would provide so much heat, you'd have to crank a window open to cool off. Engine temp would be "only" around 70C (158F), it would never reach operating temp off 100C(212F)..even with the cabin heat truned off, engine temp would never go above 70C(158F).

4 days ago we starting getting into the -20C range (-4F) ouside, with the wind its about -35(-31F). First day driving in -20C weather, I noticed when trying to warm up the truck, engine temp started rising to 100C(212F) and the cabin heat was FREEZING..I had no heat from HVAC. I gave blips to the throttle and noticed heat started from the HVAC and engine temps shot right back down to 70C(158F)..at that point I didn't think anything.

Next day I started the truck and let it run to get warm, same concern happened, no hot air from HVAC and engine temp started to rise. I checked the coolant level in the resevoir, slightly low but still had coolant present near bottom, I topped up the resevoir and blipped the throttle, noticed I started to get heat and the engine temps started to lower near the average 70C(158F). At this point, I checked for any possible external leak, none from engine bay nor cabin, I suspected some air had got into the system and that was my concern related to no heat and engine overheating. Drove the truck around all day and no more problems, heat was great in the cabin and engine temp was not rising to overheat.

Yesturday I thought all would be ok now, but when I started the truck..all the same concerns were present..but this time engine temps started reaching 130C(266F), and HVAC air was freezing cold. I checked the resevoir level and all was good. I felt the heater core hoses, "both" are cold. I checked the radiator hoses, top is hot and lower is cold. I slowly opened the radiator cap and coolant was seen backflowing into the resevoir. Temp outside was to cold to do anything so thats far as I got with diagnostic.

Today outside temps are not bad, -2C (+28C). I went out to test the truck. I checked coolant level in radiator, is full right to cap, resevoir is full to hot line..started and warmed up the truck, opened the rad cap and rev'd the engine, no bubbles seen at all. I checked the engine and HVAC temps, all is working fine!!??? Engine temp stays at the 70C(158F) that it always has been around, cabin temp air from HVAC is hot just like I need. Heater core hoses are obviously hot, lower rad hose is still cold but then again my engine temp is only 70C(158F). So at this point I am stumped..

1: Why is my engine temp only 70C(158F), could be a bad sender or stuck open stat?
2: do you think I had just air in the system all this time, and this is why I had no heat and overheating?

 

Sounds like a bad tstat to me!! Simply and cheap place to stay.

 

would be worth a check of that tstat forsure. i dont think it controls the heater core much tho.

 

If it were me I would replace the T stat (with a factory temp) and your rad cap with new and properly burp the system when you are finished. Both are cheap and a great place to start with symptoms like you are describing.

 

^^^^ all good advice.

 

I would also backflush the system, as well as the tstat.

 

I have a 2001 jimmy and I am having sorta the same problem. I am getting no heat to my cabbin but it goes to the red and says check gauges I have replaced the t-stat and ran hot water thru it to try to flush it out some. There was an orange mud like substance in it. So I am not sure what to do next plz help

 

keep flushing, also you can get a cooling system cleaner. prestone i think makes it at the automotive stores.... you run it in your system for a while with just straight water and it cleans it out then you can replace it after a flush with antifreeze.

old trick i do is run straight water with 4 cups of CLR in the water for a day or 2 then dump it and flush it with water again, then dump that and put recomended coolant in.