coolant temperature hot

Sharpz · January 4, 2021

I am sure this has been asked several times, but after doing a search it wouldnt pull anything relevant up for me, so I apologize in advance. So last week while making a trip to pick up a mattress less then 30 miles away, my 2004 gmc sierra 1500 z71, with the 5.3 gave me no issues on the drive to the mattress store. I parked, shut the truck off and then went in, to do paper work and pay. I had to pull around back of the warehouse and didnt notice any issues. I shut the truck off, loaded up, and then proceeded to leave, at that time I noticed the temperature gauge climbing past 200. It continued like that until it nearly maxed out and the coolant temperature hot warning came on. I shut the truck off and popped the hood, the hoses seemed to be intact, no coolant leakage, and the truck didnt smell hot. After about 5 minutes I started the truck up, and the gauge was down to around 220, and I was able to drive for about a mile before coming to a light, and then it started to climb again. I pulled into a gas station and shut it off again I checked the hoses, all were hot but not burning hot as I would expect, bottom, top, and inlet and outlets of heater core. I started the truck back up, and it was at normal operating temp just about 200, and was able to drive home with no other issues. In a week of short driving, less then 60 miles a trip, I never had another issue.

So bring me today. Because I couldnt find any issues with the coolant system itself I assumed the problem was the coolant sensor, so swapped it out today. Started the truck and everything worked as it should. I made a short trip around 24 miles, and the truck ran good, not cooling issues at all. I stopped at boomarangs to grab a burger, about 30 minutes inside, when I started the truck less then 4 minutes of driving time in town, the temperature gauge started to climb past 200, and did so until the coolant temperature hot warning message came on. I shut it down, and waited 5-10 minutes and was about to go a few more miles, before it did it again, each time I looked under the hood, and could not find any issues at all. After 2 times of doing this, I was able to get up to 65 and had no rise above 200, until i got back home in 30mph zone, then the temp started climbing again. bottom hose, was hot but not too hot to touch, same with top hose and inlet and out let heater hoses. The bottom hose, water pump and fan clutch are less then 8 months old, are not leaking and the top hose doesnt seem to be collapsed and if I squeeze it I can force water to through faster then it would normally move. When I put the water pump in 8months ago I bled the system good, and have had no issue until this last week. I get good heat from the heater, and havent seen any coolant leaks, so dont think its the heater core. When I got home I let the truck cool down then took the coolant overflow top off, and let the truck run for about 10 minutes to see if it had air in the system, the temp at idle didnt go past 200. Any ideas of what could be causing this would be appreciated. Sorry this is so long, but I figured the more information the better.

2004 gmc sierra 1500

5.3 automatic, 275k

Edited January 4, 2021 by Sharpz
engine size

CamGTP · January 4, 2021

I'd be looking at a possible faulty thermostat. That's the cheapest solution at the moment.

Sharpz · January 5, 2021

1 hour ago, CamGTP said:

I'd be looking at a possible faulty thermostat. That's the cheapest solution at the moment.

Thanks, tomorrow I plan on doing a flush of the system, and will change out that thermostat. Ive never had one that would work sometimes and sometimes not, so it will be a new one on me if this is the case. Thanks for the idea.

dsr611 · January 5, 2021

Stepper motor for the temp gauge possibly going bad? That gen of GM products are notorious for stepper motors in the instrument panel going bad.

Sharpz · January 6, 2021

So yesterday I decided to change out the fan clutch because it seemed to spin easier then I would imagine it should, but after a test drive the issue still wasnt resolved. Id bought a new thermostat and new coolant. I drained the system, flushed my radiator with a garden hose, filled the system back up with just water and let it run through for a while. Then I redrained the system, removed the thermostat, and filled back with water. Again let run for a while, before draining it again. Installed the new thermostat, filled with regular prestone antifreeze, ,bled the system and so far it seems like everything is good to go. I am going to take it for a longer test drive, if I dont update this post, consider the issue fixed. I hope this helps someone. Thanks for the helpful tips.

22 hours ago, dsr611 said:

Stepper motor for the temp gauge possibly going bad? That gen of GM products are notorious for stepper motors in the instrument panel going bad.

redvett · January 6, 2021

I dont think you had a hot engine at 240F the SES would have set and at 250F PCM would have started to drop cylinders to protect the engine. My bet is on the coolant sensor or the yellow/black wire having coolant gotten through the sensor to the wiring. Inspect the plug/pigtail for coolant and when you change the thermostat only use 187 F only.

Sharpz · January 6, 2021

2 hours ago, redvett said:

I dont think you had a hot engine at 240F the SES would have set and at 250F PCM would have started to drop cylinders to protect the engine. My bet is on the coolant sensor or the yellow/black wire having coolant gotten through the sensor to the wiring. Inspect the plug/pigtail for coolant and when you change the thermostat only use 187 F only.

I suppose you could be right. This issue was intermittent and I did change out the water coolant sensor thinking that was the issue. The problem still continued after I did the swap, although I didnt see any issues with the wires, if it happens again I will splice in a new plug. I drove it around 50 miles, with a lot of in town driving, stop go, etc, and the temp gauge never moved past 200. For now I am calling it solved. Thanks for the good info. I did think it was odd the truck didnt auto shut down. When the water pump went out it shut down almost immediately.