Water temperature very low, but cooling fans running continuously on high

[cklein7385]

**UPDATE** Fixed the problem. In my case I ordered and installed new (OEM) thermostat and water temp sensor. I cannot confirm if both failed, or just one of them. The parts were pretty inexpensive and an easy fix that would have taken about an hour or less to complete. I did, however, also replace the upper and lower radiator hoses, and serpentine belt (as they were all original) so it took me a bit longer to complete the entire repair. Also, just some additional information... after the truck sat for 3 days, I started it to move it into the garage to do the work. Almost immediately the fans started running on high again. The fans also ran for about 5 minutes again after shutting the engine off (it had only been running a few minutes). Hope this helps someone else.  

   

 

2016 GMC Sierra; Crew Cab; 5.3L; 6 Speed Trans; 2WD; SLT; 120,000 miles; No mods (other than a cold air intake to the original air filter box);

 

This morning I had been driving on the Interstate for about 20 minutes when I noticed the water temperature gauge was reading cold (by cold, I mean it had not moved from the "peg"). It was cool outside, about 55 degrees F, but even in colder weather (20-30 degrees F) there is typically not a problem with getting mostly normal water temperatures. My first though was the thermostat was stuck open.

 

But then I heard the electric cooling fans running at very high speed (so fast that I could hear the fans running in the cab while driving on the highway with the windows up). The air conditioner was off, and I even shut the climate control off just to make sure the fans were not running because of the AC compressor. This seems to mean the ECM “thinks” its overheating. Also, the voltage gauge was reading 14+ volts to keep the fans running at that high speed.

 

There was no check engine light, warnings or other abnormalities. The truck was driving and sounded normal (other than the fans running high and the temp gauge not reading).

I stopped, left the engine running, and felt the upper and lower radiator hoses. They were both cool (not cold) to the touch and had little to no pressure. The coolant overflow was full, so the system has coolant. There were no signs or smells of overheating (but the fans were running hard).

 

I got back home and shut the engine off, but the fans stayed running for about 5 more minutes. The fans have never stayed running after engine shut down, even on the hottest North Texas summer day. So again, I feel like the ECM "thinks" its overheated.

 

A double failure seems unlikely (the failure of the thermostat and temperature sensor), but I have ordered a new OEM thermostat and sensor to install. My confidence that these will solve the problem is slim, even though it seems like a stuck thermostat. But I don’t know why the ECM was running the fans like it was overheating. I suspect there is something else going on, more than just a simple t-stat and sensor change.

 

Anyone have any insight on this? If this is a common problem with a known fix, I have overlooked it.

 

Thoughts? Suggestions?

Edited March 28, 2022 by cklein7385
FIXED THE PROBLEM - SOLUTION

[2015 High Miler]

Generally, its the thermostat going out, but it could also be a cracked radiator tank. That is what happened on mine. I found out about the thermostat during the search regarding the running fans and cold temps. 

 

  GM truck running fans

 

Thermostat is not too hard to do.

 

good luck

Edited March 23, 2022 by 2015 High Miler

[CamGTP]

The temp sensor failed, that is why the fans went to full power.

[lonestardiver]

I had the same happen on my 18 GMC 6.2 recently after a cold snap.  Next day and ever sine it ran fine.  I suspect it was a stuck thermostat.

[cklein7385]

On 3/22/2022 at 9:41 PM, 2015 High Miler said:

Generally, its the thermostat going out, but it could also be a cracked radiator tank. That is what happened on mine. I found out about the thermostat during the search regarding the running fans and cold temps. 

 

  GM truck running fans

 

Thermostat is not too hard to do.

 

good luck

Thanks for your reply, I appreciate it. I replaced both the thermostat and water temp sensor and that solved the problem. 

[cklein7385]

On 3/22/2022 at 9:56 PM, CamGTP said:

The temp sensor failed, that is why the fans went to full power.

 

Thanks for your reply, I appreciate it. I replaced both the thermostat and water temp sensor and that solved the problem. 

[cklein7385]

On 3/23/2022 at 7:23 AM, lonestardiver said:

I had the same happen on my 18 GMC 6.2 recently after a cold snap.  Next day and ever sine it ran fine.  I suspect it was a stuck thermostat.

 

Thanks for your reply I appreciate it. A few days later I started the truck and was still having the problem. I replaced both the thermostat and water temp sensor and that fixed it. Glad your thermostat seemed to "fix" itself.  

[lonestardiver]

I can’t say it is fixed.  I suspect it is a sign the thermostat is going out or the temp sensor may be heading that way.  Haven’t any issues since then but I am keeping an eye on it.

[Mark St-Onge]

On 3/22/2022 at 10:56 PM, CamGTP said:

The temp sensor failed, that is why the fans went to full power.

I had the same issue and I replaced the thermostat than everything was working fine, which wouldn’t be the temp sensor

[Beaglerbob]

I’m 600 miles from home. Need to get home to fix it. Will it damage the vehicle to drive it home with this problem of low temp/fans running on high?

[Scott Avanzino]

Thanks for having this resource and to cklien7385 and others! 

 

I am back on the road so far so good.

 

This same exact condition (everything mentioned in thread) occurred yesterday while driving my 2014 GMC Sierra SLE 5.3 L V8.

 

Left house 12/3/2024 to run an errand 15 miles from home - everything normal but had to stop for extended period for train crossing and noticed loud noise after a train passed - gauge temp 160 degrees and determined cooling fan was running at high speed, voltage 14.5 V on gage. Was 39 degrees in AM warmed up to 50 by the time I left. First real cold snap here in south MS.

 

Pulled over and googled and found this thread.

 

Hoses cold, coolant in expansion tank. Turned heat up to full blast and it was blowing hot air so figured I had coolant circulating and it would be safe to turn back and hit nearest parts store. Halfway toward home air went cold and check engine light went on. Pulled over and called for ride. No maple syrup smell indicating coolant leak and fluid in expansion tank. Fan running for 5 minutes after shut off. No visible leaks or odd smells but wasn't taking chances on vehicle with 175,000 miles.

 

Luckily I keep tools and spare parts in my truck. Had coolant and spare thermostat but wasn't sure if it was right one. Checked out you tube videos and additional threads on this board while waiting for ride and locating parts at Advance Auto. 

 

Bought 2 gallons dexcool, new temp sensor and new 207 degree OEM thermostat and housing - $130 total. Returned to vehicle 30 mins later, released a little pressure on cap on white plastic expansion tank slowly with gloves and towel. Barely a hiss and no fluid overflow, coolant temp warm but not dangerously hot. Hose to thermostat housing from radiator felt limp and empty and cool as well.

Disconnected ground on battery and cleaned terminal and stub with sand paper and greased and reconnected (I was told this would reset check engine light at parts store), removed air lines (squeeze the pinch releases top and bottom simultaneously and pull back quick connects toward cab) and loosen 6" hose clamp on air handler, pull and peel back to access thermostat housing and water jacket temp sensor underneath the unit (driver side of engine). Disconnected 2" radiator hose clamp with channel locks, removed hose from front of housing and dumped warm coolant into plastic paint bucket I had - about a quart came out. Disconnected 3/8" clamp and hose on top thermostat housing and wire harness on sensor maybe 1" to left of thermostat housing. Removed and reinstalled temp sensor and aluminum crush gasket with 19mm box wrench. My sensor threads came with thread dope on them so I didn't tape it. Removed three 10mm bolts on thermostat housing, pulled off inspected and found thermostat stuck in open position. Cleaned up old rubber O ring and wiped the base, squeezed lower hose to force coolant and debris out. Reinstalled new unit and gasket - with just a 1/4 turn past snug as I noticed the base threads are in cast aluminum. Fortunately, everything I purchased matched the old and I only got a little grease and dirt on my hands, no burns or cuts or lost tools.

 

Added new dexcool to expansion tank (about a gallon) and started up truck. Engine light off and truck started warming up as normal in idle above 160 degrees and heat was hot fairly quickly. Headed for home with needle reaching just a hair under 210 degrees - normal operating temp.

 

Hope this help someone else! I am very much appreciative and for the moment relieved.

 

 

 

[Tom Chillson]

Make sure you clear the ECT code after replacing the thermostat and ECT. Once I did that, the temp gage read correctly and fans stopped running immediately.  Good to go.