Engine runs too cool

12-05-2017, 08:53 PM
I'm new to the forum so forgive me if this has been asked about before. I replaced a leaking radiator and since then the engine only gets to 180 to 185 degrees. Before the radiator I replaced the water pump so that's new and it reached 210 degrees normal operating temp fine after that but only started after replacing the radiator. I've replaced the temp sensor and refilled the coolant using an Airlift vacuum filler. The system held a vacuum just fine but still get gurgling noise from under dash so I suspect there is still air in the system but don't know if that's enough for a 20 plus degree drop in temp.Changed thermostats 3 times now each 187 degree rating. One other thing is rear heat now comes out cold but that shouldn't make the engine run cool I think. Any help would be greatly appreciated. Thanks
2001 Yukon XL 4X4 5.3 litre engine

12-05-2017, 09:57 PM
The rear heat being cold is a result of the engine running cool, its not the cause.

The gurgling you're hearing could be from the heater core having air in it. That could also result in your other issues.

12-06-2017, 04:20 AM
Sounds like you still have air in the system.
What was the degree rating on the original thermostat? Running a 187 degree one you shouldn't see 210. With rear heat I would suggest a hotter thermostat.

12-06-2017, 05:06 AM
You will still see 210, or even more, with a 187 thermostat. 187 is just when the stat opens. The fans won't come on until at least 210.

12-06-2017, 09:15 PM
I'm going to try and run a second vacuum but wait until I can do it on a cold engine. Unfortunately I need my truck for work every morning so I'll do it Sunday and see what happens. 187 degree thermostat is OEM and I couldn't even find a 195 degree one for this engine. There is only a slight gurgling sound when started but after warm up it goes away. It just sounds like it has to be air in the system.

12-08-2017, 03:52 PM
So I found this info on the Alldata website and wondered if anyone has come across this info before. The radiator that I installed was from Carquest. I'm still going to run a second vacuum on the system but I pretty much don't hear any gurgling anymore. If that dosen't work I'm considering getting an OEM radiator and trying that. I really don't feel like giving up and taking it to my mechanic but if that dosen't work I may have to admit that the truck was smarter than me this time.

Wrong Radiator Causes DTC P0128 on 2006 GMC Sierra

Tech Tips

Vehicle: 2006 GMC Sierra 1500, 4WD, V8-5.3L, VIN T, Automatic Transmission
Mileage: 174,009
Problem: This vehicle came to the shop because the temp gauge intermittently dropped to 165° F after engine had completely warmed-up. The MIL (malfunction indicator light) was on.
Details: The tech connected a scan tool and pulled a P0128 DTC (Engine coolant temperature sensor does not meet the minimum thermostat regulating temperature in the pre-set amount of time). The technician first replaced the ECT (Engine coolant temperature) sensor and the thermostat with OEM parts but the problem remained. On advice from the Tech-Assist consultant, the technician checked the actual engine temperature with a non-contact thermometer (laser infrared type) and compared that temperature with the scan tool’s ECT PID. It was the same and below the required threshold temperature.
Confirmed Repair: Because the technician had already replaced the thermostat, the next possible cause was that an incorrect radiator had been installed. NOTE: Some replacement radiators may be missing a restricted orifice that is built into the radiator tank, which is located in the radiator hose fitting that feeds coolant to the surge tank. The engine may not reach operating temperature or it may set DTC P0128 due to excessive coolant flow to the radiator surge tank.
NOTE: If the actual temperature is correct and the ECT sensor PID is incorrect then check the electrical circuit and the connector pins for the ECT sensor. If the circuit and connector are good, you may have a bad “New” sensor.
The tech confirmed that there was no restrictor in the radiator fitting and that the radiator had been recently replaced. He replaced the radiator with a different brand that had the restrictor and the problem was solved.

01-21-2018, 12:35 PM
Well I finally got around to fixing it. I looked inside the small hose fitting on top of the radiator that feeds coolant to the surge tank on an OEM radiator and there is a small pinhole inside. Then I looked inside the Carquest radiator that I had installed and inside that fitting the hole was as large as the fitting diameter. So I had to find a way to make that hole a pinhole. What I did was take a valve stem cap off of my truck and I drilled a tiny hole in it very carefully with the smallest drill bit I have and then I gently tapped that into the hole with a lite mallet and a piece of wood too not mushroom it. The cap fit in about half way so that's why I tapped it in. The cap fits in the fitting very tight and won't come out. Put everything back together and now it's been two weeks and everything has worked as it should. Before I did this I could see coolant flowing back to the surge tank when I revved the engine but now there isn't coolant flowing back through this hose. Temp gets to between 200 and 210 and the truck runs great. I hope this can help someone out who experiences the same issue. It was pretty tricky to diagnose why this was happening until I found that All Data site. If you ever buy an aftermarket radiator check inside that fitting and make sure it is a pinhole and not wide open.

01-21-2018, 05:23 PM
EXCELLENT feedback!! Thank You.

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