Engine Slow To Heat Up After Coolant Replacement

Beamie · December 20, 2020

I recently replaced my coolant (Dex Cool to Dex Cool) and since then the truck ('08 Tahoe) seems very slow to come up to temperature. Normally it runs at 210 on the gage. Now it seems to hover around 185 to 195. It eventually makes 210 after about 10 miles of driving where it used to be pretty quick to get there. Could they be related to the coolant replacement? Would measuring the coolant temp in the fill reservoir be the same as at the temperature sensor?

CamGTP · December 21, 2020

Probably the thermostat sticking open.

Likely unrelated to the coolant exchange and it just happened at roughly the same time you did this work.

Just so you know the gauge may read 210 but it's not really 210. The cluster is about 10-15 degrees off. So that 185-190 you are probably seeing on the gauge is less than that in reality.

A handheld scan tool would read the coolant temp in the drivers side cylinder head. Otherwise a temp gun on the heater core hoses or the lower radiator hose is a good place to check.

Colossus · December 21, 2020

Did you replace the thermostat? Essential that you do that if you did not.

tlaw91 · December 21, 2020

+1 on replacing that thermostat

Now ur truck gets 2 coolant flushes lol

Grumpy Bear · December 21, 2020

16 hours ago, Beamie said:

I recently replaced my coolant (Dex Cool to Dex Cool) and since then the truck ('08 Tahoe) seems very slow to come up to temperature. Normally it runs at 210 on the gage. Now it seems to hover around 185 to 195. It eventually makes 210 after about 10 miles of driving where it used to be pretty quick to get there. Could they be related to the coolant replacement? Would measuring the coolant temp in the fill reservoir be the same as at the temperature sensor?

I see you live in the upper northeast. Been cold there lately, right? So there are some other considerations. The part that is telling us something is that is does reach temperature. Just takes 10 minutes longer. I live in northern Illinois. See this happen every year when it turns cold. Summer time (anything over 45 F) and it's up to heat like a jack rabbit. Colder it gets, the longer it takes and the lower the 'warmed up temp". Swing on my truck is about 8 degrees F. If I make that same comparison between 30 F and 100 F the steady state temp will be as high as 15 degrees and 15 minutes different. System is just more efficient in the winter. If you did the coolant swap around the same time as the cold snap it could be faking you out. It happens, right. We don't hawk that gauge any harder than we do right after a service.

Another possible is the new coolant isn't 50/50. I've got a few bottles over time that were as low as 40% glycol. That much has quite an effect on cooling and freeze point. Check it. Yes, it happened during the glycol shortage some years ago but ever recently I've got bottles +/- 5% solution different.

Beamie · December 21, 2020

Good point on the coolant. They were new unopened premix bottles but I did not run a test on them. Will check it out now.

New comment: You were right about the coolant. Test on what's left in the bottle and also in the overflow tank shows -20 degrees. The info on a full strength bottle claims -34 degrees using 50/50 mixture. I don't know what ratio the -20 degrees result equates to but that is a couple of formulas I'm going to figure out. They list 70/30 mix as -84 degrees.

I am assuming that more water in the weaker mix results in more heat transfer.

Edited December 21, 2020 by Beamie

It's Tim · December 21, 2020

Drain coolant, replace hoses & thermostat, refill.

garagerog · December 21, 2020

Beamie, the only thing I haven't seen mentioned so far was if you're sure you got all of the air "burped" out of the coolant system after your coolant change. If you have an air pocket you will get errant dashboard temperature readings.

Beamie · December 21, 2020

18 hours ago, CamGTP said:

Probably the thermostat sticking open.

Likely unrelated to the coolant exchange and it just happened at roughly the same time you did this work.

Just so you know the gauge may read 210 but it's not really 210. The cluster is about 10-15 degrees off. So that 185-190 you are probably seeing on the gauge is less than that in reality.

A handheld scan tool would read the coolant temp in the drivers side cylinder head. Otherwise a temp gun on the heater core hoses or the lower radiator hose is a good place to check.

Interesting thing, the upper radiator and heater hoses are heating up at about 175 degrees on the dash gauge. The lower radiator hose is cold. Today it's 35 degrees out and the gage did get up to 210 in a reasonable time at idling with the heater turned full on but no fan running (significant heater action in the cabin). The lower radiator hose was slightly warm near the thermostat but cold at the bottom of the radiator. The cooling fans never came on.

Ran the truck a couple miles on the road, gauge holding at 210, and still one end of the lower radiator hose is just warm and cold at the radiator end. Again, cooling fans not running, at least when parked, idling, after the ride.

Can the system really hold the temperature at 210 (gauge) without circulation?

Would this indicate that the thermostat is not opening?

There have never been any overheating issues according to the dash gauge.

Edited December 21, 2020 by Beamie

Beamie · December 21, 2020

1 hour ago, garagerog said:

Beamie, the only thing I haven't seen mentioned so far was if you're sure you got all of the air "burped" out of the coolant system after your coolant change. If you have an air pocket you will get errant dashboard temperature readings.

I haven't seen any specific instructions on that other than turn on the heater, fondle the hoses, run and refill when the level drops in the bottle. I've also given it more than a few miles on the road afterwards. There doesn't seem to be pressure in the overflow bottle when the cap is removed. Should there be?

Beamie · December 21, 2020

3 hours ago, It's Tim said:

Drain coolant, replace hoses & thermostat, refill.

Nice time to live in VA!

The original change was on an odd 60 degree day but, since the day after, it's gotten cold and wet up here for elective surgery.

Edited December 21, 2020 by Beamie

Grumpy Bear · December 21, 2020

47 minutes ago, Beamie said:

Interesting thing, the upper radiator and heater hoses are heating up at about 175 degrees on the dash gauge. The lower radiator hose is cold. Today it's 35 degrees out and the gage did get up to 210 in a reasonable time at idling with the heater turned full on but no fan running (significant heater action in the cabin). The lower radiator hose was slightly warm near the thermostat but cold at the bottom of the radiator. The cooling fans never came on.

Ran the truck a couple miles on the road, gauge holding at 210, and still one end of the lower radiator hose is just warm and cold at the radiator end. Again, cooling fans not running, at least when parked, idling, after the ride.

Can the system really hold the temperature at 210 (gauge) without circulation?

Would this indicate that the thermostat is not opening?

There have never been any overheating issues according to the dash gauge.

It's never really without circulation. There is always some sort of minimum flow bypass and yes, on a cold enough day it's enough. It all sounds pretty normal to me. :dunno: