Engine running too cool - The 1947 - Present Chevrolet & GMC Truck Message Board Network

JMD · 10-25-2020, 11:17 PM

Hi all,
I have a MarkIV 454 in my ‘68 K10. It has a 180* thermostat, cheap 2 core radiator, and 2 temperature sending units on the back of the intake manifold. Now that the weather has cooled way off, I’ve noticed that the engine temp is dropping down to 140-150* at highway speeds. If the truck is just idling in park it will warm up to 180+ but it takes a very long time. I don’t believe it’s a sending unit issue because both gauges read within 5* of each other at all times.

I’m guessing the thermostat is stuck open, but before I go through the hassle of ripping into and replacing the thermostat I wanted a second opinion. Thoughts?

**Stocker** · 10-26-2020, 12:37 AM

My guess is same as yours -- t-stat stuck open. Check the specs for that engine though, I suspect a 180* might be too cool. A 190* or better yet a 195* might be a better choice, especially in Michigan in winter. And it won't make your engine overheat in summer.

gmc684x4 · 10-26-2020, 12:38 AM

It can also be your electric fan on to much as well as a stuck tsat
my trucks only get a 195 stat year around

BigBird05 · 10-26-2020, 09:27 AM

180 Deg stat was the norm for years. Then it was raised to reduce emissions.

geezer#99 · 10-26-2020, 09:41 AM

If you have two temperature sending units in the back of the intake are you sure you’re not running way hotter than what your gauge says.

72c20customcamper · 10-26-2020, 09:51 AM

Quote:

Originally Posted by geezer#99

If you have two temperature sending units in the back of the intake are you sure you’re not running way hotter than what your gauge says.

I agree . I have all the mechanical sensors in the head , temps can be 20 to 30 degrees hotter in the head . My Chevelle reads 205 when the electric fans kick in they are Controlled by a sensor in the radiator that kicks on the fans at 195 water temp .

JMD · 10-26-2020, 08:46 PM

Where is the stock sending unit location? If I understand the cooling system correctly, there shouldn’t be any coolant flowing through the upper radiator hose until the thermostat has opened. So if the thermostat is stuck open and I start the truck after it’s been sitting all night, I should feel coolant moving through the upper radiator hose, right?

**LockDoc** · 10-27-2020, 12:03 AM

Quote:

Originally Posted by JMD

Where is the stock sending unit location? If I understand the cooling system correctly, there shouldn’t be any coolant flowing through the upper radiator hose until the thermostat has opened. So if the thermostat is stuck open and I start the truck after it’s been sitting all night, I should feel coolant moving through the upper radiator hose, right?

Driver side head by #1 plug.

LockDoc

**WorkinLonghorn** · 10-27-2020, 12:13 AM

Quote:

Originally Posted by JMD

Where is the stock sending unit location? If I understand the cooling system correctly, there shouldn’t be any coolant flowing through the upper radiator hose until the thermostat has opened. So if the thermostat is stuck open and I start the truck after it’s been sitting all night, I should feel coolant moving through the upper radiator hose, right?

Correct. You will feel it getting warm way before the 180° is reached.

**Steeveedee** · 10-27-2020, 10:37 PM

Quote:

Originally Posted by LockDoc

Driver side head by #1 plug.

LockDoc

That's on an SBC, right? Because my (70) BBC has the temp sender on the passenger's side, and the wiring appears to be stock.

**LockDoc** · 10-27-2020, 11:33 PM

Quote:

Originally Posted by Steeveedee

That's on an SBC, right? Because my (70) BBC has the temp sender on the passenger's side, and the wiring appears to be stock.

Yup, Brain fart on my part...

..

LockDoc

**Steeveedee** · 10-27-2020, 11:54 PM

Quote:

Originally Posted by LockDoc

Yup, Brain fart on my part...

..

LockDoc

Well, I never noticed that the sender was on the driver's side on the SBC, so there you go.

And to think that I worked on these things for many years. I worked for my BiL starting at age 15 ('67) on the weekends doing basic mechanical work. I know that they were all pretty much stock in those days, and we only fixed things, and didn't do anything custom.

geezer#99 · 10-28-2020, 09:00 AM

Do a search Steve.
Some bbc had the temp sensor between #3 and #5.
69 L78 did.
Factory valve covers had a clip on them to hold the wire for the sensor.

The tall deck motors had it too.

http://67-72chevytrucks.com/vboard/s...d.php?t=437017

10-26-2020, 12:37 AM	#2
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,809	Re: Engine running too cool My guess is same as yours -- t-stat stuck open. Check the specs for that engine though, I suspect a 180* might be too cool. A 190* or better yet a 195* might be a better choice, especially in Michigan in winter. And it won't make your engine overheat in summer. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

10-26-2020, 09:27 AM	#4
BigBird05 Registered User Join Date: Aug 2019 Location: Cheraw Posts: 1,300	Re: Engine running too cool 180 Deg stat was the norm for years. Then it was raised to reduce emissions.

10-26-2020, 08:46 PM	#7
JMD Registered User Join Date: Jun 2013 Location: Grand Rapids Michigan Posts: 291	Re: Engine running too cool Where is the stock sending unit location? If I understand the cooling system correctly, there shouldn’t be any coolant flowing through the upper radiator hose until the thermostat has opened. So if the thermostat is stuck open and I start the truck after it’s been sitting all night, I should feel coolant moving through the upper radiator hose, right? __________________ My build thread: http://67-72chevytrucks.com/vboard/s...96#post7439296

10-28-2020, 09:00 AM	#13
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,777	Re: Engine running too cool Do a search Steve. Some bbc had the temp sensor between #3 and #5. 69 L78 did. Factory valve covers had a clip on them to hold the wire for the sensor. The tall deck motors had it too. http://67-72chevytrucks.com/vboard/s...d.php?t=437017 Last edited by geezer#99; 10-28-2020 at 10:06 AM.

10-25-2020, 11:17 PM	#1
JMD Registered User Join Date: Jun 2013 Location: Grand Rapids Michigan Posts: 291	Engine running too cool Hi all, I have a MarkIV 454 in my ‘68 K10. It has a 180* thermostat, cheap 2 core radiator, and 2 temperature sending units on the back of the intake manifold. Now that the weather has cooled way off, I’ve noticed that the engine temp is dropping down to 140-150* at highway speeds. If the truck is just idling in park it will warm up to 180+ but it takes a very long time. I don’t believe it’s a sending unit issue because both gauges read within 5* of each other at all times. I’m guessing the thermostat is stuck open, but before I go through the hassle of ripping into and replacing the thermostat I wanted a second opinion. Thoughts? __________________ My build thread: http://67-72chevytrucks.com/vboard/s...96#post7439296

10-26-2020, 12:38 AM	#3
gmc684x4 Registered User Join Date: Jul 2012 Location: Whitehorse yukon Posts: 1,218	Re: Engine running too cool It can also be your electric fan on to much as well as a stuck tsat my trucks only get a 195 stat year around Last edited by gmc684x4; 10-26-2020 at 12:49 AM.

10-26-2020, 09:41 AM	#5
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,777	Re: Engine running too cool If you have two temperature sending units in the back of the intake are you sure you’re not running way hotter than what your gauge says.



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