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Old 10-25-2020, 11:17 PM   #1
JMD
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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?
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Old 10-26-2020, 12:37 AM   #2
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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.
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Old 10-26-2020, 12:38 AM   #3
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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.
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Old 10-26-2020, 09:27 AM   #4
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Re: Engine running too cool

180 Deg stat was the norm for years. Then it was raised to reduce emissions.
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Old 10-26-2020, 09:41 AM   #5
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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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Old 10-26-2020, 09:51 AM   #6
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Re: Engine running too cool

Quote:
Originally Posted by geezer#99 View Post
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 .
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Old 10-26-2020, 08:46 PM   #7
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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?
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Old 10-27-2020, 12:03 AM   #8
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Re: Engine running too cool

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Originally Posted by JMD View Post
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.

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Old 10-27-2020, 12:13 AM   #9
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Re: Engine running too cool

Quote:
Originally Posted by JMD View Post
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.
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Old 10-27-2020, 10:37 PM   #10
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Re: Engine running too cool

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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.
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Old 10-27-2020, 11:33 PM   #11
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Re: Engine running too cool

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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.....

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Old 10-27-2020, 11:54 PM   #12
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Re: Engine running too cool

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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.
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Old 10-28-2020, 09:00 AM   #13
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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.
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