Temp Light Coming On...why ? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

john68c10 · 08-21-2021, 03:09 PM

Okay,

68C10- 350sbc

Truck has been sitting for about 6 months while I've been working on suspension & steering.

Took it around the block (5 min) today and the temp light came on. About 1 year ago, I put in a new temp sensor.

The radiator was about 1/2 gallon low. Lower and upper radiator hoses were hot, and both heater hoses too.

Fan is spinning, shroud is on.

I'm thinking if the thermostat was stuck then both of the hoses wouldn't be hot would they ?

Water pump ?

Honestly, I'm tired from working on the truck a lot over the last few months and don't want to spend much time on this right now. Where would you start ?

**jabborabbo** · 08-21-2021, 03:31 PM

The temp light in my ‘67 GMC started coming on when I was at idle after driving the truck…turns out there was an exhaust manifold leak right above the temp sender (250 L6 engine)….

BigBird05 · 08-21-2021, 03:57 PM

I would unplug the sender and see if the light stayed on. Then I would get a temp gun and check upper hose, lower hose and cylinder head at temp sender.

john68c10 · 08-21-2021, 05:04 PM

The light does not stay on when unplugged. Radiator fluid gets steamy hot at open radiator cap within 2-3 minutes of running.

Heater blows very hot.

So, coolant IS flowing.

Since it's the newest part, should I just put in another temp sensor and see ?

don t. - 72gmc · 08-23-2021, 06:07 PM

Quote:

Originally Posted by john68c10

Radiator fluid gets steamy hot at open radiator cap within 2-3 minutes of running.

Fire engine back up when hot and cap is off.
Is coolant flowing thru radiator?
A simple test to check the water pump is working, t-stat is open.

HTH...good luck,
don t.

RustyPile · 08-21-2021, 06:39 PM

Depending on radiator size, coolant 1/2 gallon low is about 1/3 - 1/2 radiator content. Hot air coming out of the heater is not an indication of good water flow as the water through the heater bypasses the radiator.. Hot radiator hoses isn't fool proof either.. You could have a partially restricted radiator.. The average temp sender turns on the light at ~ 225 degrees. Fill up the radiator and stick an oven thermometer in the radiator and get a temp reading..

rockyrivermark · 08-21-2021, 07:16 PM

Get a $12 IR gun and verify engine temp at thermostat housing
I’d start there.
RRM

08-21-2021, 03:09 PM	#1
john68c10 Registered User Join Date: Sep 2007 Location: Cookeville, TN Posts: 53	Temp Light Coming On...why ? Okay, 68C10- 350sbc Truck has been sitting for about 6 months while I've been working on suspension & steering. Took it around the block (5 min) today and the temp light came on. About 1 year ago, I put in a new temp sensor. The radiator was about 1/2 gallon low. Lower and upper radiator hoses were hot, and both heater hoses too. Fan is spinning, shroud is on. I'm thinking if the thermostat was stuck then both of the hoses wouldn't be hot would they ? Water pump ? Honestly, I'm tired from working on the truck a lot over the last few months and don't want to spend much time on this right now. Where would you start ?

08-21-2021, 03:31 PM	#2
jabborabbo Senior Member Join Date: Apr 2013 Location: Podunk, Texas Posts: 773	Re: Temp Light Coming On...why ? The temp light in my ‘67 GMC started coming on when I was at idle after driving the truck…turns out there was an exhaust manifold leak right above the temp sender (250 L6 engine)…. __________________ "It is hard to search for a black cat in a dark room, particularly if it is not there. Especially if this cat is smart, brave, and polite." Sergei Shoigu, Russian Minister of Defense

08-21-2021, 03:57 PM	#3
BigBird05 Registered User Join Date: Aug 2019 Location: Cheraw Posts: 1,300	Re: Temp Light Coming On...why ? I would unplug the sender and see if the light stayed on. Then I would get a temp gun and check upper hose, lower hose and cylinder head at temp sender.

08-21-2021, 05:04 PM	#4
john68c10 Registered User Join Date: Sep 2007 Location: Cookeville, TN Posts: 53	Re: Temp Light Coming On...why ? The light does not stay on when unplugged. Radiator fluid gets steamy hot at open radiator cap within 2-3 minutes of running. Heater blows very hot. So, coolant IS flowing. Since it's the newest part, should I just put in another temp sensor and see ?

08-21-2021, 06:39 PM	#6
RustyPile Registered User Join Date: Dec 2017 Location: Elkhart, Texas Posts: 1,861	Re: Temp Light Coming On...why ? Depending on radiator size, coolant 1/2 gallon low is about 1/3 - 1/2 radiator content. Hot air coming out of the heater is not an indication of good water flow as the water through the heater bypasses the radiator.. Hot radiator hoses isn't fool proof either.. You could have a partially restricted radiator.. The average temp sender turns on the light at ~ 225 degrees. Fill up the radiator and stick an oven thermometer in the radiator and get a temp reading..

08-21-2021, 07:16 PM	#7
rockyrivermark Registered User Join Date: Apr 2011 Location: Fairview Park, Ohio Posts: 1,031	Re: Temp Light Coming On...why ? Get a $12 IR gun and verify engine temp at thermostat housing I’d start there. RRM __________________ My build page http://67-72chevytrucks.com/vboard/s...d.php?t=764662 My cabinet building site http://www.cbcabinets.com



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