Why is it so hot! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bac1755 · 03-20-2009, 12:30 PM

I have a stock 305 from the mid 80's in my 71 GMC why does the coolent temp rise so high after I turn the truck off? I just put this engine in the old one never did this, and this engine is in alot better shape then the old one.

ChevLoRay · 03-20-2009, 12:37 PM

My school bus does this. It will rise from 200 degrees to almost 240 in the space of 5 minutes. It cools down in less than a minute after I restart it. Does yours also cool down as quickly?

If not, I'd be looking at the fan clutch.

bac1755 · 03-20-2009, 12:45 PM

It has electric fans on it, and yes it cools down after i start it. I had to put a heat spacer between the intake and carb cuz it would boil the gas out of the carb.

Andy4639 · 03-20-2009, 12:49 PM

I would say it's normal for thew motor to heat up after you turn it off. The heat has to go some place and with the water pump off and cooling fans off then all it can do is go up in temperature until it starts cooling again.
You could have senor's on one of the fans to turn on/off for a few minutes after the key is off like the late model cars.

**mr48chev** · 03-20-2009, 12:59 PM

Quote:

Originally Posted by Andy4639

I would say it's normal for thew motor to heat up after you turn it off. The heat has to go some place and with the water pump off and cooling fans off then all it can do is go up in temperature until it starts cooling again.
You could have senor's on one of the fans to turn on/off for a few minutes after the key is off like the late model cars.

+1, not really a big deal. The coolant isn't circulating to cool the engine so a bit of heat builds up.

bac1755 · 03-20-2009, 01:00 PM

I might try some sort of relay on the fan.

piecesparts · 03-20-2009, 01:16 PM

The heat sensor is on the head (just above the cylinderA) so if you stop the water flow, the heat from the now stagnant system will increase to a higher temp. A temp of 240 degrees is not a bad thing, as long as you are not lifting the rad cap relief. Also, when you start back up the temp should come back down. Are you sure that your temp sending unit is reading right? Is it the one that should be working with the gauge that you have?

**El Jay** · 03-20-2009, 01:52 PM

That's pretty much normal.
My not so new engine does the same thing.
There's just no coolant circulation, so the temperature goes up. As soon as you restart the engine, you coolant going through the radiator.

Longhorn Man · 03-20-2009, 04:24 PM

this is normal.
hooking up the fan to run with the engine off will do nothing but shorten the life of the battery and fan.
The engine cooling, and radiator cooling are not the same system.
Cooling the coolant in the rad, with the engine off, will do nothing to cool the engine, since there is no circulation to get the coolant in the engine.
People keep saying on here that late models will continue to run the fan after the engine is off, but I have yet to see one personally. The fans may kick on when the engine is off, but the ignition is on... but not with the ignition off.
This iss perfectly normal, you just either never saw it before, or you never had a mechanical guage hooked up on one before.

**El Jay** · 03-20-2009, 04:33 PM

Pretty much.
But if the fan is wired "hot", with a sensor at the radiator (usually aftermarket) & a relay; sometimes after turning the engine off you can get some thermosyphoning that will cycle the fan on & off.

Longhorn Man · 03-20-2009, 04:42 PM

true, but not enough benifit to off set the damage and cost of wearing down the battery quicker.

**El Jay** · 03-20-2009, 04:54 PM

True 'nuff, Andy. I'm not disputing what you said.

I've just seen a couple where the installer just basically took a hunk out of the upper radiator hose & installed a sensor (kind of like the old retrofit smog control crap) and the fans would cycle on & off as the hot water thermosyphoned out of the block & into the radiator; and vice versa.

With modern smog controlled engines sometimes running 220+, I've never gotten real concerned about water temperatures when the engine was shut off, assuming it wasn't running hot beforehand.

How goes things in "Oh-Aitch-Ten"?
I have to outside in the sunny, 70 degree stuff & service the Chevy so I can run it up to Sacramento tomorrow.

bac1755 · 03-20-2009, 04:59 PM

So the fan relay is a no go. The old engine was so full of crap"looked like mud" it might not have been showing. If its normal then i'll leave it alone.

Tx Firefighter · 03-20-2009, 05:06 PM

I'd bet money that you also have the wrong cooling temp sensor in the engine too. Late model 305s like you have use a smaller threaded hole than your old original engine did. There is no way to screw the original sensor into the new head where it belongs. If you have a sensor screwed into the head, it's surely the wrong one. Late model sensors read different values than the early ones did. Been there and learned that lesson.

Longhorn Man · 03-20-2009, 05:47 PM

very good point kevin

bac1755 · 03-20-2009, 05:48 PM

I have aftermarket autometer gauges, it reads fine just after I shut it down it will rise in temp.

bac1755 · 03-20-2009, 05:51 PM

The guage set came with a bushing reducer for the threads. and i put it in the same location i have always put them.

03-20-2009, 12:30 PM	#1
bac1755 Registered User Join Date: Jan 2009 Location: Denison, Texas Posts: 245	Why is it so hot! I have a stock 305 from the mid 80's in my 71 GMC why does the coolent temp rise so high after I turn the truck off? I just put this engine in the old one never did this, and this engine is in alot better shape then the old one.

03-20-2009, 12:37 PM	#2
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Why is it so hot! My school bus does this. It will rise from 200 degrees to almost 240 in the space of 5 minutes. It cools down in less than a minute after I restart it. Does yours also cool down as quickly? If not, I'd be looking at the fan clutch. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

03-20-2009, 12:45 PM	#3
bac1755 Registered User Join Date: Jan 2009 Location: Denison, Texas Posts: 245	Re: Why is it so hot! It has electric fans on it, and yes it cools down after i start it. I had to put a heat spacer between the intake and carb cuz it would boil the gas out of the carb.

03-20-2009, 12:49 PM	#4
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,945	Re: Why is it so hot! I would say it's normal for thew motor to heat up after you turn it off. The heat has to go some place and with the water pump off and cooling fans off then all it can do is go up in temperature until it starts cooling again. You could have senor's on one of the fans to turn on/off for a few minutes after the key is off like the late model cars. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

03-20-2009, 01:00 PM	#6
bac1755 Registered User Join Date: Jan 2009 Location: Denison, Texas Posts: 245	Re: Why is it so hot! I might try some sort of relay on the fan.

03-20-2009, 01:16 PM	#7
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: Why is it so hot! The heat sensor is on the head (just above the cylinderA) so if you stop the water flow, the heat from the now stagnant system will increase to a higher temp. A temp of 240 degrees is not a bad thing, as long as you are not lifting the rad cap relief. Also, when you start back up the temp should come back down. Are you sure that your temp sending unit is reading right? Is it the one that should be working with the gauge that you have?

03-20-2009, 01:52 PM	#8
El Jay Gone to greener pastures Join Date: May 2007 Location: Gateway to the Delta Posts: 7,354	Re: Why is it so hot! That's pretty much normal. My not so new engine does the same thing. There's just no coolant circulation, so the temperature goes up. As soon as you restart the engine, you coolant going through the radiator. __________________ '69 Chevy 1/2 T LWB Stepper: Daily Beater '72 GMC 3/4 T Fleet: Another Daily Beater '72 Plymouth Gran Coupe: ? "Ah women. They make the highs higher and the lows more frequent." Friedrich Nietzsche "Never kick a fresh turd on a hot day." Harry S. Truman GUN CONTROL: Never having to say, "I missed you." Always fire two warning shots into your attacker's chest area before putting a bullet between his eyes. Paraphrased from Louis Awerbuck

03-20-2009, 04:24 PM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Why is it so hot! this is normal. hooking up the fan to run with the engine off will do nothing but shorten the life of the battery and fan. The engine cooling, and radiator cooling are not the same system. Cooling the coolant in the rad, with the engine off, will do nothing to cool the engine, since there is no circulation to get the coolant in the engine. People keep saying on here that late models will continue to run the fan after the engine is off, but I have yet to see one personally. The fans may kick on when the engine is off, but the ignition is on... but not with the ignition off. This iss perfectly normal, you just either never saw it before, or you never had a mechanical guage hooked up on one before.

03-20-2009, 04:33 PM	#10
El Jay Gone to greener pastures Join Date: May 2007 Location: Gateway to the Delta Posts: 7,354	Re: Why is it so hot! Pretty much. But if the fan is wired "hot", with a sensor at the radiator (usually aftermarket) & a relay; sometimes after turning the engine off you can get some thermosyphoning that will cycle the fan on & off. __________________ '69 Chevy 1/2 T LWB Stepper: Daily Beater '72 GMC 3/4 T Fleet: Another Daily Beater '72 Plymouth Gran Coupe: ? "Ah women. They make the highs higher and the lows more frequent." Friedrich Nietzsche "Never kick a fresh turd on a hot day." Harry S. Truman GUN CONTROL: Never having to say, "I missed you." Always fire two warning shots into your attacker's chest area before putting a bullet between his eyes. Paraphrased from Louis Awerbuck

03-20-2009, 04:42 PM	#11
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Why is it so hot! true, but not enough benifit to off set the damage and cost of wearing down the battery quicker.

03-20-2009, 04:54 PM	#12
El Jay Gone to greener pastures Join Date: May 2007 Location: Gateway to the Delta Posts: 7,354	Re: Why is it so hot! True 'nuff, Andy. I'm not disputing what you said. I've just seen a couple where the installer just basically took a hunk out of the upper radiator hose & installed a sensor (kind of like the old retrofit smog control crap) and the fans would cycle on & off as the hot water thermosyphoned out of the block & into the radiator; and vice versa. With modern smog controlled engines sometimes running 220+, I've never gotten real concerned about water temperatures when the engine was shut off, assuming it wasn't running hot beforehand. How goes things in "Oh-Aitch-Ten"? I have to outside in the sunny, 70 degree stuff & service the Chevy so I can run it up to Sacramento tomorrow. __________________ '69 Chevy 1/2 T LWB Stepper: Daily Beater '72 GMC 3/4 T Fleet: Another Daily Beater '72 Plymouth Gran Coupe: ? "Ah women. They make the highs higher and the lows more frequent." Friedrich Nietzsche "Never kick a fresh turd on a hot day." Harry S. Truman GUN CONTROL: Never having to say, "I missed you." Always fire two warning shots into your attacker's chest area before putting a bullet between his eyes. Paraphrased from Louis Awerbuck Last edited by El Jay; 03-20-2009 at 08:35 PM.

03-20-2009, 04:59 PM	#13
bac1755 Registered User Join Date: Jan 2009 Location: Denison, Texas Posts: 245	Re: Why is it so hot! So the fan relay is a no go. The old engine was so full of crap"looked like mud" it might not have been showing. If its normal then i'll leave it alone.

03-20-2009, 05:06 PM	#14
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	Re: Why is it so hot! I'd bet money that you also have the wrong cooling temp sensor in the engine too. Late model 305s like you have use a smaller threaded hole than your old original engine did. There is no way to screw the original sensor into the new head where it belongs. If you have a sensor screwed into the head, it's surely the wrong one. Late model sensors read different values than the early ones did. Been there and learned that lesson. __________________ I'm on the Instagram- @Gearhead_Kevin

03-20-2009, 05:47 PM	#15
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Why is it so hot! very good point kevin

03-20-2009, 05:48 PM	#16
bac1755 Registered User Join Date: Jan 2009 Location: Denison, Texas Posts: 245	Re: Why is it so hot! I have aftermarket autometer gauges, it reads fine just after I shut it down it will rise in temp.



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03-20-2009, 05:51 PM	#17
bac1755 Registered User Join Date: Jan 2009 Location: Denison, Texas Posts: 245	Re: Why is it so hot! The guage set came with a bushing reducer for the threads. and i put it in the same location i have always put them.

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