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Old 01-06-2018, 04:59 PM   #1
Gregski
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71 350 running hot coolant backup up problem

hello, I hope you can help me out with my 1971 GMC 1/2 ton basket case running hot, that I put together, it has a crate Mr. Goodwrench long block 350 SBC in it bought in the mid 90's but its not a roller engine its a flat tappet cam, with a 350 transmission

I don't know if it ran before I got the truck I don't know if it overheated before I got it either, the engine was out of the truck and I put it in, the transmission was already married to it, I have no idea why the engine was removed from the truck to begin with or if it came out of a different truck

the engine has an Edelbrock Performer intake on it and a 1407 Edelbrock 750 CFM carb with an HEI distributor with the vacuum hose connected to Ported vacuum, which pull 0 Hg at idle, the manifold vacuum pulls 21 Hg at idle, I am not sure what the timing is set at at the moment but the truck starts easily and idles good and smooth

I do not have a heater core in it, so there are no hoses going to the heater core, there is also no bypass rubber hose from the intake to the water pump, I mention this because I thought the stockish SBC heads flow coolant internally through the intake manifold and so you don't need the bypass hose like you do on the Vortec heads

I have a brand new 195 thermostat, currently I am only testing with water and not with 50/50 coolant mixture

outside temp today was in the high 58 F degrees, when I test drove the truck it was showing 215 F so I cut the test drive short

without the thermostat in the engine the water comes out the top rubber hose that runs from the thermostat to the driver side on the radiator, I disconnected it from the radiator so I could easily see if water was coming out and without the thermostat it does, this makes me believe the water pump is good and spinning in the right direction

I even back filed the engine with water by pouring it in that top radiator hose and down the thermostat housing and it took a bit of water, I was trying to eliminate an air bubble, which I thought I done did, water even started coming out the filler hole on the radiator so I thought mission accomplished, but no

with the new thermostat installed, the water topped off and the truck running, after a while it starts to spew water out the filler hole on the radiator, I leave the cap off to see what in the world is going on in there and thats when water comes out the filler hole, and my digital brand new temp gauge starts climbing 195, 205, 215, ... 225, etc so I shut her off and put my head in my lap, lol

sorry for all the words, but I am stumped and want to answer as many questions ahead of time as I can, as I know there are a lot of experienced wrenches on this forum and I value your feedback and need your help

Last edited by Gregski; 01-06-2018 at 05:10 PM.
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Old 01-06-2018, 07:13 PM   #2
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Re: 71 350 running hot coolant backup up problem

My '70 C10 is set up similarly and I'm not running the bypass hose from the water pump to the Edelbrock manifold either. I do get a little heat soak sometimes partly due to having an aluminum vs. phenolic riser between the carb and manifold - but I don't experience overheating of the coolant.

The first thing I would try would be to figure out the existing initial timing (after first verifying the TDC mark on the harmonic balancer is accurately reflecting true TDC)..It will still start and run good at idle even if at zero TDC.

Because with modern gas as you know if your initial timing is not sufficiently advanced it will cause exactly the symptoms you describe.

Also less important and I do understand why you are running just plain water while you sort this out but as you know plain water will boil at 212F - so maybe a little antifreeze just to raise the boiling point will be a little safer for the motor.

I noticed you have well over 4,000 posts on this forum so I'll be surprised if I'm going to be of any help to you but at least hopefully this will encourage others to chime in.
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Old 01-06-2018, 07:29 PM   #3
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Re: 71 350 running hot coolant backup up problem

Ummm, why are you leaving the cap off ? before you reinvent the wheel let it cool completely then try adding a gallon of straight anti freeze, clean water till full and using a new radiator cap you should be fine . It's a pressurized system ,No cap no pressure .
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Old 01-06-2018, 09:45 PM   #4
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Re: 71 350 running hot coolant backup up problem

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Ummm, why are you leaving the cap off ? before you reinvent the wheel let it cool completely then try adding a gallon of straight anti freeze, clean water till full and using a new radiator cap you should be fine . It's a pressurized system ,No cap no pressure .
Thanks, I was only leaving the cap off to look for flow after I determined there was a problem, I tested with the cap on and drove it with the cap on of course.

With cap on after idling for 10 minutes the Temp gauge reads 215 F. Same goes with a 50/50 coolant mix with cap on.

I have ran dozens of cars on plain water while troubleshooting and when all things are dialed in water works just fine, of course then I add the coolant for drivability but I am not putting in the green stuff to shoot all over my engine and garage floor while troubleshooting.
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Old 01-06-2018, 10:04 PM   #5
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Re: 71 350 running hot coolant backup up problem

215 not hot
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Old 01-06-2018, 11:19 PM   #6
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Re: 71 350 running hot coolant backup up problem

It shouldn't be running 215F at that temp. Pulling a 26' travel trailer on level ground, I don't get that hot, this time of year. The bypass for a SBC is inside a water pump passage, not like a big block, so it doesn't matter what intake one is running, given the old style heads.

You need to let it settle down, and get it full of water, for starters. It should be full when you run water into it, especially without a thermostat. You have something else going on, if it overheats at idle. Do you have a clutch fan?
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Old 01-06-2018, 11:34 PM   #7
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Re: 71 350 running hot coolant backup up problem

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Originally Posted by harpo231 View Post
215 not hot
I disagree, I appreciate the feedback and please note I am not trying to argue. I have a 1974 GMC as well 350 as well it has a 180 thermostat with Vortec heads and even on 100 F days it purs at 185 degrees max unless something is wrong then it starts to climb

In the January 2014 Hot Rod magazine Hot Rod To The Rescue article called "Malcom Dunk's '65 Chevy El Camino Overheats the guy brings it in because the car cruises at 210-215 degrees... etc. etc.

Its a good read, they fix it with an aluminum radiator and an electric fan so it runs at 180, I think that's a bit drastic for my case, but I will find a happy medium somewhere in the neighborhood of 195.
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Old 01-07-2018, 12:49 AM   #8
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Re: 71 350 running hot coolant backup up problem

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Do you have a clutch fan?
No I don't.
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Old 01-07-2018, 12:40 PM   #9
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Re: 71 350 running hot coolant backup up problem

Do you know the condition of the radiator?
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Old 01-07-2018, 01:31 PM   #10
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Re: 71 350 running hot coolant backup up problem

Mine runs in 185 Fahrenheit range too but I could make it overheat within 15-20 minutes of start up and just at unloaded idle by simply adjusting my initial timing to say 8 degrees ATDC instead of my normal 12 degrees BTDC
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Old 01-07-2018, 01:43 PM   #11
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Re: 71 350 running hot coolant backup up problem

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Do you know the condition of the radiator?
good question, it came out of a 1969 donor GMC truck, from a guy that I bought the 12 bolt rear end, is it the original one I doubt it, it is a metal one though, very OE like copper/brass material not the plastic aluminum type

I flushed the heck out of it forwards and backwards and it does not appear to be clogged and the cooling fins seems decent and straight

I pressure tested it before putting it in, it was installed to get me by until I can get a brand new one, well the place that sold me a radiator for my '74 said they don't make one like it for this '71 so I am still getting by with this one

did some 15 PSI pressure tests of the system last night and the only tiny leak I can see is from the thermostat housing gasket area so I will work on that today
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Old 01-07-2018, 01:48 PM   #12
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Re: 71 350 running hot coolant backup up problem

just some pics for you so you know what we are dealing with

I would pump it up to 15 PSI and check on it after 30 minutes make any tightening of the hose adjustments and re test, sometimes the tests went for as long as 2 hours, it does leak down slowly I assume its from that thermostat housing gasket

I am using the dime a dozen blue thermostat paper gasket with orange gasket sealer on both sides of it just enough to make it nice and tacky, not too much
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Old 01-07-2018, 02:12 PM   #13
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Re: 71 350 running hot coolant backup up problem

Just a few thoughts of some other causes of a hot temp if you sure about the radiator, t-stat, waterpump, ect... Maybe the engine is bored out .060? bad temp sender or gauge? is it an electric sender or mechanical? where is it it installed? You can drill a 1/16 hole in you T-stat. I took it a step farther and drilled two and my 400 never gets above 180* on a hot day.
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Old 01-07-2018, 02:19 PM   #14
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Re: 71 350 running hot coolant backup up problem

Quote:
Originally Posted by Gregski View Post
I disagree, I appreciate the feedback and please note I am not trying to argue. I have a 1974 GMC as well 350 as well it has a 180 thermostat with Vortec heads and even on 100 F days it purs at 185 degrees max unless something is wrong then it starts to climb

In the January 2014 Hot Rod magazine Hot Rod To The Rescue article called "Malcom Dunk's '65 Chevy El Camino Overheats the guy brings it in because the car cruises at 210-215 degrees... etc. etc.

Its a good read, they fix it with an aluminum radiator and an electric fan so it runs at 180, I think that's a bit drastic for my case, but I will find a happy medium somewhere in the neighborhood of 195.
OK just sayin cuz he said 215 after idling for several minutes
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Old 01-07-2018, 02:31 PM   #15
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Re: 71 350 running hot coolant backup up problem

all good thoughts, thank you

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Maybe the engine is bored out .060?
highly unlikely as it was bought as a crate replacement engine from Chevrolet, but you never know

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Originally Posted by bautino95 View Post
bad temp sender or gauge? is it an electric sender or mechanical? where is it it installed?
brand new digital Intellitronix gauges from Summit as well as brand new electronic temp sending unit, I know even new can be off, I will try to somehow test them or verify them with another gauge set, water temp sending unit installed in driver side cylinder head between the #1 and #3 cylinders

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You can drill a 1/16 hole in you T-stat. I took it a step farther and drilled two and my 400 never gets above 180* on a hot day.
I assume you are running a 180 thermostat, I am running a brand new 195 Duralast thermostat from AutoZone just for this reason it comes pre drilled with a hole in it and I really like it, works great in my other cars.
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Old 01-07-2018, 03:56 PM   #16
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Re: 71 350 running hot coolant backup up problem

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With your 195 degree thermostat you just added 15 degrees to the normal operating temp of your engine. 180 degree thermostat was original.
Where are you getting this information from? When I do a search for an OE replacement thermostat for a 1971 truck on AutoZone I get 195 degrees, I do the same for my 1974 and get the same result, I do it again for the latest 1986 square body and still get 195 as the direct replacement.
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Old 01-07-2018, 06:12 PM   #17
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Re: 71 350 running hot coolant backup up problem

I would double check those temps with an IR gun
They are pretty cheap and nice to have around.
Shoot right at the T stat housing.
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Old 01-07-2018, 06:51 PM   #18
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Re: 71 350 running hot coolant backup up problem

Did the engine sit around with the intake or thermostat housing off? I had a bunch of engines in a shed become apartment complexes for mice. Like to never get all the nests out of the water jackets. A nest sure would cause a flow problem. I would think you should have found some junk in the thermostat as many times as you have had it out, but just a thought.
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Old 01-07-2018, 07:33 PM   #19
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Re: 71 350 running hot coolant backup up problem

Fry a dif temp gauge ??
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Old 01-07-2018, 09:20 PM   #20
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Re: 71 350 running hot coolant backup up problem

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Where are you getting this information from? When I do a search for an OE replacement thermostat for a 1971 truck on AutoZone I get 195 degrees, I do the same for my 1974 and get the same result, I do it again for the latest 1986 square body and still get 195 as the direct replacement.
The 195 degree thermostats came about when ECM engines came out. The most common thermostat in V8 engines up till the early 80s was the 180 degree thermostat. I was also in auto repair starting in 1981 so I changed a lot of them. It's possible the 195 you have in there is not working properly. Get a quality 180 and install that. Check your engine timing - retarded can cause overheating to some degree. Move your hand around different areas of the radiator to check for cool spots - a partially clogged radiator core is possible. Feel the upper and lower hoses for temperature differences. If you grasp the upper hose after the thermostat has opened (use a rag or something to prevent burning) you can actually feel the water passing through as it scrubs against the inside of the hose. Check to see if the fan is pulling air - is it the correct rotation? Water pumps that are intended for serpentine systems (like my 1990) have pump impellers that are designed to work in a counter-clockwise rotation (and so is the radiator fan). Perhaps the wrong pump was installed.
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Old 01-08-2018, 10:18 AM   #21
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Re: 71 350 running hot coolant backup up problem

First, get a quality 180 degree thermostat. If it does not already have them, drill two 1/8" holes in the plate of the thermostat. Get your timing properly set and try it again. Switch to manifold vacuum for the vacuum advance, ported will retard the timing at idle which is not what you want for both mileage and cooling.

Is the temp sending unit in the head or in the intake manifold coolant passage? The ones in the head will always read hotter. Also, the gauge and temp sending unit must be calibrated to one another. Using a mismatched sending unit with a given gauge can provide inaccurate readings.

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Old 01-10-2018, 12:26 AM   #22
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Re: 71 350 running hot coolant backup up problem

Thank you for taking the time to make suggestions I value them.

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The first thing I would try would be to figure out the existing initial timing (after first verifying the TDC mark on the harmonic balancer is accurately reflecting true TDC)..It will still start and run good at idle even if at zero TDC.
Well after 5 hours spent trying to verify true TDC marks (see link for sister post below) I think I can tell you what my initial timing is/was.

Finding True Top Dead Center - TDC not so easy?

My initial timing was 19 degrees @ 650 RPM. That was not intentional when I set it, I was trying to get the truck running and move it from one location to my house.

I have since reset it to 12.5 degrees at 850 RPM with a manifold vacuum of 19 Hg. Not sure this is where I will keep it, but I will let the engine cool down then test it again.

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Also less important and I do understand why you are running just plain water while you sort this out but as you know plain water will boil at 212F - so maybe a little antifreeze just to raise the boiling point will be a little safer for the motor.
There was some anti freeze left over in the motor for sure, so it wasn't 100% water. Also the pressure in the cooling system adds 2 degrees per 1 PSI so at 15 PSI that's 30 additional degrees before we start to make bubbles and steam.

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I noticed you have well over 4,000 posts on this forum so I'll be surprised if I'm going to be of any help to you but at least hopefully this will encourage others to chime in.
Ha ha, I bet half of them are questions, lol.
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Old 01-10-2018, 01:05 AM   #23
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Re: 71 350 running hot coolant backup up problem

That truck came with a 180 degree T-stat from the factory. I don't care what Autozone's website says.

Try a different radiator. You can "flush" a radiator all you want at home. It can still be clogged. Either that one might need to be rodded out, or you need a new radiator.

You set initial timing with the vacuum advance disconnected and the port on the carb capped off.

That fan shroud is ill fitting too.

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Old 01-10-2018, 01:44 AM   #24
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Re: 71 350 running hot coolant backup up problem

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That truck came with a 180 degree T-stat from the factory. I don't care what Autozone's website says.
All parts houses list the 195 degree thermostat as a direct OE replacement, Rock Auto, NAPA, etc. I just through AutoZone out there cause their website is easy to use. Are they all wrong? lol

Plus then there's the manual, this is for my '74 but the engine is the same generation 350 SBC... on page 72 it reads:

"A 195* thermostat is standard equipment on all models except ..."
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Old 01-10-2018, 01:45 AM   #25
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Re: 71 350 running hot coolant backup up problem

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You set initial timing with the vacuum advance disconnected and the port on the carb capped off.
Correct that's what I did, the vacuum gauge was hooked up to manifold vacuum to aid in optimum initial timing setting, you hunt for the highest vacuum and find that sweet spot. Not the only way to do it, but it's one way to do it.

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That fan shroud is ill fitting too.
I agree, and am working on it
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