[35benzman]

I have a custom car with a 1976 Camaro 350 engine. I've had continuing overheating problems. I first had the radiator replaced. Then I had white smoke coming out the tail pipes and replaced both heads (which were cracked). After that, the engine still seemed to be running a little hot so I replaced the water pump with a high efficiency one. Also replaced thermostat and radiator cap and the hoses between the engine and radiator and from the radiator to the overflow tank.

The current problem is when the engine heats up, coolant flows into the overflow tank but never goes back into the radiator. Also the temp gauges read 220 or more on a typical drive at 70 outdoor temp (the thermostat is 180 degrees).

I took the car to a radiator shop and they said:
- CO test with gas analyzer and also with liquid shows no CO
- water pump checked for correct flow direction
- fan correct
- air flow good through radiator
- timing ok (at 8 btdc)
- surface test of radiator ok
- pressure test on radiator from cold to thermostat opening. Blew the seal of the tester at 20 lb.

The conclusion of the radiator shop is:
- there is an internal leak in the block or heads
- recommend that the engine be replaced

My questions:
I do not understand how the block or heads can have a leak and yet the two CO tests pass??
I do not understand how it is that the coolant pumps out to the overflow tank but does not return when the engine cools down??
The engine backfires some if I punch it before it warms up - after it is warmed up it is ok. Not sure what this is or if it might relate to the cooling problem?
How do I tell if all cylinders are firing ok? It would seem like a dead cylinder could contribute to the overheating - but shouldn't cause the coolant flow back problem??
Is it time to bite the bullet and buy a new engine? If so, is a crate engine the way to go?


Any thoughts will be appreciated.

[Joe G]

A combustion leak detector, like this one from Lisle, will help you determine whether or not combustion gasses are finding their way into the cooling system.


http://www.amazon.com/LISLE-75500-COMBUSTION-DETECTOR-BLOCKS-HEADS-GASKET/dp/B0002KPZ2Q

Also, a cooling system pressure tester is not meant to be used in the way you described. They're lucky the tester seal blew before the radiator let go.

Maybe there are other schools of thought out there, but the way I was taught to use a cooling system pressure tester is to install it in place of the rad cap with the engine cold and NOT RUNNING. Then, using the hand pump, you pump up the pressure to whatever the pressure rating is for the correct rad cap. It'll be something between 7 and 16 PSI depending on what you have for a radiator. If the pressure holds steady, leave it for 10-15 minutes and see if it continues to hold pressure. A pressure drop of 1-2 psi is normal due to the expansion of rubber hoses, but anything more than that would indicate a cooling system leak.

You can also use a cooling system pressure tester to confirm that the radiator cap is functioning properly.

I do not understand how it is that the coolant pumps out to the overflow tank but does not return when the engine cools down??

Make sure the coolant recovery tank hose is drawing from the bottom of the recovery tank. Also inspect your radiator cap to make sure the center part is not stuck.


Other thoughts:
Does it have a proper fan shroud?
Does it have a fan clutch? And if so, is it working properly?

Hopefully other members will have more to add.

Hope this helps.....

[boothboy]

Something else to think about. Is your catalytic converter plugged? A bad converter sure can heat up a engine. You might want to get it tested. Also have you checked the timing?

[35benzman]

Thanks for the insights. I learned something and have a few things to look into, based on the insights. I read on a similar problem that locating the temp sensor on the intake manifold (as I have it) gives better accuracy than mounted on the head (as GM originally placed it). I also read that fans without clutches (as mine is) are good.

I do have a good fan shroud and I have tested the radiator cap return valve.

I remain puzzled by the non return of coolant coupled with the lack of CO finding and the positive result of the pressure test (although I'm unsure that the test was performed correctly. It's a radiator shop that did it - they should know what they are doing - geese!

I may try the pressure test myself and the CO test or have these done by another shop.

[Jumpin Jim]

Hi,
I had all of those symptons on my 350 and it turned out the block had a very poor repair done to it.
The engine was built for me and ran fine for a while, used a bit of coolant and did puff out white stuff from exhaust but I ignored it.
It got steadily worse, began running really hot and finally hydro locked.
I did go through all of the things you have and played with the cooling system, upgraded with large radiator and overflow tank to no avail.
I don't suppose a CO2 check would work as coolant was escaping into cylinders and not the other way around.

It looks like a chunk of the block between the centre cylinders broke and a braze repair was done, which slowly broke down.
This is plain to see, but maybe you have just a hairline crack that opens when block is hot





The only good news is the mill is now being used by a drag racer

cheers
Jim

my first post in tech - shame it's bad news!
the engine guy has since immigrated so no comeback

[35benzman]

Thanks, Jim. I think I'm loosing a little coolant - although not a lot. Maybe half a cup after driving 50 - 60 miles. But then my toyota pickup hasn't lost a drop in 5 years!

Did you replace the engine once you identified the problem?

[T-bucket23]

Originally Posted by 35benzman

Thanks, Jim. I think I'm loosing a little coolant - although not a lot. Maybe half a cup after driving 50 - 60 miles. But then my toyota pickup hasn't lost a drop in 5 years!

Did you replace the engine once you identified the problem?

8* before is not enough initial timing and can lead to overheating. I would try at least 12*. Does it do it standing still as well as going down the road.
Do you an an A/F fuel gauge available. Running lean can also push the temperature up.

[Jumpin Jim]

Originally Posted by 35benzman

Thanks, Jim. I think I'm loosing a little coolant - although not a lot. Maybe half a cup after driving 50 - 60 miles. But then my toyota pickup hasn't lost a drop in 5 years!

Did you replace the engine once you identified the problem?

Hi, yeah I really went for it and got an all forged 383 short block and swapped all the parts over myself.
First time I'd been into a V8 so was an easy, albeit expensive way to learn.
I've just replaced the twin point no vac distributor for an MSD magnetic pickup distributor and it has transformed it.

Runs fine with no heat issues, just needs a dyno tune

[DoubleVision]

I agree with T bucket. Also check that the mechanical advance is working. I would also connect the vacuum advance to a manifold full time vacuum source, one that pulls vacuum at idle.
With vac advance plugged, and you've confirmed the mechanical advance is functional, bump the timing up to 12 degrees before top dead center. Lock distributor. Connect vac advance to a manifold source. Right away it'll idle up. If it does not the vac advance is ruptured. If it idles up, move the idle back down to normal speeds. Recheck timing, it should be 18 to 24 degrees before top dead center at idle. This is how I've did all my engines, and as a result not only does low end throttle response and power pick way up, it also runs considerably cooler. Idle and off idle mixtures are lean, the leaner the mixture the slower the burn, this is why it needs more spark advance.

[ssmonty]

FWIW,
I had an intake gasket on my Chevy truck that leaked coolant. It would'nt return coolant from the recovery tank because the system lost pressure after it heated up.
If your coolant level keeps dropping you have a leak somewhere. May be external and it only does it when its hot, and vaporizes before you can see it?
Doubt it. I'd get someone else to do the pressure test while its hot, and see if it holds for awhile after engine is shut off. You might see a difference in the spark plugs appearance on affected cylinders to get you in the neighborhood of the leak.
Is there any moisture near the heater core? Under the floor mats? Its got to be going somewhere. Perhaps the equipment they used for checking CO2 needs calibrated? If both heads were cracked perhaps the block is too?

[35benzman]

As I mentioned at the top, I've had an on going problem with the 1975 Chevy 350 engine in my custom auto; in summary:
Engine overheated
- replaced radiator core
White smoke coming out exhaust
- replaced both heads that were found to be cracked
Still running hot
- replaced water pump with high performance water pump
- replaced all hoses
- also put in water wetter
Failed pressure test
- had radiator repaired
Coolant flowed to reservoir but not back to radiator
- radiator fill neck had been squished down to give hood clearance;
in doing this, the seal was damaged
- replaced radiator fill neck
What else?
- new 16 lb radiator cap
- new 180 thermostat
Currently problems:
- when the engine warms up, about a quart of coolant gushes out to the
coolant bottle; then, after awhile some goes back to the rad and so on.
Because it is so much, I can only fill the coolant bottle to about an inch
above the add line (near bottom of bottle) - or else the bottle will overflow.
- I did a 35 mile cruise, chugging up the freeway, a lot of it on a gradual
incline. The next morning the coolant level was down about 3/4 cup.
These are the two problems I am currently dealing with. What have I done
so far?
- pressure test ok
- HC test of coolant ok
The car runs pretty good, no back firing, no run on. The temp gauge reads about 210 - 220 on the freeway or chugging through traffic. But when I check with the temp gun and it appears that the temp gauge is reading 10-15 degrees high. The temp gauge sending unit is mounted on the intake manifold near the rad hose connection (originally it was mounted in the drivers side head).

What could it be?