Just got a 74 project for dirt cheap due to, among other things, it is overheating.

I started by pulling out the thermostat and flushing out the cooling system. I started the car running and kept flushing out the system. Knowing it would probably overheat I only had plain water in the system. The other problem I have is the temperature gage is not working. I plan to get a temporary one tomorrow but I couldn't resist starting the car today.

With the radiator cap off and thermostat still out, I let it idle in the driveway. I let it run that way for about 10 minutes. I stuck a hand held thermometer into the radiator where the cap was off. It was about 170 F. Again on pure water.

I could look into the radiator and see that most for the cross tubes were clear and flowing water.

As far as the fans go, this is not a stock car, all the emissions stuff has been removed. No A/C. The main fan is on a long shaft and there is no fan clutch. On the front of the radiator is an electric fan which appears not to function in any way.(see pic below)

After running awhile on the driveway, there appeared to be a few seconds of muddy looking water pass by. I first thought it was the chocolatey oil and water mix I have seen before on the oil side of a blown head gasket. I immediately assumed the worst, either a blown head gasket or a cracked block. I flushed that batch of water out with the hose and it never appeared again. There has never been any water in the oil.

More idleing in the driveway gave us confidence to take it on a test ride. We put on the radiator cap and drove to the gas station a couple of miles away and got gas. Everything was OK at that point. Drove around for about 10 minutes in the neighborhood, went home and checked everything. Again everything seemed OK. Drove around for another 10 minutes, driving somewhat harder, then came home

As we shut the car down, the hissing began. The radiator cap was spewing steam out the side. I hosed down the front of the radiator and it was quite hot. The whole engine seemed hot and some drops of water that hit the intake manifold seemed to boil.

Another weird thing was that the overflow tank didn't seem to be working. Water or steam didn't seem to be going there from the radiator. After everything cooled down I check the line and i seemed clear.

So this brings up a couple of questions.
Why is the engine doing this? Is it a dreaded problem with the engine? Head gasket? If it is the head gasket, is there a way to test that?

Or could it be something simpler, ie older radiator, bad radiator cap, nonfunctioning electric fan?

Also, what is the normal operating temp on a 74 350cid in around 90F air temp.

Any suggestions would be appreciated.




Glenn in San Diego

 

You might pull your intake and start there...

 

Glenn,
I also have a '74 but I have for decades been a big fan (no pun intended!!!) of electric fans and have fitted them to almost all of the cars I've owned, including the Vette.

For the Vette, I built a custom shroud that, apart from an area of about one square inch next to the hot water inlet, forces ALL of the air to go through the opening in the shroud and therefore the electric fan, a sixteen incher. The fan is controlled by a sender that fits between the fins in the radiator via that one inch square hole.

When I got the car, I drained all the water and also took out the two drain plugs on either side of the block just above the pan rail. I suggest you do that and you'll soon know how much gunk has accumulated in the bottom of the water jackets. You could then stick a hose in the holes and blast the gunk out, but there's not much room there. If there is a lot of gunk in there, that will affect your cooling.However, most of the heat is in the heads and the above won't help a lot.


One other area is the radiator cap. There are basically two types. one for cars WITHOUT an overflow tank and the other for cars WITH an overflow tank. The latter type usually has TWO rubber gaskets so it is possible that a previous owner has fitted the wrong cap to the car and while it might look good, it is not allowing the system to funtion as GM planned.

The only other area I can think of is your timing. If it is retarded, the car will run hotter, especially at idle when there is no mechanical or (ported) vacuum advance.

Keep plugging away, you will eventually find the gremlin in the system.

Regards from Down Under

aussiejohn

 

I went and looked at the radiator cap
It has only one rubber gasket, and is stamped 16 LBS



So is this the wrong cap for an overflow tank system?

Thanks
Glenn in San Diego

 

thats the right cap, but that doesn't mean that it works, replace it, they are cheap. Did you use some type of cooling system flush? You can try that, I would also get the rad tested, and when you refil it you have top make sure all the air gets out of the system. That aux fan should be conected to a sensor in one of the heads. There is a sensor in one head for the gauge and the other head for the fan, or should be anyway.
As far as testing for a head gasket, if you keep having air in the system it could be coming from a blown gasket. Rather than spend a week screwing around on it, I think I would just pull the heads, clean it up and put it back together.

 

Well today I went and got a new cap, slapped it on, and drove around for about 30 minutes. No overheating. At least no water boiling. But I need to let it cool down so I can look in the radiator to see how much water is left in there. Will be taking it in to my vette mechanic for a more detailed evaluation of the car on Thursday.


Glenn in San Diego