Hunting Thermostat

[KirkV]

I have a problem in my 302 that I cannot quite put my finger on.I had a leaking stat gasket on the goosneck so I pulled the stat and gasket and replaced both while I had it apart. My motor temperature is hunting from 180 to 210, and I thought I may have a bad stat. It would heat up slowly and then rapidly fall off as the stat opened up. You could actually feel the heat buildup and release when the stat opened. So I purchased a new stat and gasket and installed it correctly with the sensor toward the block and just idled the engine to warmup temps. PS it has no weep hole tp put in the 12:00 position.

It appeared to do the same thing, now I have not taken it out yet and driven it and also let me state that the fan is on all of the time on this engine and I have not installed a fan controller, but do not know if it would help or hurt.

Whew, any suggestions?
Thx Kirk

[jhv48]

If you drill a small hole (3/8") in the 12 o'clock position on the thermostat you will notice a much less pronounced spike and fall on your temp gauge.

Your fan should be temp controlled to come on automatically around 195 degrees.

[Rick Parker]

What temp rating is the thermostat? Sometimes the radiator actually can provide more cooling capacity than necessary for a 302 unless it is being RPM constantly on a track or "Worked" such as climbing in the mountains. In your case I would suggest installing a thermostatically controlled fan switch. It should not, nor is it necessary for it to run constantly.

[Gaz64]

Originally Posted by jhv48

If you drill a small hole (3/8") in the 12 o'clock position on the thermostat you will notice a much less pronounced spike and fall on your temp gauge.

Your fan should be temp controlled to come on automatically around 195 degrees.

+1.

I never fit any thermostat without a bleed hole or a jiggle pin.

[tcrist]

Originally Posted by jhv48

If you drill a small hole (3/8") in the 12 o'clock position on the thermostat you will notice a much less pronounced spike and fall on your temp gauge.

Your fan should be temp controlled to come on automatically around 195 degrees.

Joey,
I believe you ment three 1/8" holes?

[tcrist]

Originally Posted by Rick Parker

What temp rating is the thermostat? Sometimes the radiator actually can provide more cooling capacity than necessary for a 302 unless it is being RPM constantly on a track or "Worked" such as climbing in the mountains. In your case I would suggest installing a thermostatically controlled fan switch. It should not, nor is it necessary for it to run constantly.

Rick,

The way it is now would not be any different than a car with the fan attached directly to the water pump like the old days. Would it not?

But he is probably going to wear out the fan motor.

[Barnsnake]

You may have air in the system. It is even likelier since you don't have a "burp hole" in the thermostat. You might want to loosen the temp sender in the intake manifold with the front of the car as high as you can get it (any steep slopes around?) This will release the air pocket in the engine.

Are you running a coolant recovery tank?

Do you have a thermstat bypass hose? If not, you need one.

[KirkV]

Ok guy's,

So I too thought a 3/8" hole seemed a little large, what is the correct size hole, or holes that should be there? I do not have a thermostat bypass hose and need some help there.

I can get the engine up a slope and verify all of the air is out of the system though and will try to do it soon. I take it you think the pump may be cavitating?

The overflow sump is there and it has a 16 pound radiator cap I believe that would blow to it if it were to blow.

I will try to verify all the air is bled out before going too much further.

[DAVID GAGNARD]

Originally Posted by KirkV

Ok guy's,

So I too thought a 3/8" hole seemed a little large, what is the correct size hole, or holes that should be there? I do not have a thermostat bypass hose and need some help there.

I can get the engine up a slope and verify all of the air is out of the system though and will try to do it soon. I take it you think the pump may be cavitating?

The overflow sump is there and it has a 16 pound radiator cap I believe that would blow to it if it were to blow.

I will try to verify all the air is bled out before going too much further.

1/8th inch hole is the correct size and I like to drill 4 in the thermo flange, just make sure one of them is at 12:00 high when you re-install the thermostat, you'll need this more so since you do not have the bypass hose......I don't have the bypass hose on my race car (331 cu in) and have 4, 1/8th holes and have had no problems at all........
It's not unusual to have air pockets and I've had to run the engine up to temp and then let it cool down, doing this 3 or 4 times to get all the air out of the system....
Drill 4 holes in an X pattern,re-install the thermostat, fill radiator and crank it up and let it come up to temp with the radiator cap off and add water as needed to fill it as the thermostat opens and water is circulating,once full let it cool down, check your fluid level and put the cap on and take it for a drive, let it sit overnight and re-check the fluid level. by then the air should be purged and everything should be fine.......
If all is working as designed, your temp will come up and when your thermostat opens, you should see a drop in temp as the cooler water goes thru the engine and cools it down some, then after a few minutes of running, it should come up to operating temp and stay there, maybe you'll see a 5 to 10 degree swing bewteen highway driving and city driving, but that should be about the max..

David

[KirkV]

Thanks David

[Barnsnake]

You really do need a bypass hose if you have a thermostat.

Without the bypass the coolant in the engine does not move unless the thermostat is open. The coolant around the cylinder heads and the upper parts of the cylinders can reach or exceed the boiling point while the coolant exposed to the thermostat is still relatively cool. When the coolant at the thermostat finally heats by convection and the thermostat opens, a gusher of REALLY HOT coolant heads to the radiator (and past the gauge sender). That coolant is replaced with the relatively cold coolant that has just been sitting in the radiator and the thermostat closes. The cycle then repeats. This will result in your indicated temperatures going up and down like a yo-yo. It probably also puts some considerable thermal stresses on the hotter parts of the engine.

If you put enough holes in the thermostat to eliminate this phenomenon, it will probably be unable to control the engine temperature when the ambient air is cold.

The reason for the drilled hole in the thermostat is to allow air to escape from the engine when filling the cooling system. A single 1/8" hole will pass plenty of air and do the job nicely if it is at the top.

In short, if you have a functioning thermostat you need the bypass hose.

[Rick Parker]

In short, if you have a functioning thermostat you need the bypass hose

AMEN

Thankyou Barnsnake

[Snakebit]

Rick, I second that!