Technical SBC Overheat until thermostat opens. Help!

Hello. I have been looking at the message boards for about a year now, but this is my first post. My name is Bryan, and I have been building my first rat\hot rod for the past year. It is a 31 Model A sedan, with a sbc 350. I have everything working, and have been driving it, but have an odd issue that I cant figure out. When I start it cold the temp gauge doesn't really move and then all of a sudden it shoots way up. Sometimes it makes the gauge go almost all the way around, and then it comes right back down to 160. For the rest of the drive it is fine and never seems to go above 180. The engine is a fresh build with new radiator, pump, and thermostat. I had a bit of a oil leak on the manifold valley, so while fixing that, I put in another brand new 180 thermostat thinking I just got a bad one. Took it for a ride, and the same thing happens. It acts like the thermostat is sticking, or there is an air pocket. I cant believe that 2 thermostats would be bad and do the same thing. The fluid level is always good, and I never seem to get anything in the catch tank. What else could cause this? It has aluminum heads so I am worried that it is getting too hot initially and will cause issues. Any help would be great. Thanks.

 

Where is the temperature gauge sender?

The usual cure is to drill a small hole in the thermostat, about 3/32" diameter, to prevent an air pocket.

 

The sender is in the manifold right by the thermostat housing. It is a mechanical gauge...

 

Get a high volume thermostat or drill a hole like squirrel said. The new thermostats are made for fuel injection engines that have to warm up quickly. I fought the same deal until a SW rep told me what the problem was.

 

Words of advise above^^^^^^ but DONT use the rodent word here!

 

HA. Ok got it. Thanks for the input. I will give that a try!

 

He's got a hot rat.

But seriously. Is there anything on or in the opening header of the HAMB, that indicates rat rods are discussed or fixed here?

 

Drill a hole in stat so air can get out. Another thought, is the stat in upside down?

 

Just one 3/32 hole right in the valve plate? I don't think it is upside down. Spring side is down in the manifold.

 

I would check to see what temp the car is actually running. It sounds like a gauge issue to me. You can get a infrared thermometer at most tool or home improvement stores. I would get one and check to see what temp the engine actually is.

 

I will check it with the infared but in the garage when running the top hose stays cool until it opens and does the same thing with getting real hot first.

 

do you have the heater core port blocked off at the pump and manifold ?? if so you should run a loop from the manifold back down to the pump so the water will circulate instead of stagnate against the thermostat ( the thermostat will open quicker as it will see warmer water quicker ) and it will allow the thermostat to open easier as it will have less head pressure against it also

 

Yes both are blocked off. You just run heater hose from the manifold to the pump? I don't think I have seen that before. Is that common?

 

right now the water is not moving till the thermostat opens , this is what the heater core ( bypass hose ) does is allows the water to move thru the water jackets so it heats evenly and not spike then open plus the pump will not cavitate

 

I had a problem one time witch a Chevy motor In my A, I drilled a hole in The thermostat,
Still would over heat , after dealing with it Sevreal times , I took the thermostat
out filled the motor threw the intake , let it set for a few Hrs, came back, No water in the intake , filled again let set , topped off ,installed thermostat, reinstalled Hose ,
Filled Radator, Fixed ,,,
There was a air pocket that just would not burp out , until I did the way I describes ,
I even tried to Vac fill , That was a No Go,

 

normally it goes thru the heater core then back to the pump the heater core sees water movement all the time and allows the bypassing . the drilling of the hole does allow the air to purge and some water movement ( abeit a little )

 

I had an OT 70's Buick that did the same thing. It had an idiot light, so I added a mechanical gauge in addition to the light. Just for good measure. The temp would spike to about 260 when the thermostat would open. At that time the heater core would get bubbles in it. I just
Learned to live with it. Put 110k on it like that. Also had a TBI v6 blazer that did the exact same thing. Never bothered to drill the hole in the thermostat.

 

Before you do anything check your actual heat there's no ducking way that your car goes from ice cold to boiling hot and then operates totally normal.

Your gauge is bad or has some weird quirk that's making it go all nutso in the first few minutes.

Runs and drives just fine and stays around 180, no over flow from the rad and isn't showing any other signs of actually being hot.

I'd bet you a cheese burger your gauge is wrong. If it didn't act weird would you even think it was over heating?

Does it puke? Ping? Lose power? Smell like a scalding hot cast iron skillet? If your answering no you car isn't over heating.

 

It seems to get real hot to me. When it does it in the garage you can hear it when it finally goes into the radiator and sounds boiling hot. Driving you obviously cant hear it but do get a smell of coolant when it spikes like that. It is just like the thermostat sticks.

 

Ok. I guess an easy test would be to take them out and drive it and see if it acts differently.

Seems like such a weird thing for it to do.

You can also test your thermostats in the kitchen to see if they are working. Throw them in a pot of water and boil them, put a thermometer in the pot and see what temp it is when they pop open.

Figure that at least lets you know for sure they are working and at what temp

 

Well if you're gonna pull the stat to drill the hole in it. I would do what 19eddie30 said while you were in there.

 

Try a robertshaw thermostat. My experience with this issue has been a bad headgasket once, and bad parts store thermostats multiple times.

 

It does sound like an air pocket. On my first fire up of a new engine, I take a heater hose off at the firewall and fill the engine with coolant that way.

 

There is a air pocket under the thermostat. Try the small hole ,It will fix your problem .

 

Whatever it is please post the fix! It's a bummer to be trouble shooting and find a thread that discribes exactly what your facing but it never says what happened in the end.

If you think it's the air bubble but don't have a heater hose that's up high I've parked my car on ramps and pulled the rad cap and let it run for a little while, the air bubble should run to the high spot.

 

In my current O/T build the radiator fill neck is lower than the thermostat housing so I used two short upper hoses with a spliced in filler neck between them that allows it to loop up higher than the engine. The difference in filling the cooling system is almost another gallon of coolant. I always drill a small bleeder hole in new thermostats before installing just in case.

 

Yeah, every engine running a thermostat needs a heater bypass in some form or another. There are several methods.
Some heater control valves bypass when closed, so the heater hoses are always as hot as the engine coolant. That scheme would require the heater hoses to be hooked up.
Some heater control valves shut off completely, so a bypass must be provided in/on the engine.

According to the 1955 SAE paper the original 265 engine had an internal bypass. That does not mean later engine did not do it differently

 

I agree with Stimpy. My simple evaluation is your temp sender is not in circulating water when the thermostat is closed. When the thermostat opens it gets hit with steam giving the spike, then gets hit with normal circulating water.

Ensure flow when thermostat is closed, heater block off and thermostat drilling, consider moving your temp sensor to get full flow at all times for more accurate reading.

 

I see... I have 2 senders on my Ford. The mechanical one is in the intake, towards the front. The factory temp gauge is in the cylinder head. Most factory small blocks had them in the head.

 

O/T BMWs have a bleeder screw in the base of the thermostat housing, which is opened whilst filling the radiator (cold)
It accomplishes the same thing as 19Eddie30 does removing the thermostat. (displaces the trapped air with water)
Typically logical for the 'Ultimate Service Machine'...