SBC tempature sporadic??? help!!

Hi, i have a 350 chevy in an old 1954 international pickup ,it has some performace parts, and the readings on my temp gauge are all over the place from 180 to 220 but usually around 190-200. the temp every now and then will spike. it changes very quickly, like in 10 seconds it will go from 190 to 220 and back. this even happens in 20 degree weather and driving at a constant speed at 40 mph.:banghead:

i have had two different radiators, two different mechanical temp gauges, two different thermostates, three different fans, and a new waterpump. my latest combination is an aluminum 2 1/2 inch thick aluminum radiator 19 in by 21 in and 180 thermostate, and flex fan.

could it have over heated? could there not be enough flow?
the oil didn't look murky and the antifreeze wasn't either

If you saw it go up to 220 and then down to 190, that was probably the point at which the thermostat opened.

yes but it will happen long after the engine is warmed up many times

I've got the 5.7 in my '95 PU, with a huge radiator (trailer pkg), 180° thermostat. While the actual reading on the IP gauge is typically dubious (who knows what the tick marks really represent!), once it is warmed up I will see it bottom out at 170-175°, then slowly rise to 185-190°, then drop fairly fast back down. Then it would repeat the cycle again.

I don't know why it does that, and I have not owned a truck like this when it was new to compare to, but my off-the-wall guess is that the radiator is probably more than enough for both our trucks, cooling it down so fast that it is causing cycling of the thermostat. Remember that a while back alot of radiators were still copper and brass thus didn't radiate heat as well as these new and more efficient aluminum ones.

I would however suggest checking your coolant temp sensor to make sure it is working and isn't fouled up. I had much greater temp swings similar to yours before I replaced the radiator, t-stat (original 195°), heater core, and the sensor.

Also check to make sure that any of the bypass hose fittings used are steel and not the cheap cast zinc kind. These can corrode away internally and pieces can break loose and cause blockage of the passageways. A magnet test will tell you if they're steel. If so, unscrew them from the water pump and the rear of the intake manifold and if you find them corroding from the inside out or the threads are almost gone, you should flush your system and pump to get the pieces out.

Since I did all this, the fluctuations are greatly reduced. Hope this helps!

well the motor only has 4000 miles on it and a brand new water pump so i don't think that it has corroded much. maybe there is some other crud floating around in there
i have had two different new mechanical gauges and they both did the same thing

but i just thought, i don't have a heater and i just blocked off those hose fittings. could this matter? i wouldnt think so because most race cars dont have heaters

I do not think that would matter.

Check to make sure your thermostat is not upside down. The spring faces the engine on the 'hot' side.

Also make sure there are no air pockets in water jacket at the top of the engine. Are you using an coolant overflow tank (the kind with coolant in it) or are you relying on an airspace in the rad for expansion?
If it's the latter, is your rad a 'top to bottom' flow?
If yes, do you have at least 2-3 inches of airspace at the top of the rad?

If these are out-of-whack, the engine may be retaining air pockets around the heads and intake manifold, which can cause such temp fluctuations.

Maybe post a pic of the engine bay, if you can :)

mmmm. that would make sense because i dont have a recovery tank and the temp attaches to the top of the intake manifold.
i have a cross flow radiator. are those supposed to have a over flow tank?
if i install an over flow and fill the radiator completely when cold, will the air pockets be filled?
it seems someone told me to keep adding coolant until the radiator starts to get warm in order to accomplish this
thanks for your help

mmmm. that would make sense because i dont have a recovery tank and the temp attaches to the top of the intake manifold.
i have a cross flow radiator. are those supposed to have a over flow tank?
if i install an over flow and fill the radiator completely when cold, will the air pockets be filled?
it seems someone told me to keep adding coolant until the radiator starts to get warm in order to accomplish this
thanks for your help

Cross flow rads can be used with an airspace at the top for expansion. (My '79 Mustang had such a system).

But the top or the radiator must be at least 3-4 inches higher than the top of the intake manifold. This will allow the engine to purge all it's air into the radiator, over time.

If the rad is a bit too low, you can make your cooling system a 'closed' type so long as you are using a modern radiator, and not the original 1954 rad style.

Just make sure the radiator's cap has both an inner and outer gasket and has a spring-loaded drop valve.
Then you run the overflow hose into a recovery tank. Keep the tank 1/3 full and make sure the hose is immersed in it. Over time, the system will purge itself of air and fill completely.

If the top of the radiator is lower than the top of the engine, make it a closed system and bleed the air from the engine as you fill the radiator completely. Imo, the best way to do this is to drill a tiny hole in the flange of the T-stat. This will not affect warm-up, but will allow air to escape as you fill the rad. Squeeze the upper rad hose as you slowly fill the rad to purge all the air.

Also, check that T-stat's orientation, just in case.