Ron Miller

My '66 has a nondescript 350 ci replacement block which I've run for several years now without a problem. However, when the engine is started cold and starts to warm up, the temp gage has always moved past the mid-scale mark to very near the hot mark at the right before the thermostat opens. Then, it'll swing back well past mid-scale to the cold side before again moving past mid-scale to the hot side.

The oscillations dampen out, finally settling in and stabilizing as it should at near the middle of the gage. I've checked the gage against temperature readings at both the block and the thermostat housing with an IR gun, and the gage is about as accurate as you could wish.

I know this isn't hurting a thing, all is well, but I recently thought I'd correct the matter and I replaced the old thermostat with a new GM AC Delco thermostat. It didn't cure a thing, and if there was any change at all it would be that the new thermostat may magnify the issue slightly. My local mechanic (who's pretty good, by the way) doesn't know what causes it either, although he says he has two Chevy trucks that will do the same thing.

If I didn't know better, I'd almost think the thermostat is installed backwards with the pill away from the block, rather than facing the block. But, that's not the case.

Any ideas??

Mike Ward

I think you'll find that they all do that to a certain extent. Mine does. I cured it by not staring at the gauge during warm up.

Ron Miller

For sure a lot do, but not all. My '65 with the original block, 327/300 hp temp hand moves smoothly up to its final position without going past more than the width of the temp hand.

Go figure.

Powershift

Ron:

Try drilling a 1/8 inch vent hole in the thermostat body. This should reduce/eliminate any air pockets under the sensing pellet.

Larry

landshark 454

Is your heater core hooked up, or is there a hose to and from the inlet and outlet on the engine for the heater core? Is there a bypass hose running from the intake to the water pump? Not having at least one or the other of these will cause this problem. The high performance small blocks, and all big blocks have a bypass, which prevents your symptom.

larrywalk

Landshark 454 is right on!

Mike Ward

The OP said he has a replacement 350 ci engine. These usually have a water pump with an internal bypass. Irrespective of whether coolant is allowed to flow through the heater or not, there is typically a slight overshoot on the temp gauge as the thermostat opens.

MikeM

The only time I've ever seen the temp gauge overshoot the normal mark to any substantial degree on warm up is when the cooling system is low on coolant or there was an air pocket in the cooling system.

WAG, next time you do a cold start, loosen the temperature sending unit to bleed off any air or ??? then tighten. Monitor results during the warm up.

The only other thing I can think of is your heater hoses are disconnected and your internal water pump bypass is plugged. The latter is not likely.

Mike Ward

Mike- you would know this. In the case of a simple air pocket, does the eventual coolant flow push the air to the highest spot in the system, or does it remain trapped in situ.

Maybe I've answered my own question- is the highest spot on a C2 the thermostat housing?

32361ARTHUR

My 383 does the same thing and I replaced the thermostat and sending unit. After it stabilizes it is fine. I have a heater but it is turned off. I never tried having the heater turned on during start up but I can't see where that would make a difference. Just for information my 383 engine has a later model 350 roller block and it has the short water pump. I notice that if I let it warm up for about ten minutes at an idle it minimizes the fluctuation compared to if I drive it away with a shorter warm up. If it is an air pocket causing this what would be the easiest way to get rid of it?

Ron Miller

I've been out for a while and just now able to review all the comments. Heater hoses are hooked up, the heater works in the winter time.

I appreciate all the comments, I think the thermostat already had a small air bypass hole in it, but I'll recheck and try all suggestions once I'm able to get to it (considering all my other job priorities).

Thanks much!

Ron Miller

Well, I've had a little time to dig into this matter, and wanted to update you guys on what I've found. Thanks to all who offered suggestions I've been able to eliminate most of the problem.

Turns out there was some blockage in one of the heater hoses which wasn't allowing the coolant to circulate as designed. In turn, the thermo would overheat, open, shut down, etc., etc.

All is well now, still goes slightly above the 210 mark where it settles peacefully, but nothing like what was happening before! Thanks again, all the help and comments always help in thinking out these types of problems. (This is a great site!!)