Coolant temperature flucuates

Just an update, I replaced the thermostat three times and burped the air out each time. It stil kept fluctuating. I put a vacuum tester on the cooling system system and it would not hold a vacuum. I finally found it was dripping water out the lower weep hole on the water pump. After replacing the water pump it would hold the vacuum properly. I guess it was getting air in the system thru the weep hole maybe, anyway after replacing the water pump the temp is rock solid even though a bit lower than it should be with the 180 thermostat. Maybe because I drilled one hole in it.

Thanks for updating the thread with the solution! That always helps the google searchers when they have a similar problem!

Failed water pump, heat and pressure leaks water. As coolant heats makes pressure( like a pressure cooker) at same time loses same pressure out leak.
Drilling another hole in the thermostat is unnecessary. The first time after you drive more than 15 minutes the system will open and fill any voids. All you have do is after shutdown and cool down is check coolant and add if needed.

GoToGuy said:

Failed water pump, heat and pressure leaks water. As coolant heats makes pressure( like a pressure cooker) at same time loses same pressure out leak.
Drilling another hole in the thermostat is unnecessary. The first time after you drive more than 15 minutes the system will open and fill any voids. All you have do is after shutdown and cool down is check coolant and add if needed.

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The hole in the thermostat is NOT un-needed. It helps balance the thermostats operation by preventing the coolant from dead heading in the system. It really helps control temperature fluctuations in the engine by always allowing a small amount of coolant to flow to the radiator, preventing localized hot spots and localized overheating, thus steam pockets. With a hole the engine will run at a much more consistent temperature. The thermostat will open and even close as needed in response to engine temperature the whole time you are driving.

This is one area a LS and just about every other car manufacturer has changed for the better with a coolant inlet controlled thernostat and massively sized bypass ports that continualy flow a high volume of coolant through the engine all the time. My 8.1L for example has a 1" bypass hose and circulates coolant all the time, regardless of thermostat position gallons of coolant are circulating every second.

If not a dedicated bypass it's usually the heater circuit. Thermostat closed is not a dead head situation on our engines. Big and small blocks have bypasses.

454cid said:

If not a dedicated bypass it's usually the heater circuit. Thermostat closed is not a dead head situation on our engines. Big and small blocks have bypasses.

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Never said they were dead headed, but rather dead headed to the radiator. Without a small amount of coolant circulating through the radiator itself the temperature fluctuates a good bit.

L31MaxExpress said:

The hole in the thermostat is NOT un-needed. It helps balance the thermostats operation by preventing the coolant from dead heading in the system. It really helps control temperature fluctuations in the engine by always allowing a small amount of coolant to flow to the radiator, preventing localized hot spots and localized overheating, thus steam pockets. With a hole the engine will run at a much more consistent temperature.

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Yep, there's been discussions about this around this forum, especially when you mix TBI stuff with Vortec. In my case, I put TBI heads and water pump on a Vortec style 377 cu in block with no bypass hose. The 1st 3 times I started it up, the gauge went up to ~230° while the ECM was seeing ~170° before the T-Stat opened. I even had an 1/8" hole drilled in it. Then we pulled the T-Stat and I drilled 2 more 5/32" holes in it. Now the gauge reads ~10° higher than the ECM sees while warming up and when at full operating temperature and the 180° T-Stat open.

If that works for you, great. You overlook one fact, hundreds of thousands small block Chevys in-service. If this was a critical problem, GM would have addressed the perceived problem.
You make a case for a cooling design flaw. Maybe you have these problems. You also operate highly modified engine setups.
Coolant Temperature readings vary widely from whatever position the sender is located.

GoToGuy said:

If that works for you, great. You overlook one fact, hundreds of thousands small block Chevys in-service. If this was a critical problem, GM would have addressed the perceived problem.

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The OP replaced a TBI engine with a Vortec engine. The bypasses aren't the same and several of us have had the problems he's having. Drilling a hole in the thermostat is a legitimate way of fixing it.

Whatever works for you is the best choice.