Hi All,
I'm having trouble tracking down the source of a erratic coolant temperature gauge issue.

Background information:

1972 Corvette, base model with AC
1969 302 SBC
Holley Terminator EFI

New Dewitts aluminum radiator
Water pump bypass passage plugged in the block.
Reverse rotation water pump (running serpentine belt drive).
NOS heater core and rebuilt box

180 degree Stewart thermostat w/holes, then switched to stock 180 degree thermostat with one 1/8" drilled hole.

Holley Terminator EFI Temp Sensor in drivers side head, operating ECU, electric radiator fans and EFI display. Stock Temp Sender in passenger side head, operating stock temp gauge (changed twice to verify operation).

Problem:

Running the above combination with the Stewart thermostat resulted in the temp maintaining steady (and otherwise operating normally) during warm months, but wouldn't allow temps above 150 degrees on the highway during cooler weather. The Holley EFI will not go into learning mode until 165 degrees, so this combination doesn't work as an all- around solution.

I changed the thermostat to a "stock" type 180 degree, but drilled one hole (since the water pump bypass is blocked). After this, the temperature bounces between 180 degrees and fully pegged out on the dashboard gauge when engine rpm is above idle, but holds steady between 185-195 degrees at idle.

The Holley Temp Sensor reads normally, the fan comes on properly at 195 degrees and the heater works fine).

The sending units are on opposite sides of the engine, and this problem didn't begin until the thermostat was exchanged.

I'm wondering if there is a cavitation or trapped air problem because of the blocked bypass and basically "stock" thermostat. I'm willing to remove the plug from the bypass passage if so. The coolant level remains full (constant) and doesn't burp.

Has anyone experienced this before?

Thanks,
John

 

I had that problem,so I changed the pressure cap.That fixed it for me.

 

How quickly does the dash gauge change from 180 to pegged?

 

I would start with making sure the T-stat is installed correctly and removing the bypass plug. Pump cavitation will cause this type issues.

Is the pump a high flow pump?

Neal

 

I would check the temp with a inferred temperature gauge.

Is the bottom hose on the radiator collapsing ?

Almost all the stats are junk. I had 4 MR G. fail within 2 years on different vehicles.

Open the cap and drain the radiator to a few inches below full. Start it and let it warm up. Check for proper flow and direction.

If you used "tap" on the thermocouples / senders there may be a ground issue.

A 180 stat starts to open at 165, and is fully open at 180. Due to the junk nature of the stats, I would put the stat into a pot of water, heat it and use a kitchen thermometer to see where it is actually opening. Keep the stat off the bottom of the pot.

I use a 1/16 hole and the 72 and 71 run cold at highway speeds during the winter. Both have all the foam on the radiator in place.

 

It changes in a matter of seconds, holds at the pegged position for varying times and suddenly drops to 180 again.

 

Thanks, I will check the cap on a pressure tester, but it is a NOS one.

 

The water pump is OEM from a 1988 TBI Camaro.

 

I haven't noticed the lower radiator hose collapsing, but I will check that out soon. My current thermostat is a parts store replacement that I modified with a hole. I cleaned the sending unit mounting hole threads with a wire brush, and did not use any type of sealer on the threads (brass on cast iron). I noticed last night that the problem doesn't occur when the engine is not running, gauge reads normally.

 

Correct me it I'm wrong, but I believe that a stock thermostat is not going to regulate your temperature correctly with a reverse flow system.

Normally the thermostat will open from the heat of the block with a normal flow system. And then the heated water from the block is allowed to exit, but it's already heated.

If you reverse that flow you may now be exposing the thermostat to really cool water from the radiator directly after it opens and so it closes in response. Then the cycle repeats over and over.

I would go back to the stewart thermostat and run a hotter one and/or use cardbord in front of the radiator to get it to stabilize at a higher temp in cool weather with the stewart thermostat.

To test the theory you could just remove the thermostat and observe the temp readings. They should stabilize if this is the case.

 

Reverse flow pump, reverse the t-stat. For the gauge, look for an intermittent wire, either at the temp sensor, of somewhere in the wire itself. Temps wont go from 180 to pegged in just a few minutes, unless you have a serious engine problem, and it doesn't appear that you do.


Bad grounds will run you crazy....

 

wouldn't it also need to be located on the other end of the system, near the water pump so that the water the thermostat see's first is heated? IE the bottom hose instead of the top hose.

Is one of these adapters being used with hoses from the manifold?





I'm not sure how guys adapt the engine to reverse flow, or if most just reverse the flow with a restrictor or some such device.

 

It's not a reverse flow system. It works normally EXACTLY like any earlier small-block Chevy except the pump rotates the opposite direction.

The temperature of the coolant around the sender can't change in seconds, so unless a big pocket of steam is hitting the sensor it's not the coolant temperature actually changing causing the gauge to swing.

Is the heater core blocked with a valve when not in use? The engine needs either the bypass port or the heater core to transfer coolant from the thermostat crossover (just before the thermostat) back to the inlet of the pump when the thermostat is closed. The fact that you changed from a thermostat with holes to a thermostat with one hole and saw a change points to there not being enough bypass available.

 

It's not reverse flow, only reverse rotation of the pump, the impeller is built to rotat the opposite direction and give the normal flow from bottom of rad through water pump, through block, into heads and then to the thermostat in the intake manifold and back to the radiator.
I was wondering if there is an electrical issue with the gauge.

 

If it's a reverse-flow pump, the T-stat has to be flipped the other way. You don't want coolant flow trying to force that T-stat flap closed. That is likely the cause of your temp fluctuation problem.

It doesn't make that much difference where it is located in the system; where it is now is fine.

P.S. If the pump is turning the opposite direction from stock C3 pump, the direction of coolant flow IS reversed. If you have a stock C3 pump being turned the opposite direction, that is another problem; the efficiency of that pump will be GREATLY reduced....and flow in revesed direction.

 

Ok I somehow read that as reverse flow system. Disregard, except taking the thermostat out to test if it's the gauge or the thermostat. That test should still work.

 

I am not trying to hijack this thread, this is a related question, but where would be the best spot to measure coolant temperature with an infared gun. And when I say best spot, I mean the place it matters most. I measured mine on top of thermostat housing??

 

NO, IT IS NOT!

The impeller and housing are just build differently so the pump still pumps coolant the same way as a normal rotation pump even though it's turning the opposite direction. The inlet of the pump is still the lower rad hose and the outlet of the pump is still into the engine block.

 

I was actually making a joke when I wrote reverse the t-stat. The water pump, on a reverse direction just has an impeller that pumps the water the same direction, from the reverse rotation. They all pump the water in the same flow. Otherwise, the entire system would have to be reengineered.


Keep the t-stat in the original orientation, but check it, like was said in water on the stove, in a pot to see if it works smoothly, and at the correct temperature.


As for the temp gun, I'd shoot the heads, the intake where water goes thru it, I'd hit each exhaust tube, the t-stat housing, each radiator hose, and anywhere you can access to see what the engine is doing. And, yes, there will be a difference from the exhaust to the t-stat housing. But each exhaust "tube" should be close to each other, the t-stat should be close to each radiator hose, etc. etc.

 

Late model Corvette water pumps are reverse flow. Better to get the cool water to the heads FIRST. I was unaware (obviously! ) of reverse-rotation pumps for C3's [apparently for serpentine belt systems]. DOOOOHHH!!