Coolant temp fluctuating up and down

[Courchained]

Measured HOW?

Infrared "temp guns" are notoriously inaccurate due to the emissivity different materials.


Nope. The concentration of antifreeze vs. water makes some difference, but not the brand.


Generally not a good idea. If the cooling system still has the OEM bypass, all that does is slow warm-up. If the OEM bypass has been blocked, you might need to drill the themostat--but why block the bypass?

Yes those measurements were with a temp gun.

 

[454cid]

I wonder if my bypass is clogged causing poor flow. Also Could my bypass even get clogged?
Side question. If my heater core lines were reversed Could that cause poor flow?

No, the heater core wouln't make a difference. Its just a loop. Also, I doubt you connected it backwards since one line is 3/4" and the other is 5/8".

Does the guage/temperature just cycle once, or does it keep on doing it? Do you lose heat from the vents? How long are you running the truck?

I'm wondering if you have a leak that you're not seeing. Sometimes they're pretty well hidden.

 

[El Tigre]

All thermostats except this last one pictured are WRONG....

 

[Courchained]

I just watched it today and from a cold start it warms up to about 198-200ish. Then drops instantly to 158-160ish. Then warms to about 185 then drops to 160ish. It continues to rise to 185ish then drop to 160ish. Up and down. Up and down. Its slow though its not a fast change.

 

[Courchained]

All thermostats except this last one pictured are WRONG....

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Is this the delco?

 

[Courchained]

I do not lose heat in the cab.

 

[Frank Enstein]

The Robertshaw style thermostat is pressure balanced to allow more steady coolant temperatures.
They are all but mandatory with a high flow water pump.

A high flow pump with a conventional thermostat can cause large swings in coolant temperature.
The higher flow holds the t-stat closed until a much higher coolant temp finally forces the t-stat open.

The coolant bypass is good during warm up to allow coolant circulation before the thermostat opens.

The problem with most bypasses is they remain open after the thermostat opens short-circuiting the cooling system.

If you do not have a heater control valve that shuts off the flow of water to the heater core the bypass isn't needed as the heater core effectively becomes the bypass.

For race applications or high output street applications I recommend this style of thermostat.

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They are just a Robertshaw thermostat with 3 3/16" (4mm) bypass holes. This allows enough flow to be an effective bypass but without the short circuiting of a traditional bypass.

My 94 Suburban diesel's GM thermostat is just a Robertshaw with a push on bypass. You may find that the bypass can be removed and moved over to the new thermostat so you can have your cake and eat it too!

Meziere a manufacturer of racing electric water pumps (In my opinion the best ones) recommends drilling 3 holes just like Stewart has in their t-stats.

A trick I learned a long time ago is to drill a 1/16" hole (A smaller hole will get clogged and a larger hole can slow warm up significantly) in the non-moving part of a thermostat to purge the air out of a cooling system very quickly. You don't even have to wait for the t-stat to open.

Too much coolant vs. water can cause overheating. A good rule of thumb for mixing coolant is 0 degrees F on a temp tester or 10 degrees lower than the vehicle will ever see. Even if you are someplace that NEVER freezes having some antifreeze will prevent corrosion and lubricate the water pump seals.

Bottom line as long as your coolant temp stays within 5 degrees + or - of the thermostat opening point you're just fine.

Close the hood and go get ice cream!

 

[El Tigre]

The '94 motor is totally different from '96-up Vortec which REQUIRES two stage stats. just like the LT1's of same era. SBC style thermostats will not work properly regardless who says they do.

 

[GoToGuy]

The small hole punched in flat plate of the thermostat was not placed for flow purpose. 50 years ago you would gave seen small blocks with Stant or Murry thermostats with an eighth inch hole and a dogbone looking piece installed. The small hole was to ensure no airpockets formed under the thermostats during flush, drain and fill. This small hole is to burp any air. Not any cooling or flow. Take your thermostat out, pan of water on stove, good liquid thermometer. Toss the thermostat in cold water, put on stove, crank up heat with liquid thermometer. Watch the pot, wait for magic at stamped temp on thermostat. Record open at temp to full open. AND record temps for closeing to fully closed. Pass or fail. Go or No Go. Note a failed open thermostat can cause water temps way too low constantly.

 

[Schurkey]

The small hole punched in flat plate of the thermostat was not placed for flow purpose. 50 years ago you would gave seen small blocks with Stant or Murry thermostats with an eighth inch hole and a dogbone looking piece installed. The small hole was to ensure no airpockets formed under the thermostats during flush, drain and fill. This small hole is to burp any air. Not any cooling or flow. Take your thermostat out, pan of water on stove, good liquid thermometer. Toss the thermostat in cold water, put on stove, crank up heat with liquid thermometer. Watch the pot, wait for magic at stamped temp on thermostat. Record open at temp to full open. AND record temps for closeing to fully closed. Pass or fail. Go or No Go. Note a failed open thermostat can cause water temps way too low constantly.

THANK YOU!

There's any number of thermostats that have "jiggle valves" in the rim. They are there exactly as you said--to make cooling system servicing easier. The ONLY reason was to bleed air after refilling the radiator. And--this is important--the jiggle-valve is intended to close and BLOCK THE HOLE as soon as the water pump develops pressure. Therefore, warm-up time isn't extended by having a hole in the thermostat.

Similar to the jiggle-valve, Robertshaw thermostats have a tiny vent stamped into the metal. Again, air passes freely so refilling the system is a snap--but very little coolant flows so warm-up time isn't affected much.

Compare the stamped, V-shaped Robertshaw vent to the drill size often used to pop holes in the thermostat flange:

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