Temperature gauge questions

[Ol Blue 2018]

I have a stock 1987 V10 with a TBI 350. This motor was installed in 1999 by Chevrolet at a local dealership.

I was getting a reading from the temp gauge in the dash that seemed a little high, so she got a new 195 degree thermostat, 16 lb radiator cap, fan clutch and gauge sending unit and the engine coolant temp sensor next to the thermostat.

This did not change the temp value on the dash, so I installed an aftermarket electronic temp gauge and the coinciding sending unit. Now i have an accurate reading of the actual temp.

My goal: have the original, factory temp gauge work well enough so I can trust it is accurate within reason. I am thinking of running the new gauge for a couple weeks, then removing the needle from my stock gauge and placing it back on an matching it to what I know the temperature is. Sort of “recalibrate” it. And swapping back the correct sending unit.

My question 1: what should my temperature be ideally ?

The engine never steams or pushes coolant past the 16 pound cap into the resevoir.

My question 2: what temperature d o the lines on the original gauge represent?

i.e. is the mark between 210 and 260– is that 235?

Here are some pics that hopefully explain more.

I would love to hear thoughts. Thank you.

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[yevgenievich]

Appears to be running a bit hot. As for calibration, instead of removing the needle, verify voltage and ground on the cluster(12.5v-13v), verify the sender resistance correct with temperature change(there are charts online), verify the resistance of the wire between the cluster and the sender(1 ohm or less), and last check the resistor on the back of the gauge it self. The color of the resistor on the back of the gauge actually indicates what sender should be used as well. There are charts online for that as well.

Also can verify that thermostat does actually open at 195*. With cooling system operating in good order, 45mph cruise should not have temperatures any different than idle.

 

[Ol Blue 2018]

Oh and i clipped off 4” of bad wire at the business end of DK green

 

[ajd89]

My 87r10 tbi 350. Always ran a needle under the second line on the factory gauge... Well other then when it didn't have enough coolant in the water and it froze up and redlined the temp gauge lol

 

[Ol Blue 2018]

Appears to be running a bit hot. As for calibration, instead of removing the needle, verify voltage and ground on the cluster(12.5v-13v), verify the sender resistance correct with temperature change(there are charts online), verify the resistance of the wire between the cluster and the sender(1 ohm or less), and last check the resistor on the back of the gauge it self. The color of the resistor on the back of the gauge actually indicates what sender should be used as well. There are charts online for that as well.

Also can verify that thermostat does actually open at 195*. With cooling system operating in good order, 45mph cruise should not have temperatures any different than idle.

Thank you. Excellent info.

 

[yevgenievich]

but also, from what the two gauges reading I do not think much of a recalibration is really needed. The main point for the gauge is to indicate abnormal operation and it is on the safer side which would be fine.

 

[hatzie]

Those temps look right where you want them to be.
You want the engine to run above the boiling point of straight H2O to boil off any condensation and blowby water along with any light petroleum aromatics from blowby in the crankcase and blow it out through the PCV system.
The engine efficiency actually rises along with the engine temps up to just under the point where lubrication and metallurgy begins to fail. This is one of the reasons why the military funded a bunch of research into adiabatic diesel engines with little or no cooling using ceramics and exotic lubrication in the 70's and 80's.

50:50 coolant has around 220°F boiling point but that number rises 3°F for every PSI of cap pressure. This means that even 240°F isn't the end of the world as long as it isn't rising quickly.
50:50 coolant with a well sealed cooling system and 16lb cap shouldn't actually boil til it gets to around 265°F. Hoses and other parts will probably let go before you get there but 220°F isn't much to worry about with a properly sealed cooling system.

Modern vehicles and even some sports cars of the same vintage as our trucks actually run hotter than these antique trucks to take advantage of efficiency improvements at higher engine temps. They utilize an expansion tank on the pressurized end of the cooling system rather than an overflow bottle.

The best coolant mix for freezing resistance is actually 65% anti-freeze to 35% water but glycol isn't good for heat transfer efficiency. Most folks don't need more than the -40° C/F freezing temp of 50:50. The 50:50 heat transfer is better than 65:35 and the 265°F boiling temp that you get with 16PSI on 50:50 mix is just fine.

You do want to be sure that the temp sender matches the year of your gauge. There are at least four and probably many more senders used in the squares between 1973 & 1991. The earliest version called Nailheads are obsolete and not correctly reproduced by Delco any longer. The mass market aftermarket attempts at making a "similar" replacement that doesn't act like the original drives the Corvette restorers up the wall. There is at least one that is correct but you'll pay more than one from the Borg.

 

[Ol Blue 2018]

@hatzie
Thank you for your input. Im going to dig into the paperwork for the 1999 engine.


I wonder if the 87 sender is not approprite in this case.

Edit: either way, im in the ballpark and may be splitting hairs. Just want optimal conditions. Thanks again

 

[hatzie]

@hatzie
Thank you for your input. Im going to dig into the paperwork for the 1999 engine.


I wonder if the 87 sender is not approprite in this case.

Edit: either way, im in the ballpark and may be splitting hairs. Just want optimal conditions. Thanks again

The 87 sender makes the dash gauge match the aftermarket unit reasonably well. You don't have a 1979 & earlier truck so the Nailhead sender isn't an issue for you. I'd call that a win and not worry about it.