Author Topic: Temp gauge not working  (Read 2245 times)

Offline Danny C

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Temp gauge not working
« on: January 15, 2020, 08:00:15 PM »
Hello new to the forum.  Just posted in the welcome section with what I have done so far to my 1980 C20 2wd pickup.

My current issue is my temp gauge was not reading (stays pegged at cold).  I removed the terminal at the sender and grounded it out and it pegged to hot so I assumed the sender was bad.  I bought a new sender and replaced the old nail head style sender with a tang style.  It was the one the parts counter guy recommended so I am assuming the ohm range is appropriate.  I reused the same electrical connection as it seemed to fit fine.  Just replaced the sender and let it idle for 15 min.  Let me interject here that my radiator is cracked (that will be my next job this weekend hopefully) and I lost a good amount of coolant during the sender swap.  I would say my radiator was half to 3/4 full of coolant.  Because of the leaking radiator I am assuming I am not able to build up any pressure in the radiator.  Anyway, with a warmed up engine the temp gauge was still reading cold.  Maybe a hair above dead cold. I really could not tell but I think it rose a bit above where it was before I started the engine.  My infared  on the engine was reading 120-140 on the top of the heads, 200 where the spark plugs are and where the temp sender is, and 450ish at the exhaust manifolds.  Something else curious was the driver side seemed 10-20 degrees cooler than the passenger.  Maybe obstructed coolant flows?  Any thoughts on why my gauge isn't moving?  what temps should I be seeing on the infared and where should I be taking those temps.  I thought it weird that the heads were so cool.

Another random question, it seems the radiator only has one tube (an overflow) to the coolant reservoir that looked like it had an overflow to atmosphere.  I will have to double check that but wouldn't that negate the ability to pressurize?

Forgive my lack of knowledge on the 350, I am mechanically  inclined on many things but this is my first real deep dive into truck engines.    I am sure this is the first of many questions.  So far this forum and youtube have been life savers.

Thanks,
Dan

Offline bd

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Re: Temp gauge not working
« Reply #1 on: January 15, 2020, 09:36:39 PM »
Did you refill the cooling system before warming the engine and checking the temperature?  Verify the engine temperature with a direct reading thermometer.  The engine may be running too cold to register significantly on the gauge.

Your truck employs a semi-closed cooling system.  By design, the coolant reservoir is open to the atmosphere and never pressurizes. 

The radiator pressure cap contains a low tension one-way check valve and a separate ~15 PSI pressure relief valve.  During warmup, the check valve closes and seals the 1/4" tube as engine coolant heats and expands, building pressure in the radiator, which effectively isolates and protects the reservoir from pressure buildup.  Rather, pressure buildup is contained within the closed engine-radiator system.  During cooldown, the check valve opens when the cooling system pressure drops below ambient, drawing coolant from the reservoir back into the radiator as the coolant contracts and creates a vacuum.  This cycling of the check valve allows the cooling system to self-purge, eliminating air pockets and resulting hot spots. 

If the cooling system pressure rises above ~15 PSI, the pressure relief valve in the cap opens and allows coolant to surge into the reservoir, trapping it rather than allowing it to escape the system onto the ground.  The overflow vent on the reservoir simply allows the coolant to ebb and flow back-and-forth between the reservoir and the radiator unincumbered.  However, if the reservoir is overfilled with coolant, the excess will be forced out of the reservoir via the overflow until the coolant reaches a manageable level.

Does all of this make sense?  When you replace the radiator, you should similarly replace the radiator cap.  But be sure to procure the replacement cap by application so that you use the correct style and pressure.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Danny C

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Re: Temp gauge not working
« Reply #2 on: January 15, 2020, 10:04:14 PM »
Wow, thanks bd.  Interesting, I had no idea about the radiator system.  yes that makes sense.  I will make sure I get the correct radiator and now cap.  I would not have even thought about the cap before.  Cap actually looks new and may have been replaced recently before I got the truck. 

I did not completely  refill the coolant before testing as I did not have any coolant on hand and did not want to just use  water as the temps may get cold here in the next day or two and I didn't want to freeze anything.  If anything I would think that would have increased temps and caused the gauge to go hot.  I was careful to watch temps as to not over heat. Could my t-stat be stuck open?

As far as engine temp.  I was using a temp gun and right by the sender the temp got to around 200F.  I would think that is enough to at least register on the gauge.  I am wondering if even thought the gauge moved when I grounded it, maybe the gauge is still bad enough to where it will not read the range of the sender and only moved when it got a complete ground.

Offline bd

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Re: Temp gauge not working
« Reply #3 on: January 15, 2020, 11:21:51 PM »
"Testing" engine temperature with the cooling system low essentially is invalid since the sender has to be 100% emersed in coolant that is circulating effectively.  You would be better off using a contact thermometer emersed in circulating coolant than a remote IR.  Remember that the factory sender location is in the cylinder head between two hot exhaust runners.  The right place to begin is with a full cooling system and a reliable, precision measuring tool.  Yes, the t-stat could be stuck open or a 160 °F t-stat might be installed.  Yes, gauge calibration could be off.  But some factory gauges are "cold-blooded," meaning that they don't even register below ~120 °F.
« Last Edit: January 16, 2020, 08:30:08 AM by bd »
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Danny C

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Re: Temp gauge not working
« Reply #4 on: January 16, 2020, 03:08:04 PM »
That all makes sense.  not that I think about it, with the fluid levels low, I was most likely getting no circulation and cooling.  I would still think that as long as the sender was submerged, it should have read a higher than normal temp, but who knows.  Either way, a new radiator is the next step. 

Speaking of a new radiator.  I called up the local shop to see if they had one in stock and realized I do not know enough about my current one to order.  I will be pulling it out soon to take dimensions.  I assumed I needed an AC one as my truck has AC but I also know my truck has 4 radiators/condensers.  Not sure what they all go to as I have not tracked the hoses.  A full size one directly in front of the fan (assuming main engine coolant radiator), another full size one right in front of the first one (AC Condenser?), and two small ones (2' x 8") next to each other in the very front (oil cooler or trans coolers?).  If I am on to something here then maybe I do not need a radiator with AC capacity.  Either way, pulling my old one should give me a definitive answer.

Offline Danny C

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Re: Temp gauge not working
« Reply #5 on: January 21, 2020, 08:11:29 PM »
I just wanted to follow up with my solution.  I replaced the radiator (cu730) and tstat (195F) and filled with new fluid.  That did the trick.  gauge is reading accurate.  BD I think you were right about the system not being completely full did not allow an accurate temp reading.  Thanks for the advice.  Now on to all the other items...

Offline bd

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Re: Temp gauge not working
« Reply #6 on: January 21, 2020, 08:17:24 PM »
 ;)     8)
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)