Engine Temp Again
Thread Starter
Instructor
Joined: Oct 2014
Posts: 190
Likes: 37
From: Jensen Beach Fl
There you have it, that’s my temp sensor, and it never reads higher than 154 on the temp gun, we just checked the temp gun against a probe in the kitchen this morning, and his spot on. going outside now to test it again in real time and see what’s going on. I’ll let you know.
Thanks,
Powe
Thanks,
Powe
Thread Starter
Instructor
Joined: Oct 2014
Posts: 190
Likes: 37
From: Jensen Beach Fl
Ok, just finished my second round of testing…. the thermostat kicks in at 160, my wife held the temp gun and I ran the engine at 1500 RPMs, everything read normal compare compared to the heat gun to the temp gauge inside the car… then suddenly the gauge needle inside the car started going straight toward the red zone, not really fast, but slowly moving up, but the temp gun on the engine stayed the same at 160°.. on the temp sensor and the thermostat area.. and the big hose going to the radiator. The hottest that the temp gun ever got was at 194° when the gauge on the dash was in the red zone at 280°. It was only there momentarily I let the engine keep running at 1500 degrees and slowly, the gauge reversed itself and went back to the normal position at a little under 200°.
So this is beginning to look a lot like a SENSOR issue. any thoughts on this new test?
So this is beginning to look a lot like a SENSOR issue. any thoughts on this new test?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
ETRADIO,
As I understand it, any dash gauge that reads zero is not getting any voltage.
Any gauge that runs in the normal range is getting correct voltage & a good GRD source.
Any gauge that pegs-out is getting correct voltage but a "poor" GRD.
Its resistance that brings the gauge needle down to proper levels.
The senders are a "One-Wire" unit. So the GRD-ing wire are the threads in the casting.
A common error on sending units whether oil pres or temps is, a poor metal to metal contact during installation.
That's why you should never use a thread sealant or thread tape on the first few threads of any sending unit.
You want just enough to seal but not interfere with grounding.
Is that corrosion I see around the threads of the sender?
H-m-m-m-m-
As I understand it, any dash gauge that reads zero is not getting any voltage.
Any gauge that runs in the normal range is getting correct voltage & a good GRD source.
Any gauge that pegs-out is getting correct voltage but a "poor" GRD.
Its resistance that brings the gauge needle down to proper levels.
The senders are a "One-Wire" unit. So the GRD-ing wire are the threads in the casting.
A common error on sending units whether oil pres or temps is, a poor metal to metal contact during installation.
That's why you should never use a thread sealant or thread tape on the first few threads of any sending unit.
You want just enough to seal but not interfere with grounding.
Is that corrosion I see around the threads of the sender?
H-m-m-m-m-
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,135
Likes: 4,245
From: US-PNW
Sounds like a solid test to show that the engine is not overheating and it's an electrical gauge/sender issue. The sender works on resistance change with heat, but that's all I know about them. I'm not usually a guess and go sort of guy with repairs, but I think I'd just put a new sender in the manifold to see if that fixes it. Taking the gauge cluster apart to test the gauge is way more complicated and it will still be there if the sender swap doesn't fix it.
Drifting
Joined: Jun 2025
Posts: 1,715
Likes: 597
When the sender is positioned relatively low (as is OE in production vehicles), it is nearly always submerged in coolant (as it should be). Maybe your cooling system has entrained air that's intermittently trapped near where your sender is positioned in manifold; and that's contributing to errant/erratic signals to gage ?
Have you attempted to raise nose of car at an angle, and then purge any air ? Sometimes it helps to remove thermostat-water neck from manifold and fill block.
Thread Starter
Instructor
Joined: Oct 2014
Posts: 190
Likes: 37
From: Jensen Beach Fl
Good idea, and yes I did that, I’ve ran it a bunch with no issue at all, but this just started two days ago, out of the blue. The coolant is absolutely full as it can get….Great information, thank you.
Powe.
Powe.
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Race Director
Joined: Mar 2017
Posts: 11,949
Likes: 4,507
From: Virginia
Did you drill a tiny hole in the thermostat?
I think your problem is likely electrical. If the sensor wire shorts to ground, it will erroneously give you a very high reading. And this can happen anywhere along its length.
But, wouldn't it be funny if all you needed was this:
https://www.corvetteforum.com/forums...at-or-not.html
I think your problem is likely electrical. If the sensor wire shorts to ground, it will erroneously give you a very high reading. And this can happen anywhere along its length.
But, wouldn't it be funny if all you needed was this:
https://www.corvetteforum.com/forums...at-or-not.html
Thread Starter
Instructor
Joined: Oct 2014
Posts: 190
Likes: 37
From: Jensen Beach Fl
Absolutely never make that an option. Jeezzzz…I would hope not. I’ve checked the wiring. I’ve checked the gauge. There’s nothing wrong with the gauge or the wiring, it’s absolutely has to be a sensor issue, but not a big deal, easy fix.
Thanks,
Powe.
Thanks,
Powe.
Race Director
Joined: Mar 2017
Posts: 11,949
Likes: 4,507
From: Virginia
I hope it's an easy fix. See if you can take a few resistance readings of the sensor (from the spade connector, to the body/engine ground). If the resistance drops during one of your high temp excursions, perhaps it is the sensor. If the resistance doesn't change, you may have to look elsewhere.
Good luck!
Good luck!
Thread Starter
Instructor
Joined: Oct 2014
Posts: 190
Likes: 37
From: Jensen Beach Fl
Thank you, I will definitely jump on that next. You’re right, that “easy fix” was tongue in cheek, this car is always one step forward and two steps back…lol. Just when I think I’ve got it fixed,,Boom…oh well, good thing I’ve cars…. And working on them.
Thanks again,
Powe. 😎👍
Thanks again,
Powe. 😎👍
Melting Slicks
Joined: May 2021
Posts: 2,271
Likes: 1,316
From: Fleming Island, FL
Do you have the intake manifold to water pump coolant bypass kit installed on that engine? Vortec heads aren't drilled for coolant bypass and you won't have coolant circulation in the engine when the thermostat is closed creating hotspots.
Le Mans Master
Joined: Jan 2001
Posts: 6,741
Likes: 1,383
ETRADIO
I've done a bit of testing of temp senders and gauges with my adjustable resistor that I got from Willcox.
Go read up on this. I suspect that you either have a failed/failing sensor or a failed/failing resistor in the back of the gauge.
I don't know where you would find an adjustable resistor since Willcox is gone but Ecklers does list the same thing....but I don't think they have it. Go read this thing and see what you think. Within that link you will see the post for the Ecklers adjustable resistor (looks just like Willcox's --- but I don't think they really have, but you can call them). I linked the post where I did the testing of a 3/8" Wells SU109 Temp Sender...but you can see before for where I dialed in my old 1/2 original temp sender.
See what you think.
https://www.corvetteforum.com/forums...post1608841610
I've done a bit of testing of temp senders and gauges with my adjustable resistor that I got from Willcox.
Go read up on this. I suspect that you either have a failed/failing sensor or a failed/failing resistor in the back of the gauge.
I don't know where you would find an adjustable resistor since Willcox is gone but Ecklers does list the same thing....but I don't think they have it. Go read this thing and see what you think. Within that link you will see the post for the Ecklers adjustable resistor (looks just like Willcox's --- but I don't think they really have, but you can call them). I linked the post where I did the testing of a 3/8" Wells SU109 Temp Sender...but you can see before for where I dialed in my old 1/2 original temp sender.
See what you think.
https://www.corvetteforum.com/forums...post1608841610
Drifting
Joined: Jun 2025
Posts: 1,715
Likes: 597
Thank you, I will definitely jump on that next. You’re right, that “easy fix” was tongue in cheek, this car is always one step forward and two steps back…lol. Just when I think I’ve got it fixed,,Boom…oh well, good thing I’ve cars…. And working on them.
Thanks again,
Powe. 😎👍
Thanks again,
Powe. 😎👍
Seems this (or related) issue is carrying on in 2 or 3 similar threads. None of this is easy but it ain't rocket science.
Seems you retain OE '75 heads. If so, pull the sender from the driver's '75 head and have local machinist turn it down and thread it to fit current Vortec head.
So long as your temp gage system WAS working OK when '75 heads were in service, then the above remedy should work exactly as it had..
If you don't have the old sender from '75 heads: do as previously suggested. Buy a new temp sending unit for '75 vette and have a local machinist turn it down and thread it to fit your Vortec head.
OR check with Lectric Limited : they used to offer a NOS larger 1/2" NPT sender that they'd turned down to fit the 3/8" NPT sender-hole in Vortec heads.
*** correct NEW replacement gage sender for '75 vette is Standard Motor Products P/N TS-6 (NOT TS6T NOT) sender is larger 1/2" -18 NPT
replacement elec connecter is SMP P/N S2292
Get a TS-6 + S2292 and have local machinist turn down & rethread as above.
Last edited by Rebelyell; Jun 13, 2025 at 04:36 PM.
Drifting
Joined: Jun 2025
Posts: 1,715
Likes: 597
the following is verbatim quote from GM on these matters:
*"Any small block engine, regardless of year, that uses Vortec heads, will require an external coolant bypass line from the intake manifold to the 5/8" hose nipple on the water pump (passenger’s side). Suggested routing is from the 3/8 NPSF boss on intake manifold to the water pump".
If the Vortec bypass issue has yet to be resolved, suggest it will pay OP dividends to do so. OE L31 engines are installed by GM via what is essentially same scheme. JMO, but I think this solution addresses the core issue more directly than does drilling t'stat flange.
* the above quote is verbatim in OE install instructions for Both 350 HO (x576) and CT350 (x602) crate motor(s).
https://www.chevrolet.com/content/da...e-12366576.pdf
https://www.chevrolet.com/content/da...e-88869602.pdf
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Sender have only "one wire" for signal; but they are not just a 'one wire' system. For that gauge to read properly, the gauge and the sender must be connected to a common electrical ground. The gauge is connected to ground via the metal console housing (IF it is grounded properly per the factory connection): the sender is connected to ground via the metal case being in direct contact with the engine block. If you have teflon tape or other non-conductive material on the sender's threads which prevent good contact to the engine block OR if the engine block and the gauge console plate are not connected together [electrically], the gauge will not read properly.
Thread Starter
Instructor
Joined: Oct 2014
Posts: 190
Likes: 37
From: Jensen Beach Fl
I have tested mine now several times and it’s spot on. So far so good, When I replaced the thermostat I did noticed that someone had installed a regular thermostat and not a high flow, I replaced it with a 180 high flow thermostat when I was replacing the temp sensor. Since I have done these two things I have not had a problem since & my heat gun reads exactly almost to the number of what my thermostat in the car reads, on a long trip I reach about 197 198 close to 200° but it never gets hotter than that and according to my gun that’s spot on. If I had a huge difference in the two readings I may be concerned, but right now and so far I’m a happy Vetter!
Drifting
Joined: Jun 2025
Posts: 1,715
Likes: 597
I have tested mine now several times and it’s spot on. So far so good, When I replaced the thermostat I did noticed that someone had installed a regular thermostat and not a high flow, I replaced it with a 180 high flow thermostat when I was replacing the temp sensor. Since I have done these two things I have not had a problem since & my heat gun reads exactly almost to the number of what my thermostat in the car reads, on a long trip I reach about 197 198 close to 200° but it never gets hotter than that and according to my gun that’s spot on. If I had a huge difference in the two readings I may be concerned, but right now and so far I’m a happy Vetter!
Thread Starter
Instructor
Joined: Oct 2014
Posts: 190
Likes: 37
From: Jensen Beach Fl
I will check when I have a chance, I’ve been bedridden for the last 28 days, waiting for a reprieve, then I’ll get back to it. I’ve had a couple of short reprieves, lol…I run around the neighborhood in my Vette. I have a really good Vette Mechanic, and I sent him the info you sent me, but there’s little I can do right now, I appreciate you sending me that information, and I will let you know what I find out, cheers my friend,
Thanks again
Powe.
Thanks again
Powe.