Just barely cruising around the neighborhood, about three or four blocks into my little journey I noticed the temperature was staying at 200, then suddenly I started to go into the red near the end of my trip, I’m probably cruising around at 20 25 mph, when I saw this happening, I let off the gas when I did it went back to 200…..pulled into the drive grabbed the temp gun, everything was normal??? Sounds like a bad fan clutch. Any thoughts ,

Keeping in mind my engine is a CHEVY CT602 RACE ENGINE. but this has never happened before.

 

The CT350/602 is based on Chevys other crate engines, but with a camshaft designed for a 2-6K RPM range. Its cooling needs shouldn’t be different than any other small block.

It could be a bad clutch fan or a failing thermostat, but like what was said in your other threads ignition timing can have a huge effect on engine operating temperatures. What is yours set to?

 

This I do not know, I will check it on Monday.

 

It still doesn't sound like a cooling issue but... Can you post a photo of the gauge face, water pump, fan, and shroud?
At that low speed, your engine is relying on the fan to move air. When you are taking a temp gun reading, are you looking at various cooling surfaces like the radiator, hoses, and water pump? That the temp drops when letting off the gas sounds backwards to me -- higher revs allow more active cycling of the coolant and fan/air movement.

If it were timing, then I'd think you'd see higher temps all the time and not this swing from normal to hot.

If you are indeed getting higher than normal operating temps, I'm thinking the gauge is wonky or getting a wonky signal from the temp sender. I don't have any tips for testing the gauge or sender. I'm pretty sure there was a thread or three on testing the sender's resistance across different temperatures.

 

I just did another test, I checked the fan clutch, no wobble, no excessive spinning, the fan does stop when the engine stops, what I mean by that is that the fan comes to a dead stop when you shut off the engine.

Now this is the part of this leading me to think it may be a faulty thermostat, I started the engine and let it idle at 1500 RPMs, The gauge needle on the dash started to slowly rise above 200 and head toward the 280 mark, which is the orange or red zone, I shut down the engine and checked everything immediately with the temperature gun and everything was normal.

Next, I restarted the engine and let it run at 1500 RPMs for five minutes much longer than before and it never reached 200° until I let my foot off the gas at which time it only slightly reached 200.

so the problem is very intermittent. Sounds like a faulty thermostat.

 

Most likely, there is an intermittent wiring problem with the temp sender wire. If that wire shorts out to ground, the temp gauge will go to 'full hot'. Check the wiring at the sending unit and then along it's path in the engine compartment. You may find where that wire is rubbing something metal and has worn thru the insulation. The only reason why the temp gauge would show proper operating temperature, then immediately go full-hot, then back to normal would be a rub-thru short in the sender wire.

 

It doesn’t go immediately to hot….. It slowly rises…. Then I can restart the engine and I never do it again… Then I can restart the engine again and it slowly climb past 200 and slowly go toward the 280 mark.

 

Try this thermostat
https://www.summitracing.com/parts/e...MaAk4WEALw_wcB

 

When you say thermostat, are you meaning the thing MelWff linked to, or are you referring to the electrical gauge circuitry?

I don't think it's a cooling system overheating issue. This sounds electrical to me because each time you've checked with the temperature gun it's normal even though it's just had a "high-heat incident" -- yes? If so, then give 7T1vette's comments some thought. Find the sender on the block (should be on driver's side mid-way at the head's mounting surface - there may be a similar sender on the pass. head, but that's for another use) and check that the gauge wire is firmly clipped to it.

 

I’m talking about the gauge this featured in this photograph. I did trace the wire out from the engine bay all the way inside the automobile, there’s absolutely nothing wrong with the wire anywhere along It’s route. Now it could be the sending unit itself. But I’ll find out Monday when I start ripping things apart….again.

Thanks again for everyone’s help. It’s greatly appreciated. I have found out that this in particular Corvette is one step forward two steps back… I think that just might be the nature of these beasts.

 

Its Two Steps Forward and One Step Back. That's why we keep them.

If it was the other way around, it would get a For Sale sign.

 

At one point a few years ago I was fighting a very similar issue.
Everyone on here will tell you all the common things to check and replace. The high flow thermostat linked above is indeed a excellent suggestion. Checking for the spring in the lower hose. Making certain ALL the packing is around and over the radiator so all the air must go through it is HUGE. And on and on.
On mine. When I rebuilt my engine, I installed new aluminium heads. So I thought it was a good idea to install a new sensor for the temp gauge in the head.
And most of the time, my gauge was reading around 195 which I thought was a little off, but pretty close. But then when it would go up, it didn't go up a little, it went up a lot!
I went out and bought a IR temp gun. That was interesting. If you point it at different things, you get different temperatures. Which is right?
after chewing my finger nails completely away with worry and trying absolutely everything from running the engine richer. To various assorted timing changes. I mean you name it, I tryed it.
Then I dug out the old original temp sensor. I'm a pack rat.I don't throw things away. Anyway installed the 45 year old sensor in the head.
gauge reads 180 all the time. If it goes up a bit sitting at the lights. It goes up a bit, not a lot. Then my fans kick in and I watch it go back down.
The aftermarket sensor was apparently really close ohms wise right on the normal temp. Maybe off a hair but as the temp went up a little the ohms changed a lot. So it was hard to detect the problem.
And by the way, I'm still running the factory original sensor, and my finger nails look fine.

 

Have you done the simple tests of the gauge?

The sending unit is just a variable resistor. If you short the connector at the sender to ground, you should read a very high temperature.

You can manually check the resistance of the sending unit.

You can also confirm all temps with an IR gun.

Sadly, the Willcox Corvette website is gone, but Ernie's videos persist on YouTube. RIP Ernie.

 

Wow!!! Now wouldn’t that be interesting if I have the same issue, but this engine was put in around 2021. I’ve just finished the interior, and boom! Maybe it is the sensor, did yours gradually creep up? Mine just starts creeping up, then let off the gas and it goes back down, engine gauge reads nearly 280! The temp gun reads 154 at the hottest part, right at the thermostat. Now the manifolds are always super hot, but the hottest part of the engine I could find was 154. Then I’ll start the car and it runs normal and never goes up? I haven’t gone to chewing my nails yet, but rubbing my head bald is taking it’s toll, I’ve looked at parts of this engine and read more than I thought I ever would. Back when I was able I would just start taking things apart and cleaning and checking, but it takes me about 4 hours just to wash it…lol. My neighbors are use to it, the see a man laying down on the ground by his car, they say, “That’s Powe washing the wheels on his car!” I’m one of those never give up no matter what, I appreciate you taking the time to share that amazing story with me, great insight..and very informative.

Thanks again,
Powe.

 

CT350 602 race crate is essentially same motor as 350 HO crate: Both have iron Vortec heads which have a small hole for coolant sending unit (same HFT cam as well). OTOH, 1975 sbc heads have a much Larger hole for much Larger temp sender. That hole is between spark plugs #1 & #3 driver side Or #6 & #8 pass side.
Most small senders have different resistance than the Large sender that WAS matched to OE dash gage.
That mismatch is rather common and does cause gage error.
Perhaps you have a sending unit that's Not a correct match to your dash gage ?
FWIW, Lectric Limited used to offer a correct resistance Large sender that is turned down and rethreaded to fit the smaller hole. If you retain OE '75 heads ? Have that sender turned down & rethreaded ?
* an OE CT350 has a "left-turn" oval track oil pan which is Not recommended for a street vehicle. Could be bad mojo in a high-G right sweeper.

 

I changed the oil pan and pump. I also installed a dip stick.

Thanks for sharing,

 

Like I said, those IR temp guns can be confusing. The best place I found was right on the thermostat housing right next to the hose.
For whatever its worth. I remember someone on here, I think it was Craig also from Australia tested numerous temp sensors and finally found one he found to be acceptable.
other's have reported trying numerous different sensors with different results.
One of the biggest issues we have in this hobby is finding quality replacement parts.
For the price of a sensor, wouldn't hurt to try one just to see the difference. And when it gets hot get out immediately and shoot that thermostat housing for temp!.

 

That’s exactly what I did, 154. Both times. The entire engine was never above 200, the only thing that was super hot where the manifolds at 560.

I have a really good Temp gun, I’ve tested it next to a probe and it’s always spot on.

Thank you.
‘Powe.

 

Interesting thread.

The high flow thermostat linked above seems like an excellent idea for Corvettes like mine that run hot at idle. BTW: I put in a high-volume water pump years and years and years ago. I read somewhere that they flow high volume at speed and at idle flow less water. I guess that is in the impeller design.

I was doing an HPDE at Summit Point with my son. He came in and the engine was hot. Like you, we hit it with a temperature gun, and it was in range. The coolant level was fine. I wanted to change the sender, but as mentioned that can be matched to the gauge. So, I took it out to check it, put it back and took care to wrap and check the wires going to the sender. They are near the exhaust. Fortunately, it never happened again.

 

Isn’t this the temp sensor? There is not one anywhere on the sides of the block.