My 1975 C3 is overheating almost instantly. By the time I fire it up to the time it take me to get to the front of my neighborhood (less than a half mile) it over heats. I just purchased the car and am going through it to fix the little things. At first the temp gauge would sit at 100° for like 10 minutes, then in less than a half a second would shoot all the way up to 280°. So I changed the thermostat, no fix, took thermostat out, no fix. I then changed my temperature sensor (not in factory placement but at the coolant bung on top of the motor drivers side bank). After changing the sensor it read smooth and wasn’t all or nothing as it was before. The problem I’m having now is that the water temp gauge spikes to over 280* over the course of one or two minutes. Even with the gauge reading 280, I dipped my finger in the radiator and the water was cool. I rev’d it a little and it looked like coolant was passing through the radiator. The car reads good oil pressure as well. Any information or thoughts yall might have would be greatly appreciated.

 

ddickey99
Doesn't seem like you are really overheating. Either your wiring or your gauge is/are bad..... I would suspect the leads having some sort of problem after the vehicle starts "rumbling".....

Don't throw away your old temp sensor either.

sounds like a false reading. go get yourself one of those temperature guns at harbor freight...... they're cheap and work fast..... and you can then check the performance of your sender and/or gauge.

Here....watch this....everything you need to know to fix this....should be easy to narrow it down....

 

no, I’ve tested all that. Do you think I might have a blocked radiator or bad water pump? Coolant cap is also holding as it depressurizes when I remove it.

 

What indications besides the dash gauge do you have for the engine overheating?
Can you touch the top of the intake manifold? How about the upper & lower radiator and heater hoses?

You believe the coolant is moving, but is cool to the touch, and if you can touch the manifold and the hoses... your engine is fine and it's the gauge, sender, or wiring that's leading you astray. I think this is further supported by the gauge sitting at 100°, then suddenly shooting to 280°. That's just not the way an overheating engine behaves.

 

yeah everything’s hot asf after about 15 minutes. The radiator does take longer to get hot than the gauge. Is there any chance the might be a resistance issues? I’ve run all the tests on the gauge and there’s nothing to indicate it’s the guage. It really just seems like there’s nothing recirculating. Could it be an issue with the the location of my water temp sensor? I know the stock location is below the valve covers. My location is pictured.

 

that’s how it was over heating before I changed the temp sensor. After I changed the temp sensor it moved like a gauge should, the gauge would move linearly at idle and would creep up faster while driving. The gauge would go all the way to 280 after about 2 minutes of driving. Before changing the sensor it would snap from 100-280 instantly after 15 or so minutes of driving. So the gauge is definitely respond better/ right than before by far.

 

The sender on top of the intake shouldn't be an issue at all - the 327s and 427s had them located there. I believe it moved to the heads when the 350 and 454 came out.

I'm thinking an engine hitting 280° would be just about seized!
A laser temp gun would be really handy to help diagnose a true over-heating engine...
Just out of curiosity, did this issue spring up out of nowhere or has it been getting worse over time?
Was anything done to the engine that may have triggered it?

 

no I took delivery of the car last week. I don’t know. It was my neighbors car, he took pretty good care of it of it. It sat for a year before he took it to a consignment dealer where they did some work on it converting it from efi back to a four barrel carb. I know running an engine lean can cause over heating but it fires right up and runs good. It take about a full second longer to start when hot. Also no. I don’t think the coolant is boiling. It’s not coming out of the engine at least. To be honest I’ve never seen coolant boil or know what it would look like. What should I measure with a temp gun. The block? Also what temps should I expect on the areas you want me to insoect

 

I will tell you what, Mr. Dickey.

If that engines cooling system was TRULY running at 280, it would blow the Rad Cap right through that fiberglass hood.
Steam everywhere, the smell of glycol, perhaps a ruptured hose or two. Valve cover breathers weeping oil. Fuel percolating out of the carb.
None of that is happening.

Ironically, if the temp gauge single wire fell off the sender, the gauge would immediately jump to 280.

Ideally, normal temps are from 170ish to 200*.

 

Get an IR temp gun. From Amazon, from Harbor Freight, wherever. It should be $25- or so.

Shoot the upper radiator hose when "hot". Then you'll know!

I like using Stant radiator caps, since you can open them manually (with a thick glove and towel!) to vent to the overflow tank, and check that the system is pressurized.

 

Like Bikespace said, take readings on the upper rad hose - also the intake at the base of the thermostat housing. Then go to the lower rad hose/water pump. No matter where you measure, at most you should see ~220°F, most likely you'll see ~200°F. Overheating would be very obvious: steam and coolant belching out. You'd hear it too - boiling inside the radiator.

 

new update. I’ve come to realize that it must be an electrical issue. I understand that the water temp sensor and gauge work based off resistance. And I have come to realize that even when the car is off with the ignition in the on position the temp gauge starts climbing. I figure that somewhere in the electrical is an issue that is drawing power on the same circuit? I’m not really sure.

 

I suggest starting a new thread with the subject: "troubleshooting a '75 temp gauge"
That will get new attention to helping solve the problem.

 

So now you don't know that it's NOT overheating.

You should still get a temp gun, and check it out. It's a really handy diagnostic tool.

 

There inexpensive. And absolutely maddening. Everywhere you point it will show a different temperature! It's really an interesting guessing game!
I have found shooting it at the thermostat housing was the most reliable in this type of issue. Shooting the upper rad hose was useless in my case. Perhaps because I run a stainless steel upper rad hose.
Everyone on here for years preaches to get a IR temp gun.
So I did. WOW, Everywhere you point it. You get a different temp! A very easy to misinterpret tool.

 

Very true, but it doesn't matter that there are multiple-degree changes between readings. He's looking for an overall averaging sort of reading or swings that are way higher showing a severe hot spot - like a thermostat not opening.

 

I would start troubleshooting your engine by going to the electrical system. Verify that your engine is grounded properly to the chassis. I frequently like to measure the voltage at the battery first and then move to the engine and use it for the ground signal and measure the battery voltage again. For the positive side I use the horn relay on my C3. The numbers should be the same or very close.

Check your battery for the chassis ground if the readings are not that close together. Grounds on the C3 can be a big issue. The engine has a ground strap that connects it to the chassis, if that strap is broken or missing then your gauge will have no ground reference. I would even check to be sure that your center console is grounded as that could be part of the problem.

It has to be an electric issue that is causing your gauge to act like it is. Follow that path and the problem will appear....

On my 1968 C3 with it's BB I use the cylinder head sensor location (between cylinders #1 and #3) versus the intake manifold holes that can be used instead. The intake manifold when in use will have some air in the intake manifold and this can keep the sensor out of the coolant mixture where it would appear to cool down. Then some HOT coolant hits it and bang you see a temporarily higher temperature right away. For a continuously smooth signal I use the lower hole and it gives me a good idea of how hot the engine really is.

I would suspect that an engine with both cast iron block and heads would survive a brief hit of 260* (F) where the aluminum headed cast iron block might not survive. I have worked on engine that would periodically hit 260-270* and they would be fine after they cooled down. My DD with its aluminum heads saw 235* and both heads were trashed right away.

 

Radiator measurement

Thermostat housing

70 MPH reading

Cheap Amazon IR gun readings.
1975 L48.
Hot day (105*) here in Phoenix, Arizona.
Have you flushed and drained the system to be sure no blockage occurs after running the motor? When I bought my car six months ago, the PO had mixed Dex-cool and Prestone 50/50 mix together, due to several leaks in the heater hoses. It took 2 flushes to get the "snot" out of the system.
I disconnected the heater hoses and plugged the inlet outlet connections for them. No need for a heater here.
Suspect an electrical problem somewhere. Hope it isn't the gauge.... PITA to remove. Sounds like your electrical connection to the sender is grounding after a few minutes.

 

one other thing you could do if you do not want to purchase a temp gun is to buy a $20-$30 temp gauge and hook it up and see what temps it's reading compared to your factory gauge.
Pat

 

ddickey99
To determine if the lead to the sensor is the problem I would just run a new temporary lead from the gauge to the sensor. Out the window, under the hood, and hook it up. That will eliminate the lead if it is not the problem. You can then start searching for bad grounding issues.

Good thing is that you're not really overheating.

I'd get the temp gun. They come in really handy. I just got one this year but really..... shoulda gotten it earlier.