You, or your motor?

'

Personally I like to swim upstream against the crowd, so here goes.

I don't think anything's wrong with your motor/cooling system, other than perhaps a too-sensitive temperature gauge. One that responds TOO rapidly. Or perhaps your sender is mounted in the wrong location, or a less-than-ideal location.

I think what it's doing is absolutely normal. Temp hits 240 briefly, (actually closer to 220) thermostat opens, a cool blast of water hits the head and drops the temperature, t-stat briefly closes, rinse, repeat until the temp stabilizes throughout the cooling system. Mind you, that at 10PSI or whatever the cap holds, 220 is NOT boiling.

I'd make sure it was burped well and forget about it. 50/50 coolant/water. Maybe test your radiator cap or just replace with a new STANT.

If I had to diagnose SOMETHING I'd say your temperature gauge is A) Too sensitive, and B) As you've already posted, perhaps reading 10-20 degrees too high.

So how do you like that 383 stroker? Where do you live? I want to come drive it before deciding if I am going to put a 383 with Vortec heads in MY '68.

Cheers!

 

An engine with a 180*F stat won't go from 180 (once the stat opens) to 250*F in seconds for no reason. In fact, NO engine will do that, nor will a flaky gauge.But, a widely varying resistance in the sending unit signal will do that. I really don't care what the 'real' cause is for this problem; I just want to OP to figure it out. But some things are just not very likely.

 

You're in territory that's unfamiliar to me. Is AIRGAP a description or a brand? Can you give an example of an AIRGAP manifold, a URL? Just so I can keep up with technology and you youngsters!

Thanks!

 

Weiand Team G and Edlebrock rpm airgap are examples of airgap type intakes.

 

A quick discussion of "sticking thermostats."


See all the crud on the thermostat shaft, held between his fingers? That'll make it bind. Probably calcium (hard tap water) + rust.


Working @ NISSAN, almost every time I saw / diagnosed a "sticking thermostat" it's because a DIYer was battling overheating and decided to "flush" their cooling system.

They go buy a quart of caustic flush @ AutoZone (ACID) pour it in, warm up the motor, drain and refill with coolant without so much as a rinse or a little baking soda to neutralize any leftover acid.



Here's what I BELIEVE goes wrong. A cooling system is like a battery. Coolant is in contact with several dissimilar metals --- aluminum heads, cast iron block, steel impeller, brass radiator, etc. (Which is why some guys with aluminum radiators install caps with "sacrificial" anodes )

How do you construct a battery? Dissimilar metals plus an electrolyte --- aka "acid. " Tap water with a little leftover acid from a flush will do just fine.

What a battery's potential does is cause metal ions to move from one surface to another, gaining or releasing an electron in the process.

In a brand new cooling system, after it's run awhile, everything has plated out, the battery has "gone dead," All the metal that's gonna move already has, surfaces are coated with slight layers of oxidation, etc. That and anti-corrosion inhibitors have minimized metal movement. (Coolant contains inhibitors which are a sort of "anti-electrolyte" which is why you need to refresh them with a coolant drain & refill every couple of years. )

Etch all that metal with strong acid and you've now got nice fresh metal surfaces, and the battery starts back up again.

One of metal's favorite places to plate out seems to be on the thermostat shaft, which can cause it to stick.

That is my UNPROVEN THEORY of HOW it happens, all I know for a fact is that it DOES happen. Flush a radiator or coolant with acid and 2-5 days later you've got a sticking thermostat.

9 /10 the REAL problem (with Nissans) was an insufficiently burped cooling system!

I personally, never, ever flush a cooling system. I'll take a radiator to a radiator shop for a professional flush, if it needs it.

In my DECADES of diagnosing and correcting overheating, I've only seen ONE vehicle that a flush "fixed," But I have seen a lot of thermostats which began to stick a few days after a caustic "flush."

Main thing is to use 50/50 coolant, and make sure the other 50% is DISTILLED water. Continually topping up with (hard) tap water will clog your radiator over time.

An occasional top up or emergency? No big deal. Use whatever you got.

 

https://www.corvetteforum.com/forums...-manifold.html

After a 10 minute search it turns out one of the best articles is right here on our site!!! LOL.

For the rest of you, SOME "Airgap" manifolds have coolant crossovers, some have exhaust crossovers, some don't.

Quote:

Originally Posted by wadenelson

From a Mustang forum: Neither the RPM nor the Air Gap have provisions for the rear

 

I also have a ‘72 with a BluePrint 383 that seems to exhibit a similar problem, although my gauge doesn’t jump as high (it also reads at 15 degrees hotter than actual temperature at warm up as measured with an infared gun). I had just installed a Standard brand temp sensor that I purchased at one of our shop suppliers , as the car didn’t have one when I purchased it. I used a vacuum type cooling system filler that I had at the shop. This system eliminates trapped air in the system. I also have a new aluminum radiator installed with electric fans. I’m suspecting the problem is in the new sending unit location. I had to install it in the intake manifold. The correct location is in the cylinder head, but the thread diameter there is too small. Supposedly they do that because of a combination of things. Later cars had smaller diameter temp sensors. And aftermarket gauges also are smaller, although every one I’ve seen comes with an adapter so I don’t understand that. I have not been able to find a smaller temp sensor that measures the same resistance required to make our temp gauges read accurately. As a last ditch effort I have just ordered a new temp sensor from Ecklers just so I can eliminate the possibility of a defective or wrong sensor. If there is no change then the problem has to be either the sensor location, or the gauge itself. I hope this helps you.

 

Depending on the temperature coefficient of the sender, you may be able to fix this by adding a small resistor in-line with the sender.

Measure the sender's resistance both hot and cold and guess as the resistor value you need --- 2 ohms, 10 ohms, whatever. Cut the sender wire and alligator clip the resistor in, try different values till the gauge reads where you want it. You can buy an assortment of low-value resistors on Ebay for $10.

STORYTIME: Had a Jeep Cherokee where the owner SWORE it was overheating after some work one of our guys did on it. Me and the shop owner drove it up and down mountain sides, AC on full blast, no overheating, NADA After the high speed run we even let it idle for 30 minutes, no puking coolant, no steam, nothing. Laser thermometer confirmed there was no problem. Customer just didn't like the needle anywhere past the center of the gauge. Soldered in a resistor and sent him down the road, 100% happy. END OF STORYTIME

 

since it is a new stat, could it be binding due to the manifold or cover housing mismatch?

 

BattyMantis
It's fantastic. You'll be happy as can be. I'm assuming you have the stock gauge since I cannot really tell from your statement.
Here is my story:
https://www.corvetteforum.com/forums...-question.html

It's fantastic. I lived with an inaccurate gauge for decades. You'll love it. My temp sensor is on the intake manifold also (since I have Vortec heads with the little hole for the temp sensor there).

 

I had a similar problem with a 350.

I agree with a previous statement "I'd make sure it was burped well"

I had to buy this and finally got it to burp

 

Another vote for.... ^^^THIS^^^ Works really nice to get all the air out.

 

What water pump are you using?
Do you have the heater core lines connected or blocked off?

I've seen this type of behavior with high flow pumps and the lack of enough bypass flow to keep the pumps from cavitating while the T-stat is closed.

Neal