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PostPosted: Wed Nov 29, 2017 4:29 pm 
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Joined: Mon Jan 28, 2013 8:48 am
Posts: 47
Location: OH
In the past couple of days I've noticed the temp gauge on my '72 240Z (83K miles) has been acting erratically when engine is at normal operating temperature. I should preface this by noting I did have the cooling system flushed & new coolant installed within the last 150 miles.
With the engine at normal operating temp the gauge will show normal readings and then on an approx. 5 mile, 50MPH cruise the needle will steadily move to indicate a lower operating temp. Today on the same route the needle actually moved to the area where it would be on cold engine start-up. After awhile needle will move back to the normal range. Today the needle did much more fluctuation than it did yesterday. Today while moving cars around when I got home (engine idling at normal operating temp) the needle was indicating the cold start temp condition. Placed my hand on upper radiator hose & top of thermostat housing (with engine running) and both were warm as they should be for a fully warmed up engine. Engine runs great the entire time, no difference in its running when this occurs.
So, since I recently had a coolant change do I have an air pocket(s) in the system & it needs to be "burped" or might the coolant temp sending unit be exhibiting signs of its coming failure? Prior to all this and the first 100 miles after the coolant change the temp gauge was acting normal.
In advance, Thanks for your thoughts & help.

Best,
Chris


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PostPosted: Wed Nov 29, 2017 9:36 pm 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 14779
Location: CT
Hi Chris. I would buy either possibility, but since you had the system emptied and then filled again and it operated fine before that, I would lay more money on the air bubble theory. The engine should be run with the radiator cap off until the thermostat opens. That will gush a lot of very hot coolant all over the engine and you, leaving the top 1/3 of the radiator empty with hot water surging into it from the top hose. Slowly refill the empty top 1/3 of the radiator and recap it. I have always found a way to get scalded during this operation. :?

You could also wiggle the temperature sensor wires and see if you get a different reading. Replacing that is actually straightforward and cheap (screw out/screw in). You could pull the wire off the top and examine the spike connector (at least on my car) for corrosion; scrub it with a toothbrush soaked in baking soda/water solution and it will be clean again.

Interesting subject ~ let us know what you find?
Frank

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1970 240Z


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PostPosted: Sun Dec 03, 2017 8:38 pm 
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Joined: Fri Dec 23, 2016 7:11 am
Posts: 678
Good advice Frank. I believe you are right.

The only bit I would add is that I have seen thermostats go bad occasionally immediately following a drain and refill of the cooling system. Couldn't tell you why that is though. Something to consider.

I think the air bubble or temp sending unit are more likely causes though.

Jay


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PostPosted: Sun Dec 03, 2017 11:08 pm 
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Joined: Mon Jun 30, 2008 2:16 pm
Posts: 1181
Location: Ansonia, CT
Hi Chris,

I've experienced the faulty thermostat syndrome as well after a coolant flush as Jay mentioned. And by the way, you should actually remove the thermostat when you flush the coolant system as there is very little flow that moves through a stuck or closed stat.

Another easy check would be to actually stick a meat thermometer in the top of the opened and depressurized radiator and read the temperature of the coolant as it enters from the upper hose with the engine running and "warm". If it reads cooler that normal, your thermostat is probably stuck open. If the coolant is hot, its probably the sensor or faulty wire connection. Either way, cheap and simple fixes. :)

And always be careful to avoid getting burned by hot coolant! :oops:

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John Kish
1971 240Z - original owner


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PostPosted: Fri Dec 15, 2017 12:31 pm 
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Joined: Fri Jun 23, 2017 5:34 am
Posts: 62
Not sure how ghetto you want to get but...

In the past when I question my thermostat I just hack out the spring and valve and basically leave it as plate with a hole in it. That hole diameter is important though because it helps slow water flow by creating some pressure on the water pump. This helps to stop cavitation on the pump. Never just remove a thermostat.

To refill systems I:
*Fill them from the highest point.
*Put everything together.
*Start the motor.
*Take off the radiator cap.
*Get wet.
*and wait... wait... wait...
*Once the thermostat opens water will either attack you, flow out nice, or start to go down in radiator, keep it topped off if it goes down.
*Once the water pours out for a couple seconds consistently, not a burp, shut the cap. Have fun cause there is going to be a mess. As the water expands is going to keep coming out the over flow tube once the system goes over the cap pressure. So have a pan ready and keep the cat and dogs away.

All that said... totally not how you do it if you have a working reservoir. If you have a working reservoir. You can do all that. It will be cleaner as the expanding water should go in the tank, not the ground. But the most important part is choosing the six-pack you are going to drink. Because after you have let the car completely heat up and give it some nice slowly increasing revs to like 5k rpm. Shut her down crack some beers and watch the reservoir level as the engine cools down. The reservoir must never go empty. So it nice to see what it does. If reservoir the level doesn't go down during cooling. There could be a bubble that only heat cycles will get rid of.

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Bruce - Early 260Z - 305 Light Blue Metallic


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