OK, so my 1982 280ZXT was running pretty HOT on the way up to our last meeting...pretty hot is probably an understatement - it was 2/3 up the gauge...

So, I make it home afterwards and find there is a slight drip from the radiator drain. I also find that the float in in coolant reservoir is stuck in the high position, giving me a false indication that there is plenty of coolant in the reservoir when in fact there was almost none. So I figure the overheating is a result of insufficient coolant stemming from the drip at the drain.

I examine the drain cock and find it has been broken off (plastic/nylon type). I manage to get the rest of the thing out and I drain and flush the coolant. I install a new drain cock, new coolant/distilled water 50/50 mix and retest.

Now the car is running too COOL. It barely gets a 1/4 of the way up the gauge (~140 degrees). The temp does move - when idling at a red light it goes up a bit and when driving on the highway it comes back down.

I suspect the thermostat, so I pull it expecting it to be in the stuck OPEN position, but it was fully closed. I test it in hot water and it opens when the water hits 180 degrees as it should.

So, what are the next things I should be looking at? Keep in mind that it was running hot right until I changed the coolant and the drain cock. Faulty gauge, faulty temp. sensor, something else?

- Jim

Something else.

Unless you knew exactly what you were doing when you re-filled the radiator, there's a good chance that you left a big air pocket in your cooling system.

An air pocket will compress right behind the closed thermostat and apply "air temperature" to your temperature sensor, rather than true water temperature. When the thermostat finally opens, the air bubble will move to the top of your radiator (you'll hear bubbling). But unless you quickly refill the (empty) reservoir at that moment, the bubble will remain in your system and your radiator will remain less than full.

When the car is COLD, open the radiator cap and check the fluid level. If you're lucky, the air bubble (empty space) will still be at the top of the radiator and you can simply top it off.

If the radiator appears full, run the car with the cap off until it warms up. You will see a surge and splashing when the thermostat opens. Stand back and don't interfere. After the gurgling and hot liquid splashing stops, you can step in while the car idles and carefully pour warm coolant into the reservoir or radiator until the level reaches the full point. Recap your radiator and mop up the mess.

Your temperatures should then look actual, since the air pocket will have been purged from your water jacket and your temperature sensor will be reflecting coolant temperature, not air temperature. Your water temps were probably always normal or slightly above normal, but the sensor was only reading hot air temperature.

We had a full discussion about this in Tech Talk awhile ago.

http://forums.ctzcc.com/viewtopic.php?t=7529

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1970 240Z

Thanks Frank...as usual your help is clear, convincing and thorough...as soon as I get a new gasket, I'll reinstall the thermostat and follow your procedure.

- Jim

OH! Well, as long as you still have the thermostat off, you could pour as much coolant as possible directly into that port, thereby reducing the size of your air pocket considerably. That would save time and trouble when you did the full operation, since the air pocket will have been reduced in size dramatically. You could even use a turkey baster squeeze tube thingy, like i do. Just don't put it back into the kitchen for food use after that. Wives have killed for less.

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1970 240Z

The therm. housing is already full to the top with coolant...would that be expected if there had been an air bubble trapped behind the closed thermostat?

No, but there still could be a lesser air bubble in the rear of the block, near #6 cylinder.

But that wouldn't affect the temperature issues you described. It's your thermostat's job to keep the engine operating at the prescribed temperature as consistently as possible. If it got too cold, the thermo should simply close until the engine heated up. If it got too hot, it should simply stay open until the engine cooled off.

If your radiator is full, and your thermostat housing is full, and your reservoir is at the full/cold mark, I'd say your water jackets are at least full enough to let you operate the car under any normal circumstances.

You could hold a kid's thermometer against your upper radiator hose (wear gloves) after it warms up and compare that reading to your dashboard temp gauge. Like John T always says, these old gauges were never very accurate to begin with, and time certainly doesn't help them. Shut the engine off to avoid the fan when you check that way, pleaZe.

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1970 240Z

Q: Where should the "180*F" mark be on your ZXT gauge?

On the S30 gauges it's dead-center btwn the 120F* low mark and the 250*F high mark. (The difference btwn 250-120 = 130*. Divide that into halves and it shows 65* on either side of the center mark. 120 + 65 = 185*F.

If your car ran hot while dripping coolant, and is now running cool, you should try to determine "how cool" your car is. (No snide comments).

Do the math and figure out where the 180*F point should be, then compare that to where your needle usually points. If you're not too far off, I wouldn't worry about it. If it's drastically off (1/4 of the gauge or more), it would be a matter of concern and correction. You've wisely tested your thermostat already, so you have a known-good constant to work with.

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1970 240Z

Hi Frank:

On the 280ZX water temp gauge, the extreme points are 120 deg. and 270 deg. This would mean that the midpoint on the gauge is 195 deg.

Most of us 280ZX drivers expect to see the needle a bit to the left or right of center (that means approx. 190 deg. to 200 deg.).

Yesterday I followed your procedure: took off the rad. cap, started the car and let it idle for about 30 minutes. Eventually I saw some bubbling in the radiator which eventually subsided. This morning I drove a few local miles and temp gauge maxxed out at about 170 or so...not too far off from the 180 deg. thermostat trigger point.

So, all in all, I think everything is good - thanks to you, of course. I'm waiting for a hot day to see how high the water temp will go...but that may be many months away...

Thanks for the advice and the education!

See you next Wednesday for our first meeting back at the all-new Miller Nissan in Fairfield!

- Jim

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1970 240Z