islandflyin

Hi guys,

I am a new 996 owner and I thought I had low coolant. however when I opened the cap, air pressure was released and the coolant level went back up to max. Now after running for a few days I see that the level is back below the low line. I assume that if I release the air pressure the level will go up again.

Is this common? Should I add more than the max line so when pressure builds in the system the level will be within the low/max lines?

Thanks

islandflyin

So I take it all of your tanks don't build air pressure? Does your coolant level stay between the lines ( min/max)

Trying to figure out if I have a problem here.

Thanks for any help,

Jeff

Macster

Never hardly ever bother to remove the cap. I give the tank level a check once in a while (great while) but that's about all.

The only time the system should develop positive pressure is when the engine is running and the coolant gets hot. It is pressure that keep the coolant from boiling/forming steam pockets when the engine is at operating temperature.

If the system sheds pressure when hot (and along with shedding pressure losing water vapor) the system can develop negative pressure as the coolant cools down and the positive pressure declines.

With the engine dead cold top of the level with distilled water.

Be sure you put the cap on properly and securely. Sometimes where the cap screws and must seal gets a bit of anti-freeze residue that can interfere with the cap sealing properly. You can wash this with hot plain water and a plain old cloth towel. (I pick up a bundle of white cotton rags -- cheaply made about the size of a wash cloth -- at the hardware store or Walmart store for these kind of jobs. After awhile I just toss the rags when they get too dirty/stained.)

Before you put the cap back on my advice would be to if the part # ends in "00" to replace the cap with a new one, with a part # ending in "02" or "04". (There may be an even newer cap with a higher part #. I have not kept abreast of coolant cap releases.)

The original cap will almost certainly develop a leak that can and will have the coolant level in the tank dropping and can possibly result in the tank (the cooling system) manifesting anomalous pressure differences.

Then watch the level closely. If it drops there's a cooling system leak. After the cap the most common leak is the tank itself, almost always from a split that appears underneath along the seam of the tank.

Course, you have to give the entire cooling system a careful inspection just to be sure the leak is not say at the water pump, or with a radiator. Hoses too come in for suspicion. With the 996 the AOS can leak coolant too.

fpb111

Never remove the cap when the engine is warm only after it has been off for 4 or 5 hours. All new cars run pressurized cooling systems.

dgjks6

Maybe.

Do what Macster said. The 996 does not have a true expansion tank like my Grand Prix that I can open hot or cold. If you remove the cap when the engine is even remotely warm you will get a hiss. If it's hit you will get a splash.

The next part is not from 996 experience but Grand Prix experience. If the coolant level is low, how much coolant you see in the reservoir will change depending upon operating temp.

So as Macster said, fill it up and put on a new cap and watch.

Hurdigurdiman

The pressure relieve valve on top of the tank may have something to do with it. MAY being the operative word. If you can lift that pressure relieve valve to the open position and close it by hand without much force, then I take it that the system is working correctly. I haven't checked my level, when up to working temps, since changing a leaky water pump and changing the original thermo to a lower temp opening type. Before the change I would find an egg cup full of coolant mixture under the pump on the garage floor and needed to top off the expansion tank about once a week. Not had to top off the tank since the change. If you are not having to top off the expansion tank, then I would think your system is working correctly. If you have, then you obviously have a leak somewhere. I would NOT top over the max line. That is only asking for trouble if the relieve valve is jammed and not opening. That would split the expansion tank for certain as the coolant expands at working temps. BTW a water pump on these cars lasts for approx 70,000 miles. The water pump maybe developing a slight leak around the bearing as mine was.

islandflyin

Very helpful guys!

Thank you, I topped up today and will watch. The previous owner had a new tank installed last year with the current one so I am guessing there was a bit of air in the system. I have never seen a leak on my garage floor or seen any high engine temps.

Thanks again.

J

fpb111

996 can trap large air pockets in the cooling system. There is a relief valve built into the blue cap on your expansion tank. That allows pressure relief. That is why when the cap fails you will often see evidence of dried coolant in the tray under the cap.

The "Expansion" valve on the black "cap", with a square rod and bale attached, above and behind the blue cap is really a vacuum break/purge valve that allows one to purge air out of the system as it cools.
When the engine is hot the square rod rises taking the metal bale with it. If you snap the bale up it will hold the rod up breaking the vacuum as the engine cools, allowing the tank to equalize pressure and air to escape.

islandflyin

This is what I need to know! Thanks, I appreciate the help!

J

fpb111

Also coolant should be at or just above the "Min" line when cold and at the "Max" line when hot. If you fill to the "Max" when cold it will "Burp" coolant out of the pressure relief in the Blue cap until there is some "air cushion" in the tank.