I am having an intermittant issue with coolant overflow out the overflow tube shortly after shutting off the car.

I am in central Texas. I am running 2 year old G05 Coolant, 50/50, with water wetter. Reservoir cap is a year old. Reservoir is a bit yellow and I have a new replacement on hand. I am running the dual electric fan system.

Temperatures reach to just below the second white line while stopped and are typically mid range while driving.

Thermostat has also been replaced in the last 2 years.

I have read in other posts that the cap and condition of the reservoir itself are frequently cited as potential issues which is why I bought the new coolant tank.

What else should I check?

 

Start with a new cap.
Was it run hard and got exceptionally hot when you had these episodes ?

Edit: I read to my mind 'Reservoir cap is old'.

 

Cap and reservoir are also likely culprits. At the same time, ensure that all hose clamps are tight too. Air may be getting in the system.

If what was recommended to you above doesn't solve the issue, go to Sears and get a cooling system pressure tester. Hook up the pressurizer to the reservoir and pump it up to 10-11 psi. Walk away for about 30 minutes, then return. If the pressure still remains, you can rule out internal leakage. However, if the pressure drops and you see no external leakage, this could be a sign that the head gasket has failed on one of banks and is causing the compression to be leaked out of the bores, pressurizing your cooling system to push out antifreeze.

Don't ask me how I know....

 

Autozone carries a loan out of this if you don't want to invest (there is a $$ deposit though)

 

Behr OE cap?

 

Cap came from 928'sRus so pretty sure it is. I have another on the way.

 

Hopefully that will fix it....

Make sure the reservour isn't overfilled, when cold. If there is too much coolant, the expansion of the coolant from cold to hot will reach the top of the tank. Fluid will be pushed out, until the "proper" level is reached.

 

I drove the car today to the Austin monthly gathering with no changes made.

OAT was 90ish and I was running the AC. Made several stops and did about 50 miles of stop and go in town driving with no overflow at any stop despite the temp running at the second white line - I hate intermittent issues!

I still plan to replace reservoir, cap, thermostat and drain and replace coolant when I get a chance.

I assume if it were a head gasket that I would not see the issue intermittently?

 

I can't see the T-stat being an issue if the engine is not running hotter than usual, but, sure, if it's old, go for it just on principle. I'd just replace the cap if it were me. If the overflow is just going out the overflow tube, the cap is responsible for stopping that. What you are seeing doesn't surprise me. When the engine is running and the coolant is circulating, the coolant temp at the bridge and the system pressure are lower than just after you stop the car and all the heat stored in the block is transferred into the coolant without the benefit of the radiator. This is why if you remove the coolant cap from a running motor, not much happens, as the coolant is below its boiling point, but if you then stop the motor, the coolant will begin to gush out of the reservoir in a minute or two.

 

Problems with higher than spec temps during normal running can be due to the old radiator just filled up with scale and whatnot. Replaced the radiator in my 944 with a brand new one and it was night-and-day difference.