Hi New here, recently purchased a 1999 986. Have two possibly related issues with the cooling system.

First the light is blinking on the dash next to the temp gauge. Coolant level is fine, temperature is fine. Could this be related to the below, or another issue.

Next the annoying one the coolant reservoir cap does not tighten, the thread starts to grip for about an eight turn so it will stay in place snug-ish but if I tighten any further pops off. Certainly not tight enough to hold if any pressure in the cooling system. The cap is the blue 996.106.447.04 part. I saw some other threads saying this is a replacement for 996.106.447.00, and the original fits better. The seller noted that the coolant tank was recently replaced. Figured that the cap used for the new tank is not compatible. Appears to be new, clear new looking plastic. Should I be looking for any original 996.106.447.00 or is there another aftermarket cap that fits the replacement tank.

Thanks,

 

Get a new cap. They come in blue and black. Not sure what the difference is but I think the original is blue. Strange it won't tighten. Hopefully it was an OEM tank they installed. That's a PITA job.

Flashing light means low coolant level. The sender/sensor is at the bottom of the tank. Possibly they forgot to plug the harness into the sensor when they replaced the tank. Pull the carpet back that should be obvious. It's small two prong connector.
I cannot recall if a new sensor was included with the tank or if I had to transfer it over from the old tank. If so also possibly they forgot to transfer it over and your new tank has no sensor. If that is the case unfortunately you have to remove the tank to install the sensor as it snaps into the bottom. Beyond that maybe a bad sensor on the new tank.

I don't know if aftermarket tanks are a thing, but if so, certainly possible a knockoff tank was used and this could be the issue for both problems too.

EDIT - I just read elsewhere that a fault in the engine compartment cooling fan can also cause the coolant gauge light to blink, so that's another thing to look at if the sensor checks out OK.

Cut pasted from Renntech post:

Four functions of the coolant warning light:

1. Engine coolant level too low

— light flashes slowly (0.5 Hz)

2. Engine compartment temperature too high

— light flashes slowly (0.5 Hz) (engine compartment blower might be faulty)

3. Engine coolant temperature too high

— light is lit; pointer on the right

4. Temperature sensor at water outlet faulty

— light flashes rapidly (1 Hz); pointer on the right

Note

The temperature warning in point three is indicated if the conditions "engine coolant temperature too high" and "engine coolant level too low" are present simultaneously

(1Hz = 1 flash per second. 0.5Hz = 2 seconds per flash)

So it might be the temperature sensor is faulty (if the light is rapid flashing).