Strange Engine/Cooling Issue 996

I have a 1999 996 C2 coupe, which currently has about 90k miles on it. Not first owner, car is a daily driver, well, it was until an issue came up that has me and others puzzled. I recently had service on the car to replace the clutch and to address a check engine light that was active. Shortly before the car went in for service, I noticed that as the car would warm up, right when it reached 180 degrees, that the coolant warning indicator would start blinking. Checking the coolant level was strange, you could easily see that there was a very low condition when looking at the overflow tank (which had been changed about a year ago due to the famous cracking...). Once cold, the level would still look about a quart or two low. When compeltely cold, I would remove the overflow cap, and there would be a large "whoosh" of pressure, and then there was a lot of coolant movement. You could hear it moving in the engine itself, and then, the coolant level would end up right where the fill level wanted it to be. Only thing I could think of was a bubble in the system somehow? Didn't know what to think about it, but given that the car had a very bad clutch, I couldn't drive it enough to really see what was going on. Took to a local Porsche shop that a good friend recommended. Focused mainly on the clutch and engine light, they did the work and replaced the clutch, spark plugs, and checked engine codes, etc. Only code they reported was a lean burn in cylinder #2. Once they reset the codes, the engine light has been off since. With that out of the way, they started to look into the coolant issue. No leaks, anywhere. Coolant pressure check was good. No real evidence of coolant in oil, or vice versa (they did an oil change). When starting the engine cold, their finding was that the coolant system built pressure very, very fast, inconsistent with what you'd expect from "just" the thermal expansion. They had never seen this before, and the only thing that they could theorize was causing the issue was a small crack in a head, perhaps a head gasket leak (early), or a crack in the block, allowing cylinder pressure to get into the cooling circuit. In the end, with no other real ideas on how this could be happening, so we jointly decided to throw a bottle of high end coolant system sealant in the car to see if it changed things. Keep in mind that until this point, the car had been getting minimal miles for about 3 months, as I had to save up for the damn clutch swap! Their feeling was that if this helped the situation, that something ugly was begining that would only get worse, probably fast. So, I took the car and started to put some miles on it. I got about 20 miles before the coolant light came on, blinking, but with normal operating temps (never really ran that hot, but I did notice that it had been running up around 200 or so, but stable). Now, however, the coolant was pumping out of the overflow relief on the plastic tank. By the time I got home, and let the car cool off, same issue with the response from the cap removal: whoosh, coolant movement, but now, the coolant level actually WAS low (obviously, with the coolant coming out the relief). So whatever was begining to let go, has let go. Car still runs really well, no engine light, but pumps coolant out routinely. I just checked the dipstick to see if the oil was milky, didn't look like it at all, but I realize this is not conclusive. I ran the car from cold to 180+, with the cap off of the overflow tank. Didn't see any bubbles, and the level stayed where it should, rising as the temp got hotter until it finally wanted to burp out a bit due to thermal expansion. Had the wife rev the engine up with the cap off, hot engine, and the level would drop with RPM (assuming that the water pump is just increasing flow rate), came back up when returned to idle. Now the car is sitting until I come up with an action plan. In the worst case where it is an engine or head gasket, the shop basically told me that the best path would be to replace the entire engine with a rebuild directly from Porsche, as the labor to do a head gasket (if that was the issue) relative to the whole engine was not really worth it, considering that it is a 99 with known engine issues to begin with, and further that it has 90k miles. Any and all experiences or ideas would be helpful. With the way things are in the economy, etc, I don't have the coin to pay someone to diagnose further, so I'd like to do as much myself as possible. I have a good amount of experience with engine work on older American V8 cars, nothing with the Porsche though. Help!?! Mark

Look into "bleeding your coolant system. There is a procedure you must follow to get the air out of the system.

Yeah. I agree with the last poster. I bought a 996 used from a dealer who said they had done a bunch of work on the car to get it ready for sale. A few days after I took it home, the coolant light started to blink. I took it back to them and they fixed it no problem. Said it had something to do with air bubbles in the system that cause the light to blink. I know that's not too specific, but hopefully reinforces the idea that you have air in your system that's affecting the sensor.

On the coolant tank there is a round black part looks like a cap with a metal bail pull the bail up that allows you purge the air from the coolant tank. What I do is let them idle with the blue cap on and with the bail pulled up this will allow any air out of the system allow it to run till the front fans come on. Also do this with the heater on full blast it allows all the air to escape the system. If none of this works its time to check for combustion gases in the coolant.....

I have a similar problem with my '99. I was losing about a pint, er 460ml-sorry, every month. Took it to local independent, no evidence of leakage, pressure test OK. No coolant in oil, etc. Took to dealer for $900 coolant tank swap. Problem solved for about four months then started again. Returned to dealer, tank OK, pressure test OK, no coolant in oil, etc. My solution- add coolant every two months as needed and trade car in ASAP for '05 or '06 997.

This also could be a bad cap letting coolant out the overflow hose. And air of course.

Typically, head gaskets never fail. Overheating and continued driving will crack the head. A visual oil examination isn’t good enough at this stage. Have a sample of oil sent to Blackstone Labs. Also check the coolant reservoir cap part number. They revised the caps from original and I believe the pn should be a -04 last two digits.

Ah! Just noticed this was a 2008 posting. A bit late on the fix by now 😎