Strange Engine/Cooling Problem 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

when they filled it with coolant... did they "burp" the system?

Suspect your coolant tank cap has failed - it's not working as design to maintain correct pressure in the coolant system. Get a replacement new model # cap from your local dealership - and see how that works for you. If you had a cracked head, you'd most likely see oil in the coolant - and that doesn't seem to be the case here.

I suggest you start by checking the coolant tank cap. If the part number ends in "00" then it needs to be changed to an "01" or "02".

Even if it's been replaced once, I've had the new style ones fail after a few years - so for the few bucks it costs, it's the right place to start.

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The coolant cap can be tested. Takes a couple of seconds at a dealer or indie who has the adapter.

This is a way I think you could rule out a cracked head or seeping head gasket. Ensure the engine is cold, then start the motor but do not rev the motor. Let it run for a few minutes then shut it off. It should not warm up enough for normal heat expansion but the cylinder pressure should pressurize the cooling system IF that is the cause.

Coolant needs to be tested for the presence of combustion gasses. There are consumer-grade kits for this.

Guys, thanks for the help on this one. Had been travelling last few days with no ability to respond to the responses above. I tried replacing the cap yesterday evening with a new one. Old unit ended in "01," as did the new one from Porsche (new one had a manufacture date of 05/08 on the plastic parts bag). At first glance, it seems to have worked, I filled with water to the correct level, and then ran the car for a full warm up, and thereafter a five mile run (not enough time to have done more last night). Previously, this would have resulted in significant discharge, but last night? Not a drop. Will check levels again (cold) this evening and retest, but it seems at this point that this might have been the issue. Easy to look past the simple things, and I now wonder a bit about how deeply the shop looked into this. Also, I had replaced the cap less than a year ago when the tank was replaced, so it went bad in less than a year, which surprises me, but I'll take a $22 cap over a teardown ANY day. Hopefully this solves it. If so, I will likely do a full flush of the coolant and refill, as with all of the water I added in this exercise probably diluted the ratio (even though it doesn't get below freezing where I live, I have recently witnessed a very nasty failure on a different application by using the improper coolant in conjunction with an aluminum head vehicle, never realized how critical the composition is....). I can tell you, that seeing this was an extremely welcome outcome, I'll make sure it is settled, but this couldn't have come on a better day (tax day for those of us that file extensions...., and it was my birthday yesterday, nice present!!!). Thanks again team, very much appreciated. Mark

Glad it worked out. Happy birthday

Awesome...please give us an update in a few days. I replaced my cap with the one ending in 01 about two years ago, 40k miles ago, maybe I'll buy a new one for good measure.

You acted like me, start at the worst and work back to realize it was just a blown fuse...... It happens though, especially when hearing the problems some have had.

Where is the Pic Police? Paging Tim..

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Mean 69, Hopefully this solved your problem. Did somebody call me??? OK, here goes..... Ahem... < Tim clears his throat > Mean 69, Welcome to Rennlist. It is customary for Newbies to post pics of their cars shortly after posting for the first time. It "sounds" like you have a nice P-car, but we can't tell because we can't see it. So.... come on, Newbie, post the !@#$%^&* pics! Remember, this is all in good fun, and once again Welcome to Rennlist!

Ha!!! Understood, damn newb's! I'll get a few shots to post of the car this weekend, desperately needs a bath. Good news continues, drove the car to work today and an additional ~80 miles, looks like it needs a slight top-off, but there is no leakage, and no overheating, etc. It is so nice to drive the car again, my "other" dd is a Dodge RAM extended cab, long bed, diesel pickup, if you live in So Cal you know what a pita it is to deal with a huge vehicle. I hate to ask a question before I have investigated on my own, but is there any magic in flushing the coolant system out (home garage)? One central drain outlet on the car, or do I need to find a way to get it out of the front mounted rad's too? Thanks, Mark