Cooling system leak

So I recently did a bunch of work on my '99 C4 Coupe which stated when the AOS gave out. Got everything back together and the car has been running like a champ for the last few weeks. I use the car as my daily driver but most of my driving is between 40-60mph... I don't frequently get out of the major interstates to do 80+ mph. Yesterday I was able to get out and get it up to speed... averaged 80-90mph in light traffic. The car was running beautifully. In my rear view mirror I could see the car behind me using his windshield wipers every few mins... which I thought was was weird. Then the 'low coolant' light stated blinking and eventually the temp started to rise... maybe around 200 or so. I got to my destination and could smell the coolant on the engine and saw a big puddle under the car. GREAT! The coolant container was completely empty. I dropped in a gallon of 50/50 mix that I bought at a nearby gas station so I could make it home. After about 10mins of driving the low coolant light came on again and the temp started to slowly climb. This time I stopped at Walmart and picked up 2 gallons of distilled water.... dumped 1.5 in and headed home. This time, the temp didn't budge and the low coolant light never came on.... almost as if it had fixed itself. I got home and put it up on the left and found coolant everywhere underneath making it hard to tell where it came from. It was still dry near the back so probably not the water pump... most was on the drivers side. My suspicion is that the coolant container has a crack in it (seems to be a known issue) but that it only leaks under extreme pressure from high speed driving. Anything else to check before dropping a few hundred dollars and replacing the container? Just had it out when I did the AOS and it looked OK... Thanks, -JRW

Your story is similar to what happened to me when my waterpump died. When the engine was fairly cool, it didn't leak enough to be noticeable. But once the temp got up and the system pressurized, it was enough force to let the coolant out. In my case, the water was escaping from the water pump shaft (bad seals, lots of play in the shaft once I removed it). Have you gotten under the car to look for signs of dried coolant yet? If your car is a '99 and you're still on the original coolant tank, I would definitely start with that since they're a well-known issue. Even if it's not the tank leaking right now, it's good preventative maintenance.

I've only owned the car for about 6 months. The previous owner had it about 6 years but only did about 10k miles during that time. He also stated that he had done a water pump and AOS during that time.... but of course I was doing an AOS a few months after buying it, so who knows. My guess is that it is the original tank and I've had it out before so replacing it shouldn't be a big deal. It was just so hard to see where the coolant was coming from and then it wasn't leaking when I had it on the lift. Maybe I'll just start there and then go for a high speed drive! -JRW

I just had the same problem on my 2002. It was the water pump.

Hmmm... maybe I should just do the water pump then. I checked out the guide and video on rl.org and it looks fairly easy. I'll order a new pump and gasket from suncoast... should I just do the thermostat too while I'm in there? Thanks, -JRW

The water pump is good preventative maintenance, you might as well do it if it's the original one or has more than 60k miles or so on it. Easier to change it at your leisure than be forced to do so when it craps out on you somewhere.

don't forget to check/replace lines and hoses also. as already stated above, do da pump

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You can do a simple pressure test. Be sure the cooling system is full and has no air pockets. Start the engine and drive the car around town but stay close to home until the engine is up to temp. Ideally you would like to hear/feel the radiator fans come on. At home, raise the rpms up to around 1500 and hold the rpms there until you hear the fans come on. Shut off the engine. The cooling system experience high pressure (high for a cooling system) and any leak or leaking item will show itself. Be careful when you are around the back of the car and the engine compartment lid is open. If the coolant tank is bad finally fails outright it could spray out hot coolant. It would be nice if you could view the water pump but my WAG is the coolant tank is the culprit. That's a lot of water loss in a short time to come from a bad water pump. The seals would have to be almost gone, if not entirely gone and I think the thing would be making noise. If it proves to be the coolant tank if you do the water pump at the same time is up to you. And the same applies to the tank if the water pump proves to be bad. The water pump in my 02 Boxster lasted 172K miles but not every water pump is so blessed with that long a life. The coolant tank lasted longer, I might add. Sincerely, Macster.

I posted a few weeks ago that I was having a simular issue with my 99' c4. The car was running fine. I haven't driven it in a few weeks since the initial post, so I decided to have it towed to a Maryland shop (Autobahn Pro) who determined that there is antifreeze in the oil and as a result, I believe he said " the number "1" head is all likely warped. He suggested that I take it to the dealer for a second opinion, but thinks I may need a new/rebuilt engine costing approximately 4/5 thousand. I need to make a decision soon, so any feedback would be greatly appreciated. Thanks

UPDATE! So I decided to replace the expansion tank in hopes of curing my problem. As we all know, its not a hard but its a major main in the ***! After I got the new tank in and connected the coolant lines, I noticed that the small line that goes to the AOS was disconnected from the AOS at the back of the engine.... WTF?!?! I just replaced the AOS so this was obviously my fault. Not sure if this was where the coolant was leaking from or if I pulled it off during my struggle removing the old expansion tank. THANKFULLY, I was able to reach in back and attach the line and tighten the clamp without the major surgery that was required when replacing the AOS. Everything seems to be fine now. I'm still getting the low collant light on ever couple of days and need to fill the tank as the system bleeds its self, but nothing is leaking so I'm happy! -JRW

JRW, I have a leak that appears to be coming fromt the top of the AOS. Fresh coolant is dripping down the AOS belllows so I'm thinking the leak maybe coming from the same location as yours. How did you access the hose and/or clap?

Sorry for the hijack, but I was preparing a new post when I read yours

No problem... I really just wanted to update the post so everyone would know the result if they found the thread later in a search. Anyway, I am a tall skinny guy... which means I have long skinny arms... which are required to do this. First, the top of the AOS where the coolant line connects seems to be fragile. I broke it off when I was taking out the old AOS. If its cracked or broken, you have to replace the entire unit... have fun! If the hose came off cleanly (like mine did), you should be able to reach in above the intake and slide the hose on. I used a clamp that tightens with a screw instead of the stock clamps. After attaching the hose and sliding it all the way onto the AOS, I reached in with a small socket and tightened the clamp ... the screw part of the clamp should be on top of the hose with the screw facing the drivers side of the car. This was on a '99 6-speed coupe. Not sure if other years and/or models will allow this. Good luck, -JRW

Did you go in through the alternator opening or from another point in the engine bay or from underneath?

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I went under the intake (above the alternator) to feed the hose through. Then I reached in over the intake to put the hose on. There are other lines above the intake... carefully slide them towards the passenger side of the car to buy yourself another inch or two or room. During this process, I scraped the back of my arm against the top of the car.... that's how tight it is. I'd start by seeing if the hose is connected at all... a little tug on it at the expansion tank should tell you that. You can also see it with a flash light. If its not connected, pull it back through the engine compartment and see if its just the hose... hose and clamp... or worse yet, hose, clamp and part of the broken AOS. -JRW

Thanks JRW for your time and willingness to help I'm heading back outside to the garage to verify either a broken line...but I suspect it is the AOS. I've not worked on the back of the engine, so the odds of a hose being the cause of the coolant leak is minimum. In addition, the AOS is a known failure possibility and all of the post’s I’ve read on Rennlist and Renntech point to the AOS failure.

FWIW to others reading this thread and wondering how long an AOS lasts: 1999 C2 6-speed (in case you didn't see my signature) My personal experience with the AOS is that I had to replace it in 2006 when the car was 7 years old and had 24,000 miles on it. I had do it again here in 2012 when the car had 84,000 miles on it. So the second one (updated) lasted 60,000 miles. A single data point, not a guide post. In 2009, I elected to replace the coolant resevoir rather than wait for it to fail. The cars was over 10 years old and had 46,000 miles. In late 2010, I elected to replace the water pump rather than wait for it to fail. At that time the car was pushing 12 years old and had over 60,000 miles on it. I also elected to have the bearings and pulleys replaced as well since they were doing the sepentine belt. My theory is that I would rather replace something proactively, than wait for it to fail at a very incovenient time and location. The XXX and YYY will be replaced with the proper upgraded parts when I do the clutch. Like most stuff that goes wrong on these cars, whatever it is can happen at any mileage, any age, and can happen again at any mileage and any age, or may never happen (again) at all, if ever.