Coolant overflow - 986 Forum - The Community for Porsche Boxster & Cayman Owners

I am losing my damn mind here A month ago, on first drive after extensive modifications (none to the cooling system) I was running hot and spewing coolant out the overflow tube. I drained the system, replaced the TStat with a new BW low temp TStat, refilled, burped, all the good stuff. While the system was empty and the hoses unclamped, I stuck my scope up in the water pump (original pump) - impeller blades looked fine. Took the drive belt off - no play in the bearing. I also put a new cap on for ****************s and giggles (.04 version, replacing a .04 version, just because). Also, convinced that I had cracked a head during the mods, I tested the expansion tank for exhaust gasses - none present. After that, no more overheating! Back to running on the 1 of 180. But I am still spewing coolant out of the overflow tube! Looking at the caps in the trunk, when the system is hot, everything looks fine and there are no leaks. There is also no coolant in the trunk anywhere. What the hell is going on here? It's like the system is over pressurizing. I'm at a total loss here. I've ordered a pressure tester to see if I can find any leaks in the system, but I don't think that's the issue. I've also ordered a new burp valve and a new cap, purely because I don't know what else to do. Is there a valve in the overflow tube that could have gone bad and now the thing leaks anytime the system is pressurized? Does anyone know exactly how that works? I've scoured the e-katalog and fsm and there's really no explanation of it in either of those documents. Any insight here would be greatly appreciated. I don't have any hair left to pull out. Best, Ike Sent from my POCOPHONE F1 using Tapatalk

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Have you tried letting it burp without topping it off to see where it settles or if it settles????? Interesting that you run on the 1 of 180. Does you needle move at all???? You could hook up your OBDII scanner and watch what the temp is doing during a drive. The 1 on 180 would be very low temp om my car. Like 150-160. The needle should actually move dependent on condition. I know you tested for it but it sounds like a head gasket leak. If you have not done it the next thing to do is pressure test the system.

You still have air in the system. Perhaps try the lisle funnel? Saved my bacon more than once

Blue- I was worried that you would say that. I'm still worried about the head too. Pressure tester is on the way. Yes, my needle works. With the low temp TStat, I have found (with both ln and bw) that high speed runs on wide open roads with no stop and go results in a coolant temp around 160ish. Stop and go is different, 180ish. Shutting down immediately after a fast run will also run the coolant temp up to 190. Lemans - What a Lisle funnel? Is it supposed to reduce bubbles while pouring fluid? I truly don't think there is air in the system. If there was, the symptoms would be overheating followed by overflow. The anatomy of the overflow tube confuses me. According to the fsm, the system should be pressured to 1.3 bar. How does it do that if there is a tube stuck out the top of the tank without some sort of restrictor valve on it? Sent from my POCOPHONE F1 using Tapatalk

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Have you checked your bleeder valve? I was dumping coolant but not running hot- a bad o-ring in the valve was the culprit. Easy cheap fix.

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Your temps run lots different then mine. I have a low temp. thermostat. Takes longer to get to operating temp but doesn't operate much different then it did with OEM thermostat. Have you watched the coolant level in the overflow tank while letting it run up to temp. Usually with a bad head gasket coolant level in the overflow tank will rise and burp. Just like you have air in the system, but it just never stops doing it. What is the color of your exhaust at WOT and deceleration.???? Anything look odd?? Like whiteish???? or steam??? Once you do a pressure test you will have a better direction to go. Could be something simple like the burp valve.

If worried about a HG drain your oil, is it intermixed? coolant intermixed? If not it isn't going to be HG

I would drain the coolan and then fill. Use the tool in this video, always does a great job. https://www.youtube.com/watch?v=IwELkpDMuV8

Similar issue with 98 Boxster for the past year. Pre-Covid - I noticed coolant coming from overflow. Took it to indy mechanic Replaced the cap with OEM cap - Performed pressure test, burped valve, etc...and everything passed with flying colors. Kept it their overnight - mechanic drove it around the next day, performed the same tests with same results. Occasionally small amounts of coolant started coming out of overflow again - Indy mechanic doesn't want to charge me the labor to investigate more or replace the coolant reservoir. I have to respect that. I top it off at least once a month.

Blue, when I ran the OEM TStat my temps were cosntantly in the 190ish range. Never past the 0 but they are definitely lower now than what they were. Mind you that was also under a very different driving circumstance - then I was in a big city, deep south so hot as balls always, with very little room to run. Now I'm in a cooler climate and air is usually running through my radiators at 90-110 mph when I am running systems tests Yesterday I ran twice with the burp valve open and no overflow. Definitely steam coming from the top of the tank though, hard to say exactly from where. Slightly sweet smell in the cabin with the top up (toward end of the drive). Fun note - over 60 mph we can drive in the rain with our tops down! Thanks science! This morning I ran it with the burp valve closed and again no overflow. Slightly sweet smell coming from top of tank but no steam. Same smell from passenger rear wheel well but no overt fluid loss or steam. Despite losing what looked like at least a liter of coolant two days ago on my driveway, the coolant reservoir is still completely full. No funny looking or smelling exhaust under any circumstances since I have put high flow cats on. I recently drained and flushed the system, and did oil change. No evidence of intermix on either side. With that being said, some years of subaru h-6 (i forget which generation) have head gasket failures and they dont usually see intermixing. New burp valve is on the way so that will be getting swapped out for good measure. Pressure tester is on its way and I will post results. Maybe all I have done is overfilled the coolant reservoir (although I'm not sure how I would have been able to physically do that) and the dump tube was just doing its job. If that's the case I will be thankful for the mistake.

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That is why I asked if you had allowed it to find a settle out point. The issue could be as simple as the coolant needing expansion room. Or the burp valve could be faulty. A system pressure test should tell all. The Idea that if you don't have intermix of some kind then you don't have a bad head gasket does not prove out in my experience. What I have seen mostly with a bad head gasket is either a whiteish (steam) exhaust or the overfill tank acting as if there is air in the system. Or both. You get air being pushed back into the coolant system by the piston. So the overflow tank gurgles and air pressure pushes coolant out. Sounds like you have a great driving area. I have The Columbia River Gorge and lots of back roads in the Gorge area. Canyons, Mountains, flats. On the back roads I see very few other cars. So I can play or just cruise Let me know what you find with the pressure test. I am always interested in results and outcomes.

Hi Ike! I agree with not being able to determine HG failure through intermix reliably. My '09 Impreza has a HG leak on bank 1 with no symptoms other than a minor oil leak. That said, without combustion gases present in the coolant, it seems unlikely that your current coolant issue is related to a head gasket. Anyway, having just changed out that coolant tank myself a few months back, there's only 3 real possible ways for coolant to get to that overflow tube. 1) Coolant cap 2) relief/burp valve or 3) (VERY unlikely) a crack in the top of the black plastic. I think the burp valve will fix it for you! Pullin' for ya over here!

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Fan, dude! You've seen inside the belly of the beast! Can you please explain to me how the overflow valve communicates with the main tank/coolant system? I hate not understanding stuff, and I can't find any diagrams or descriptions of what that tube actually does.

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Hey Ike! Yea, I have the Katalog as well and it doesn't go much into it. So far as I have gathered, it can (mis)function like a relief valve. I pulled the old one off of my original tank because it was working fine. It truly just bolts to the top of the reservoir with a small passage that allows it to act like "the highest point of the cooling system" similar to an old radiator cap so you can burp the system properly. However, the gaskets have been known to fail and there were a couple of threads (I cannot find them right now) that showed the valve allowing coolant out onto the top of the tank and that coolant would make it's exit out of the overflow tube. Here's a pic of my old one: that black plug in the center is rubber and the spring force holds it closed until you use the metal tab on top to open it to evacuate air. Best I can tell from what I've heard, the rubber on the center plug gets tired or, as mentioned earlier, the rubber gasket can fail. The rubber gasket isn't shown in the pic because I disposed of it, expecting to order a new one if I ever needed to swap this valve in. Let me know if you need more pics. I disposed of the old tank or I'd have taken a picture of the port where the expansion valve connects to the tank as well. Good luck!

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Sorry fan, I think I misworded my question (although I appreciate the knowledge on the burp valve too! My new one is coming in the mail so the more I understand about it up front the easier that job will be). You mention the gaskets - are you referring to the two o rings that sit between the valve housing and the tank? My main intrigue is actually the overflow tube (not valve) which is the open ended tube that comes off the tank (if I remember correctly it's the lowest hose coming off the manifold) and opens to atmosphere. If the system is pressurized then how does this tube not just constantly do gas and coolant? You may have alluded to it acting as a drain for the top of the resivoir tank... Does that mean that it's only purpose is to dump coolant that comes out the burp valve or around the tank cap? Thanks again for the time (and effort! Love the pics) in helping me! Sent from my POCOPHONE F1 using Tapatalk

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Hey Ike! My bad, sorry yes I misunderstood what you were looking for. When I said gasket, I did mean the o-ring as well. Memory isn't always 100% haha For the coolant overflow hose, you hit it on the head. It's one job is to allow coolant that gets on top of the reservoir to flow to the outside of the vehicle. Here's another pic I took: Sorry, it's basic, I didn't want to open Photoshop. But, for orientation, brown arrow is oil fill, orange arrow is coolant fill and florescent yellow is the hole that eventually ends with the coolant overflow tube. It is not under any pressure other than atmospheric unless the cap or valve leak, then it's whatever pressure the rubber seal between the rear trunk lid and reservoir lip can maintain... not much above ambient I'm guessing. If you're familiar with the way the reservoir is connected to the rest of the system, then it is the bottom short hose that goes through the firewall and then it simply routes 90 degrees down near the passenger rear wheel well once it's through the firewall. No problem on the pictures. Worth a thousand words and all that. Let me know if I left something out. Best of luck!

fan...mind=blown. i have looked on top of that tank dozens of times and never noticed that before. now i realize why though - i have to cock my head sideways up under the trunk strut to see it. im kinda blown away right now with this info. this whole time I thought that tube acted as an overflow drain for the closed system. now i realize that all the fluid i have seen literally pouring out of that tube has either come up through the burp valve, under neath the cap, or out of cracks on top of the tank! I'm amazed that much fluid went through that hose and never once overflowed into the trunk compartment. This makes me much more confident in the decision to replace the burp valve since I really dont think there are any cracks (pressure tester will arrive soon so will know for sure at that time) and its definitely not the fill cap. Thanks again fan! I owe you!

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Hey Ike! Happy to help! That port is very difficult to see from the rear trunk access. Removing the old reservoir made it much easier to identify! I'm hoping that replacing that expansion valve will fix your issue. Like you said, there's only 3 ways for coolant to get to that hose and with a new cap and a presumably functional coolant reservoir, there isn't much else it can be! You owe me nothing, you and a few others got me through my fuel injector/MAF issue a few months ago, I'm just happy to be able to pitch in! Keep us informed on your progress! Good luck!

Pressure tester came in today. At 0.4 bar the burp valve pops like a turkey timer. I think we have an answer! New valve is on its way. Hopefully this is the end of this issue. Interesting side note - while pressurizing the chamber I notice a drop in the coolant level which immediately rebounds upon releasing the pressure. Water is a non compressible fluid, so what gives? The hoses maybe expanding? Anyone else ever notice this? Sent from my POCOPHONE F1 using Tapatalk

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Proper testing gives a proper diagnosis once again Yes I have seen coolant level drop when pressurizing systems. Hose expansion along with the small amount of air between coolant level and the top of the tank that is compressing.