Jump to content

The RennTech.org community is Member supported!  Please consider an ANNUAL donation to help keep this site operating.
Click here to Donate

Welcome to RennTech.org Community, Guest

There are many great features available to you once you register at RennTech.org
You are free to view posts here, but you must log in to reply to existing posts, or to start your own new topic. Like most online communities, there are costs involved to maintain a site like this - so we encourage our members to donate. All donations go to the costs operating and maintaining this site. We prefer that guests take part in our community and we offer a lot in return to those willing to join our corner of the Porsche world. This site is 99 percent member supported (less than 1 percent comes from advertising) - so please consider an annual donation to keep this site running.

Here are some of the features available - once you register at RennTech.org

  • View Classified Ads
  • DIY Tutorials
  • Porsche TSB Listings (limited)
  • VIN Decoder
  • Special Offers
  • OBD II P-Codes
  • Paint Codes
  • Registry
  • Videos System
  • View Reviews
  • and get rid of this welcome message

It takes just a few minutes to register, and it's FREE

Contributing Members also get these additional benefits:
(you become a Contributing Member by donating money to the operation of this site)

  • No ads - advertisements are removed
  • Access the Contributors Only Forum
  • Contributing Members Only Downloads
  • Send attachments with PMs
  • All image/file storage limits are substantially increased for all Contributing Members
  • Option Codes Lookup
  • VIN Option Lookups (limited)

Coolant boiled over

Recommended Posts

I was out with my wife in our 2003 Boxster with 79,250 miles yesterday driving to three locations each about 10 miles apart. We were late to the first appointment and we took a back way with winding roads. It was a beautiful day and we made good time. It was the second nice drive on windy roads for me of the day. The car was running great.

When we pulled up to the last location my wife notice a bunch of fluid pooling at the curb. Sure enough, coolant was boiling over and running down the overflow tube. I popped the engine cover and couldn't see anything wrong. 20 minutes later there was still some coolant in the expansion tank and I drove with the coolant light blinking to a Seven Eleven 3 blocks away and bought a 24 pack of half liter spring water bottles. I went through a little less than 4 liters and then took an easy way home of roughly 10 miles. With all the covering off the engine certainly sounded different but it didn't sound troubling. It did seem like the Tiptronic was keeping the car in lower gears a bunch, but that might have just been that I'm not used to driving it like an old lady with triple cataracts I stopped half way home and there was plenty of diluted coolant left even as I could see a trickle coming out by the wheel. Not surprisingly, roughly the same amount of water made it onto the driveway within an hour after we parked at my house. Not once did the water temperature gauge seem high.

When I started surfing the boards, I learned that my version .01 coolant cap could be the culprit and should be replaced. I have also read up on this possibly being a head gasket failure which would be much worse. I did pull the oil cap and found more than a few drops on the underside. I have attached a photo but that's after I cleaned up roughly half of the moisture with a paper towel to see if it was colored (it was brownish, maybe a little yellow). I then looked at the dipstick and the Mobil 1 seemed to be a bit watery and maybe a tiny smidge yellow but I might be paranoid at this point. That being said, it didn't seem like a milkshake. Another point to make is that I bought the car used in 2008 with 28,500 miles from a Buick dealership. They had flushed the coolant and replaced it with standard yellow antifreeze which hasn't been a problem (until now?).

The worst part is that I live in Austin and work in San Antonio. We have three cars but my wife is driving the 2000 Suburban b/c her 2007 MB E320 Bluetec is leaking enough oil that it drips on the engine and creates some awful smell and a little smoke. Not good. It looks like I'll be spending $200 to rent a car tomorrow through Thursday unless I get some confidence that my Porsche can make it the 80 highway miles without risk of damage.

What do you think might be ailing my beautiful, until now trouble-free car? What diagnostic tests can I do myself to see if the head gasket is shot?

  • Drain some oil from above using my topsider? Open the oil drain plug to let a little out?
  • Cinch down the coolant cap harder?
  • Drive driving it again to see how the engine is running? Maybe look for moisture in the exhaust?

Thanks in advance,


P.S. I did install a new water pump (no broken vanes) a couple months back after the bearing went. And I installed a low temperature thermostat at the same time.



Link to comment
Share on other sites

  • Moderators

You need to be extremely careful with the cooling systems on these cars; the M96/97’s do not take well to being overheated.

Neither photo show any signs of an intermix issue; if you had one, the oil and/or coolant would look more like hot chocolate, and they do not. That said, the “-01” cap is a suspect. If the car were in my shop, I would pressure test both the cap (to see what pressure it opens at), and the cooling system to see if there are any leaks. If the cap and cooling system checked out OK, I would then pull the plugs, looking at each for signs of “steam cleaning” that would be evidence of either a head gasket (not common) or cylinder head cracking (more common). I would also run a leak down test on each cylinder, and possibly bore scope them as well looking for the same signs.

Assuming that the inspections found nothing obvious, I would replace the cap, drain the entire cooling system, mix fresh coolant with distilled water and refill the system under vacuum.

Link to comment
Share on other sites

From what you say i would suspect an air lock from the water pump change, plus i would change the cap as an introduction of air from a defective seal will cause boiling of the system and of course the rapid expansion of the water resulting in the overflow.

Water boils at 100c but for every pound of pressure the boiling point is raised by 2 degrees, therefore, in a sealed system - with no air pockets, all is fine but if the cap seal fails the introduction of air will allow the water to instantly boil, an air pocket not bled out when you did the water pump does circulate, if the pocket of air reaches the block around the head or cylinders the lack of coolant will allow more heat to generate in a localised area - when water then fills the gap the heat instantly boils the water, creates steam and overpressurises the system - hence the boil over.

Never heard of a head gasket fail on a Boxster so that unlikely

If a head gasket has gone then combustion gasses will pass into the water jackets and pressure/boil the system almost immediately after starting, If you remove the cap when the engine is cold, get someone to start the car whilst you watch the water in the expansion tank, get them to rev up and down the engine - If the gasket has failed then within 30 to 40 seconds water will push up like a volcanic eruption and it will overflow.

Link to comment
Share on other sites

UPDATE: I received a new revision 4 version of the coolant cap for $25 delivered from Sunset Imports (they are great). I installed the cap and after a few days of perfectly normal/spirited driving I have yet to have another boil over. So far, so good.

My next move is to flush and replace the poorly mixed coolant. I'll report back in a week or so to confirm that the car has made it past this drama.

Thanks for all the assistance!


Trevor in Austin

Link to comment
Share on other sites

So it looks like either an air bubble or defective seal on the cap, one thing for sure is that you dont have any head issues as it would have boiled up again.

Not the easiest of cars to bleed out air but the process i have always found works is:-

Jack up the rear of the car - assists air back to the expansion tank as air travels uphill in water.

Remove the cap and also open the vent

Fill up your coolant and then start the engine with cap off and vent open

Allow to tickover to get to normal opperating temperature (thermostat then opens and full flow of water - and any air)

Let it tick over for another 5 minutes to purge any trapped air - a few blips of the throttle will increase water flow and push any air towards the vent.

Switch off, close vent and replace cap.

Chexk water level a couple of times over the next day or so

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.