951 plastic coolant reservoir bulge

[chrischrischris]

@CarBot OK, you have convinced me to do the block test. It looks like it tests for carbon dioxide gas in the coolant. These test kits are inexpensive, so I will order one.

[CarBot]

OK, you have convinced me to do the block test. It looks like it tests for carbon dioxide gas in the coolant. These test kits are inexpensive, so I will order one.

Good plan. The CO2 block test is a nice quick sanity check for combustion gases in the cooling system, and it’s cheap insurance given how much that tank ballooned.

A couple practical tips so you get a trustworthy result: do the test with the engine fully warmed up, and be careful not to suck actual coolant into the tester (coolant contamination can skew the fluid). On a turbo car, it can also help to repeat the test after a short drive where you’ve seen the system build pressure, since a small leak can be more apparent under load.

If the test stays negative, that points you back toward cap/tank/venting issues. If it goes positive, stop driving it hard and plan next steps before you put a new reservoir at risk.

When you run it, post what kit you bought and the result and we can help interpret it.

[chrischrischris]

@CarBot The expansion cap is relatively new. I took a look at it today and it seems to be OK. It is a very simple mechanism and I don't see how it could fail, but I will replace it anyway. I checked the pressure release opening in the tank and it is not clogged. The tank is Genuine Porsche and is the "02" part number revision. I don't know when it was introduced but the date stamp shows "05" in the center of the circular stamp. So if that is 2005 then the tank is 20 years old at this time and I can understand if the plastic is not as firm as when new.

[CarBot]

The expansion cap is relatively new. I took a look at it today and it seems to be OK. It is a very simple mechanism and I don't see how it could fail, but I will replace it anyway. I checked the pressure release opening in the tank and it is not clogged. The tank is Genuine Porsche and is the "02" part number revision. I don't know when it was introduced but the date stamp shows "05" in the center of the circular stamp. So if that is 2005 then the tank is 20 years old at this time and I can understand if the plastic is not as firm as when new.

A 2005 date-stamped genuine tank being deformed in 2026 is believable. Twenty years of heat cycling next to a turbo motor will absolutely age that plastic, and once it starts to creep it can balloon more quickly.

Caps can still fail even though they look simple. The spring can weaken, the seal can harden, and the little valve can stick or not seat consistently. Also, if the cap is the wrong pressure rating (or an off-spec aftermarket one), it can let the system run at a higher pressure than intended. Replacing it with the correct 944 106 257 00 spec cap is still a good move.

Given what you’ve already checked, I’d do three practical things. Replace the tank and cap as a set. Do the CO2 block test you mentioned after the new parts are on, ideally after a full warm-up and a short drive. Then keep an eye on how quickly the system builds pressure from cold; if the upper hose goes rock hard within a couple minutes of a cold start, that’s a clue you’re getting combustion gas pressurization even if you don’t have other symptoms yet.

If your block test is negative and the new tank stays stable, you can probably chalk this up to age-related plastic creep plus normal system pressure. If it’s positive, it’s time to stop chasing caps and tanks and look at the underlying cause before the next reservoir gets stressed.