951 plastic coolant reservoir bulge

[CarBot]

Yikes! The humans agree -- that's not normal! Is that a Porsche part or the new, cheap AliExpress knock off? I've never seen a Porsche tank do that, and I've blown more head gaskets than John McEnroe. The fact that it blew up like that before something else burst makes me wonder if it was just defective.

That amount of ballooning isn’t normal, agreed.

A genuine Porsche/Behr tank can still deform with age, but what you’re showing looks more like either a weakened/heat-aged tank that finally gave up, a cap/venting issue that let pressure climb too high, or a non-OE quality tank. I can’t confirm from the photos alone whether it’s genuine; usually the OE ones have clear molded markings/part numbers and the plastic quality looks more consistent. If Chris can post a close-up of the molded part number and any logos on the tank, that’ll answer the “Porsche vs aftermarket” question quickly.

Even if it is defective plastic, I wouldn’t assume it’s only the tank. The right move is still to replace the reservoir and cap, then verify the system isn’t over-pressurizing. A cooling system pressure test and a combustion gas (block) test are the quickest ways to make sure you’re not about to balloon the next one.

Also worth checking that the small return/bleed hose to the tank isn’t blocked and that the cap seat/neck on the tank isn’t damaged or distorted, since either can mess with how the cap regulates pressure.

[chrischrischris]

It is the Genuine Porsche tank. Since it is not yellowed, I'd assume that it has been replaced at some time and is not the original 1987 tank. But I don't know the age and I've never seen it balloon like that before.

The venting cap is pretty simple, correct? When pressure overcomes a spring, a vent hole is exposed which directs gas through the hose adjacent to the cap, right? It is hard to imagine this mechanism failing.

Is the 1.5 bar the best cap for the turbo? I think earlier cars used the 1.0 bar.

[chrischrischris]

I will replace the cap with febi bilstein Radiator Expansion Tank Cap 06568 which is rated at 1.4 kpa.

@CarBot does that sound good to you?

[CarBot]

I will replace the cap with febi bilstein Radiator Expansion Tank Cap 06568 which is rated at 1.4 kpa.

does that sound good to you?

I’d double-check that rating before you buy it. 1.4 kPa would be essentially no pressure at all, so I suspect you meant 1.4 bar (about 140 kPa). If it’s truly 1.4 bar, that’s in the right neighborhood for a 951 cap.

That said, I’d personally stick with an OEM Porsche/Mahle-Behr cap with the correct bar rating for your year, because the cap is the pressure regulator for the whole system and a wrong spec cap can cause exactly the kind of tank deformation you’re seeing.

Also, with a tank that ballooned that much, I wouldn’t only replace the cap. Replace the reservoir too, then do a quick block test for combustion gases and a cooling system pressure test so you don’t immediately distort the new tank. If you tell me your model year and whether it’s an early or late 951, I can sanity-check the correct cap pressure spec.

[chrischrischris]

@CarBot You are right, I meant 1.4 bar or 140 kpa. Is there a difference in cooling system pressures among different model year 951 cars? I thought it was the same for all 951 models.

[CarBot]

You are right, I meant 1.4 bar or 140 kpa. Is there a difference in cooling system pressures among different model year 951 cars? I thought it was the same for all 951 models.

1.4 bar makes sense, and yes, you’re basically right that the 951 cooling system is the same concept across the run. In practice, Porsche did use different cap pressure ratings across years/markets and parts supersessions, so it’s not always “one cap fits all” even if the engine is the same.

The safest approach is to match what Porsche specifies for your exact model year and whether it’s early/late (and any updates your car may have). A 1.4 bar cap is commonly used on 951s, but I don’t want to tell you “definitely correct” without your year and ideally the Porsche part number you’re cross-referencing.

Given your tank ballooned badly, I’d also make sure you’re not masking an over-pressurization issue. If the system is getting combustion gas pressure, even the correct cap won’t prevent weird behavior, and the next tank can deform too.

If you reply with your model year and the part number on your current cap (or what the catalog says for your VIN), I can sanity-check whether 1.4 bar is the right target for your car.

[chrischrischris]

@CarBot The catalog for my 1987 car gives part number 944 106 257 00

[CarBot]

The catalog for my 1987 car gives part number 944 106 257 00

For an ’87 951, 944 106 257 00 is the expansion tank cap part number Porsche lists in a lot of catalogs, and it’s the right reference to use. That cap is typically the higher-pressure version used on the turbo cars (commonly sold as about 1.4 bar). So yes, aiming for a 1.4 bar cap that cross-references correctly to 944 106 257 00 is reasonable.

If the Febi 06568 you’re looking at explicitly cross-references to 944 106 257 00 and is truly 1.4 bar, it should work. I still tend to prefer an OEM Porsche/Mahle-Behr cap here just because the cap is the system’s pressure regulator and you’re already dealing with a tank that’s deformed.

One more practical note: with the amount of bulge you showed, I’d replace the reservoir as well, not just the cap. And after the new tank/cap are on, it’s worth doing a quick block test for combustion gases and a cooling system pressure test. That’s the best way to make sure you don’t balloon the next tank if there’s an underlying over-pressurization issue.

[chrischrischris]

@CarBot I think I would see other symptoms if I had a cylinder head leak.

[CarBot]

I think I would see other symptoms if I had a cylinder head leak.

You often do see other symptoms with a head gasket or cracked head on a 951, but not always right away. Sometimes the first obvious sign is the cooling system getting over-pressurized, especially if it’s a small leak path that mainly lets combustion gas into the coolant under boost or higher cylinder pressure.

If you want a practical way to settle it without guessing, a block test (sniffer test) at the expansion tank and a cooling system pressure test are the two quickest checks. They’re cheap compared to another tank and they don’t require tearing anything apart.

Things that tend to go along with a combustion-gas pressurization issue are the upper radiator hose getting very hard quickly from a cold start, repeated coolant loss with no visible external leak, bubbling in the tank/return line, or pushing coolant out the overflow after a spirited drive. But you can have only one or two of those early on.

Given how much your tank ballooned, I’d still do the tests once you’ve replaced the tank and cap. If both tests are clean, then you can be pretty confident it was a cap/tank issue rather than a cylinder head leak.