951 plastic coolant reservoir bulge

[chrischrischris]

Hey @CarBot is it common for the genuine porsche 944 coolant reservoir to expand under pressure? Mine has a bulge on top that is pushing the hood out of alignment.

[Tom]

Hey @CarBot is it common for the genuine porsche 944 coolant reservoir to expand under pressure? Mine has a bulge on top that is pushing the hood out of alignment.

You need to select CarBot from the drop down list after you start typing @carb... like this: @CarBot please answer Chris's question about the coolant tank...

[CarBot]

You need to select CarBot from the drop down list after you start typing @carb... like this: please answer Chris's question about the coolant tank...

A little “growth” over decades from heat cycling is pretty common on 944/951 expansion tanks, but a bulge big enough to push the hood out of alignment isn’t normal and I’d treat it as a warning sign.

Normally the system runs around 1 bar (about 15 psi) with the correct cap. If the cap isn’t holding the right pressure or isn’t venting correctly, the tank can see higher-than-intended pressure and deform. The other big cause is combustion gases getting into the cooling system (head gasket or cracked head), which will over-pressurize the tank quickly, especially from cold, and can balloon it.

Practical checks that usually sort this out fast are to verify you have the correct pressure cap and that it’s in good shape (many problems are just a bad/incorrect cap), then do a cooling system pressure test and a block test for combustion gases. Also pay attention to whether the upper hose goes rock hard very quickly after a cold start, or if you’re pushing coolant out/seeing repeated “mystery” coolant loss.

If the tank is already bulged, I’d replace it sooner rather than later. Once they deform, they’re more prone to cracking and making a mess. If you post a photo of the bulge and tell us what cap you’re running (rating/brand) and whether it pressurizes quickly from cold, I can help narrow it down.

[Tom]

You need to select CarBot from the drop down list after you start typing @carb... like this: please answer Chris's question about the coolant tank...

A little “growth” over decades from heat cycling is pretty common on 944/951 expansion tanks, but a bulge big enough to push the hood out of alignment isn’t normal and I’d treat it as a warning sign.

Normally the system runs around 1 bar (about 15 psi) with the correct cap. If the cap isn’t holding the right pressure or isn’t venting correctly, the tank can see higher-than-intended pressure and deform. The other big cause is combustion gases getting into the cooling system (head gasket or cracked head), which will over-pressurize the tank quickly, especially from cold, and can balloon it.

Practical checks that usually sort this out fast are to verify you have the correct pressure cap and that it’s in good shape (many problems are just a bad/incorrect cap), then do a cooling system pressure test and a block test for combustion gases. Also pay attention to whether the upper hose goes rock hard very quickly after a cold start, or if you’re pushing coolant out/seeing repeated “mystery” coolant loss.

If the tank is already bulged, I’d replace it sooner rather than later. Once they deform, they’re more prone to cracking and making a mess. If you post a photo of the bulge and tell us what cap you’re running (rating/brand) and whether it pressurizes quickly from cold, I can help narrow it down.

I think we need a picture of it. They all bulge a little over the years, but I can't even envision one so bloated that it prevents the hood from closing right. I'd be curious if you've confirmed the hood issue is really being caused by the tank or maybe by something else....

[CarBot]

I think we need a picture of it. They all bulge a little over the years, but I can't even envision one so bloated that it prevents the hood from closing right. I'd be curious if you've confirmed the hood issue is really being caused by the tank or maybe by something else....

Yep, agreed a picture would help a lot here. They do tend to swell a bit with age, but “hood alignment” level bulge is unusual.

A quick sanity check is to put a straightedge across the top of the tank and compare it to a known-good one (or even photos of a new tank), and also look for witness marks on the underside of the hood insulation or paint where it’s contacting the tank. If there are no rub marks, the hood misalignment may be coming from the hood bump stops, latch adjustment, hinges, or the hood itself rather than the tank.

If it really is contacting, I’d still focus first on why it’s ballooning. Wrong/bad pressure cap is the easy one. The more serious one is over-pressurization from combustion gases; that usually shows up as hoses getting very hard quickly from cold, repeated coolant loss, or bubbling in the tank. A cooling system pressure test and a block test will tell you a lot.

If you can share a photo plus what cap you’re running and whether it pressurizes quickly after a cold start, we can narrow it down fast.

[chrischrischris]

I looked back at my purchase records for the car and think that the cap is Mahle Behr MAH-CRB17000P

[chrischrischris]

Photos of bulging reservoir

IMG_8050.jpeg (1.71 MiB) Viewed 241 times

IMG_8049.jpeg (1.56 MiB) Viewed 241 times

[chrischrischris]

@CarBot did you see the responses?

[CarBot]

did you see the responses?

Yes, I saw them, and I also saw Chris’s photos and the cap info (Mahle Behr MAH-CRB17000P).

From the photos, that tank is more than the normal “old 944 tank” swell. The top is clearly ballooned, and if it’s close enough to affect hood fitment I’d stop trusting that reservoir. They can split without much warning once they’ve deformed like that.

Next practical steps are to confirm the cap is the correct pressure rating for the 951 and that it’s actually regulating pressure. Even a good brand cap can be wrong spec or defective. If you can, have the cap pressure-tested or swap in a known-correct OEM-spec cap.

At the same time, I’d do a cooling system pressure test and a combustion gas (block) test. The reason is simple: a bad cap can deform a tank, but combustion gases will keep over-pressurizing the system and will just wreck the next tank too. A big clue is whether the upper radiator hose gets rock hard very quickly after a cold start.

If Chris can confirm whether there are rub marks on the underside of the hood and whether it pressurizes quickly from cold, that’ll point us toward cap-only versus over-pressurization from the engine.

[Tom]

did you see the responses?

Yes, I saw them, and I also saw Chris’s photos and the cap info (Mahle Behr MAH-CRB17000P).

From the photos, that tank is more than the normal “old 944 tank” swell. The top is clearly ballooned, and if it’s close enough to affect hood fitment I’d stop trusting that reservoir. They can split without much warning once they’ve deformed like that.

Next practical steps are to confirm the cap is the correct pressure rating for the 951 and that it’s actually regulating pressure. Even a good brand cap can be wrong spec or defective. If you can, have the cap pressure-tested or swap in a known-correct OEM-spec cap.

At the same time, I’d do a cooling system pressure test and a combustion gas (block) test. The reason is simple: a bad cap can deform a tank, but combustion gases will keep over-pressurizing the system and will just wreck the next tank too. A big clue is whether the upper radiator hose gets rock hard very quickly after a cold start.

If Chris can confirm whether there are rub marks on the underside of the hood and whether it pressurizes quickly from cold, that’ll point us toward cap-only versus over-pressurization from the engine.

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.