Doghouse oil cooler rupture due to stuck relief valve

I've never heard of one actually sticking (was it seized in the bore?), but I have heard that a stiffer spring (in that front passageway) blows coolers.

Give that piston a GOOD visual inspection....   If you see ANYTHING that looks like corrosion or pitting, replace it.....   The original VW pistons were hard anodized to prevent this problem... 

Bruce,  how did you discover the ruptured cooler?  Was there a leak and this caused you to pull the cooler?

I wouldn't put that piston back in there at all.

Get a new one from CB or Berg - the old one is certainly out of round or it wouldn't get stuck up in there.

Like Al, I've never heard of this before, either.

New is cheap and piece of mind....

A friend's dog house oil cooler expanded like a balloon - right before the catastrophic engine failure. High output oil pump, stuck relief valve and waaaaay too many revs = bad ju ju! 

Al....Yes...It was lodged up at the end of the bore. I found a dowel stick and shaped it to have an interference fit, drove it up into the inside bore of the piston/valve and then pulled and twisted it until it cane out. There were some score marks on the sides of the piston/valve. not really deep, knarley scoring but enough to be concerned since it didn't have them when I installed it 500 miles ago in the new crankcase. It wasn't stuck really tight but enough that I was concerned that I would pull the dowel stick out without the Piston/valve on the end.

Frank P......The oil was running out of the bottom of the engine between cyl 3 and 4 and at the front of cyl 3. About $60.00 of Brad Penn oil.

I pressure tested the cooler by making a block-off plate out of aluminum that has holes matched to the cooler mounting bolt holes, also one hole indexed to one of the cooler oil passage holes. I tapped this hole with 1/8 NPT threads so I could mount an air pressure regulator and pressure gauge. The regulator is set for 20 PSI. I then install two of those red "doughnuts" used to seal the cooler to the bracket on the crankcase. This way there is no escape for the air pressure except through a leak. I plunge the assembly into a bucket of water and look for air bubbles. There were a lot of them and it was obvious that it was ruptured.

So now I'm concerned. I'm going to use the same piston/valve (It's a CB Perf.) but a new one but still, what's going to keep this from happening again ? This time it happened in my driveway. What if it happened on the road somewhere and I don't see that little dinky red light on the dash light up......?  I still can't understand why oil pressure doesn't blow that little piston/valve down where it's supposed to be next to the top of the spring. When I removed the spring retainer plug there was no spring pressure helping to push the retainer plug into my hand like the rear (pulley side) pressure relief retainer. This is pretty scary considering the consequences if I don't find a suitable explanation/resolution. Come on guys, give me more of what you think or know............Bruce

Bruce, I can only add that my brand shiny new VS was delivered with this 'feature' too.

It soon took out the external oil filter mount in what was quite a spectacular event, according to the shop where that happened. They were trying to figure out why the oil lines were 'blowing up like balloons' when the engine was running.

And then, all of a sudden, they found out.

The flywheel side is the pressure relief. The pulley side is the cooler bypass. The bypass valve feeds the bearings whether it is open or closed, it doesn't regulate the oil galley pressure. But if the relief valve sticks closed, the only outlet for the oil is through the bearings. Or exploding the oil cooler as you've found out.

The force on the piston really isn't that great. The piston top is 0.296 in^2. So if you had 150 psi, that's 44 lbs of force on it. However, when the piston is at the top of the bore, the area actually exposed to pressure is only 0.125 in^2, so 150 psi only puts 19 lbs of force on it. (This difference in area between when the piston is at the top of the bore and when it's farther down creates hysteresis so the relief valve opens at about 60 psi, but closes at about 25 psi. It also makes it very important to cut the bore sealing surface properly if it needs to be resurfaced. If it's improperly cut flat instead of beveled, it could take 170 psi before the relief valve opens.)

JUSTIN H...Thank you for your reply ! It was very helpful and informative ! So with spring pressure and the amount of "stuckness" of the flywheel pressure relief valve being more than the oil cooler can stand....the oil cooler looses. The bore diameter which is (5/8") works out to your calculation of 44 PSI, using your hypothesis  now that makes sense. Just today i was working the piston/valve up and down in the bore to insure it doesn't stick again at the top of the bore and by using a depth micrometer, I was able to recognize that the top of the bore IS tapered., being deeper at the center, near the oil inlet hole than at the outer edge of the bore, thus allowing, as you say, pressure to enter onto the top of the piston/valve and act on the top of the entire piston/valve diameter.

Justin..are you an engineer or just know a lot about this stuff ? I was really worried about putting in another oil cooler (lots of work) and have it blow up again because I didn't really understand/solve the problem in the first place. My background directs me to fully understand what happened before I can implement a repair or modification. Thanks again ! Bruce

The taper does not reach the full diameter of the piston. The oil pressure can only act on the area of the piston actually exposed to the oil. The stock spring force is about 7.5 lbs, so if there was no taper, the piston when sealed would only be exposed to the area of the galley which means the valve would open at 170.4 PSI (7.5 / 0.044) . If the entire piston top was exposed to oil, the valve would open at 25 PSI (7.5 / 0.296) which is too low. The taper does three things, first it changes the exposed area of the closed sealed valve so the valve opens at a reasonable 60 PSI. Second, just after the valve opens, the entire surface area of the piston is suddenly exposed so the force on the piston jumps from 7.5 lbs to 18 lbs making it snap fully open quickly rather than doing so gradually. And third, with the entire piston surface now exposed, it won't close again until the pressure drops to 25 PSI. Of course proper operation is dependent on the spring being correct. An old weak spring or too strong aftermarket spring can both be detrimental.

The best details on the dual-relief oiling system are here: http://www.thesamba.com/vw/for...ewtopic.php?t=280293