QOTW #22 – Can you explain Aston Martin PCV Valve failure?

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Welcome to BamfordRose and another question of the week. This week, it’s another widely seen issue of PCV valves on V12 engines. This will be the same failure for a O4 DB9 510 horsepower V12 V DBS, any V12 engine of that Ford derived error.

PCV valves or positive crankcase ventilation is a one-way valve that allows air from within the engine. So, your crankshaft has been rotating, connecting rods Pistons have been going up and down. This creates a volume of air that needs to be moved away from the engine or out of the engine. If it was to stay inside the engine then there’d be a big parasitic loss for the engine to overcome, which will zap performance.

So, all this air which is normally created needs to get out of the engine. But the environmentalists don’t like gases like that outside the engine. So back in the 70s when this first bit of legislation came in, you had to feed all of your breathing gas back into the internet manifold. So, the engine can consume its own gases, which are laden with hydrocarbons because it’s been inside the Sun Barrett.

So that’s been going on since the 70s, nothing new there. This one-way valve, what it does, is allows that gas to move from the bottom end of the engine into the inlet manifold and the one-way valve stops any oil from being carried across in the air. Because as that the crankshaft is whipping up, moving from outside some, it’s going to pick up some glob juice of oil. And the one-way valve just stops that oil going through that manifold. Obviously, you don’t want to burn oil in this recirculated air, that is not bad for a number of reasons.

What happens on the Aston is that these valves repeatedly fail, their rubber beak type valves within a rubber pipe. And what happens is they go brittle over age. As they age, they get brittle and then they stick open, so now there is no one-way valve. The engine is consuming a little bit of oil that’s picked up with a normal breather gas.

Even in the worst case, if the valves are stuck open, I don’t think all consumption is going to go anything over half a liter per thousand miles. But you can just see on cold start, with a blue smoke, and that’s a telltale sign that the valves have failed. Unfortunately, a few cases, if people are doing high mileage, they don’t regularly check their oil. And if you’ve done a few thousand miles at half a litter. You’ve earned half oil consumption. Then you’ve only got a drop from the maximum mark, which is about twelve and a half liters, down to the tip of the dipstick which will be about seven liters. It’s at that point, if you’ve only got seven liters in the sum going down a motorway at two or three thousand rpm, the rear began bearing for cylinder six and twelve is the last in the oil feed circuit. The dry engine seizes up. Yeah, that’s best avoided.

The PCV valves should be checked at service. The easy way to do this is to look at the air filter. Because if the air filter has got staining of oil on the element, the element has acted like blotting paper. Then you know that PCV valves have failed. Because the circuit goes from the engine to the inlet manifold, pre throttle. The gravity will mean that the oil drops down into the airbox. Therefore, you know, if the air filter has got some oily witness marks on that, the PCV valves have failed.

In these, the inlet manifolds change to change them and there’s one way of buying the whole kit. It’s about five and half hundred quid of Aston. Or individually you can go and find those valves and just try and graft them into the pipe work. Just clear up engine breathing. Probably heard people say, if an engine has worn out, how it’s breathing? That’s something separate.

What will happen there is the combustion gas on top of the piston, or an engine is warm or breathing, it’s going to seep past the piston rings. That’s going to go into the sump. There’s going to now be some positive crankcase pressure because combustion gases have been forced into some. That’s going to force that air and oil out of the block, into the PCV valves. So that is an engine that’s breathing. Because it’s born out. But that’s not the normal recirculation of air that we’re talking about, the positive crankcase ventilation system does its normal job.

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