Okay the pcv valve topic has been done many times but never wanting to miss a good photo opportunity a pair of really knackered valves presented themselves today and the camera was close by as always.

 

The breather system relies on these one way valves been fully operational to prevent oil laden air been drawn into the combustion chambers and causing exhaust smoke at tick over.

 

The two valves were completely gone and was clear to see the problem, however sometimes they can appear to be ok but be weak or debris preventing them closing all of the time, replacement is the best option given they are not too expensive.

 

Jeff TT

 

 

 

 

 

What they should look like....

 

Edited March 4, 201411 yr by JeffTT

Jeff, funny you should post this, I think mine need replacing! Will contact you later....

So are they supposed to hold themselves closed when held upright?

So are they supposed to hold themselves closed when held upright?

 

Yes spring loaded.

 

Jeff TT

Ok, just to bump an old topic:

 

I've just changed my PCV valves as I was getting occasional smoke from the O/S tailpipe whilst idling. I've changed them for a secondhand pair that rattle nicely, but doing some reading on here now has me a bit confused.

 

In Jeff's tech stuff above, he states that the valve is spring-loaded so, whilst rattling it will get it to make a noise, it'll default to the "closed" position when you stop shaking it. That's exactly what the two I've just taken off do.

 

However, in another thread (that I don't have the link for right now), Ian Greenfield stated that he got a new pair from MJP and they weren't spring-loaded inside!

 

So which scenario is correct please? I haven't had chance to take the car out since changing them so I don't if they've made any difference to the smoking or not. But should I be concerned or not that the valves I've just fitted have no tension inside them...?

They are spring loaded, albeit not very strong and do rattle when you shake them from side to side, the spring holds it open while engine is off

 

This may help http://www.300zx.co.uk/forums/showthread.php?176698-pcv-valves-explained-part-1

Ok. The ones I've taken off have one end "blocked" by default by the plunger. This plunger can be depressed using a screwdriver, but snaps back into the "closed" position as soon as that pressure is released.

 

So according to Jeff's info (and the fact that it rattles freely when shaken), that valve should be ok, yes? Whereas the two I've just fitted rattle nicely but the plunger will move without pressure from a screwdriver and doesn't snap closed by default.

 

Confused!!

Default is open as shown in the pic above, albeit by about 5-10mm

The new ones are a couple mm open and they do rattle. I have replaced mine a couple times with no joy to cure smoking issue. So just replaced turbos and is all good

It doesn't matter how exactly they spring. It's just a non return valve. It's function is to allow crankcase gasses to be scavenged and keep crank case pressure as low as possible whilst in negative boost or vacuum situations. It is then to close once positive pressure is applied to the manifold to stop boost or positive pressure being applied to the crank case and oil galleries. At that point the system is reversed with excessive pressure going out the intake cm cover vents.

 

The issue is especially with turbo cars, excessive crank case pressure by which I mean not a lot, distrupts the ability to drain oil efficiency allowing oil to pass into the exhaust and causing smoke, more technically steam as the oil evaporates. If you have excessive crank case pressure you will most certainly suffer from smoke esp with a free flowing exhaust.

 

So all they need to do is pass air from the hose joiner end to the threaded end and stop flow when the reverse is applied. Put it to your lips and blow check the functionality of it as a non return. If it fails permanently open or closed its duff.

 

If, if the PCV is functioning correctly and you still have smoke it's not the PCV. Checking compression and the vacuum readings on the oil galleries throughout the Rev range should reveal the cause. Unless of course the turbo is what is at fault. What kind of smoke are you experiencing mate?

What kind of smoke are you experiencing mate?

 

White/blue smoke, only ever at idle. There's no smoke on throttle or gear changes. It's only ever at idle, and not all the time either.

Valve stem oil seals....?

The pcs valves which are not spring loaded are non genuine ones, personally when I bought some of these to try I chucked them in the bin as cannot see why Nissan would have spring loaded them and after market did not...... the price tells the story after market are cheaper but appear to be incorrectly equipped, Buy from Nissan and buy from the UK as ones from certain states in the US are different according to the emissions control laws there.

 

Old ones that still snap shut would normally be deemed to be ok and the cause of the tick over smoke be elsewhere, however its worth changing them and the rubber pipes including the two front ones as they have velocity reducers in which can get blocked, the old pipes will most likely fall apart / crack when removing which can be the cause of air leaks and the PCV system not working correctly, further more after fitting new valves you need to cover at least 300 to 400 miles before all residue will be completely removed and only then can you make a decision on if they have cured the issue or not.

White/blue smoke, only ever at idle. There's no smoke on throttle or gear changes. It's only ever at idle, and not all the time either.

 

Same as mine and it was a turbo fail

White/blue smoke, only ever at idle. There's no smoke on throttle or gear changes. It's only ever at idle, and not all the time either.

 

Check compression first and foremost mate. Also does your exhaust contain a H or X pipe? There is a documented set of cases where running two single lines causes smoke at idle. The school of though is that the frequency causes a sort of sucking or pumping effect on the rear seals of the turbo.

I have a suggestion for you which I will PM as its a touch unorthodox.

The pcs valves which are not spring loaded are non genuine ones, personally when I bought some of these to try I chucked them in the bin as cannot see why Nissan would have spring loaded them and after market did not...... the price tells the story after market are cheaper but appear to be incorrectly equipped, Buy from Nissan and buy from the UK as ones from certain states in the US are different according to the emissions control laws there.

 

Old ones that still snap shut would normally be deemed to be ok and the cause of the tick over smoke be elsewhere, however its worth changing them and the rubber pipes including the two front ones as they have velocity reducers in which can get blocked, the old pipes will most likely fall apart / crack when removing which can be the cause of air leaks and the PCV system not working correctly, further more after fitting new valves you need to cover at least 300 to 400 miles before all residue will be completely removed and only then can you make a decision on if they have cured the issue or not.

 

 

I would have agreed with you when I first looked at them Jeff but I've been running them for a few years now without issue. They are a very simple non return valve and that function which is what matters works. The opening and closing of the PCV is far in a way controlled by engine vacuum/boost it would work without a spring full stop. Same as the non returns on the clutch and brake booster.

The non spring loaded ones are from the Z31 and by coincidence fit the Z32. lots of peeps using them successfully, guess it depends on the individual engine and turbo condition.

 

 

Jeff