1.4T PCV Explained

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Where to start? I think it's best to explain the entire theory because it makes it easier to understand a system when you know what it's suppose to do/

PCV=Positive Crankcase Ventilation

Why is positive crankcase ventilation necessary?

Well the biggest reason is blow-by. Heat expansion coefficient dictates that the piston rings must have a small split in them to allow them to expand when they experience rapid changes in heat.

The crankcase isn't actually entirely sealed from the cylinder where the combustion takes place. As the piston comes up on the compression stroke, some gas/air mixture will always be allowed to "blow-by" the ring gaps and enter the crankcase. Also combustion blow-by occurs during the combustion stroke.

The PCV system is designed to not only actively remove those hydrocarbons from the crankcase where they will rapidly compromise the engine oil's integrity and attack gaskets and seals, but also it is designed to return those gases back into the cylinders in an attempt to burn them, rather than release them into the environment as emissions.

You can read about the history and how it was actually GM that first instituted the system for widerange in civilian vehicles by searching PCV on wikipedia.


A traditional PCV on an N/A engine is quite simple and effective. Fresh air is allowed into the crankcase, and since the intake is almost ALWAYS under some vacuum..... that vacuum is allowed to pull that fresh air through the crankcase removing the volatile hydrocarbon blow-by with it. A one-way or check valve is used to prevent any pressure or fuel mixture from backfeeding through the PCV. This is the PCV valve most of us are accustomed to.

Now we come to turbocharged engines and are faced with a new obstacle....
Boost.

When the intake is no longer under vacuum, but instead boost pressure, a checkvalve would close and stay closed for the duration of the time boost is occurring. ALSO, boosted fuel mixture has a much greater probability of blowing by the piston rings.

As a result, most PCVs on turbo'd applications, have a more complex two stage system. A vent for idle and low throttle operation, and a secondary vent for WOT and boosted operation.

There are a variety of styles used, but I'm just going to jump straight into ours.

Our PCV on the LUV/LUJ draws crankcase gases through the intake manifold (IM) through a port in the center between the two middle intake runners.

The crankcase gases exit the case through the head. The head collects the gases from block directly and also through passages in the valve cover.

Here is where the gases leave the head and enter the IM:

Here are some of the passages it collects vent gases through:

Now to explain the two processes. The first is the PCV during idle and part vacuum, like mild throttle situations and cruising.



Idle

(please laugh at my elementary drawings..... I lol'd)

This is a good time to show a close up of "checkvalve" in our PCV. Looking up the PCV runner in the IM from the view of the gases leaving the crankcase, you encounter a bright orange orifice. This orifice allows gases to be "pulled" into the intake path and essentially burned with the fuel mixture.

This portion is pretty consistent with a traditional PCV system. But now let's see what changes when you dig into the throttle and create boost.

WOT

(laugh it up)

Now the IM is pressurized, and that little orange "diaphragm" is pushing against those little holes. That stops boost from escaping back into the crankcase, but now the crankcase gases need a new exit.

Look again and notice the port at the top of the IM before the orange check.

That is this big nozzle:

It runs via a tube directly to the inlet of the turbo compressor:

There is a considerable amount of suction coming from the compressor, and that is what pulls the crankcase gases (which are substantially greater during high RPM boost) from the PCV.

I hope that was comprehensive. I've wanted to do this write-up for a while because the PCV comes up a lot in troubleshooting discussions. Especially when it comes to "puff of smoke" problems.

I expect that topic to be addressed soon, but this thread can be references where PCV is concerned.

 

[$ub$onic]

Excellent write up

 

[Preloader]

Thank you. It's a lot easier to see it when you have the parts in your hand.

So now it should be easier to understand why having a bad seal on the oil cap or dipstick will cause problems.

There isn't a fresh air entry point in our PCV, so if air is allowed to enter, anywhere, it will pass through the crankcase with so much velocity, that it will pull oil with it.

And that air will also have bypassed the MAF, so the ECU will not have compensated for it and it'll run like dirt.

 

[Preloader]

Also this may help explain the "rupture disc" in the valve cover.

Let's say the orange check valve fails..... Suddenly the crankcase will be exposed to 14-24psi boost. Not a good thing!

The rupture disc would fail (or succeed technically) making a huge mess in the engine bay, but saving the engine in the process. I think Martyr Disc is a more appropriate name.

 

[Papasonic]

Nice explanation of the crankcase vent system.
Is there any circumstance whereby the crankcase vapors/blow-by would be vented to atmosphere? On my 1.4 turbo; upon a hot restart, there is a strong odor of crankcase vapor that enters the cabin. It occurs at almost every hot restart, and goes away soon after engine starts, and doesn't re-occur again until another restart.

 

[Preloader]

No. There is no circumstance within the hardware that should be allowing that.

It may be worth investigating the seals at the injectors.

Caveat..... When the car is shut off, technically the crankcase fumes are vented to the turbo inlet and could escape out through an SRI. But highly unlikely the almighty SAB would allow that under its iron fist rule.

But once started, all crank gases should be subject to consumption through the intake tract only.

 

[Preloader]

Now to get into the " my car blows clouds when I decelerate" theory.

If it is evident that oil is dumping into the compressor from the PCV tube..... This thread should help with visualizing where to look for problems.

If compression test, valve cover, and turbo are ruled out. It is my theory that the next place of concern is that little orange diaphragm.

Remember, that tube going to the compressor is coming right from the intake manifold at the sit point of the crankcase. So if the orange diaphragm is bad, boost is splattering oil all around the crankcase, and once you decel, and stop boost, the vacuum will immediately pull all that chaotic oil up into the PCV runner. And eventually into the turbo or combustion chamber.

First it should be confirmed that no unmetered "fresh air" is entering at high oil grabbing velocity, through the oil cap, dipstick or valve cover gasket. But the next thing to check would be that orange disc, which should be possible to test, if the IM is removed.

 

[Papasonic]

I also agree that there should be no vent to atmosphere, but somewhere there is a breach that allows the vapors to escape under the conditions I mentioned. There are other owners on this forum that also complain of the same problem.
My dealer service tech claims that the odor comes from sealants in the hvac vents. He doesn't get it! The odor is definitely crankcase vapor. I believe under certain conditions, the crankcase pressure exceeds design limits, and finds a vent to atmosphere somewhere under the hood; I just have yet to determine where.

 

[Preloader]

Interesting, and now I recall a previous discussion involving you and that issue. I'll try to consider causes in my future PCV studies.

I also really want to get to the bottom of this "puff o' smoke" problem. I know it's becoming an epidemic. And there has to be a simple decisive cause that can be revealed.

I'm thankful I have not contracted the "puff" yet, but I want to be prepared.

 

[Papasonic]

I don't have the puff of smoke issue, but agree that is a higher priority issue than an odor during a hot restart. The puff of smoke translates into higher oil consumption. My 1.4 doesn't consume any measurable amount of oil; but I have a stock tune, low miles, and don't ever abuse the powertrain.

 

[MPFab]

Nice write up Pre!
The vapor you smell at hot start could be attributed to the EVAP system. It has a solenoid that allows fuel vapors from the gas tank to be re-ingested by the engine via the intake manifold as pressure builds in the tank. The vapors are filtered through a charcoal canister that sometimes resides under the hood, but the more recent trend seems to be to put them closer to the fuel tank. If that system is malfunctioning, it could be venting tank vapor and that would cause your smell.

 

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Ahhhh yes, I forgot all about that EVAP system. You can see the solenoid in this pic:

That is very likely a place of interest for troubleshooting the start-up odor.

 

[smorey78]

this is why the OIL CATCH CAN is not as effective on the 1.4 as it is on other cars.

 

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I agree that an oil catch is not very necessary on our set-up. And people experiencing tons of oil coming through the WOT portion of the PCV to the turbo, are generally not going to fix the problem with adding a catch can.

 

[rcmn0022]

Taken from Oil Everywhere!!!!!

There is a check valve located in the connection at the turbo inlet. The check valve is closed whenever there is vacuum in the intake manifold. This allows the PCV to remove crankcase pressure at idle and deceleration, and under load, when the PCV closes, this check valve opens to allow the suction from the turbo to take over, removing crankcase pressure.
Make sure that is functioning. If it isn't, crankcase pressure is building up at WOT. Too much crankcase pressure will not let the turbo drain oil back into the pan causing the bearing housing to overfill. The oil will then go past the turbine shaft oil seal and out into the exhaust where it makes smoke.
To test, pull the line of the intake manifold and blow into it. You should be able to. Now try to suck through it. You should not be able to.

 

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That is a great diagnostic tool for "exhaust smoking".

But if the "charge tubes are filled with oil", could that oil have come through the compressor side seal?

 

[rcmn0022]

That is a great diagnostic tool for "exhaust smoking".

But if the "charge tubes are filled with oil", could that oil have come through the compressor side seal?

I'm going to guess and say likely. But for the purpose of diagnosis, I'd check the other spots first.

Eliminating the easiest checks first before attempting to disassemble the entire engine(or in this case, the turbo)

 

[Preloader]

Testing the particular check valve being discussed should fall into the category of eliminating easiest checks first.

 

[rcmn0022]

Testing the particular check valve being discussed should fall into the category of eliminating easiest checks first.

That is most definitely an easy check. I guess I could have mentioned that would be one of the easy steps for the people that aren't as inclined on diagnosis. Thanks for having my back on this

I'd check that valve first as how MPFab suggested to check the valve.

I'd check that there is vacuum on that large line next. Simply get some to Rev the engine and you should be able to see the line expand slightly from a lack of vacuum

I'd then go on to check that the valve that blocks the boost is working. I'm not sure the easiest way to check that valve though. I'm thinking of a way that seems way more complicated than needed, so any input on this would be great.

 

[Preloader]

That is a good order of operation for troubleshooting.

As far as testing the orange diaphragm, I don't see anyway without removing the intake manifold. And then you could seal the PCV runner, and try to pull vacuum on the nozzle that goes to the compressor.

If it holds, then the diaphragm is good.

OR...... You could try to pull vacuum on that nozzle with the IM installed. But it won't hold hold strong because of he piston ring gaps. But should be discernible over an immediate no vacuum hold type failure.