i am just about to remove and inspect my DPF, i,ve read that some DPF are putting excessive back pressure in sump and bypassing up the rings , causing all sorts of sump and cam shaft pressures.
This sounds like a bunch of hogwash to me.
For a moment, let's assume the DPF actually can cause additional back pressure. So let's see where that extra pressure would be. We'll go backwards through the exhaust system to figure out what might be affected.
First, you'd have the exhaust pipe between the DPF and the turbo. That's all pretty good metal there. A bit of increased pressure isn't likely to cause any leaks. The weak point is probably the flex pipe, but that weakness is in it's bracket and not the pressure it needs to hold.
There might be a piece of the exhaust gas treatment system in here as well. Perhaps the SCS cat?? I'm not sure. At any rate, it's another wide spot in the pipe. No connection to the sump or cam. Maybe some impact on the way it treats the exhaust stream.
Next thing back is the turbo. A bit of extra exhaust pressure there might have a small effect the turbo's performance, but that's it.
Then we've got another bit of piping and the exhaust manifold. Nothing interesting there.
Now we're back to the engine proper - the head, valves, cylinder bore and piston. Finally, a potential connection to the sump and camshaft. What is the biggest pressure in this area? The power stroke by a huge margin. But let's just look at the compression stroke.
With a compression ratio something north of 15:1, we're looking at a minimum of 220 psi at the top of the compression stroke using normal atmospheric pressure. We then need to add in the additional air from the turbo. I think a Sprinter's maximum boost roughly doubles atmospheric pressure, which would roughly double the pressure at the top of the compression stroke. Then you have to add in the pressure from burning the fuel.
At this point, I was going to take a WAG at what that pressure might be. But then Mr. Google saved me from myself and turned up this chart of pressures in a cylinder.
http://performancetrends.com/Definitions/Images/Cylinder-Pressure-Lrg.gif Granted, that's for a gasoline engine and not a diesel. (You can tell that from the label showing where the spark occurs.) It's also not a turbocharged engine, from the pressure below atmospheric during the intake stroke. But in spite of these differences, it's still useful for our purposes.
From the chart, you can see that pressure peaks at over 700 psi. I wouldn't be surprised if a turbo diesel engine's peak is over 1000 psi. But we'll stick with that 700.
Let's leave this for a moment. How does a sump or crankcase get pressurized? That would be from gasses getting past the piston rings, of course. And when are those gasses going to get past the rings? Is that going to happen when pressures are from 200-700 psi during the power stroke? Or when pressures are 50-100 psi during the exhaust stroke?
Now, let's talk about the pressure sensors in the exhaust system. A common test of those sensors is to turn the ignition on, but don't start the engine. Then get the actual readings from the sensors. Since they're all exposed to atmospheric pressure in this test setting, they should all read the same, give or take a bit of tolerance. So what is that tolerance? IIRC, it's 5 millibars.
Drat, different units. Mr. Google to the rescue again. 1 psi is about 69 millibars. (68.9476 if google is to be believed.) Stated the other way, 1 millibar is about 0.0145 psi.
Our spec is 5 millibars, which is about .0725 psi.
That's how accurately your Sprinter is measuring pressures in your exhaust system. Get beyond that difference, and you can get spurious readings and probably a check engine light. But more importantly, get far enough away from the expected values and your car is going to notice. How far is far enough? I have no idea, and I'm getting tired of referring to Mr. Google. (OK - I did one search and didn't find anything. Plus this post is getting too long anyway.) Let's use a swag and say that 10 psi is far enough. The system is certainly capable of measuring much smaller pressure differences (by a couple orders of magnitude).
Back to exhaust back pressure and the questions I left hanging a couple paragraphs back. When is blowby going to occur? It's mainly going to occur during the power stroke. If the rings are really bad, you might get some during the compression and exhaust strokes as well. (And with a turbocharged engine, a bit even during the intake stroke is possible.) A 10 psi increase in exhaust back pressure is barely a blip in the big picture. That's not going to cause any significant increase in crankcase pressures.
On the cam side, the only time exhaust pressure comes into play is in opening the exhaust valve. That happens with cylinder pressures in the 100 psi range. If anything extra pressure in the exhaust system behind the valve would reduce the pressure difference between the cylinder and exhaust port, making the valve easier to open.
In short (really, after this book length post????) I find the claim that sump or cam pressures are affected by a DPF to be silly.
vic
