Strange observation

I was tinkering with my TDI today I noticed something to me seemed strange.I started the car and while it was idling I took of the oil fill cap.there is a steady flow of hot air just like putting your hand over a tail pipe.That can't be normal.I recently put on a new PCV valve.Any thoughts on this?

Enough to cause the cap to wobble around when loose but not fly off? Perfectly normal.

OK, That checks out.The cap bounces around but doesn"t fly off. I've had this car 10 years and never noticed this.Thanks for the information.

Yes, that surprises many people. I once was selling a car and the buyers brought another guy who refused to speak english. They said he was a diesel mechanic. The guy pulled the fill cap while the engine was running and claimed the air blowing out was proof that the engine was run out. I told them to bring a real mechanic next time and if they wanted a discount to go play in the traffic. (Well, the exact verbiage wasn't that polite.)

Cheers,

PH

So don't take the cap off with the engine running then. TDI's are somewhat of an oily mess internally.

There is a limit. to the amount of blow-by we think is acceptable. Also, if the blow-by is addressed at the proper time, you usually can take an engine many more miles without what I call a Major, which means pulling the block and boring to the next oversize.

If the oil cap is placed on the oil fill hole, and it blows off, that is my 'Red Neck Check' for Too Much Blow-by. At that point, we usually see wear to the point the engine needs some serious attention.

If the oil cap wobbles violently, but stays in place, you have reached a decision point. If the cylinder head is removed and the pistons punch out, we ball hone to deglaze and refit with new rings, rod bearings and if needed, main bearings and reassemble. This is quite common with engines in the neighborhood of 300-400k.

On the other hand, if the goal is to drive it till it drops, then go another 100-150k and when it won't start, go find another car.

I can tell you from my own personal experience what I had to do. I suddenly noticed a fog of oil smoke, only in city traffic at 240,000 miles. I could drive 1000 miles and not use oil on the highway. So, I decided to see what was wrong.

When I pulled the pistons, #2 oil control ring had failed. So, following my own advice, I reringed. Two things I noted. The cylinders cleaned up with a 280 grit ball hone real fast and easy. And on reassembly, there was NO blow by. Power was up, but then, I did port the head, put in my Stage II cam and upped the injectors and tune. It's at the point where I'm tearing up my clutch and have to be careful with it until I get some free time... heh, like that's going to happen.

how about smoke coming from that cap? is that normal?

my car burns 1L per 5000 km. sometimes at the 5000 mark, ill check dipstick 2 minutes after driving and itll read as low. so I pop the oil cap off to throw a liter in, and smoke comes out. not much though.

is that normal?

When you figure that blow-by is from combustion gasses, then smoke would not be unreasonable. On the other hand, what does it look like? The ones I have observed look like a slight mist, and are kind of hard to detect that it is smoke. Might be more definite if it is cold out, and you can see your breath, for example. If you have a definite mosquito-vanquishing cloud coming out, and the cloud persists (unlike water vapor that dissipates), then you probably have a problem that needs attention.

Cheers,

PH

If there is blow by, there is going to be some smoke. More blow by, more smoke. Eventually, you are carrying oil mist with the oil and that gets very messy.

There is always a question, what leaks and what burns? And then, there is the question, is it from the rings or is it from the turbo?

A large amount of pressure coming from the oil cap is a definite sign of blow by. A volume of oil coming from the turbo usually will cause oil to pool at the intercooler. You can suffer some oil burn from rings, but a turbo that has reached the the point it's leaking that much oil is subject to runaway. That will hydro-lock your engine and then, it's not just rings, it's bent rods and pistons with pieces of turbo blade stuck in them.

In a way, what is acceptable becomes a personal issue. I don't like a machine that burns oil and solve it. It's hard to argue, it's easier to pour oil into an engine than rebuild it. Our opinion is the longer you delay re-ringing, the more taper in the block. Limit of taper is .0015", or roughly .038mm. Past that, rings will have difficulty seating and block needs honed.

We will be marketing pistons that are repaired to work with marginal engines. We will be posting new information on the vendor's section soon.