Blowby -- What is it?
 

I've read and searched many threads. I have read that "my car has little to no blowby" My question, What is blowby??????

well it is illegal in indiana
 

along with marrying your sister.

just kidding

it is the combustion product that escapes past the rings, into the crankcase and back up through the oil drain tubes into the valve cover.

it is easily guaged by removing the oil filler cap with the motor running. little or none is good. if it blows smokey air out like a locomotive, that is not good.

it is a reliable indicator of ring sealing. it is not measurable scientifically though some here have tried to figure ways to do so. it is a judgement call. some blowby even quite a bit will not prevent a car from giving good service, but if you find one that has a really lot, figure on a rebuild to achieve reliable starting, especially on a diesel which depends on good compression for good starting.

tom w

Thanks for the info!

Forgive me for picking a nit here, but I respectfully submit that blowby certainly is scientifically measurable. It's simply not a practical necessity for most of us to have the equipment on hand to do so.

Would this be a neccesary check on a vehicle being looked at for purchase, or are there other factors that would cause blowby that might be easily remedied?

Thanks,
Dunl

Do people have standard ways of measuring it, then (other than a judgement call), or are you just saying a way could be developed?

If they are out there I'm curious about what the measurable ways are.

Thanks

There are ways to measure it. Try googling "blowby measurement".

Some of the instruments you may find are obviously intended for use in a research and development environment. Other methods might be more accessible to those interested in performing such measurements on the cheap.

compression test
 

I thought higher compression = lower blowby and vice versa.

My favorite way of judging blowby is to loosen the oil filler cap and watch it for movement. If there's no movement (make sure the cap isn't sticking) then that's excellent. If there is, try raising the RPM to eliminate engine vibration as a factor. A little movement is ok, but if the cap flies off like a bat, it's time to run!:eek:

So do people normally check this when buying a Mercedes?

I did.

When you rev the engine the blow by will decrease, so don't rev too much when checking.

Actually we do have and use a method of establishing the quantity of blowby. Removing one end of the breather tube and obstructing it totally with your finger while timing the seconds till engine quits. It is a very useful way of guestamating the remaining life left in these engines. The actual time to exceed is something like 10-12 seconds before engine dies. The engine quits because the built up back pressure shuts off the injection pump. I will not consider an engine with a time of less than 10 seconds myself but the true range of times is buried in the archives somewhere. Of course perform the test with the engine up to operating temp. I also would be very interested in the time a freshly rebuilt engine just broken in gives as have missed the mentioning of it or perhaps has never been posted to my knowledge. Maroon this is about as practical as it gets as it is almost absolute and I prefer it to the oil cap test even though it is also a good test.. Neither gives actual volume but both indicate to some extent reality with the tube test perhaps a little less subjective as I can describe the run length better than an exact description of how much the oil cap is moving. The common expression is dancing by the way. I want all my 123s to be 10 second or better cars. Twelve seconds is a lot better for example. Well have to go down to the workshop and turn out some loaded oil caps. Need to melt and conceal about a half pound of lead to each one..Watch out for loaded oil caps coming to an ebay auction near you. I thought I had seen it all in life. Then ebay surfaced. I can almost a visualise it now. Good for a million miles and the oil cap does not even move when loosened.

Mine puffs out quite a bit of blowby with the cap off (makes it dance if you loosen it) and yet it starts within 6-7 cranks at 0F, so I wonder how much of a problem it really is, it probably has to get reeeeal bad before major problems occur from it. It doesn't puff out any more now than it did 30,000 miles ago, so who knows......our '83 has essentially no blowby (nothing out of filler-hole with the cap off) and yet it doesn't start any better in the dead of winter than mine does.

Blow by
 

...is what most cars do to my 240D on the freeway.:)


Tom

I once read about somebody here doing this test and he blew something. I don't remember the details but you could probably look it up if you want. Interestingly, my Jetta at first glance has what appears to be a lot of blowby. The cap dances violently and I have to rev it quite high to make it stop moving and there's a quite a bit of smoke coming out of the oil filler hole. I don't know about the smoke but the cap dancing is definitely due to the engine vibration and I really don't think such a new engine would be worn out. It starts instantly in 40 degree weather even without using the glow plugs. Perhaps cold starting ability, assuming the battery and starter are in good shape, is the best indicator of compression and blowby.