| [ Total Views: 908 | Total Replies: 3 | Thread Id: 1712 ] |
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 matara
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| posted on November 12th, 2002 at 08:51 AM |
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Blowby
A friend came round in his Superbug at the weekend, and asked me to look at his car. We opened the lid, and he took the breather pipe off the oil
filler, and a steady stream of smoky air was coming out. The car is a 1600DP and goes fine, no loss of power, although it has a habit of cutting out
at lights (which I think is because the air going into the carb/air filter is smoky and reduces the o2 available to the carb).
Is this crankcase pressure caused by worn rings? I'm guessing that the rings let explosive gases blow by and into the oil and out through the
breather??? The wierd thing is there isn't any sign of blue smoke at the exhaust???? Mmmmm?????
Any ideas?
Thanks
Steve
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Secoh
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| posted on November 12th, 2002 at 08:59 AM |
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excessive blowby is caused by worn compression rings. the oil control rings may still be fine, preventing oil to pass into the combustion chamber,
but still allowing compression into the crankcase.
If it aint broke,
Make it go faster!!  |
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Baja Wes
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| posted on November 12th, 2002 at 01:18 PM |
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some blowby is perfectly normal.
Trying to get a feel for what is normal and what is excessive is the tricky part.
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555bug
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| posted on November 12th, 2002 at 01:31 PM |
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throw a vacume gauge on it and have a look at the readings..that is the best way to determine what going on without riping it apart..unless you have
access to a leak down tester |
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aussiebug
A.k.a.: Rob Boardman
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| posted on November 12th, 2002 at 03:06 PM |
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You could also try a compression test - usually worn rings will give low results.
A fresh 1600 will usually give 130PSI or higher (and all cylinders should be within 10psi worst to best).
As the engine ages the compression gradually drops, until at 100psi or lower it's time to start organising a rebuild, even though the car may
still be driving just fine.
The lower the compression pressure the more blow-by you'll get. As has been said by Wes, you'll always get SOME blowby - the pistons/rings
never make a perfect seal, but so long as there is little/no oil in the blowby the exhaust will stay reasonably clean, because the fumes have been
recycled back through the engine via the breather tube up to the air cleaner. If the blow-by contains a lot of oil fumes you will get a smoky exahust
and you'll also eventually clog up your heat riser so you'll end up with carby icing problems on cool/humid days.
Re the cutting out at the lights...that could be caused by:
1. a too-low idle speed - should be about 850rpm
2. Incorrect timing - should be 7.5BTDC with a vacuum distributor for example.
3. Air leak in the carby (throttle shaft) or inlet manifold (rubber boots or head/manifold joint).
4. Tight valves. These give a proportionally higher amount of leakage at idle (compared to cruising), so may not be noticable until the engine is
idling.
Probably more reasons, but that's enough for now.
A good tune-up can help eliminate obvious problems, and help identify the less obvious problems.
There are complete tune-up procedures on my web site if needed.
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OvalGlen
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| posted on November 13th, 2002 at 09:07 PM |
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and points / coil / plugs , etc
Regards,Glenn>
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aussiebug
A.k.a.: Rob Boardman
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| posted on November 15th, 2002 at 12:03 PM |
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Hi Geln,
Re the mention of the coil.
Funny thing, but my 1970 bug is still running on it's original coil, with 248,000 miles on the engine. The coil is bare metal - no blue or red
coil for me!
Maybe I'm just lucky, but I've always wondered why VW folks are always talking about replacing the coil.
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70AutoStik
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| posted on November 16th, 2002 at 06:37 PM |
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A faulty coil is a common problem in older cars. The original coil is a quality unit, so breakdown of the cap is rare; usually they fail due to
breakdown of the insulating/cooling oil - especially if one or more condensers have failed. The fault only occurs at high voltage, so the engine
often runs and the fault is only found when power and smoothness appear on it's replacement. The Bosch GT40 (red coil) costs less than an
original (if they're still available) and covers a lot of sins in an old worn motor due to it's higher output (fouled plugs, poor mixture,
bad fuel...)
[Edit:] Back to the original question, I have had engines with excessive valve-guide wear that have produced excessive blow-by. Stock engines are
supposed to have their valve guides checked/replaced at about 100k - wild cams and high compression can shorten this period significantly.
[Edited on 20-11-2002 by 70AutoStik]
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