L Jet vacuum leak pursuit '74 1.8, How does this work? Options

[nihil44]

I have bragged to my carburettored friends about how the '74 1.8 L Jet starts off the key even after a lay off of a month or so. Lately it has required two cranks to start. I suspected a vacuum leak as I pulled a plug and the pluf looks like the mixture is lean but the exhaust tips are black



I made a smoke maker based on this YouTube site

THE BEST SMOKE TESTER YOU CAN BUILD!! / THE MR. FUSION MINI BUILD - YouTube

Applied smoke into the intake system and this is what resulted.



Over the years I have read about ensuring the oil filler cap has a good O ring and cap seal otherwise unmeasured air will enter the intake system downstream from the Air Flow Meter and cause a disturbance in the A/F ratio determined by the ECU.

There is a factory hole in the oil filler cap as revealed by the smoke test. I have 2 filler caps and they both have the small hole.

How does this work if the L Jet intake system is supposed to be closed?

I was expecting the smoke test to reveal a vacuum leak in the intake boot or elsewhere and that would be the 'aha' moment. Not so lucky. However I would like to confirm that the intake system is vacuum leak free.

Would appreciate some explanation from the collective brain trust.

A word of caution: If doing a smoke test, perform it outside or in a well ventilated garage otherwise it will set off the smoke detectors. How would I know this?

David

[wonkipop]

@nihil44 there are about 5 posts to read in that samba thread.
ignore 75% of them.

the type 4 engine running L jet in the bus (and other vws) does have crank-case vent using a valve. the valve is on top of the rectangular baffle chamber just like our oil filler tube.

unlike D jet or the bus/kombi pcv valve, we do not have a valve on our L jets.
its a straight oil filler tube with the vapor hose out the side.

why VW wanted a valve on the kombis i don't know.
but they did.
that valve closes when the engine is switched off and vapors stay in the crankcase.
maybe it had to do with the orientation of all the inlet components in the engine bay.
ie oil could condense in the inlet air duct and run down to the AFM unit.
i really don't know.

for some reason they were confident with no valve in ours and valves in other model type engines.

they cobbled together the components for these two different set up using plastic sub components. that much is clear.

the central part of the oil filler cap on our cars is the same as the top section of the pcv valve in the type 4 kombi. its a much bigger pcv valve than it is for d jet engines on those kombis.

and in there is a membrane valve that works off crankcase pressure pulses. releasing vapors to the intake system for burning during combustion. its pretty simple.

the pinhole is necessary to the membrane being able to be pushed up and then return to the closed position at rest and seal off the crankcase from the induction system.

the pinhole is not an opening to the atmosphere in the induction system.

the pinhole is not a crankcase pressure relief valve, there is a membrane in the way.

so in my view that pinhole is not meant to be open in the 914.

or in the kombi.

and if you can force smoke out of it.
- that means
in a kombi the membrane has failed - and your pcv valve is kaput.
or in a 914 something has given out in the top of the cap that normally seals the hole.