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]

i read that samba thread from end to end.

not that it directly helps much because the L jet on buses or 412s had a different set up for the crank case vent.

separate oil filler tube and cap off to the side.

but i noticed something - which i have noticed more and more looking for parts etc.
VW seemed to assemble a lot of different function components from common sub mouldings. eso. plastic parts.

the top section of the oil filler cap on the 1.8 appears to be the same as the top part of the crankcase ventilation cap on similar type 4 bus engines.
in that situation it definitely has a pin sized hole right through.
underneath it is a membrane set up with a spring.
it works like a positive pressure valve.
the pin hole seems to be to allow air to escape as the membrane is pushed up by crankcase pressure towards this cap piece. the hole does not have any direct connection to the crankcase or the induction inlet tract. its just to allow the membrane to move upward and then to let air back in as the membrane relaxes.

the system on the 914 L jet is nothing like that. we don't have any valve or membrane in there. its just a crankcase vent to the inlet tract. i think there are some kind of baffles to the side in the little rectangular box below the oil fill tube. i don't think ours even has a valve in it down in that box section like the d jet cars. d jet owners would know more about what is in theirs. but i think ours works as a straight vent to the intake air duct.

i'm thinking that VW used that standard moulded piece combined with a ring grip section bonded to it to make the oil filler cap on our cars. but plugged the hole which was in the standard casting for the upper top piece?

but the plugs might have fallen out of yours or failed?
maybe all they did was goop some epoxy in there.
i've tried looking at mine from the bottom or the inside and i can't see anything.
its not gunked up with oil goo or anything, its pretty clean in there..
and as far as i can tell that hole does not go right through. its sealed i believe.

makes sense, because you do not want that hole - it is unmeasured air.
its a very small hole for sure but then most of the ones that cause problems are pretty small holes too.

thats what i think.

others here may know better.

that would accord with djway's oil cap smoke test too.