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

[Van B]

@wonkipop
I did some part number tracing (because you’ve pulled me into this vortex of obscure oil cap research) and the only 914 to ever use this cap was the US/CAL 1.8L

All other models, including the AN engined Europe 1.8L used the standard cap. What they had instead is the PCV in the little black box that the filler neck bolts too.

So, rather than trying to repair NLA parts, why couldn’t We just seal the cap (or buy a sealed cap) and then find a parts store PCV that can be put inline on the hose that runs between the fill neck and the intake boot?

[wonkipop]

@wonkipop
I did some part number tracing (because you’ve pulled me into this vortex of obscure oil cap research) and the only 914 to ever use this cap was the US/CAL 1.8L

All other models, including the AN engined Europe 1.8L used the standard cap. What they had instead is the PCV in the little black box that the filler neck bolts too.

So, rather than trying to repair NLA parts, why couldn’t We just seal the cap (or buy a sealed cap) and then find a parts store PCV that can be put inline on the hose that runs between the fill neck and the intake boot?

we probably could.
but first i have to stop referring to it as a positive crankcase vent valve.
it is a PRV valve - in the L jets. Pressure regulating Valve. there is a difference.
works the opposite way to a typical pcv in the D jets.
they are trying to pull air through and they inject air to do that. they can.
we can't. its more about balancing the pressure in the crankcase in ours.
back and forth. and never letting the vacuum draw from the crankcase become too strong because it will go looking for that air at the weakest point.
kind of the opposite of too much pressure and blowing the seals and oil out.

so we have to have the right valve.
which is what i think most modern valves are.
more or less the same way of working as the L jet ones.

the question would be matching the valve to the engine.
working with the spring in the way that the cap valve does with its spring.

------

correct - the AN twin carb engines have the same valve as the D jets.
though i don't think they have vented heads. thats a d jet thing i believe.

you are right, the one on ours is only on the EC L jet engine.
something like 15,000 - 20,000 engines at most.
chickenfeed to VW.

------

just started mine to do the puff test.
could not discern anything holding my palm over it.
all i could feel was air current being pulled into fan making it difficult to know what might be going on anyway.

did yours noticeably puff?

used my finger to plug the pin hole - nothing happened. idle or revs did not change.

watched the vid nihil44 was talking about. pretty neat smoke machine. will have to make one its so good.

----
only thing about a replacement in line versus original is the spring strength in relation to vacuum. the way i was thinking of it only the membrane gets replaced. and you know it works because you have retained the original spring. these things are a kind of universal design for a lot of cars but suspect they are tuned through spring strength?