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]

well it's a good thing those caps are NLA...
I suppose I better start searching for one of those too. I know for sure mine vents on occasion when the engine is cold.

[wonkipop]

well it's a good thing those caps are NLA...
I suppose I better start searching for one of those too. I know for sure mine vents on occasion when the engine is cold.

@Van B and @nihil44 .

i think i have satisfied myself what the original valve does.
its not so much a positive pressure relief valve -
its more an excessive vacuum control valve.
its there to stop over strong vacuum situations where inlet vacuum might be high enough to cause too much suction -----> leading to sucking in the valve cover gasket as people like to talk about happening? meant to stop that?

i'm guessing that the typical high vacuum situations on the inboard side of the throttle plate are not the problem as the high vacuum is in the intake plenum but not the intake boot (ie idle or de-accleration) - but perhaps it can happen at cruise - part throttle which is a high vacuum situation where the vacuum is also in the intake boot (also operates the advance can in the distributor in that situation but no other).

with no membrane this control would be lost in that situation.

a lot of the rest of the time i don't think there is so much vacuum getting through to that valve. sure its strong vacuum at idle but that is inboard of the throttle valve and this connection is outboard of that. and at full throttle there is not a lot or no vacuum really, its pretty atmospheric.

most of the time its going to want to be open. except at cruise/part throttle?
then it might want to be very restricted or almost closed. could also be sucking oil up the crankcase vent pipe at times like that too.

i mentioned the mg smokestack behaviour above.
it must be very prevalent in the southern hemisphere.
you can buy a rebuild diaphram for them!
check it out from this heritage mob in new zealand.
(a very similar dimension to the 1.8 cap - 75mm).
https://www.classiccarparts.co.nz/index.php...product_id=1421

led me to ebay and numerous rebuild diaphrams for all sorts of later model vws. must be a whole cottage industry of folks rebuilding their pcv valves.

https://www.ebay.co.uk/itm/283483223754

some of them look similar to the diaphram inside the bus unit from the 70s.
which i assume is what it is like inside ours.

might have to go in search of a second hand filler cap for a 914 and ship one in.
doesn't matter if its kaput.
then go test the water with these various diaphrams if i can get it apart.
i believe if i were to retain the original spring in the one we have, there is a chance you might be able to rebuild the pcv.

i've fixed up bosch 3 port fuel pumps when they are supposed to be unfixable sealed units. it is probably worth a hack at this.

i'll be smoke testing the one i have when i get the car in the workshop.
i can't feel any puffing through it, but you never know.

edit

here are some more promising leads much closer to the mark.
need some diameters.


https://www.rkxtech.com/blogs/news/replacin...valve-diaphragm

https://www.rkxtech.com/collections/pcv-val...alve-repair-kit