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Fri Jul 25, 2025 6:35 pm

Just an fyi. I’ve been chasing vacuum leaks. Recently did a smoke test and found smoke around the Aar but couldn’t pinpoint where it was coming from. I checked the rubber 90 and the hose connections and they seemed ok. Finally grabbed some bubbles and soaked the aar itself. Found a leak on the body itself.

It was leaking where it looks like the body is crimped together. In red above.

Fri Jul 25, 2025 7:32 pm

Throw some sealant on the crimp.

As far as smoke tests go you need to plug each leak as you go. Others will soon pop up. Then fix them permanently

Fri Jul 25, 2025 7:46 pm

aeromech wrote: Throw some sealant on the crimp.

As far as smoke tests go you need to plug each leak as you go. Others will soon pop up. Then fix them permanently

I have the aar out now to give it a good cleaning and try to seal it up.

Fri Jul 25, 2025 7:47 pm

There are several products you can easily buy to plug the leaks

Fri Jul 25, 2025 9:53 pm

while that is a reasonable fix, they are still available used and some NOS ones for say Porsche show up from time to time at a reasonable price. Maybe seal that one and keep an eye for a really good one at a good price to replace it before they too are impossible to find.

Sat Jul 26, 2025 9:38 pm

I've never taken one apart but there are rebuild kits on ebay for $42. The instructions mention rivets holding the halves together. You might check that.

Sun Jul 27, 2025 12:01 am

Those AAR valves were also fitted to Fords over here in the UK. Much cheaper secondhand than the same thing with a Porsche label on it..

Sun Jul 27, 2025 3:52 am

Due to the high air flow ratio to very small vacuum depression value, the L jetronic system is not really effected by minor leaks keeping within factory recommended A/F ratio tolerances, especially at higher rpm's and as a new part the AAR ,due to it's dry press fabrication could of let some smoke by .
Something important to note, the L flap is spring floated and bottoms out on AFM lower metal cover plate every time you hit a bump in the road wearing both the L flap and the plate over thousands of miles, allowing air to bypass the AFM internally.
This is much more incidental than the minor leak and should be addressed in priority if you are looking for a perfect to factory blueprint
settings.
As regards the leak, super glue could be ideal for perfecting the seal, bear in mind the glue could ingress and stick up the AAR and therefore should be applied with the AAR positioned so as to take advantage of gravity to keep it from ingressing too deeply.

Mon Jul 28, 2025 1:21 pm

Nitramrebrab72 wrote: Due to the high air flow ratio to very small vacuum depression value, the L jetronic system is not really effected by minor leaks keeping within factory recommended A/F ratio tolerances, especially at higher rpm's and as a new part the AAR ,due to it's dry press fabrication could of let some smoke by .
Something important to note, the L flap is spring floated and bottoms out on AFM lower metal cover plate every time you hit a bump in the road wearing both the L flap and the plate over thousands of miles, allowing air to bypass the AFM internally.
This is much more incidental than the minor leak and should be addressed in priority if you are looking for a perfect to factory blueprint
settings.
As regards the leak, super glue could be ideal for perfecting the seal, bear in mind the glue could ingress and stick up the AAR and therefore should be applied with the AAR positioned so as to take advantage of gravity to keep it from ingressing too deeply.

Interesting. I’ll have to take a look at the afm. I corrected the leak on the aar and my symptoms continue (fluctuating idle 100-300rpm) I’ve found no other vacuum leaks in the engine compartment. I’ll be testing my brake booster next.

Mon Jul 28, 2025 7:40 pm

i have taken them apart and repaired non-functional ones.
they function just like a choke element.

Tue Jul 29, 2025 6:11 pm

Globedog12 wrote: Nitramrebrab72 wrote: Due to the high air flow ratio to very small vacuum depression value, the L jetronic system is not really effected by minor leaks keeping within factory recommended A/F ratio tolerances, especially at higher rpm's and as a new part the AAR ,due to it's dry press fabrication could of let some smoke by .
Something important to note, the L flap is spring floated and bottoms out on AFM lower metal cover plate every time you hit a bump in the road wearing both the L flap and the plate over thousands of miles, allowing air to bypass the AFM internally.
This is much more incidental than the minor leak and should be addressed in priority if you are looking for a perfect to factory blueprint
settings.
As regards the leak, super glue could be ideal for perfecting the seal, bear in mind the glue could ingress and stick up the AAR and therefore should be applied with the AAR positioned so as to take advantage of gravity to keep it from ingressing too deeply.

Interesting. I’ll have to take a look at the afm. I corrected the leak on the aar and my symptoms continue (fluctuating idle 100-300rpm) I’ve found no other vacuum leaks in the engine compartment. I’ll be testing my brake booster next.

Check your fuel pressure. You have rising rate fuel pressure regulator. Make sure the pressure is its lowest at idle and steady.

Check your timing. Does the timing light show timing rising and falling with the idle change.
If so, disconnect vacuum advance and check again.

It could be a worn TB or could be the seal/o-ring between TB and plenu.

Make sure your throttle plate is closing all the way.

Ray

Wed Jul 30, 2025 11:00 am

I’ll verify fuel pressure and timing. As for the tb I recently had it refurbished so I have faith that is as good as it can be. I did have a leak between the tb and the plenum that I fixed

Wed Jul 30, 2025 2:06 pm

Rpm fluctuation not as bad as I had stated. Currently fluctuates between 875 -925. Maybe a little more. The timing tracks right with the rpm. Up and down a couple degrees. Hose disconnected and plugged. Static fuel pressure (engine not running flap opened to turn on pump) is 46 pounds and at idle 40. So pretty high. I might revisit the tb gasket. I have not seen any smoke when testing but spraying some brake cleaner in the area and my rpm shoot way up.

Wed Jul 30, 2025 3:21 pm

since you don't know the cam, that is a hard one. The webcam 142, or any other cam with its duration will lope a little at idle. That is part of the reason VW stopped using that cam - it was a little too much for the average street car. So, if the problem is cam driven, you won't get away from it. I am not saying that is the cause.

Thu Jul 31, 2025 12:50 am

Globedog12 wrote: Nitramrebrab72 wrote: Due to the high air flow ratio to very small vacuum depression value, the L jetronic system is not really effected by minor leaks keeping within factory recommended A/F ratio tolerances, especially at higher rpm's and as a new part the AAR ,due to it's dry press fabrication could of let some smoke by .
Something important to note, the L flap is spring floated and bottoms out on AFM lower metal cover plate every time you hit a bump in the road wearing both the L flap and the plate over thousands of miles, allowing air to bypass the AFM internally.
This is much more incidental than the minor leak and should be addressed in priority if you are looking for a perfect to factory blueprint
settings.
As regards the leak, super glue could be ideal for perfecting the seal, bear in mind the glue could ingress and stick up the AAR and therefore should be applied with the AAR positioned so as to take advantage of gravity to keep it from ingressing too deeply.

Interesting. I’ll have to take a look at the afm. I corrected the leak on the aar and my symptoms continue (fluctuating idle 100-300rpm) I’ve found no other vacuum leaks in the engine compartment. I’ll be testing my brake booster next.

There is a compensating screw that can be used to make up for the internal wear on the AFM. Internal wear really only effects idle and the screw will only effect idle . Before you adjust it remove the large square cap on the AFM screw then the adjuster screw cap, spray some WD 40 down the adjuster screw thread as there is a rubber o ring seal that can be damaged if rotated dry.
Unscrew(anti clockwise) half a turn first to make sure it isn't already fully screwed in. Now screw in (clockwise) and you should see the copper potentiometer arm go anti clockwise..
Screw in slowly taking note of how many turns .
If the idle rpm's goes up when adjusting the screw keep turning the screw until they no longer go up, then turn back ever so slightly the screw.
Once you have found the sweet spot for the screw re-adjust the idle back to what it was. Once you have readjusted the idle check the AFM screw is still in its sweet spot. By screwing the AFM in then out no more than half a turn looking for it's smoothest idling position, which is usually the fastest position but not necessarily .
This will perfect your idle as regards the AFM but it could also be compensating for another underlying issue so keeping note of the AFM screw position could be useful.

.

Thu Jul 31, 2025 5:09 pm

Nitramrebrab72 I’m not quite to that point yet but I’ll keep this in mind. Still trying to eliminate any other issues.

Fri Aug 01, 2025 9:27 am

Flip off the AFM plastic cover , check it turns smoothly by hand, you can also check if there is a vacuum leak or if air is bypassing the AFM. Start the engine and hold the inertia weight in your right hand and slowly move it until you have a smooth running engine.
If moving it anticlockwise helped it means you are running lean probably due to a vacuum leak.

If it did not help then it is not a vacuum leak related problem.

Fri Aug 01, 2025 9:56 am

Because it was very elusive - I know of a bus with a tear in the S-Boot that was 'closed' until the engine was running and the vibration opened up the slit.

It was not evident until the S-Boot was removed and flexed.
It could not be detected with it in place because it was in one of the inner folds.

A very long shot - but maybe.....?

Fri Aug 01, 2025 4:58 pm

andrewtf wrote: Because it was very elusive - I know of a bus with a tear in the S-Boot that was 'closed' until the engine was running and the vibration opened up the slit.

It was not evident until the S-Boot was removed and flexed.
It could not be detected with it in place because it was in one of the inner folds.

A very long shot - but maybe.....?

I actually replaced mine with a new one.

Fri Aug 01, 2025 5:02 pm

Nitramrebrab72 wrote: Globedog12 wrote: Nitramrebrab72 wrote: Due to the high air flow ratio to very small vacuum depression value, the L jetronic system is not really effected by minor leaks keeping within factory recommended A/F ratio tolerances, especially at higher rpm's and as a new part the AAR ,due to it's dry press fabrication could of let some smoke by .
Something important to note, the L flap is spring floated and bottoms out on AFM lower metal cover plate every time you hit a bump in the road wearing both the L flap and the plate over thousands of miles, allowing air to bypass the AFM internally.
This is much more incidental than the minor leak and should be addressed in priority if you are looking for a perfect to factory blueprint
settings.
As regards the leak, super glue could be ideal for perfecting the seal, bear in mind the glue could ingress and stick up the AAR and therefore should be applied with the AAR positioned so as to take advantage of gravity to keep it from ingressing too deeply.

Interesting. I’ll have to take a look at the afm. I corrected the leak on the aar and my symptoms continue (fluctuating idle 100-300rpm) I’ve found no other vacuum leaks in the engine compartment. I’ll be testing my brake booster next.

There is a compensating screw that can be used to make up for the internal wear on the AFM. Internal wear really only effects idle and the screw will only effect idle . Before you adjust it remove the large square cap on the AFM screw then the adjuster screw cap, spray some WD 40 down the adjuster screw thread as there is a rubber o ring seal that can be damaged if rotated dry.
Unscrew(anti clockwise) half a turn first to make sure it isn't already fully screwed in. Now screw in (clockwise) and you should see the copper potentiometer arm go anti clockwise..
Screw in slowly taking note of how many turns .
If the idle rpm's goes up when adjusting the screw keep turning the screw until they no longer go up, then turn back ever so slightly the screw.
Once you have found the sweet spot for the screw re-adjust the idle back to what it was. Once you have readjusted the idle check the AFM screw is still in its sweet spot. By screwing the AFM in then out no more than half a turn looking for it's smoothest idling position, which is usually the fastest position but not necessarily .
This will perfect your idle as regards the AFM but it could also be compensating for another underlying issue so keeping note of the AFM screw position could be useful.

.

Started to take a look at this tonight but unfortunately someone drove my adjustment screw all the way in and stripped it. If I understand you correctly I am currently bypassing the max air. Not that this helps me because I can’t adjust at all.