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SGKent 
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Joined: October 30, 2007
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Location: at the beach
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 Posted: Sat Aug 02, 2025 12:56 pm Post subject: Re: 2.0 FI vacuum leaks |
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do you have a late 1976 or an early 1976 system? I am guessing late. The early had the electric EGR and switch I think. Late has the mechanical EGR same as 1977, and 1978. Early 1976 is like the 1975.
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Globedog12
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Joined: April 11, 2024
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Location: Atlanta, GA
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| Posted: Sat Aug 02, 2025 12:58 pm Post subject: Re: 2.0 FI vacuum leaks |
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| SGKent wrote: |
| do you have a late 1976 or an early 1976 system? I am guessing late. The early had the electric EGR and switch I think. Late has the mechanical EGR same as 1977, and 1978. Early 1976 is like the 1975. |
Late. I have the mechanical egr.
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SGKent
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| Posted: Sat Aug 02, 2025 5:38 pm Post subject: Re: 2.0 FI vacuum leaks |
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018 or 020 are probably the appropriate one. Both are identical other than the numbers.
L-Jet was used on a lot of cars and there are a plethora of NOS units for some models out there. I wish I knew if those systems were also compatible.
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Globedog12
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| Posted: Thu Aug 07, 2025 6:34 pm Post subject: Re: 2.0 FI vacuum leaks |
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| Nitramrebrab72 wrote: |
You don't need smoke to determine if air is by passing the L flap and smoke is not a 100% fool proof test.
The L flap is connected to the potentiometer which in turn determines richness by it's position , if there is a vacuum leak air will bypass the flap, but not only, it can bypass the flap if it is worn(this gets compensated with the screw any wear here will normally only slightly effect idle) or if the backfire safety valve is stuck open in the flap, bearing the above in mind, there is a simple way of testing the correct position off the flap and check no air flow is bypassing the AFM and that is to remove the plastic 2 1/2" by 2 1/2" cover, and with engine running and your hand on the small inertia weight manually move it ever so gently and slightly anti clockwise, if the idle increases and stabilises doing so then the engine is running too lean at idle possibly due to the potentiometer being in the wrong position, caused by a) air by-passing the flap from an external a vacuum leak, b) air by-passing the flap due an internal leak in the AFM(sticking or broken valve or worn or broken L flap)
If that didn't do it try clockwise, if that helps it means you are running to rich or possibly the spark is too weak.
If clockwise or anticlockwise don't make any improvements and make it worse then it is likely not to be vacuum leak/air by-passing related it could also be your manual intervention making up for another fault that is why it is always advisable to know the condition of your leads, plugs, distributor rotor and cap, visually checking spark from a plug could be useful info to have in diagnosing your fault.
Also check the flap moves freely by hand engine off, (go gently careful not have the copper arm get damaged ) if it is sticking it could have the effect you are experiencing. |
So turning the potentiometer counterclockwise increases idle and clockwise it begins to stumble. I’m not moving it much only 1/16” or so. If I understand correctly I am running lean. My flap moves freely and easily. From this I’m thinking a few things. I have unmetered air either because of internal Afm wear or vacuum leaks elsewhere. I can’t find any vacuum leaks in the engine compartment. My brake booster held 10 inches vacuum and degraded to 5 inches overnight. Could be my test setup. I’m guessing brake booster is good. The potentiometer is pretty touchy so I s hard to tell if increasing enrichment settles idle. Warmed up running condition right now I still get about 50 rpm variation. My throttle valve is fully closed and recently refurbished by sparxwerks. Although not perfect bill says I should be able to tune just fine. Any other places to look for a vacuum leak that a smoke test might not show? I’m trying to eliminate all other possibilities so I can say it is the Afm itself passing air without the flap moving. Im going to recheck valve cover gaskets, oil filler cap and clamp and maybe oil breather gasket. Anything I’m missing? Unfortunately as I mention before the bypass screw on the Afm is stuck and stripped closed so is it is a possibility that my flap is moving ever so slightly that I can’t see it?
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SGKent
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Joined: October 30, 2007
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| Posted: Thu Aug 07, 2025 7:56 pm Post subject: Re: 2.0 FI vacuum leaks |
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I would not have messed with the settings on the wiper arm without a properly running AFM and an O2 sensor. It is very sensitive.
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airschooled
Air-Schooled
Joined: April 04, 2012
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Location: West Coast, USA
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| Posted: Thu Aug 07, 2025 9:01 pm Post subject: Re: 2.0 FI vacuum leaks |
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[quote="Globedog12"]So turning the potentiometer counterclockwise increases idle and clockwise it begins to stumble. I’m not moving it much only 1/16” or so. If I understand correctly I am running lean.[quote]
You are running lean at that RPM at that temperature in that moment. Once you touch the throttle and speed builds up, mixtures change completely.
Your previous post mentions timing haivng a 3° spread or so, which is common in engines with play in the distributor drive gear, distributor, or crank end play. Usually there's a little of each, but 3° isn't enough to cause performance issues.
It is enough to cause a 50rpm fluctuation at idle.
| Quote: |
| My throttle valve is fully closed and recently refurbished by sparxwerks. Although not perfect bill says I should be able to tune just fine. Any other places to look for a vacuum leak that a smoke test might not show? |
Bill @ Sparxwerks has a much higher standard for throttle bodies than I do, but even the leakiest throttle bodies don't fluctuate at idle on non-O2 sensor engines in my experience. They either leak at a given temperature or don't. Yes it's nice to have them perfect to check off your list, but I would bet money on your timing scatter causing the speed change IF it's truly 50rpm.
Are you seeing it on a meter or listening? Can you post a video of the fluctuations? Are you confident that your measuring tool is capable of discerning a 50rpm difference?
Robbie
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Globedog12
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Location: Atlanta, GA
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| Posted: Fri Aug 08, 2025 5:05 am Post subject: Re: 2.0 FI vacuum leaks |
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[quote="airschooled"][quote="Globedog12"]So turning the potentiometer counterclockwise increases idle and clockwise it begins to stumble. I’m not moving it much only 1/16” or so. If I understand correctly I am running lean.
| Quote: |
You are running lean at that RPM at that temperature in that moment. Once you touch the throttle and speed builds up, mixtures change completely.
Your previous post mentions timing haivng a 3° spread or so, which is common in engines with play in the distributor drive gear, distributor, or crank end play. Usually there's a little of each, but 3° isn't enough to cause performance issues.
It is enough to cause a 50rpm fluctuation at idle.
| Quote: |
| My throttle valve is fully closed and recently refurbished by sparxwerks. Although not perfect bill says I should be able to tune just fine. Any other places to look for a vacuum leak that a smoke test might not show? |
Bill @ Sparxwerks has a much higher standard for throttle bodies than I do, but even the leakiest throttle bodies don't fluctuate at idle on non-O2 sensor engines in my experience. They either leak at a given temperature or don't. Yes it's nice to have them perfect to check off your list, but I would bet money on your timing scatter causing the speed change IF it's truly 50rpm.
Are you seeing it on a meter or listening? Can you post a video of the fluctuations? Are you confident that your measuring tool is capable of discerning a 50rpm difference?
Robbie |
I am seeing the fluctuation on my dash tach , digital timing light when hooked up and I can hear it. The distributor was recently refurbished end play .005.
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Globedog12
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| Posted: Fri Aug 08, 2025 5:06 am Post subject: Re: 2.0 FI vacuum leaks |
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| SGKent wrote: |
| I would not have messed with the settings on the wiper arm without a properly running AFM and an O2 sensor. It is very sensitive. |
I didn’t change any settings.
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SGKent
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| Posted: Fri Aug 08, 2025 7:43 am Post subject: Re: 2.0 FI vacuum leaks |
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| Globedog12 wrote: |
| SGKent wrote: |
| I would not have messed with the settings on the wiper arm without a properly running AFM and an O2 sensor. It is very sensitive. |
I didn’t change any settings. |
good. stay out without the proper gauges to monitor the mixture.
As I said earlier - it could be the camshaft duration causing the wandering too. My webcam 142 does it and it is normal for that Porsche cam profile to do that.
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Globedog12
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| Posted: Fri Aug 08, 2025 9:27 am Post subject: Re: 2.0 FI vacuum leaks |
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Here is a quick video. It’s pretty representative of how it idles constantly. Sounding more like something I’ll have to live with for now.
https://youtube.com/shorts/v4T1llPHDGw?si=91k9C4HuSY1dSfWJ
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airschooled
Air-Schooled
Joined: April 04, 2012
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Location: West Coast, USA
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| Posted: Fri Aug 08, 2025 10:17 am Post subject: Re: 2.0 FI vacuum leaks |
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There’s no audio in that video.
Also, vertical videos uploaded to YouTube don’t allow pause, scroll, zooming, or scrutinizing, making them functionally useless for diagnosing cars.
Robbie
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Globedog12
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| Posted: Fri Aug 08, 2025 11:09 am Post subject: Re: 2.0 FI vacuum leaks |
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| airschooled wrote: |
There’s no audio in that video.
Also, vertical videos uploaded to YouTube don’t allow pause, scroll, zooming, or scrutinizing, making them functionally useless for diagnosing cars.
Robbie |
That was the first video I’ve ever uploaded to YouTube so give me a little break. I wasn’t aware of the vertical limitations. You do see the rpm fluctuate though no?
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SGKent
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| Posted: Fri Aug 08, 2025 11:46 am Post subject: Re: 2.0 FI vacuum leaks |
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if you want to chase it further, you will need to replace the AFM with one that is not damaged, add a bung for an O2 sensor to the exhaust, and put an O2 gauge on it. That or have access to a professional smog analyzer to see what is going on. You might be able to pay a smog shop to run an analysis on it in non-test mode, and then post the results so opinions can be offered. The surging your bus experiences can be mixture, ignition, or camshaft timing related. There is also non-bung adapter Innovate makes that can be used. Colin has one if you want to hire him to tune your AFM but I would suggest getting one without a broken weight rather than trying to fix a broken one.
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Last edited by SGKent on Fri Aug 08, 2025 5:30 pm; edited 1 time in total |
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Nitramrebrab72
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Joined: November 10, 2018
Posts: 898
Location: France
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| Posted: Fri Aug 08, 2025 1:39 pm Post subject: Re: 2.0 FI vacuum leaks |
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| Globedog12 wrote: |
| Nitramrebrab72 wrote: |
You don't need smoke to determine if air is by passing the L flap and smoke is not a 100% fool proof test.
The L flap is connected to the potentiometer which in turn determines richness by it's position , if there is a vacuum leak air will bypass the flap, but not only, it can bypass the flap if it is worn(this gets compensated with the screw any wear here will normally only slightly effect idle) or if the backfire safety valve is stuck open in the flap, bearing the above in mind, there is a simple way of testing the correct position off the flap and check no air flow is bypassing the AFM and that is to remove the plastic 2 1/2" by 2 1/2" cover, and with engine running and your hand on the small inertia weight manually move it ever so gently and slightly anti clockwise, if the idle increases and stabilises doing so then the engine is running too lean at idle possibly due to the potentiometer being in the wrong position, caused by a) air by-passing the flap from an external a vacuum leak, b) air by-passing the flap due an internal leak in the AFM(sticking or broken valve or worn or broken L flap)
If that didn't do it try clockwise, if that helps it means you are running to rich or possibly the spark is too weak.
If clockwise or anticlockwise don't make any improvements and make it worse then it is likely not to be vacuum leak/air by-passing related it could also be your manual intervention making up for another fault that is why it is always advisable to know the condition of your leads, plugs, distributor rotor and cap, visually checking spark from a plug could be useful info to have in diagnosing your fault.
Also check the flap moves freely by hand engine off, (go gently careful not have the copper arm get damaged ) if it is sticking it could have the effect you are experiencing. |
So turning the potentiometer counterclockwise increases idle and clockwise it begins to stumble. I’m not moving it much only 1/16” or so. If I understand correctly I am running lean. My flap moves freely and easily. From this I’m thinking a few things. I have unmetered air either because of internal Afm wear or vacuum leaks elsewhere. I can’t find any vacuum leaks in the engine compartment. My brake booster held 10 inches vacuum and degraded to 5 inches overnight. Could be my test setup. I’m guessing brake booster is good. The potentiometer is pretty touchy so I s hard to tell if increasing enrichment settles idle. Warmed up running condition right now I still get about 50 rpm variation. My throttle valve is fully closed and recently refurbished by sparxwerks. Although not perfect bill says I should be able to tune just fine. Any other places to look for a vacuum leak that a smoke test might not show? I’m trying to eliminate all other possibilities so I can say it is the Afm itself passing air without the flap moving. Im going to recheck valve cover gaskets, oil filler cap and clamp and maybe oil breather gasket. Anything I’m missing? Unfortunately as I mention before the bypass screw on the Afm is stuck and stripped closed so is it is a possibility that my flap is moving ever so slightly that I can’t see it? |
Take note of where the idle screw is .
Now unscrew the idle screw a full turn. Try turning the potentiometer to see which way , clockwise or anticlockwise improves it, turn a second turn and do the same repeat this 4 or 5 turns and you will get a better picture of the state of the AFM if on the 4th of 5th turn the AFM is set right then it's more of an idle only mixture problem.
1/16th of an inch on the copper arm is not that bad. It's not correct but it's far from being a disaster.
Do the 5 idle turns taking note of the distance(in mm if possible and as accurate as you can) and direction turned to rectify the mixture if any.
From what you said about the AFM not sticking that is a very good sign and I am sure you will have her running on the sweèt soon.
As regards the AFM flap letting air past it rectifyable without the screw, I just fixed an L jetronic on a 2.0l Vanagon and the flap was so twisted I filed a load off the side and still got her to run sweet after some tweaks she had never ran so.smoothly with extra torque and a very smooth acceleration.
Look for a vacuum leak I believe it will be more likely that than an internal AFM leak from what your saying but it still could be an internal leak.
If you don't find one I will try and guide you to every so slightly fine tune the AFM from a distance.
There are a few options but first you must be sure it is the AFM at fault and nothing else.
The options are as follows but will depend on the test results with the 5 idle screw turns.
A) add a variable resistor to the cylinder head temp sender and wire it up at the dash so you can adjust it whilst driving to find the right extra resistance needed. Once determined measure the variable resistor and add a fixed resistor of the measured amount.
B)Adjust the copper arm ever so slightly counter clockwise
C)Loosen the clockwork spring a few teeth.
D) Block off the small bypass port in the AFM.
E) a bit of B and C
But before adjusting anything on the AFM mark with a fine permanent white marker and take some pictures .
Is there an idle stabiliser on that model? If faulty it could give a false lean result when using the clock and anticlockwise copper arm test . |
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Nitramrebrab72
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| Posted: Fri Aug 08, 2025 1:46 pm Post subject: Re: 2.0 FI vacuum leaks |
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Also the backfiring relief valve could be stuck slightly open allowing for air to bypass the L flap.
Another thing to check is that the throttle flap sender is properly plugged in and working(it's under the throttle body on most type 4's fitted with L - Jetronic) . |
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airschooled
Air-Schooled
Joined: April 04, 2012
Posts: 13524
Location: West Coast, USA
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| Posted: Fri Aug 08, 2025 5:25 pm Post subject: Re: 2.0 FI vacuum leaks |
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| Globedog12 wrote: |
| airschooled wrote: |
There’s no audio in that video.
Also, vertical videos uploaded to YouTube don’t allow pause, scroll, zooming, or scrutinizing, making them functionally useless for diagnosing cars.
Robbie |
That was the first video I’ve ever uploaded to YouTube so give me a little break. I wasn’t aware of the vertical limitations. You do see the rpm fluctuate though no? |
I don’t see the rpm fluctuate, I see a digital screen with numbers on it. 
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Globedog12
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| Posted: Fri Aug 08, 2025 7:02 pm Post subject: Re: 2.0 FI vacuum leaks |
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Nitramrebrab72 I will give that test a try this weekend without making any adjustments but I think SGKent is right. I have replacement Afm coming soon. I’m going to take my exhaust into the fab shop where I work and have them weld a bung on for air fuel mixture to see what any changes I make are doing. And to airschooled I’ll get a better video this weekend. Not sure how the sound will come out with my phone but I’ll give it a shot. I’ll get engine running with tach/dwell hooked up and take a shot of my in dash tach as well.
Thanks guys
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airschooled
Air-Schooled
Joined: April 04, 2012
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Location: West Coast, USA
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| Posted: Fri Aug 08, 2025 9:39 pm Post subject: Re: 2.0 FI vacuum leaks |
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I don’t have a link to my instructional at the moment, but in order of most important:
Hold the phone landscape.
Step away from the car for general engine sounds and inquiries.
Anything is a tripod except shaky hands.
Check your privacy settings on the video.
Post from a computer, or delete the “m.” at the beginning of a YouTube URL to embed here. Make sure there are no spaces in your embed code.
—
I would film like this for thirty seconds cold, then open the decklid for another thirty. Then warm up the bus, another minute of video open/closed. It would be the same for something like a fuel pressure test- thirty seconds of video on the gauge while you unplug the FPR vacuum hose and plug it back in, then thirty seconds of the gauge while revving.
Robbie
Link
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Nitramrebrab72
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Posts: 898
Location: France
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| Posted: Sat Aug 09, 2025 12:42 am Post subject: Re: 2.0 FI vacuum leaks |
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Other places to check for vacuum leaks:
Fuel injector to intake manifold seals.
Intake manifold to cylinder head.
The 4 rubber intake manifold hoses.
Throttle body to distributor hose .
Throttle body, check that any unused vacuum pick-up has it's bung in place.
Distributor hoses.
Fuel pressure regulator hose.
Throttle body to intermediate intake manifold( one with 4 outlets)is properly tightened down and that the seal is properly in place and not pinched up and to the side. |
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Globedog12
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Joined: April 11, 2024
Posts: 221
Location: Atlanta, GA
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| Posted: Sat Aug 16, 2025 12:41 pm Post subject: Re: 2.0 FI vacuum leaks |
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Been a busy couple weeks so not much progress but here is a video of cold and hot idle. I have a refurbished Afm to put in and a wide band af sensor and gauge on the way.
https://youtu.be/ixrTcMqGeJQ
Cold idle
https://youtu.be/yqFWG-kykng
Hot idle
https://youtu.be/InCtGJLyMcc
Tach at hot idle
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