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busdaddy Samba Member

Joined: February 12, 2004 Posts: 52681 Location: Surrey B.C. Canada, but thinking of Ukraine
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Posted: Mon Jan 15, 2024 11:04 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Usually you clamp a length of 7.5mm or 5/16" fuel hose to it. _________________ Rust NEVER sleeps and stock never goes out of style.
Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
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Good Old Dusty Samba Member

Joined: September 17, 2016 Posts: 20 Location: British Columbia
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Posted: Mon Jan 15, 2024 10:55 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Thank you T-Cash for your excellent post on testing fuel pressure testing from September 2017. I have one question for the community. In the 1978 FI system there is a T connection in the metal fuel rail between the #3 and #4 injectors. The T connection has a 7 mm bolt in it. The bolt is unscrewed to allow for the attachment of a fuel pressure test gauge. What is the type of fitting, i.e. screw thread size and thread type required to screw into the T connection?
Thanks for your feedback. |
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my59 Samba Member

Joined: August 13, 2003 Posts: 3964 Location: connecting the dots
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Posted: Sun Dec 03, 2023 3:06 pm Post subject: Re: Testing FI Fuel Pressure FAQ |
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Adding a fuel rated shut off valve at the "C" hose location makes fuel filter changes less messy. _________________ my59: Well son, my grandfather died before I got to drive it, so does that answer your question?
our79: sunroof bus w/camper interior and 2.0 FI
Other:'12 Jetta, '77 Benz 300D, and a 74 MG Midget. |
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busdaddy Samba Member

Joined: February 12, 2004 Posts: 52681 Location: Surrey B.C. Canada, but thinking of Ukraine
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Posted: Sun Dec 03, 2023 10:05 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Disconnect the green wire points from the ignition coil and cram something into the throat of the AFM so the flap is off it's stop, pump will run when the key is on. _________________ Rust NEVER sleeps and stock never goes out of style.
Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
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sblu Samba Member
Joined: April 08, 2019 Posts: 209 Location: Austin, TX
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Posted: Sun Dec 03, 2023 9:47 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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folks, its been quite a journey, but i've managed to replace all the fuel lines on the bus including solenoid valve + bypass as well as a one-way valve on the output of the fuel pressure valve. I tested the connections for the solenoid valve + bypass with 100PSI air to ensure no leaks. Now that everything is back in, I'd like to look for leaks throughout the entire system. Is there a clever way to have the fuel pump only on (to reduce any fire possibility) while i go about doing this?
Once the fuel lines are all intact, i can test the pressure as laid out in this thread, with the engine idling if needed.
thanks! _________________ 1978 Westfalia Champagne Edition Project |
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SGKent  Samba Member

Joined: October 30, 2007 Posts: 42364 Location: at the beach
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Posted: Tue Jan 07, 2020 2:10 pm Post subject: Re: Testing FI Fuel Pressure FAQ |
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I would think that the pressure in the hose between the return nipple and the FPR will equal whatever the fuel pump will put out which is about 100 PSI. When the hose is open the pressure between the pump and FPR should be about 32 - 36 - there is a chart. But once the return is sealed then the pressure will equal whatever the pump will do. I haven't tried seeing how high and hard I can push the pump, its sort of a destructive test pushing anything to its limit to see what it can do. All it tells you is what it did this time. Don't try it in a submarine. _________________ "Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin |
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busdaddy Samba Member

Joined: February 12, 2004 Posts: 52681 Location: Surrey B.C. Canada, but thinking of Ukraine
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Posted: Tue Jan 07, 2020 10:50 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Urglegrew wrote: |
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"If you pinch shut the return fuel line SLOWLY FOR ONLY A FEW SECONDS(when you are by the starter), the pressure should climb to about 4 bars or 59 psi and that will show you that the fuel pump relief valve is indeed working correctly, but your regulator is "toast". If the pressure does not rise, there is a problem with the fuel delivery system."
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If I get a spike/59 psi (my neighbor when testing said mine went up to almost 100 or so?) when pinching the hose behind the fuel pressure reg my fuel pump relief valve works but this shows that my regulator is toast? Or my regulator is toast if the pressure doesn't rise when I do this?
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"If the fuel pressure is too high, you need to shuff off the engine and remove the fuel return line from the regulator and direct it into a gas can. Run the pressure test agin and if the pressure is OK, there is something blocking the return line. Low pressure compressed air or a non-sparking wire might help clean it out. The gas pumping around the fuel rail is also what keeps the fuel pump cool." |
Does this mean if I get too high of fuel pressure when I pinch the return line, or just overall when idling? |
If you got 100psi the relief valve in the pump has issues, or it's a pump from some other car that makes too much pressure or flow. If the pressure is ~30 un pinched the regulator is working.
The second quote describes what to test if the normal idling un pinched pressure is too high. _________________ Rust NEVER sleeps and stock never goes out of style.
Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
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Urglegrew Samba Member
Joined: February 21, 2014 Posts: 76 Location: Bay Area California
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Posted: Tue Jan 07, 2020 9:38 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Quote: |
"If you pinch shut the return fuel line SLOWLY FOR ONLY A FEW SECONDS(when you are by the starter), the pressure should climb to about 4 bars or 59 psi and that will show you that the fuel pump relief valve is indeed working correctly, but your regulator is "toast". If the pressure does not rise, there is a problem with the fuel delivery system."
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If I get a spike/59 psi (my neighbor when testing said mine went up to almost 100 or so?) when pinching the hose behind the fuel pressure reg my fuel pump relief valve works but this shows that my regulator is toast? Or my regulator is toast if the pressure doesn't rise when I do this?
Quote: |
"If the fuel pressure is too high, you need to shuff off the engine and remove the fuel return line from the regulator and direct it into a gas can. Run the pressure test agin and if the pressure is OK, there is something blocking the return line. Low pressure compressed air or a non-sparking wire might help clean it out. The gas pumping around the fuel rail is also what keeps the fuel pump cool." |
Does this mean if I get too high of fuel pressure when I pinch the return line, or just overall when idling? |
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JaimeH  Samba Member

Joined: April 24, 2012 Posts: 266 Location: Canterbury, NH
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Posted: Tue Sep 11, 2018 1:37 pm Post subject: Re: Testing FI Fuel Pressure FAQ |
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Does the fuel pressure normaly fluctuate like this with the engine running? The pressure is steady with the engine stopped and the pump running with the AFM jumped.
Link
[/youtube] _________________ 1978 Transporter 2.0 FI, BA6
Special Thanks to Razor's Customs, Boscawen, NH |
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furgo Samba Member

Joined: September 06, 2016 Posts: 944 Location: Southern Germany
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Posted: Mon May 07, 2018 5:57 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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busdaddy wrote: |
A bus will run with 0 fuel pressure via gravity feed and manifold vacuum, it'll idle but dies when you step on the gas. The injectors still "fire", they just don't provide much fuel and the spray pattern is likely bad too. |
raygreenwood wrote: |
[...]. They will open and admit fuel all the way down to near 0 pressure. |
Perfect, that makes it clear, thanks!
raygreenwood wrote: |
However...understand that with an operative pressure range of about 36-42 psi....every 1 psi of pressure is worth 2.94% to 2.38% of total fuel delivery.
So being 2-3 psi off can be significant. |
In this particular case case we're talking about no running at all (unfortunately), rather than running poorly, but nevertheless a good point.
busdaddy wrote: |
Have you tried priming it through a port on the S boot when you know it's going to be hard to start?, that'll decide if it's lean or has flooded itself while sitting. A trigger sprayer for household cleaners makes an excellent primer bottle. |
Yes, that will be the next step, but before I want to check the injector solenoids and the injector pulses to rule out the electrical side of things.
In any case, my fuel pressure question has been answered. I'll keep diagnosing elsewhere. _________________ '79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
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raygreenwood Samba Member
Joined: November 24, 2008 Posts: 23074 Location: Oklahoma City
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Posted: Sun May 06, 2018 11:41 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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As BD noted....the injectors "firing" have nothing to do with pressure. They will open and admit fuel all teh way down to near 0 pressure.
However...understand that with an operative pressure range of about 36-42 psi....every 1 psi of pressure is worth 2.94% to 2.38% of total fuel delivery.
So being 2-3 psi off can be significant. Also...understand that though the "percentage" of total fuel dosage per psi is lower at higher pressures....you get an inverse curve because you HAVE more psi.
Fuel "dosage" is pressure X time. Ray |
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busdaddy Samba Member

Joined: February 12, 2004 Posts: 52681 Location: Surrey B.C. Canada, but thinking of Ukraine
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Posted: Sun May 06, 2018 9:07 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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A bus will run with 0 fuel pressure via gravity feed and manifold vacuum, it'll idle but dies when you step on the gas. The injectors still "fire", they just don't provide much fuel and the spray pattern is likely bad too.
Have you tried priming it through a port on the S boot when you know it's going to be hard to start?, that'll decide if it's lean or has flooded itself while sitting. A trigger sprayer for household cleaners makes an excellent primer bottle. _________________ Rust NEVER sleeps and stock never goes out of style.
Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!
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furgo Samba Member

Joined: September 06, 2016 Posts: 944 Location: Southern Germany
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Posted: Sun May 06, 2018 8:38 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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I'm trying to diagnose a no start situation and after double-checking there is spark and compression, I came back to this fine thread wrt. fuel pressure checks.
Randy in Maine wrote: |
With the engine idling the gauge should read about 2 bars or about 29 psi +/- 3 psi. With the engine still idling, remove the vacuum line to the fuel pressure regulator and the pressure should go to about 2.5 bars or about 36 psi +/- 3 psi. The injectors won't fire if it is <26 psi. |
This implies that if pressure is just below spec (29 - 3 = 26 psi), there will be no fuel coming out of the injectors. Do I understand this correctly? I would have assumed that if the injectors are pulsed, the nozzle would open/close at the same frequency and let some fuel out, even if the spray pattern is poor.
The reason I'm asking is because I have a theory for my current issue:
1. Drop in pressure below min spec => injectors will not fire, plus;
2. Either TTS not working (open) or also because of low pressure => CSV not firing either;
3. thus no fuel at all, no start, even with spark and compression
I'm just trying to figure out if this logic makes sense regarding fuel pressure (not intending to diagnose the full issue here, there are more checks to be done).
Essentially trying to get the fuel pressure bits clear in my head before I start pulling injectors, the pump or the FPR. #1 is the one I'm not sure about, any takers? _________________ '79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
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Tcash Samba Member

Joined: July 20, 2011 Posts: 12846 Location: San Jose, California, USA
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Posted: Tue Sep 26, 2017 12:12 pm Post subject: Fuel Injection Fuel Pressure Testing |
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To link to this post copy and paste
Code: |
[url=https://www.thesamba.com/vw/forum/viewtopic.php?p=8569192#8569192]Fuel Injection Fuel Pressure Testing[/url] |
FUEL PRESSURE TESTING
1. Disconnect the wire (usually green) from the #1 terminal on the coil that leads to the distributor. (so you do not fry the points and or the coil during testing, while leaving the ignition switch on). Unplug the CSV (cold start valve) (so it will not open during testing).
2. Connect fuel gauge to test fitting on 3-4 Fuel rail. Or between 3-4 Fuel rail and Fuel pump hose.
3. Turn the ignition switch to the 1st run postition.
4. Pressurize the fuel system. Remove the Air filter and use a chopstick to open the flap in the AFM (air flow meter) to pressurize the fuel system. Once system is pressurized, close flap.
5. Fuel pressure gauge should read 35-38 psi.
(to low check FPR diaphragm is not leaking gas. If it is replace FPR)
(to low run fuel pump capacity test. Passes test replace FPR)
(to high check for restriction in return hose)
(to high check fuel pump specifications 12v 43.5psi 34 GPH)
(to high check alternator output it not over 14.5 volts)
6. Fuel gauge reading should not drop below 14 psi in 10 minutes.
(drops below 14 psi perform Fuel Injection Leak Down Test)
7. Reverse steps in step one.
8. Start engine fuel pressure gauge should read 28-31 psi.
(to high check for vacuum leaks)
FUEL PRESSURE REGULATOR (FPR)
The FPR consist of a mechanical spring and a vacuum diaphragm acting with each other to regulate fuel pressure. The vacuum modulated diaphragm increases and decreases fuel pressure based on the engines vacuum signal and load.
Low load: when the engine needs less fuel, high manifold vacuum acts on the FPR diaphragm and opens the fuel passage to reduce fuel pressure.
High load: when the engine needs more fuel, low manifold vacuum allows the spring to close the fuel passage to increase fuel pressure.
Engine off: FPR shuts and captures fuel pressure between the fuel pump and FPR for easier restarts.
FUEL PUMP CAPACITY TEST
1. Diconnect FPR return hose.
2. Connect a length of hose from FPR return fitting to a 1 quart container.
3. Disconnect the wire (usually green) from the #1 terminal on the coil that leads to the distributor. (so you do not fry the points and or the coil during testing, while leaving the ignition switch on). Unplug the CSV (cold start valve) (so it will not open during testing).
4. Turn the ignition switch to the 1st run postition.
5. Remove the Air filter and use a chopstick to open the flap in the AFM (air flow meter) and pressurize the fuel system for 30 seconds.
6. Delivery quanity should be at least 1/2 quart.
(to low check for clogged Fuel filter, Fuel hose or Fuel tank)
(to low check fuel pump ground connection and positive voltage is at least 11.5 volts)
INJECTION LEAK DOWN TESTING
Injection Leak Down Test
1. Disconnect the wire (usaully green) from the #1 terminal on the coil that
leads to the distributor. So you do not fry the points and or the coil
during testing, by leaving the ignition switch on. Unplug the CSV (cold
start valve) so it will not open during testing.
2. Connect fuel gauge to test fitting on 3-4 Fuel rail. Or between 3-4 Fuel rail
and Fuel pump hose.
3. Turn the ignition switch to the 1st run postition.
4. It will be necessary to pressurize the fuel system before each of the
following steps. Remove the Air filter and use a chopstick to
open the flap in the AFM (air flow meter) to pressurize the fuel system.
5. Pressurize:
Pinch off the Hose between the 3-4 Fuel Rail / Fuel gauge
The pressure should not drop below 14 psi in a 10 minute period of time.
Does not hold pressure = fuel pump check valve leaking - Clean check valve and or replace pump
6. Pressurize:
Pinch off the Hose between the Fuel Pump / 3-4 Fuel Rail
Pinch off the Hose between the FPR (fuel press. regulator) / 1-2 Fuel Rail
Holds pressure = FPR leaking - Replace FPR
7. Pressurize:
Pinch off the Hose between the Fuel Pump / 3-4 Fuel Rail
Pinch off the Hose between the 1-2 Fuel Rail / CSV (cold start valve)
Holds pressure = 1 or 2 injector leaking - Pull 1-2 injectors and retest for
leaking
8. Pressurize:
Pinch off the Hose between the Fuel Pump / 3-4 Fuel Rail
Pinch off the Hose between the CSV (cold start valve) / 3-4 Fuel Rail
Holds pressure = CSV leaking - Replace CSV or in warmer climates bypass
CSV
Does not hold pressure = 3 or 4 injector leaking - Pull 3-4 injectors and
retest for leaking
https://www.thesamba.com/vw/archives/manuals/afc_f...Manual.pdf
I like this style of clamp
https://www.harborfreight.com/automotive-motorcycle/hoses-lines/3-piece-line-clamp-set-97578.html
https://www.harborfreight.com/6-in-ratcheting-bar-clampspreader-62122.html
These are a little more time consuming
https://www.harborfreight.com/automotive-motorcycl...65116.html
Good luck
Tcash |
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raygreenwood Samba Member
Joined: November 24, 2008 Posts: 23074 Location: Oklahoma City
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Posted: Mon Jun 05, 2017 7:17 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Aerosurfer......is that a Schrader valve in that brass fitting in your picture? If so....very cool!
Thats the one thing I ike about late model cars.....is schrader valves in the fuel injection ring main.
Ray |
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aerosurfer Samba Member

Joined: March 25, 2012 Posts: 1603 Location: Indianapolis, IN
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Posted: Mon Jun 05, 2017 5:33 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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furgo wrote: |
Thanks for the tip, pictures appreciated!
If I'm not mistaken, while this would work well with pre-78 buses with a long fuel hose between CSV and fuel rail, from 78 onwards that hose was replaced with another portion of metallic fuel rail, which makes it a bit tight to install the extra T (that's the case in my bus, anyway):
http://www.ratwell.com/technical/FuelHoses/FI_Kit_Inst_7879.jpg |
correct... however for testing just replace the metal pipe. on my 77 i have run both hose and metal pipe. You seem to be going for accurate restoration, but i actually prefer the hose. _________________ Rebuild your own FI Harness..My Harness
77 Westy 2.0L Rockin and Rolling Resto!
72 Sportsmobile (sold)
79 Tran$porter... Parts car money machine (gone) |
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furgo Samba Member

Joined: September 06, 2016 Posts: 944 Location: Southern Germany
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Posted: Mon Jun 05, 2017 4:18 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Thanks for the tip, pictures appreciated!
If I'm not mistaken, while this would work well with pre-78 buses with a long fuel hose between CSV and fuel rail, from 78 onwards that hose was replaced with another portion of metallic fuel rail, which makes it a bit tight to install the extra T (that's the case in my bus, anyway):
http://www.ratwell.com/technical/FuelHoses/FI_Kit_Inst_7879.jpg _________________ '79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
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aerosurfer Samba Member

Joined: March 25, 2012 Posts: 1603 Location: Indianapolis, IN
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Posted: Mon Jun 05, 2017 3:38 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Forget using the fuel port all together...
When I was testing over the years just get some extra fuel hose and T into between the fuel rail and CSV. Much more secure, and it all came with my Fuel pressure gauge kit
If for whatever Reason I ever install it again for testing, I may just leave it in permanently, and just put in a screw fitting on the T part to block it off _________________ Rebuild your own FI Harness..My Harness
77 Westy 2.0L Rockin and Rolling Resto!
72 Sportsmobile (sold)
79 Tran$porter... Parts car money machine (gone) |
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furgo Samba Member

Joined: September 06, 2016 Posts: 944 Location: Southern Germany
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Posted: Mon Jun 05, 2017 3:09 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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Thanks. To clarify, I did not mention I was intending to leave the gauge attached (other than perhaps overnight to watch for pressure loss). The AFC injection manual does not show a gauge, as far as I can tell, it recommends a particular VW gauge.
All I was wondering was whether a threaded fitting might be better. That is, gauge + short FI hose + threaded (7mm) fitting. That would make pressure testing a matter of simply unscrewing the test port cap and screwing the fitting at the end of the gauge + hose assembly.
Now the question is whether such a fitting exists and if it makes sense. I've got not much previous experience with pressure measurement, hence the question! _________________ '79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)
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Wasted youth Samba Member

Joined: July 06, 2012 Posts: 5173 Location: California's Hot and Smoggy Central Valley
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Posted: Mon Jun 05, 2017 1:31 am Post subject: Re: Testing FI Fuel Pressure FAQ |
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You could conceivably screw on a fitting. You will need to determine the correct thread pitch and size, then find the correct fitting, then marry that up to your gauge.
To me, that is a lot of hassle. I simply keep a pressure gauge with a couple feet of FI hose and a FI clamp together with my other diagnostic tools. I never leave a gauge attached. As was mentioned, that fitting is not flared or bulbous to help retain a rubber hose over a long period of time, and the normal vibration of the engine would eventually cause the consumer-grade gauges to become questionable. Remember... that's 32-35 PSIG of fuel.
Just do what the Bosch Fuel Injection manual shows... it's a temporary gauge set up for diagnostics.
If you're going to leave a gauge attached for monitoring fuel rail pressure, you might as well fab up a gauge panel and include oil pressure, intake plenum vacuum, CHT, alternator current, oil temperature, customer line vacuum, dwell meter, and a noid light bracket. Make it like an airplane...  |
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