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Testing FI Fuel Pressure FAQ
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SGKent Premium Member
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PostPosted: Tue Jan 07, 2020 2:10 pm    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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.
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PostPosted: Tue Jan 07, 2020 10:50 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

Urglegrew wrote:
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."


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?


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.
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PostPosted: Tue Jan 07, 2020 9:38 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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."


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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PostPosted: Tue Sep 11, 2018 1:37 pm    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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.
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PostPosted: Mon May 07, 2018 5:57 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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.
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PostPosted: Sun May 06, 2018 11:41 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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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PostPosted: Sun May 06, 2018 9:07 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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.
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PostPosted: Sun May 06, 2018 8:38 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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?
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PostPosted: Tue Sep 26, 2017 12:12 pm    Post subject: Fuel Injection Fuel Pressure Testing Reply with quote

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.
Image may have been reduced in size. Click image to view fullscreen.


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

Image may have been reduced in size. Click image to view fullscreen.



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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PostPosted: Mon Jun 05, 2017 7:17 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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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PostPosted: Mon Jun 05, 2017 5:33 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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.
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PostPosted: Mon Jun 05, 2017 4:18 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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
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PostPosted: Mon Jun 05, 2017 3:38 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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


Image may have been reduced in size. Click image to view fullscreen.


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
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PostPosted: Mon Jun 05, 2017 3:09 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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!
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PostPosted: Mon Jun 05, 2017 1:31 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

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... Laughing
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PostPosted: Mon Jun 05, 2017 12:59 am    Post subject: Re: Testing FI Fuel Pressure FAQ Reply with quote

I came across this old thread a while ago, but only now I've come to use the info on it. Thanks a lot for writing it.

Just a quick question: the instructions are for clamping the fuel pressure gauge to the test port in the fuel rail.

Is there not a fitting that could be simply screwed into the test port's thread instead? Particularly as the test port does not have a flare at the end to better secure the clamped hose.
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PostPosted: Mon Sep 08, 2014 7:30 pm    Post subject: Reply with quote

One addition to this thread.

Although it iw not unsafe to keep a gauge solidly connected to your fuel system.....it will be of questionable accuracy pretty quickly.

In order to be able to live in the engine compartment you need a specific type of gauge.

Virtually any gauge you can "commonly" buy will not be:

1. A high temp gauge useful for over 165F. I realize the bus engine compartment does not get too raging hot while running.....but the gauge will get plenty hot when you shut down. There are specific gauges made for high temps. Steam rated gauges are one category but you will not find any with pressure ratings low enough without special order.
Hot location gauges can be bought. You wont find one for less than about $125-150 in the pressure range you need. Steam gauges cost several times that much.

2. Shock proof gauges for high vibration areas. This will make your gauge innaccurate almost as fast or faster than the heat. Do not confuse a simple liquid filled gauge with a shockproof gauge. Standard liquid/glycerin/silicone gauges...which are fairly cheap these days only damp the needle movement to keep errant chatter from spoiling the reading you see.
A shockproof or vibration resistant gauge has a physical separation from the bourdon tube and drives the needle via a pinion. Most are also silicone filled....and most are settable and adjustable.
Good luck finding a whock proof high temp gauge with stainless bourdon tube in yiur temp range for under about $250+.

The very best thing you can do......if you want the ability to, check on the fly...would be to have a schrader valve with threaded port installed inline. You can buy or make one to just bolt in in place of about 4" of rubber fuel line. Get a decent gauge and keep it in a padded bag in the tool box.
Just screw it kn....take your reading and screw it off with only a small fuel loss. Ray
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PostPosted: Mon Sep 08, 2014 6:38 pm    Post subject: Reply with quote

dwill49965 wrote:


Yes - camping and touring around Nova Scotia logging roads and other off the beaten track places. There's a few gaps I should really plug up ...


Speaking of gaps, try tucking your spark plug boots inside your tin. I had that brilliant "aha" moment a few months ago and I swear my CHT dropped 20 degrees.
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PostPosted: Mon Sep 08, 2014 4:35 pm    Post subject: Re: Testing FI Fuel Pressure Reply with quote

Randy in Maine wrote:
"How to test your L-tronic fuel pressure"

First off you need a fuel pressure gauge that will measure between 0-60 psi. A good quality gauge looks like this one from German Supply. http://www.germansupply.com/home/customer/product.php?productid=16610&cat=300&page=1 It is a good price for a quality set up.

I like to have a hose about 3 feet long made of 7mm FI fuel line so that I can do some of the testing behind the bus where it is easier and safer to do. I would not leave the gauge on the rail (although some people do). I keep mine in the toolbox. You can sometimes find a good gauge at the FLAPS for about $50 but you might have to cut off the end of the 5/16" hose and clamp it on to the fuel rail. 0-60 psi is the preferred range.

If you are stuck in the middle of nowhere, you can make one out of a "0-100 psi water pressure tester gauge" from Home Depot and some fittings from the "air compressor isle" that will allow you to attach it to the 7mm fuel line for about $20. Take your fuel line in with you to make sure it will fit on the 1/4" barbed compressor fitting. It is a good idea to make it so that the gauge can be removed from the coupler. Use 2 good hose clamps, one for the end that goes on the fuel rail test port (between injector #4 and #3) and the other for the barbed fitting. Teflon tape will help seal the connections. These are not as accurate as a real gauge and are not designed for petroleum. For a little more money you can get one that is really designed to test fuel pressure and is more accurate. A real one will last a whole lot longer.

To test the fuel pressure generally, relieve the fuel pressure in the system by opening the gas cap to relieve pressure in the tank and applying a little suction on the vacuum line that goes to the fuel pressure regulaltor for a couple of seconds to drop the pressure in the fuel rail.

Now is the time to install the pressure gauge on the fuel rail. The fuel rail test port is a little bolt (with a 7mm head) in the fuel rail between #3 and #4 injectors. Mount it securely, but you do not have to reef on the clamp.

Start it up. 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.

Reconnect that vacuum line to the regulator and see if you get a pressure drop of 0.5 bar or about 7 psi (back down to about 29 psi) as the vacuum signal opens up the regulator diaphram. If you do not get it to drop in fuel pressure, check out the vacuum line for leaks. If that is OK, your fuel pressure regulator is faulty.

Since you are still hooked up, rev the engine up to about 3000 RPMs or so a few times and see if the pressure changes a lot or hopefully stays about 36 psi. Getting "on" the accelerator drops the vacuum signal to the regulator and tells it that you want more power as you are about to pass a semi or climb a big hill and you want more fuel to the injectors to make more power.

A dying pump or clogged fuel delivery system, will run out of gas by allowing the system pressure to drop to low. It should never go below 26 psi. It will feel like you are running out of gas, because you are. Razz

If the pressure is low on both parts of the test, either:

1) the pressure regulator is returning too much fuel to the tank or
2) the fuel pump is not delivering properly (could be a bad pump, plugged fuel filter, rust in the tank or a plugged screen in the tank but most of us do not have a screen in the tank).

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.

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.

The pump should put out about about 1 L of fuel in about 30 seconds if it is getting the 12 volts it wants. Check your pump ground, double relay, and starter connections if you are not getting correct voltage. I make sure the engine will not spark or start and just unscrew the gauge from the fitting and use that long hose to direct the flow to the gas can for the test since I am already hooked up to the test port.

To test for residual pressure when your bus won't start when warm:

With the gauge installed on the rail test port, run the engine to get it all pressured up, then shut it off. After 20 minutes the pressure should not fall below 1 bar or about 14.5 psi.

If it falls below that check:

1) all fuel line connections for leaks
2) to make sure that none of the injectors are leaking including the cold start valve and don't forget the injector seals.

The fuel pump also has a check valve to allow for only one way traffic for the fuel. To test the fuel pump check valve, run the engine and shut it off and immediatedly clamp shut the supply line from the fuel pump to the fuel rail. I have a pair of needle nose vise grips with a short piece of fuel line on each jaw to prevent real damage. If the pressure drops below the specification after 20 minutes, the fuel pressure regulator is faulty.

Remove the clamp on the supply line, start it up again and repeat the test and if the pressure is now within the specification of 14.5 psi, then the fuel pump check valve is faulty. New pump time.

One more thing...change out the fuel lines when you first get your bus so you will know when they were last done and then inspect them every year. We suggest changing them out with the correct FI fuel line every 7 or 8 years on your birthday and do them all including the vent line connections. Hate to see a nice baywindow bus burn down.

Richard's website has the fuel lines you need to come up with and where they go. Thanks Ratwell again for doing that!

German Supply has a nice diagram and has a kit that actually fits and it is of excellent quality. Thank you Scott for your good work!

Image may have been reduced in size. Click image to view fullscreen.


If you guys have something to correct or add to this, we can get one of the moderators to add it.


Randy in Maine


very nice write up.
thanks
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1FUNBUG
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Joined: June 29, 2004
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PostPosted: Thu Jun 01, 2006 8:39 am    Post subject: Reply with quote

I decided to keep the connections to and from the fifth injector flexible (i.e. hose). I couldn't fab metal lines with the flare (bump) at the end. I know Karl could have hooked me up with OE metal ones but I finally had enough hose just to swap them out. I like the metal ones under the bus do to the more harsh conditions (things flying up off the road). Its probably not a real concern but I sleep better now. Laughing
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71 Beetle, 1776, dual Kadrons
77 Westy, Camper Special 2.0 F.I.
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