Testing FI Fuel Pressure FAQ

[busdaddy]

Usually you clamp a length of 7.5mm or 5/16" fuel hose to it.
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[Good Old Dusty]

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.

[my59]

Adding a fuel rated shut off valve at the "C" hose location makes fuel filter changes less messy.
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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.

[busdaddy]

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.
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[sblu]

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!
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1978 Westfalia Champagne Edition Project

[SGKent]

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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[busdaddy]

[Urglegrew]

[JaimeH]

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
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1978 Transporter 2.0 FI, BA6
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[furgo]

[raygreenwood]

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

[busdaddy]

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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[furgo]

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.

[Tcash]

To link to this post copy and paste

[raygreenwood]

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

[aerosurfer]

[furgo]

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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'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)

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[aerosurfer]

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
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77 Westy 2.0L Rockin and Rolling Resto!

72 Sportsmobile (sold)
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[furgo]

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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'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)

Decode your M-Plate

[Wasted youth]

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...