how steady is the fuel pressure supposed to be?

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airkooledchris
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how steady is the fuel pressure supposed to be?

Post by airkooledchris » Thu Aug 30, 2012 12:38 pm

How steady is the fuel pressure usually displayed on the gauge?

At idle mine sits there bouncing between 25-33 or so, never holding a steady reading. This is a VDO gauge on the test port on the #3/4 rail.

Revving the engine up to WOT the fuel pressure does the same, bouncing around these same numbers.

You can sort of feel the pulse of the gasoline in the fuel rail - is this a sign of something being clogged and the system being unable to push it through properly?

Or might this be the FPR?

This video shows what im referring to: http://youtu.be/2U3KHn8bOWk
1979 California Transporter

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SlowLane
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Re: how steady is the fuel pressure supposed to be?

Post by SlowLane » Thu Aug 30, 2012 7:47 pm

That's a pretty rapid vibration. Mine doesn't do that at all. It just stays steady.

Can you tee a vacuum gauge into the FPR signal input and see if the vacuum signal fluctuates at the same time as the fuel pressure? If it does, then there's the source of your vibration, though I would have thought the mechanical time constant of the FPR would dampen that sort of effect out.

You could also try tee-ing your pressure gauge further downstream in the fuel ring, ideally as close to the FPR as you can get, and see if the fluctuations reduce as you get closer to the FPR. I have a theory that the three injectors doing their thing downstream of your gauge are sending pressure waves back upstream, which is what your gauge is picking up.
'81 Canadian Westfalia (2.0L, manual), now Californiated

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airkooledchris
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Re: how steady is the fuel pressure supposed to be?

Post by airkooledchris » Thu Aug 30, 2012 9:10 pm

SlowLane wrote:That's a pretty rapid vibration. Mine doesn't do that at all. It just stays steady.

Can you tee a vacuum gauge into the FPR signal input and see if the vacuum signal fluctuates at the same time as the fuel pressure? If it does, then there's the source of your vibration, though I would have thought the mechanical time constant of the FPR would dampen that sort of effect out.

You could also try tee-ing your pressure gauge further downstream in the fuel ring, ideally as close to the FPR as you can get, and see if the fluctuations reduce as you get closer to the FPR. I have a theory that the three injectors doing their thing downstream of your gauge are sending pressure waves back upstream, which is what your gauge is picking up.
Thanks for the reply, I appreciate it. Honestly it helps just knowing that this isn't a normal behavior, as I don't want to go chasing my tail for an issue that isn't there.

On to deciding what might be causing it.

I Teed into the vacuum line just before the FPR and the vacuum does fluctuate, but nowhere near as crazy as it shows on the fuel pressure gauge. I can't decide if that's because of the scale and size of the gauges or what so I shot another few video's for giggles and squirts:

http://youtu.be/8UBvSXQWlmY?hd=1

If you look really closely when the video starts you can see the tip of the needle bouncing back and forth just a hair, while the overall measurement of the vacuum slightly fluctuates between 16 and 18 (the engine was still cold.)



http://youtu.be/xBzPyLDhp_o

This one shows a better view of the fuel pressure gauge initially, then I show both gauges while giving it a bit of gas.


Right now im thinking possible vacuum leak, bad FPR, pinched fuel line and/or clogged somethingorother in the fuel system.

Will happily entertain other thoughts on this issue. I believe I have all the spare parts and tools to cross most of the above off the list if I get a few hours of freetime tomorrow.
1979 California Transporter

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SlowLane
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Re: how steady is the fuel pressure supposed to be?

Post by SlowLane » Fri Aug 31, 2012 1:21 pm

Honestly it helps just knowing that this isn't a normal behavior,
Caveat here: it might be normal behavior for a Bay Window. Mine's a Vanagon. Apples to oranges here, as the Vanagon has the FPR on the left side of the engine next to the #3 cylinder, but the Bays have the FPR on the right side next to #1 cylinder. The fuel pressure measurement port is in the same location on both models, between the #4 and #3 cylinder.

Hence my theory. The pressure-measurement port on mine is much closer to the FPR (ie. the stable pressure reference) than yours is, and with only one injector between the port and the FPR, whereas yours has three injectors pulsing away between the measurement point and the stable reference, plus a fair bit more fuel rail. That's why I suggested the second test setup with the gauge as close to the FPR as you could get.

I think it would be best for you if you could get some feedback from another Bay owner before chasing chimeras any further.
'81 Canadian Westfalia (2.0L, manual), now Californiated

"They say a little knowledge is a dangerous thing, but it is not one half so bad as a lot of ignorance."
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Westy78
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Re: how steady is the fuel pressure supposed to be?

Post by Westy78 » Fri Aug 31, 2012 1:35 pm

Just checked mine the other day and had a rock steady reading from the test port. This is with a non vacuum regulator from a 914. I could adjust the pressure by small amounts and always had a steady psi.
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SlowLane
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Re: how steady is the fuel pressure supposed to be?

Post by SlowLane » Sat Sep 01, 2012 2:39 pm

Westy78 wrote:Just checked mine the other day and had a rock steady reading from the test port. This is with a non vacuum regulator from a 914. I could adjust the pressure by small amounts and always had a steady psi.
Guess that shoots my reverse-flow-pressure-wave theory down. :blackeye:
Chris, if you temporarily remove (and plug) the vacuum signal from the FPR, do the fluctuations go away?
'81 Canadian Westfalia (2.0L, manual), now Californiated

"They say a little knowledge is a dangerous thing, but it is not one half so bad as a lot of ignorance."
- Terry Pratchett

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Amskeptic
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Re: how steady is the fuel pressure supposed to be?

Post by Amskeptic » Sun Sep 02, 2012 4:10 pm

SlowLane wrote:
Westy78 wrote:Just checked mine the other day and had a rock steady reading from the test port. This is with a non vacuum regulator from a 914. I could adjust the pressure by small amounts and always had a steady psi.
Guess that shoots my reverse-flow-pressure-wave theory down. :blackeye:
Chris, if you temporarily remove (and plug) the vacuum signal from the FPR, do the fluctuations go away?
Blame the gauge. Some gauges have good internal damping, some don't. A roller cell pump is smooth, the injector pulses are not. Since this is a grouped single fire system, the pulses are pretty severe.
Colin
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