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Fuel Pressure Measurement on a 940 Turbo? 900

posted by rob2volvos on Fri Apr 6 09:57 CST 2012
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last visit: Wed Mar 12 07:52 CST 2014

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The fuel pump is making noise and I figured one (very good?) way to investigate would be fuel pressure measurement. I can find 2 places on the car where there is a small bike-like valve (presumably for this very purpose):
a) underneath the fuel rail at the front (it's hidden and you have to crane around to notice it)
b) close to the main fuel pump underneath the car

According to the FAQs and other posts here, I'm expecting to see a steady 44psi at a) (right?) However -- what should I see at b) if the system is working correctly? I think b) is placed between the pre-pump and the main pump so I am presuming that Volvo put this here to enable a convenient way to check the pre-pump.

Can anyone comment on how I can use these valves to check the system?

Thanks,

Rob

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Fuel Pressure Measurement on a 940 Turbo? 900

posted by BobL on Mon Jul 16 22:24 CST 2012
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last visit: Wed Oct 22 21:55 CST 2014

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The 700/900 t-shooting guide on this site specs 5 to 8 PSI for the pre-pump. See "Hot StartnProblem Diagnosis and Repair"

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Fuel Pressure Measurement on a 940 Turbo? 900

posted by lucid on Fri Apr 6 11:53 CST 2012
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last visit: Fri Oct 24 12:31 CST 2014

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a) is the "regulated" pressure, and what you see depends on what the regulator "feels". With engine idling, the regulator (diaphragm) should feel a good vacuum and set the pressure down around 34.5 psi. At full throttle, the intake vacuum goes away and the pressure goes up to 43.5 psi (3.0 bar).

b) is between the filter and the regulator, I think. So it would be the unregulated output of the main pump. But I never saw a spec listed for that.

--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

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Fuel Pressure Measurement on a 940 Turbo? 900

posted by rob2volvos on Wed Apr 11 17:04 CST 2012
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last visit: Wed Mar 12 07:52 CST 2014

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Thanks for your comments, Bruce --
Just for the record and to make this thread more complete -- on my 1994 940 Turbo I can find 2 Schrader valves -- the one under the car is (I'm sure) between the in-tank pump and the main pump. I measured 13psi there after starting the car ( -- mind you the main pump was howling). With a noisy main fuel pump, the measurement at the other valve at the fuel rail was quivering way off scale -- didn't look right at all. Replaced fuel pressure regulator as per FAQs here and at the fuel rail I measured a steady 35.5 psi. Main fuel pump noise reduced considerably (always could hear a faint whine under car with this pump) and car runs better so far.

It would be nice think that Volvo set the system up deliberately with these 2 valves to trouble-shoot the fuel system easily with 2 quick measurements using a fuel pressure gauge. I'd be interested to know what fuel pressure should be produced by the pre-pump (in-tank), but like you I can't find any data on that.

Regards,

Rob

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