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Join Date: Jun 2003
Location: Fresno, CA
Posts: 7,789
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Curious about the Fuel Pump Check Valve
This is mostly to help me understand this part of the system.
I recently replaced my fuel accumulator and will replace the check valve tonight. I know that both of these keep the system pressurized, even when the engine is turned off. I'm wondering, if the check valve is defective, and system pressure bleeds down, does this mean that when the engine is turned back on, it also could have low pressure or take a certain amount of time to build back up - thinking that the fuel pump is pushing at the same time the injectors are depleting, it could take some time to build back pressure - is this correct? |
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03-30-2012, 02:35 PM
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ROW '78 911 Targa
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The pressure will eventually bleed off. there are time frames for the amount it bleeds off. My bently manual has ranges of pressure for 10 minutes and 30 minutes for minimum pressures.
The purpose of residual pressure is to stop the fuel from vaporizing when starting a hot/warm motor. When engine cools off completely the residual pressure is normally all gone.
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Dennis Euro 1978 SC Targa, SSI's, Dansk 2/1, PMO ITBs, Electric A/C Need a New Wiring Harness? PM or e-mail me. Search for "harnesses" in the classifieds. |
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03-30-2012, 02:41 PM
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Join Date: Oct 2005
Location: Capistrano Beach, Ca.
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In addition to Dennis' explanation, above, your question about time needed to re-pressurize is a good one. A healthy fuel pump should produce ~75 psi. The system is set up so not all of that pressure is necessary and is bled off by recirculating it back to the tank via the pressure relief valve in the fuel distributor and the outlet on the WUR. These two devices regulate the pressure to between ~15 psi (stone cold starting), to ~45 psi when warm running with the injectors spraying. Given this set up, even with the injectors spraying the pump does not take long to build pressure again. (BTW, I'm used to referring to pressure in bars so the psi numbers are very general.)
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L.J. Recovering Porsche-holic Gave up trying to stay clean Stabilized on a Pelican I.V. drip |
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03-30-2012, 04:38 PM
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Location: Fresno, CA
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Well, I changed my check valve tonight. The old one appears to still work as I can blow thru it one way but not the other. However, the new check valve (genuine Porsche) I can't blow thru it either way. The car started up just fine so I know fuel is getting thru.
Let's just hope the engine is running well in the morning - I have auto-x. I should know better not to change things at midnight before I need the car but I was anxious. |
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03-31-2012, 12:02 AM
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Parts replacements.........
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Tidybuoy, Have you tested and confirmed that the FA and FP check valve were defective before replacing them? Or you believe they could be the source of your problem/s? Just curious. Thanks. Tony |
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03-31-2012, 07:41 AM
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Join Date: Feb 2001
Location: South of the Mason-Dixon Line
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Tidy.....
On your '74 is the fuel pump mounted in the front or rear? The reason I ask is that I have the 73.5T with the first CIS. The issue with my set up is that the fuel pump is mounted in the rear drivers side wheel well and I have to pull gas rather then push it like on the front mounted units. With that the first CIS fuel pumps had built in (internal) check valves and subject to failure. My cold start with hand throttle is fantastic, but after driving and warm up its a hard restart. Your check valve I believe is attached at the outside the pump. I am trying to determine what you might have that was carried over form the 73.5T. I do not have the thermal time switch. Bob |
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03-31-2012, 09:47 AM
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Helge |
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03-31-2012, 11:26 AM
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Location: Fresno, CA
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Quote:
I have not checked my fuel pressures yet but I just got my CIS fuel pressure test kit so that will be next. I'm certain I'm doing this out of order and probalbly did not need to replace the check valve or fuel distributor but at least I know that these items are new and won't be wearing out soon. |
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03-31-2012, 03:03 PM
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03-31-2012, 03:04 PM
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Join Date: Dec 2004
Location: So. Cal.
Posts: 576
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Check Valve Part Number
I was just looking for theP/N of that check valve. I've replaced my lines and am replacing the pump so I thought I'd put in a new check valve rather than leave that as the only component not replaced.
Thanks,
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Randy 73.5 911 T 97' C2S 06' C2S |
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04-09-2012, 10:21 AM
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Quote:
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04-09-2012, 01:07 PM
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Location: So. Cal.
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Thanks. I assume from our host! 1.5 to 4 week wait right now. Will check for something in stock from an alternate source.
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Randy 73.5 911 T 97' C2S 06' C2S |
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04-09-2012, 02:07 PM
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Quote:
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04-09-2012, 02:36 PM
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Heinz
