Fuel Pressure Question
05-01-2007, 03:05 AM
#1
Fuel Pressure Question How long should your car hold fuel pressure after the initial prime of fuel when you first turn the key to the on position? What about after you turn the engine off? Both times my car would lose fuel pressure relatively fast. It would drop 40 PSI in the first 15 seconds and the remaining 20 PSI would bleed off within the minute. The car holds a steady fuel pressure of 60 PSI at idle. Is this normal? Any help is appreciated!
05-02-2007, 06:51 PM
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Withthe stock vane pump my fuel pressure would hold at last pressure prior to shutting off car indefinitely.
Since the switch to Racetronix geroter pump, I now have 61psi at idle, but after shutting car off, the pressure will bleed down to around 20 or so psi almost immediately, then within an hour or so, will bleed down to zero.
It doesn't have any effect on how the car runs or starts - but it is a curiousity. Not necessarily a bad thing - just different from stocker.
This has been brought up many times, everyone that upgrades their pumps seem to have the same effects. As far as I know - no one has come up with an answer as to why...
Since the switch to Racetronix geroter pump, I now have 61psi at idle, but after shutting car off, the pressure will bleed down to around 20 or so psi almost immediately, then within an hour or so, will bleed down to zero.
It doesn't have any effect on how the car runs or starts - but it is a curiousity. Not necessarily a bad thing - just different from stocker.
This has been brought up many times, everyone that upgrades their pumps seem to have the same effects. As far as I know - no one has come up with an answer as to why...
05-15-2007, 07:08 AM
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The reason aftermarket pumps do not hold pressure is because they (Walbro anyway) lack a check valve that is part of the factory fuel pumps. This is what I have come across with other threads on different boards. Like you say, it doesn't seem to have any ill effects other than when shutting the car off and immediately trying to start it again the engine will to crank a little bit longer while the pressure builds. It's just a matter of waiting a second or two before starting to avoid the extra cranking. I guess this could be a problem with an M6 if the car were to stall on a grade and need to be started immediately. Otherwise you need some quick foot work if you are trying to hold a clutch, brakes and get on the accelerator quickly to get moving and to keep from rolling in the wrong direction before everything gets coordinated. This has happened to me a few times. To me, the check valve is an emergency part but it may also have something to do if the fuel system is unexpected and suddenly opened with the engine running.