When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
After renting a Fuel System Pressure Tester I found that there is a pressure drop that is certainly contributing to these issues.
Left overnight the Fuel Pressure drops to zero,
When Cranking the system can momentarily achieve 55 PSI
Idle pressure is 48 PSI
After shut down the pressure immediately drops to 43 PSI
The fuel pressure drops 10psi after 60 minutes
My understanding is the fuel pressure should be around 55PSI when sitting and 48 PSI at idle. The fuel pressure should drop a maximum of 2PSI per hour.
Reading a post on Rennlist from Porschetech3 explained that when the fuel is hot and does not have enough pressure it boils away creating a hard start.
I’m learning about the different parts within the fuel system. Here are the items that have been checked or replaced within the last 1000 miles.
FUEL VOLUME - CHECKED OK
FUEL QUALITY - 3+ Tanks of Chevron since rebuild
FUEL FILTER - REPLACED
FUEL INJECTORS - REBUILT & TESTED (Witch Hunter)
Based on all fo your advice I think that leaves:
FUEL CHECK VALVE / FUEL NON RETURN VALVE (Are these the same thing?)
FUEL PRESSURE SENSOR
SPLIT HIGH PRESSURE HOSE ON FUEL PUMP SEND
I should also mention that the car will not let you fill past 3/4 tank in case that’s related.
Thank you for your help 🙌, As you can see from the update below, you are right on the money.
Is there a test for the check valve?
Here is the update... UPDATE
Hello Everybody!! - Thank you all for your help 🙌
Many of your pointed to the fuel system.
After renting a Fuel System Pressure Tester I found that there is a pressure drop that is certainly contributing to these issues.
Left overnight the Fuel Pressure drops to zero,
When Cranking the system can momentarily achieve 55 PSI
Idle pressure is 48 PSI
After shut down the pressure immediately drops to 43 PSI
The fuel pressure drops 10psi after 60 minutes
My understanding is the fuel pressure should be around 55PSI when sitting and 48 PSI at idle. The fuel pressure should drop a maximum of 2PSI per hour.
Reading a post on Rennlist from Porschetech3 explained that when the fuel is hot and does not have enough pressure it boils away creating a hard start.
I’m learning about the different parts within the fuel system. Here are the items that have been checked or replaced within the last 1000 miles.
FUEL VOLUME - CHECKED OK
FUEL QUALITY - 3+ Tanks of Chevron since rebuild
FUEL FILTER - REPLACED
FUEL INJECTORS - REBUILT & TESTED (Witch Hunter)
Based on all fo your advice I think that leaves:
FUEL CHECK VALVE / FUEL NON RETURN VALVE (Are these the same thing?)
FUEL PRESSURE SENSOR
SPLIT HIGH PRESSURE HOSE ON FUEL PUMP SEND
I should also mention that the car will not let you fill past 3/4 tank in case that’s related.
After renting a Fuel System Pressure Tester I found that there is a pressure drop that is certainly contributing to these issues.
Left overnight the Fuel Pressure drops to zero,
When Cranking the system can momentarily achieve 55 PSI
Idle pressure is 48 PSI
After shut down the pressure immediately drops to 43 PSI
The fuel pressure drops 10psi after 60 minutes
My understanding is the fuel pressure should be around 55PSI when sitting and 48 PSI at idle. The fuel pressure should drop a maximum of 2PSI per hour.
Reading a post on Rennlist from Porschetech3 explained that when the fuel is hot and does not have enough pressure it boils away creating a hard start.
I’m learning about the different parts within the fuel system. Here are the items that have been checked or replaced within the last 1000 miles.
FUEL VOLUME - CHECKED OK
FUEL QUALITY - 3+ Tanks of Chevron since rebuild
FUEL FILTER - REPLACED
FUEL INJECTORS - REBUILT & TESTED (Witch Hunter)
Based on all fo your advice I think that leaves:
FUEL CHECK VALVE / FUEL NON RETURN VALVE (Are these the same thing?)
FUEL PRESSURE SENSOR
SPLIT HIGH PRESSURE HOSE ON FUEL PUMP SEND
I should also mention that the car will not let you fill past 3/4 tank in case that’s related.
After renting a Fuel System Pressure Tester I found that there is a pressure drop that is certainly contributing to these issues.
Left overnight the Fuel Pressure drops to zero,
When Cranking the system can momentarily achieve 55 PSI
Idle pressure is 48 PSI
After shut down the pressure immediately drops to 43 PSI
The fuel pressure drops 10psi after 60 minutes
My understanding is the fuel pressure should be around 55PSI when sitting and 48 PSI at idle. The fuel pressure should drop a maximum of 2PSI per hour.
Reading a post on Rennlist from Porschetech3 explained that when the fuel is hot and does not have enough pressure it boils away creating a hard start.
I’m learning about the different parts within the fuel system. Here are the items that have been checked or replaced within the last 1000 miles.
FUEL VOLUME - CHECKED OK
FUEL QUALITY - 3+ Tanks of Chevron since rebuild
FUEL FILTER - REPLACED
FUEL INJECTORS - REBUILT & TESTED (Witch Hunter)
Based on all fo your advice I think that leaves:
FUEL CHECK VALVE / FUEL NON RETURN VALVE (Are these the same thing?)
FUEL PRESSURE SENSOR
SPLIT HIGH PRESSURE HOSE ON FUEL PUMP SEND
I should also mention that the car will not let you fill past 3/4 tank in case that’s related.
After renting a Fuel System Pressure Tester I found that there is a pressure drop that is certainly contributing to these issues.
Left overnight the Fuel Pressure drops to zero,
When Cranking the system can momentarily achieve 55 PSI
Idle pressure is 48 PSI
After shut down the pressure immediately drops to 43 PSI
The fuel pressure drops 10psi after 60 minutes
My understanding is the fuel pressure should be around 55PSI when sitting and 48 PSI at idle. The fuel pressure should drop a maximum of 2PSI per hour.
Reading a post on Rennlist from Porschetech3 explained that when the fuel is hot and does not have enough pressure it boils away creating a hard start.
I’m learning about the different parts within the fuel system. Here are the items that have been checked or replaced within the last 1000 miles.
FUEL VOLUME - CHECKED OK
FUEL QUALITY - 3+ Tanks of Chevron since rebuild
FUEL FILTER - REPLACED
FUEL INJECTORS - REBUILT & TESTED (Witch Hunter)
Based on all fo your advice I think that leaves:
FUEL CHECK VALVE / FUEL NON RETURN VALVE (Are these the same thing?)
FUEL PRESSURE SENSOR
SPLIT HIGH PRESSURE HOSE ON FUEL PUMP SEND
I should also mention that the car will not let you fill past 3/4 tank in case that’s related.
Your barking up the right tree for sure, thank you for your help
Here is the update.UPDATE
Hello Everybody!! - Thank you all for your help 🙌
Many of your pointed to the fuel system.
After renting a Fuel System Pressure Tester I found that there is a pressure drop that is certainly contributing to these issues.
Left overnight the Fuel Pressure drops to zero,
When Cranking the system can momentarily achieve 55 PSI
Idle pressure is 48 PSI
After shut down the pressure immediately drops to 43 PSI
The fuel pressure drops 10psi after 60 minutes
My understanding is the fuel pressure should be around 55PSI when sitting and 48 PSI at idle. The fuel pressure should drop a maximum of 2PSI per hour.
Reading a post on Rennlist from Porschetech3 explained that when the fuel is hot and does not have enough pressure it boils away creating a hard start.
I’m learning about the different parts within the fuel system. Here are the items that have been checked or replaced within the last 1000 miles.
FUEL VOLUME - CHECKED OK
FUEL QUALITY - 3+ Tanks of Chevron since rebuild
FUEL FILTER - REPLACED
FUEL INJECTORS - REBUILT & TESTED (Witch Hunter)
Based on all fo your advice I think that leaves:
FUEL CHECK VALVE / FUEL NON RETURN VALVE (Are these the same thing?)
FUEL PRESSURE SENSOR
SPLIT HIGH PRESSURE HOSE ON FUEL PUMP SEND
I should also mention that the car will not let you fill past 3/4 tank in case that’s related.
10 Psi per hour is not terrible, the other guy has a 26PSI pressure drop in an hour so two and a half times worse.
The 2 psi per hour is the Gold Standard and has been ever since Bosch invented the Electronic Fuel Injection ( 40+ years ago) . But I don't think quality control is what it used to be as I see many cars/parts that just cannot achieve this...
Some will hold strong pressure for days even weeks and some struggle to hold any pressure for 30 min , with all new parts...the quality control sucks, it's a hit or miss thing..
Fuel check valve and fuel non-return valve are the same thing and internal to the fuel pump itself.
There's no fuel pressure sensor on the M96. What you probably mean is the fuel pressure regulator (#2 in the diagram below).
You can check whether its internal membrane is fine by measuring fuel pressure at idle and compare it to the pressure when revving the engine by snapping the throttle wide open. Manifold pressure will mementarily equal ambient pressure and your fuel pressure should increase to 55 PSI momentarily too.
You could instead also remove the vacuum hose (#14) from the fpr (pinch the hose to avoid a vacuum leak) to see if 55 PSI are achieved.
Check if fuel drips out of the vacuum connection on the fpr. If it does, the membrane is ruptured.
Schematic cross section of an fpr similar to that on the M96
I don't see a way to differentiate between leaking injectors and a leaking (into the fuel return line) of the fpr easily.
One way might be to observe whether further fuel comes out of the fuel return hose on bank2 crossing over to bank1 after the initial spill when removing it from the rail on bank 2. I think it will be hard to tell, however.
Perhaps removing the fpr and applying vacuum/suction to the inner, central opening of the fpr might reveal a bad seat by allowing air to be sucked through it. If you can't suck air through it then I'd say the fpr does not leak but holds pressure as intended and the injector(s) is/are leaking, resulting in the pressure drop you've observed.
Perhaps removing the injector rail from bank 2 in its entirety would show dry injector tips thereby indicating the fpr must be leaking. Unfortunately the opposite (wet tips) may still mean the fpr is faulty too.
Personally I guess I would buy a new fpr and see if fpr drop after shut down is back to spec or at least lessened massively and then take it from there and move on to the injectors.
Last edited by hardtailer; 07-17-2022 at 09:48 AM.
The 03 996.2 doesn't have the Fuel Pressure regulator mounted on the fuel rail like pictured above. That is for the 996.1 fuel system.
The 996.2 has a return-less fuel system meaning the fuel pressure regulator is a little plastic one mounted inside the tank right above the fuel pump.The fuel pump still has the check valve and the fuel pressure regulator still holds the pressure in the line just like all systems do, it just does away with the external return line...Same principals apply.
The 03 996.2 doesn't have the Fuel Pressure regulator mounted on the fuel rail like pictured above. That is for the 996.1 fuel system.
The 996.2 has a return-less fuel system meaning the fuel pressure regulator is a little plastic one mounted inside the tank right above the fuel pump.The fuel pump still has the check valve and the fuel pressure regulator still holds the pressure in the line just like all systems do, it just does away with the external return line...Same principals apply.
True but in his first post he states his troubles are with a 2001 carrera (2WD) that's why I referred to the 996.1
07-16-2022, 10:47 PM
