[Velocity]

Any idea why my 1992 968 would randomly pressurize the fuel system?  I have a fuel pressure gauge permanently installed on the fuel rail and have noticed a few times when doing a quick oil/coolant level check (after car has been sitting for a week or so) that the fuel system was pressurized (ie, gauge read 55-60psi of pressure) and thought that seemed odd.  I thought it MIGHT be my Focus9 solid-state DME that does "prime the fuel pump" with key on-engine off prior to starting the engine, so I swapped back to the stock DME - observed the same issue.   Also thought it might have been something with opening the door and/or hood that was somehow triggering the fuel system to prime/pressurize.


So - I decided to try a test this weekend.   After driving the car yesterday, I parked in the garage, opened the hood, and observed the fuel system still had normal pressure (expected, since I'd just parked it).     I left the hood opened in the garage, and checked again that evening around 6pm - pressure was down to 0 at that point:

 

   

 

I left the hood opened and closed garage overnight.    Came out this morning around 11:30am and found the pressure reading around 12-14psi, which seemed odd (like maybe it HAD been pressurized at some point overnight but had dropped over time?):

 

   

 

I just checked it again (now 2pm) and found the system fully pressurized:

 

   

 

At no point last night or today has a door been opened or keys been anywhere near the car.   The hood was left open the entire time.   All I've done it looked at the fuel pressure gauge.

 

Is it possible that the ambient air temperature could "pressurize" the existing fuel system?     It's summer here in Texas and the temps are pretty hot.  Last night (with zero pressure in gauge) it was still 88-90F at 6pm.   Temps dropped to 74F last night.  When I first checked the gauge this morning (11:30 - with the odd 12-14psi reading) the air temp was 85F.   When I checked it this afternoon and saw it fully pressurized it was 92F.    Even if the air temp could cause fuel in system to expand slightly and create some pressure, I'd NOT expect it to get up to 56psi.

 

Any thoughts?   I'm completely baffled.

 

 

 

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[unknown4u]

Your pressureregulator at the end of the fuelcollectionpipe is shot/ or almost shot. With very warm temperatures it will be hard to start because of a vapourlock.

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[Velocity]

Your pressureregulator at the end of the fuelcollectionpipe is shot/ or almost shot. With very warm temperatures it will be hard to start because of a vapourlock.

 

Do you mean this part?    Porsche Fuel Pressure Regulator Bosch 0.280.160.287 0 280 160 287 0280160287 0-280-160-287 94411019806 | Pelican Parts

 

From what I'd read a faulty fuel pressure regulator OR a faulty fuel check valve could let pressure drop or cause lower pressure than needed - hesitation, hard starts, etc.     The problem I'm seeing is actually the opposite - the system being fully pressurized when it likely should not.     

 

I haven't noticed any hard start issues, and we're certainly getting plenty of very warm (90-100F) temperatures here.

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[unknown4u]

Yes that is the part. The pressure at the fuelcollectorpipe must be always the same. I changed the regulator in the past and pressure is equal al the time. If the pump is in good order the maximum pressure is there when you start the car.it do not has to have the symptoms as you describe. A little bit of pressure is enough that you do not have the problems of vapour in the fuelcollectorpipe.

If the pressure drops the regulator has an internal leak or the spring can be 'soft' after 30 years.

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[Waylander]

this is really interesting, I currently dont have a clue why the pressure would rise to that level, the FPR is vacuum operated and allows the fuel to level pressure at the values you are seeing, there is no other restriction in flow back to the tank

what would be interesting to try is to remove the fuel cap when the pressure is high to see if there is any tank pressure, and if the gauge drops any,

 

this could point to a charcoal filter line blockage.

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[unknown4u]

Gremlins?

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[Waylander]

30 year old gremlins,

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[Velocity]

I already have my share of other 30-year-old gremlins causing other issues with my car :whine: ... but I did confirm a bit more logical piece to this puzzle.

 

The changing readings I'm seeing on my fuel pressure gauge (again, without car being touched) are definitely related to the ambient air temperature.   I've checked several times this week and without fail:

1. Early in the morning or in the evening/night (when temps are lower) pressure is zero. 
   
2. Late morning or early evening, pressure is medium (15-40psi)
   
3. Midday, it's at the peak (55-60psi).   Temps at this peak part of the day during this time of the year are 90-100F.
   

@waylander   - removing the gas filler cap (and even depressing the filler tube "flap" when the pressure is highest at midday does NOT result in any escape of gas, pressure change or anything else.

 

So.....the question becomes - is it normal for residual fuel in the motor to "fluctuate the pressurize" at the fuel rail (where gauge is hooked up) with ambient temp change, or should any increase in pressure due to temperature change be escaping (via the charcoal canister, I'd assume)?     Where (roughly) is that canister located (so I can inspect lines going to/from it)?    I see it in some of the parts diagrams, but it's a diagram of the fuel system itself, not showing where the canister is on the car itself.

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[unknown4u]

The fuelrail is always be pressurised.

If it drops than one of the injector has a small leak or the regulator is shot.



Test is simple. Start the car. How many pressure? And wait 30 minutes. Pressure may not drop.
The fuelrail is always be pressurised.

If it drops than one of the injector has a small leak or the regulator is shot.



Test is simple. Start the car. How many pressure? And wait 30 minutes. Pressure may not drop.

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[Waylander]

There is a pressure valve in the end of the fuel pump, this is there to maintain pressure when the injectors are really hot



If there is no pressure the fuel will boil in the injectors and rail. If this happens the car can be really hard to start, so like pulling up for gas and when you get back in the car is really hard to start



But in the manual it says this pressure should be maintained for at least 20 mins, but does not mention pressure is permanently there



When you turn the key to start the fuel pump runs for a couple of secs to raise pressure, the DME will not turn the fuel pump on till 200 rpm is reached



I think it would be interesting to have more data from other 968s

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[unknown4u]

I always thought that that valve pressurises the line from pump till the fuelcollectorpipe. The fuelcollectorpipe is controled by the regulator. The gauge is in this part. That eliminate the valve at the pump which is located at the start of the line from tank till the motorcompartment.

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[Waylander]

If the non return valve in the pump is not working, the car will start from cold, once hot it will be a real nightmare to start, this system was used on Mercedes and Volvo and a few others



The fuel rail and injectors need to be kept under pressure to stop the fuel turning to vapour

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