I have a FP gauge mounted on the rail.

This week I was at events with other Porsche owners where we were looking under hoods etc.

On two occasions I noticed that after shutting down after getting engine to full temp after long drive, the pressure was dropping to near zero after about 30 minutes.

Tonight I was doing more tests and from cold was getting 3.5 bar at idle, rising to 3.8 bar at shut-off. The pressure held at this level for 30 minutes. I then idled the car till it was warm (fans cut in - is this sign engine is sufficiently hot?) and repeated leak down test. Again the pressure held at 3.8 for 30 minutes - no drop at all as per the WSM test.

Tomorrow I'm going to take her for a long drive and repeat the test. Meantime does anyone have any idea what could be causing the apparent symptoms i.e. losing fuel pressure after engine gets very hot, but pressure holding fine when cold/warm?

I don't have any warm or cold start problems. Always fires first time. New injectors so don't suspect injector leaks and this is supported by lack of starting problems.

Apologies for not having definitive symptoms - it's more suspicion at this stage. I will post results tomorrow after getting engine properly hot.

The engine is not running very sweetly though. I get 20mm Hg pressure at the FPR and dampers when I put a gauge on the vac lines going to these. When engine idling I don't get see/feel any fuel when I pull off vac lines from the FPR or dampers. Idle is steady but doesn't 'feel' smooth.

TIA for your thoughts.

 

mine did that after awhile too.( My old 5 liter part euro) heard it was a valve on the fuel pump that let the pressure out.
caused no issues though and never did anything about it.

mk

 

Since your injectors are new....that "eliminates" them....your most likely next candidate is the check valve on the fuel pump....when it fails the pressure flows back through the pump into the tank instead of stopping at the pump.....another possibility is the fuel pressure regulator is failing and returning pressure to the tank instead of holding it in the rail....easy to check by removing the return line to see how much fuel comes out (think not much when working properly).....

 

Thanks guys. I know check valve and FPR are both candidates when fuel pressure drops too fast, but would it be possible for this to happen with hot engine only and not cold?

 

Another data point here... mine fades to zero after a couple of hours (never really timed it), but it has never been an issue.

 

If the fuel in the rails gets very warm from the extended high speed run (in comparison to the idle test) the pressure could drop just because the fuel cools down to ambient temperature (takes a while - not sure how long).

 

The spec is to drop no more than 1/2 bar in 30 minutes. Leakdown above this can make warm starts take a few cranks longer and stumble for a few seconds until pressure builds. Here is a diagnostic guide.

 

I recently added a inline rail pressure guage and noticed that after a few hours the pressure will bleed off to zero is this normal or should there be some pressure in the fuel rails when the engine is cold.

 

My experience is that it's pretty normal. THe question is how quickly does it bleed off. If you don't have a problem with the car starting or running though... I wouldn't worry too much about it.

 

After sitting over night it does take a few cranks to get it started. I assumed that the fuel system would have remained pressurized up to the fuel rail.

 

I have a cold start problem but it's certainly not gas delivery...I installed new FP and valve, new filter, new Fuel pressure regulators and dampners, new Temp II, new MAF, solid vacuum pressures all around, and fuel pressure only drops by about 1 bar (from 3.5 bar) in 30 hours so I think it's safe to say the injectors are OK. I will do about anything to keep from pulling the manifold to replace stuff under there. I'm now wondering if I have a short somewhere. I hate electrical. And my approach to troubleshooting is popular with the Big Three.

H2

 

Honestly pulling the manifold is not a tough job. I actually enjoyed it and am a real newbie at working on cars - the manifold job is very satisfying and a great opportunity to indulge your parts renewal habit! It's not the kind of 928 job bedevilled by corroded/seized bolts (the only tough connections to undo are the fuel rails/dampers and that'smore about getting in a nice position to exert force on wrenches).

The tricky thing is just keeping track of where everything is connected but taking notes as you go and lots of photos addresses that, as well askeeeping parts labelled/organised as you remove them.

 

Yes....but it may be another $600 worth of WYAIT parts to replace a $90 bit. I hate doing things twice so I'd spend the extra money. Aren't the manifold gaskets quite expensive and the old ones can't be salvaged?

H2

 

Hi

I have a Fuel Pressure Transducer connected to my fuel rail...

Think of it like a fancy electronic version of yours....it sends a signal constantly to my Race Controller on dash...

I can therefore see pressures before I start and when idling and while driving...constantly.

Pressures as follows...

First thing in morning after being left all night = 37 psi/2.55 bar

On tickover = 39 psi/2.6 bar

Light acceleration = 42 psi/2.9 bar

Hard acceleration = 46 psi/3.1 bar

Hard acceleration WITH Nitrous = 52 psi/3.6 bar

Hope this helps ....

All the best Brett

 

My car is exhibiting this problem now. When the car sits for a couple of hours, it either will not fire right up, will fire and catch and sputter and die, or do both and then will fire up and run/idle fine, like the fuel system is not pressurized. When it does fire up, there is a quick, slight puff of white smoke and the car smells rich for a short bit. I replaced the fuel filter and check valve last year.

I am going to put a fuel pressure gauge on it and see what pressure I have at different points in time.

I don't know the ins and outs of the fuel system. What do the FPR, the fuel dampners, and other fuel system parts do?

Anybody have any suggested readings on how a fuel system like ours functions, parts explanations, etc?