Ever seen this fuel pressure problem?
 

I've got a NA S5 13B in my FB and am looking for some pointers with a weird fuel pressure problem.

Engine is running a TEC3 EMS, ID1000 injectors, brand new Walbro 255, and a brand new fuel filter. The rats nest has been removed and the fuel pressure regulator is subsequently connected to a nipple on the front rotor intake runner.

With the engine off, the fuel pump primes to approximately 43psi and will hold just over 20psi after it's done priming and shuts off. With the engine running, pulling approximately 15inHg of vacuum, fuel pressure will INCREASE to around 55psi. Seems like the exact opposite of what's supposed to happen.

I have verified that the fuel hoses are connected properly.

Anyone ever seen this condition before? I guess the only part to blame would be the fuel pressure regulator?

it's a property of how the walbro works. i'm not exactly sure what the pump does to raise pressure more than others with the same volume characteristics but it does.

verify that the pressure drops in vacuum and rises with the line disconnected to be sure you have a good vacuum source to the regulator. aside from that the walbros do tend to raise pressure more than other pumps so you should expect that.

what type of vacuum line are you using? some oem lines like the boost senor have resistors in them. this could cause 0 vacumm on the regulator side and vacumm on the intake side. also how are you measureing fuel pressure?

It isn't a positive pressure offset per-se, where the pressure is higher than what it was with the factory pump.

The issue is this:
- fuel pressure decreases as vacuum increases.
- fuel pressure increases as vacuum decreases.

At 0inHg (engine off, fuel pump priming) - 43psi. This is what it should be with the vacuum line disconnected, or at WOT (but will verify when I get home today).

At -15inHg of so (engine running at idle) - 50psi.

While idling at -15inHg (-7.4psi) I should see approx 36psi at the fuel rail (or on my pressure gauge) which makes for a pressure differential across the injector of 43psi.

Instead, at idle I have a pressure differential of 57psi across the injector because fuel rail pressure is 50psi and not 36psi like it should be.


What could cause fuel pressure to increase when it should decrease? Is it possible that the pressure regulator is screwed up that bad?

So this is what's up.

Fuel pressure regulator is dead. I assume it's frozen in a position. Disconnecting the vacuum line produces no change in fuel pressure.

The increase in fuel pressure is due to the voltage increase when the engine is running.

That was easy.

did you check to see if the vacuum line actually has vacuum? there's plenty of ports that don't produce vacuum signals.

I was just gonna post that!.Damn Tb ports are sneaky,I found that out with my Boost controller!
Also thought:where is the FPR installed?..it should be after the secondary rail out.

probaly the stock fpr cannot flow enough, hence the pressure rises when the engine is running and the pump is getting more voltage because of the alternator running.

2 solutions: use stock pump or use a decent fpr.

I had exactly the same issue with a TII and a AEM intank pump. once I rewired the pump the fuel pressure was over 4 bar when the relay bypassing the resistor closed and the pump was seeing a real 12-13v instead of the 9v with stock wiring