I've been continuing to have a problem starting the car. It always starts, I just have to crank it for a good 5 to 8 seconds before it finally catches. For some reason it isn't retaining pressure in the fuel system after I turn off the key. If I let the car sit for 30 minutes or more it always takes 5 to 8 seconds of cranking to get it going.

So, I noticed that the fuel system contains a "fuel pressure regulator". It has a fuel line going through it and a small plug on one side where an electrical connector plugs in. I'm thinking that might be my problem, but I'm not sure how to tell. Also, I don't even know where the doohickey is located, I've just seen pictures of it.

Other than the long crank times the car runs great, so I don't think it's a stuck injector. I ran some Sea Foam through the fuel system and that didn't help. If anything it made the problem worse. Under full throttle the engine doesn't bog down, so I don't think it's the fuel filter or fuel pump. The engine always gets enough fuel. Something is just causing the pressure to bleed off after I shut off then engine, and I'm not smelling fuel, so it must be bleeding off back into the tank. That's why I suspect this regulator thingy. I just don't know exactly what it does.

Oh yeah... it's a 2003 XK8 with the 4.2 liter engine (that's litre for you Brits).

 

Long story short, it maintains (nearly) constant fuel pressure, which is needed so that some chip somewhere can reckon the amount of fuel that will be delivered by a pulse of a given time interval.

Since the car runs fine after starting, I think the regulator is probably OK.

Can you by any chance smell gas at start time or after shutdown? I remember something about a tiny fuel leak that shows up with these cars. Maybe it was at or near the regulator; can't remember.

 

The fuel pressure regulator on my last car was situated right before the fuel rail and the original one was NOT adjustable. I ended up getting an adjustable one but this is pretty much a waste of time as the computer in modern fuel injected vehicles adjust the duration of the injector pulse to compensate (lowering the pressure will yield a longer pulse and increasing the pressure will result in a shorter pulse). As I recall, the fuel pressure in that car was about 46psi with the vacuum line disconnected and 36psi with it connected. So at wide open throttle (when you want max pressure) you get it.

If the diaphragm inside the pressure regulator goes, you will have the type of problem you are having. The other possibility is a fuel pump that is dying. This happened on my last car and the symptom was having to crank longer than usual in order to start the car. When I hooked up a pressure gauge to the fuel rail I also found that the pressure quickly dropped to zero when I turned the engine off. The pressure should hold for quite some time and loss of pressure could indicate bad injectors, leak in fuel system, bad pump, bad regulator.

Doug

 

In the X100 prior to the 2003MY the fuel pressure is held in the fuel rail and line from the tank by 2 things, the fuel pressure regulator closes at fuel pump shutdown as no more fuel needs to go back to the tank and there is a 'check valve' at or near the fuel pump itself. If either one leaks, then the fuel pressure drops and the fuel can vaporize (liquid boils at a lower temp at lower pressure) which will cause a hard restart as the injectors are designed to meter liquid (not vapor). You need to connect a fuel pressure gauge at the fuel rail and monitor the pressure to see how fast the pressure drops at shutdown. If it takes and hour or so for the pressure to drop near zero then it is probably fine and you need to look elsewhere for a fault. If the pressure drops to zero within a few minutes, then you will need to determine if the fault is the regulator or the pump check valve. Block the fuel return path and if the pressure still drops then it is the check valve in the pump. If the pressure holds with the return blocked, it is the regulator that is faulty.

The X100 2003MY has a 'returnless system' meaning that the pump speed/pressure is controlled using PWM (pulse width modulation) and there is no mechanical pressure regulator. There is a pressure sensor for pressure feedback to the control module.
If you have the capabilty to monitor the pressure electronically with a datalogger then you won't need a pressure gauge, but the system still has a check valve.

There is another possibilty that one of the injectors is leaking into the intake manifold and pressure is lost. When the engine starts, that one cylinder is flooded and the engine runs rough and some black smoke will be noticed at the tailpipe.

It's all about DIAGNOSTICS.

There are 2 ways to fix a car, Throw parts at it until it is repaired or diagnose it correctly (money or logic)

bob gauff

 

Pressure Regulator - designed to permit fuel to return to the fuel tank depending upon engine RPM. Typically mounted downstream of the fuel injectors and part of the injector fuel rail system and operated by vacuum and/or electronic controls that operate a diaphram against a spring to regulate fuel return to the fuel tank. The more vacuum (to wide open throttle), the less fuel permitteed to flow to the tank. Conversely, with less throttle (idle), more fuel is permitted to return to the tank. The fuel pump is somewhat constant in its pressure and flow performance.
Looking in the FAQ Quick Links, in the Fuel System, Microfiche G-17, I'm seeing a vacuum line to the regulator.
In your case with extended cranking but overall good performance once started I would determine fuel pressure through the fuel injector rails during cranking, while running, and after engine shut down. The regulator (preset at factory) should keep fuel from flowing back to the tank and therefore you should have enough pressure for easy engine starting. Sounds as if your extended cranking is an attempt to build pressure to trigger injectors. I also agree with motorcarman regarding a stuck open injector which may gradually be relieving the fuel rail pressure during engine off. I would not give up on a faulty injector - maybe another injector cleaner through the engine.

 

WOW! This forum is just full of awesome advice! Thank you all.

• I'm going to run another bottle of Sea Foam through the tank, but I don't think that's going to help.
• I'm not getting any smoke out of the tailpipe at startup, so it doesnt sound like a stuck injector, but, just in case it is, how does one identify which injector might be stuck? Stethescope?
• I need to do a little research to see if my 2003 has the regulator. According to the auto-parts stores it does, because they sell a fuel regulator for a 2003 XK8, but according to Motorcarman it probably doesn't. If it does, my next step will be to change it out and see it that fixes anything. It's a much easier fix than pulling the whole fuel tank.
• If none of that fixes it, I guess I'll pull the tank out. Is the check-valve a part of the pump itself, or is it a separate part?

 

You can tell if your car has a fuel pressure regulator, if it has a fuel pressure sensor, it has no regulator, it has the return-less system Bob Gauff is telling you about.