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Mine did that years ago. Made it very hard to start after it had been sitting for any length of time. I think the fuel pump runs for its 1 or 2 seconds when you turn the key on to combat your pressure at the rails dropping like this. My controller was acting up and not giving the pump the kicker signal, so I'd have zero pressure at my fuel rails when trying to start the car. It would take several seconds of cranking for the fuel pressure to come up. My guess is that the ECU "thought" the engine was running, so the pump switched to regular "on" mode, although that's only speculation. I bypassed the fuel pump controller and ran a switched, fused, 12V supply to the pump. When I wanted to drive the car I turned the ignition to ON, flipped a toggle switch to activate the pump (rail pressure would immediately jump up to normal as soon as I ran the pump), and start the car. It would start with just a bump of the starter like this. Never failed to work like this. I drove the car that way for months before deciding to go back to the FPC(fuel pump controller). For some reason the FPC decided to work OK, and it never had a problem again. No idea why. I never did figure out with 100% certainty what caused my fuel pressure to drop like a rock (I mean within a few seconds it was at zero after turning the engine off), but here are my 2 best guesses. First would be that the piece of fuel line inside the tank connecting the pump to the hardlines was either cracked, or the clamp wasn’t holing it to the fuel line tightly anymore, allowing fuel to bleed back into the tank. Second, would be a failed check valve somewhere near the pump. I *think* I confirmed to myself that there was a check valve inside the tank with the pump somewhere, but it’s been so many years that I can’t be sure anymore, sorry. I plumbed a fuel pressure gauge after the fuel filter and before the rails to watch my pressure. When running it was fine, but as soon as you turned off the engine, pressure dropped immediately. I used 2 pairs of ViceGrips and 4 Quarters. With the aid of a helper, I had the engine running so pressure was up. I used a Quarter on either side of the fuel line and pinched the lines shut with the ViceGrips and turned off the engine. This is all from memory from literally 6+ years ago, so I’m not 100% sure. I think I posted about it quite a lot when I was having these issues though, if you want to try to find my old posts on it. When I had the return line pinched off, then pinched off the supply line before the fuel filter, my rail pressure would stay indefinitely. That told me I didn’t have any O-ring leaks at the injectors, fuel rail to hose leaks, etc. Now, if I released pressure on the supply line, fuel pressure dropped immediately. As I remember, I could leave the supply line pinched off and release the return line, and the pressure stayed up in the rails. I *think* (without going back to my FSM, etc) that the FPR holds the pressure up and stops the fuel just streaming back to the tank through the return line. In summary, in my case I found that something on the supply side was letting the fuel run back to the tank as soon as the pump was turned off. With the pump running, there was enough extra supply to overcome whatever “leak” I had back into the tank, and things ran normally. Even at WOT my fuel pressure stayed OK. This was when I was still NA, so I don’t know how WOT on a TT would be affected with the higher fuel demand. Long post, but hopefully will help you some, if you would like, email me and I’ll give you any more help that I can. Best of luck,
Mike --------------------------------------------------------
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