Fuel Pressure Drops Quickly After Engine Shutoff (81 FI)

[maxm50]

Hi all,
I rented a fuel pressure tester today to diagnose a rough running condition (which ended up being a loose connector to my coolant temp sensor). Before starting the engine, I activated the fuel pump and got 40psi on the tester (no vacuum on the Fuel Pressure Regulator, obviously) After starting the engine, the pressure was steady at 30psi at idle and would blip up to about 40psi momentarily when blipping the throttle. So far, so good, right? Unless I'm missing something?

However - after shutting the car off, the fuel pressure drops VERY quickly below 20psi (within seconds) and within a few minutes is at about 15psi. After about 10-20min, it's at 0psi.

I'm not terribly familiar with fuel pressure regulators and associated components, but I found some general diagnosis instructions online (not fiat-specific) that said a rapidly dropping pressure after engine shutoff is not a good thing. Here's the text: "If fuel pressure begins to drop soon after shutting off the engine, you probably have a leaking injector, a leaking anti-drain valve in the fuel pump assembly or problems with the FPR itself."
Is this normal for an FI spider to lose all fuel pressure within minutes of engine shutoff? Or should it be holding pressure longer?

The car runs reasonably well, but the idle hunts up and down a little bit - not rock-steady smooth by any means. Also some popping on decel.

My next thought is to check for a leaking cold start injector. The fuel injectors were just rebuilt last week, and all hoses are new. Fuel pump and filter are about a year old, and only a few thousand miles. Any other things you guys would look at?

Thanks!
Max

[maxm50]

Update:
- Cold start injector is not leaking.
- With engine off, if I activate fuel pump to bring fuel pressure up to 40psi, then turn off fuel pump, then put a clamp on the fuel return line (the one coming off the bottom of the fuel pressure regulator), the pressure in the fuel rail will hold steady. It took me a few seconds of fumbling to get the clamp on, but I got it on when it was at 30psi, and it held steady there. Upon removal of the clamp, it immediately started dropping again on its way down to 0psi.

So, this tells me that whatever sealing mechanism is in the FPR is allowing fuel to leak through into the return line after the fuel pump stops. I'm assuming this is not normal operation, and I need a new FPR that actually seals properly and holds pressure. Is this correct or am I missing some unique thing about the Fiat FPR?

[maxm50]

Another update:
I spoke with Autoricambi and they confirmed that these symptoms mean my fuel pressure regulator is bad. It should actually hold 36psi (2.5bar) at idle. And it should not leak down pressure so quickly. A new one is on the way!

[tima01864]

With the fuel pressure gauge you rented, How did you attach the gauge to make your tests? Something I need to do.

[rridge]

The fuel rail should hold some pressure for at least 24 hrs. That's why the factory service manual cautions to release pressure before servicing. Immediate loss of pressure can be caused by a bad fuel pressure regulator, one or more leaking injectors, or a bad check valve in the fuel pump, which allows the pressurized fuel to reverse its flow and move back to the tank through the pump.

A shop that deals with old fuel injected cars often has a rig that allows them to test and back flush regulators and pumps that have been removed from the car. I don't know of a safe way of doing that in a home garage. Nor do I know of a safe way to testing them in car.

You might try adding a bottle of a gasoline detergent like Techron which could free up sticking injectors. Before replacing either the pump or regulator take a look inside the fuel tank. The FI fuel gauge sender unit lifts out of the top after removing six small nuts. With a flashlight pointed through that opening in the top you will see any rust and/or debris, particularly the black rubber particles shed from the lining of deteriorating fuel hoses. Installing new components in a fuel system full of debris is a waste of time and money.

[maxm50]

rridge, thanks for the tip! I'll pull the fuel sender out today and take a look in the tank. While the car has been down this week I've replaced most of the fuel lines anyway... later today I'll replace the fuel filter, add a pre-filter before the pump for insurance, and replace the last 2 remaining fuel lines in/out of the fuel tank. Thanks for the tip about fuel hose particles... I hadn't even considered that. I assumed rust was the only enemy.

I just had the injectors rebuilt last week ($35/each) through Fuel Injection Corp (they do fantastic work), so I really appreciate the tip on making sure the rest of the system is clean to protect my investment!

tima, I hooked the fuel pressure gauge up just after the fuel filter via a Tee fitting that it came with. I was replacing that hose after the fuel filter anyway, so that's why I chose that location. Many folks also attach it to the cold start injector fuel hose at the fuel rail (warm the engine up first so the CSI isn't needed).