jbeebe

Hey everyone, let's get down to it!

'92, straight-six, xl, twin tanks

-cranks for a looooong time before finally starting and idling fine. takes just as long to start on either tank,
-once started, it sputters at anything over ~20% throttle, regardless of which tank is selected
-Fuel pressure on the rail with engine off and key on: 0. with it idling: 40, opening the throttle: 42. same pressures regardless of the tank selected
-I can hear the fuel pumps priming when I turn the key

What's the first direction I should go in my diagnoses? Any ideas?

**solution/Update: new pump fixed the fuel pressure issue, then a new ignition coil got it running great

djfllmn

iTrader: (1)

@Steve83

BLDTruth

So just to clarify, when the pump primes, with the key on and engine off, you are receiving no fuel pressure at all? You should be seeing pressure when the pump primes.

jbeebe

exactly! after I hear the fuel pump primes with the key "on", there is 0 pressure at the rail. Once it eventually does start, after a lot of cranking and sputtering, there is 40 at the rail.

Steve83

The captions in this photo album explain how the fuel system works, and most of the reasons why it can fail:


(phone app link)

Pay close attention to the fuel tank levels. If one of them is INcreasing when you use the other tank, that FDM is bad. This FSA describes the problem, but the fix in it is NOT effective, and those parts probably aren't available any more:


(phone app link)

BLDTruth

This is definitely a new problem to me - never heard of the pump priming and seeing NO fuel pressure at the rail. Does it hold pressure at all when you turn it off? Or does it drop right down to zero? What happens if you pull the vac line off of the fuel pressure regulator while it is running?

Also I am unsure on the fuel pressure specs for a 4.9 - at some point in the 90's it changed. Some were 45-60 psi and some were 35-45 psi, I just can't remember what year it switched over.

jbeebe

I don't remember exactly, but I don't think it held pressure when turning it off. I took the red hose off the fuel pressure regulator and the idle changed, no fuel came out of the line.

one thing of note that I didn't remember until just now.....one time when filling up, more than 20 gallons went into the front tank before I manually stopped the pump. Could this be due to a bad "check valve" that prevents cross-flow between tanks? Could gas be pumping into the other tank instead of into the engine?

BLDTruth

Only things I can think of off of the top of my head would be bad check valves in the pump modules (unlikely both would be bad, but it can happen), bad fuel pressure regulator (vac line would smell like gas), or you have an injector that is stuck open or leaky.

Start googling those problems and good luck.

mike100

You should have 8-10 more psi than you are getting on an efi 4.9. They used 3.8 v6 injectors that supported 150 hp (but at a higher rpm). To get the things to squirt a 150 hp worth of gas at 3000 rpm, they raised up the pressure on these inline 300's.

get a long extension hose and tape the fuel pressure gauge to the windshield and go for a drive. You didn't mention if you ever checked or replaced the fuel filter.

jbeebe

I haven't replaced the fuel filter. I've never really heard of an old fuel filter causing this much havoc. As far as the fuel pressure when driving, I opened the throttle as far as I could without it sputtering, which is about 25%, and the pressure blipped to 42.