My '96 S-10 2.2L (81K miles) has had a bit of a hard time starting up lately. Takes a few seconds of cranking, and spits and sputters for a moment before settling into idle. I got around to purchasing a fuel pressure gauge and testing the system today.
The pressure gets right up to 45 PSI when the ignition is turned on, right in line with specs. However, once the pump turns off the pressure begins dropping, going below 20 PSI within about 20 seconds.
My troubleshooting research said next to build up the pressure again, and then pinch off the pressure/return lines and see what happens. In both cases, the fuel pressure continues to drop at about the same rate. What I’ve looked up tells me that this indicates leaking fuel injectors.
Does this sound like the proper diagnosis to the more experienced?
I forgot to add that the fuel filter was replaced in case it could be clogged, but that didn’t do anything.
No it is not leaking fuel injectors. It is the pressure regulator. Remove the vacuum hose from the regulator and see if there is fuel present. If there is fuel present the diaphram inside of the regulator has ruptured, the regulator will need to be replaced.
That would be a far easier job. However my understanding is that, since the pressure continued to drop with the fuel return line pinched off, one or more injectors are implicated. Supposedly a leaking regulator wouldn’t give that symptom; you’d see a pressure loss only when the inlet line (between the fuel pump and the test gauge) was pinched off. Hogwash?
Addendum: I checked in the vacuum hose for the regulator, and there was no fuel in it. I then checked again with the ignition turned on, still no fuel coming from the regulator. I suppose that means it’s okay.
Yes, that just leaves leaking injector(s) or an external fuel leak at the rail. I would think you would have spotted an external leak, so fuel injector(s) it is.
The problem may also be with the anti-drainback valve in the fuel pump.
Tester
Yeah, I’d thought about that possibility too. Would that cause pressure loss even when the pressure line (between the fuel pump and test gauge) is pinched off?
I don’t suppose there’s any simple way to determine whether it’s this or injectors, short of snaking a tiny camera down the intake manifold and watching the injectors to see which one drips, if any. sigh
If you pinched off the supply hose to the fuel rail and the pressure dropped that quick, then it’s probably a fuel injector. Why not remove the fuel rail/injectors and run the test again to see which injectors are leaking?
Tester
At the risk of sounding like a wimp, is that safe to do? I haven’t come across anything about pressurizing the rail/injectors when removed from the manifold.
Sure it’s safe. If there’s leaking injectors, they’ll spray gas for a second and then dribble gas. Just don’t be smokin’ a big stogie when running the test.
Tester
Aw crap, you just completely ruined all the fun I had planned! 
Okay, I’ll give this a whirl whenever I have a weekend day free and feel like tearing into the thing. For the time being, I imagine it’s not hurting anything to just keep driving normally.
Well, you might want to pull the oil dip stick out and smell the oil to determine if the oil is being gas contaminted. If you don’t smell gas in the oil, you can drive it until you get to fixin’ it.
Tester
I agree with the drainback valve diagnosis.
Since no one has mentioned it yet, I’ll suggest that you should never pinch an F.I. fuel line; even a high pressure, flexible hose line.
Pinching it flat is not likely going to close it off anyway no matter how tight you squash it.
Along with a pump replacement you should consider fuel line replacements or risk the truck going up in flames.
Really? That’s not cool since the Alldata diagnostic procedures I followed said that’s how you narrow down where the pressure loss is coming from, by pinching at point A and watching pressure, then pinching at point B, watching pressure, etc.
I can say that I was very quick about it, only a couple seconds at a time. No section was pinched more than once, and all returned to shape immediately. No leaks developed either. However, I guess there’s now a chance that the interior of every one of those lines has been shredded and might burst at any time. Is that what you’re saying?
OK. They make a special plier set that can be used to pinch off fuel lines. http://www.sjdiscounttools.com/atd-5465.html
Tester
Let me ask this, if I may.
How do I go about narrowing down where the pressure loss is at if I’m not supposed to be pinching off fuel lines? Somebody must have a way of doing it safely, without causing potential damage to the lines. Are there check valves or something that I need to install at multiple points throughout the fuel system?
I note the link about the pinch off pliers set but it makes reference to coolant, fuel, and vacuum lines.
To me anyway, I read this as a low pressure, soft hose application set of tools not meant to be used on high pressure F.I. lines.
To crush a F.I. line down (assuming it’s hard rubber to begin with, is going to require a lot of grunt and I can’t see putting that much hurt on a line like that.
Everyone has their own methods I guess; crushing a F.I. line down is not one of mine.
I still think Tester is correct about the fuel pump check valve. That little ball or ball seat gets one speck of grit or even the tiniest wear mark on it and fuel pressure will disappear pretty quickly.
Thanks for your input, ok. The sections of rubber fuel line (it’s metal in some places) don’t feel super hard when squeezing them between two fingers, and they didn’t take a whole lot of effort to pinch off. They came right back to shape and nothing felt odd upon a physical check/squeeze of each section, so I think they’re okay.
In any case, a fuel pump is something I’ve done before and don’t mind tackling again. On S-10’s a number of folks remove the bed and get at the pump that way, rather than dropping the tank. I’d much rather do that than tear into the intake manifold looking for a leaky injector that might not even be leaky…plus injectors run about $70 a pop while a fuel pump can be had for half that.
I’ll let everyone know what happens.