1996 4.0 fuel pressure?
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Tuned
Joined: Feb 2005
Posts: 380
Likes: 1
From: ohio
1996 4.0 fuel pressure?
96 has intermitent starting problems, will start but runs very rough or may die a time or two. Changed filter today and hooked up a guage. Pressure is 26-28 at idle, 30 before starting A search here did not haveany answers for me, what should the pressure be at idle and if mine is not right could it be causing this intermitant (more often than not) problem?Thanksin advance
Postmaster
Joined: Dec 2001
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From: UT
I'm pretty sure your '96 still has the same fuel system as my '92. 26-28 at idle is a little low, but should still run ok. 30 before starting is arguably too low, should be closer to 40. If you remove the vacuum line to the FPR, what is the fuel pressure at idle (should be 40 give or take a couple)?
Former ******
Joined: Nov 2004
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From: Salt Lake City, Utah
I'd say you have a bad regulator or a weak fuel pump. With fuel pressure than low, the injectors struggler to produce a clean spray pattern. Also the computer expects a certain flow rate from the injectors, which I'm sure they are struggling to deliver with that low a fuel pressure. The pressure before starting should be 40 - 45 psi, and the pressure while running should be anywhere from 30 - 40 preferably 35 - 40.
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Tuned
Joined: Feb 2005
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From: ohio
OK checked it agin tonight. key on engine off 26lbs no change if you turn off and back on, pumps runs as it should. Key on engine runnning about 25-6. Pulled vacuam line and had 28-30 lbs engine running. Wife droive it all day running earnads for her work said it started fiine except twice it started, then missed for 10 seconds then clears and runs OK.
Postmaster
Joined: Dec 2001
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From: UT
Unless the fuel pressure gauge is in error, those pressures indicate a problem. Does it hold pressure when the pump is off? Can you tell if gas is being returned to the tank (any flow through the return line)?
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Tuned
Joined: Feb 2005
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From: ohio
OK, gauge can be verified but I am out on business tomorrow. Yes pressure holds steady when shut off. Where is the easy place to check for return flow? And could you be specific, took quite a while for me to find the regulator, sorry but I am a 4.0 retard. Over on the 7.3 page there is some nice info like underhood shots that show all sensor locations with proper site lingo etc is there any here? Searched to no avail.
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Postmaster
Joined: Dec 2001
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From: UT
Easiest way I've found to check for return flow is to listen at the tank for gas flowing into the tank. However, in many applications, you can't hear it over the pump.
On my '92, I simply unscrew the return line from the regulator, ground the fuel pump test lead, and watch for gas to come out of the regulator. I'm ok with this, because I'm under the hood and, if any gas comes out, I can immediately shut the pump off. I don't know how to control the fuel pump like that on your '96 (might even require a higher end scanner).
Another option is to locate a rubber section of hose in the return line, get an appropriate tool that won't cut the rubber, and clamp of the return line and see if the pressure goes up. If the pressure goes up, then the regulator is opening too soon. If the pressure stays low, then your pump is too weak (or filter is clogged or something like that is going on).
On my '92, I simply unscrew the return line from the regulator, ground the fuel pump test lead, and watch for gas to come out of the regulator. I'm ok with this, because I'm under the hood and, if any gas comes out, I can immediately shut the pump off. I don't know how to control the fuel pump like that on your '96 (might even require a higher end scanner).
Another option is to locate a rubber section of hose in the return line, get an appropriate tool that won't cut the rubber, and clamp of the return line and see if the pressure goes up. If the pressure goes up, then the regulator is opening too soon. If the pressure stays low, then your pump is too weak (or filter is clogged or something like that is going on).
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Joined: Jan 2009
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Is black smoke coming from tailpipe? Could be a sign of a leaking injector. But I'd rather prove out the fuel pressure problem first. Key on engine off pressure should be 40psi as suggested, running @ 30psi. You said you already changed the fuel filter so it's either a worn out pump or a stuck regulator.