92 F-150 SWB, 5.0L EFI, M5OD

I've been chasing a heavy throttle sputter for a while now. It acts like it's trying to run out of fuel when I start up a long steep grade (well, as steep as it gets between Kentucky and Central Alabama). Small short hills, no problem, just when the throttle has to stay open for extended periods.

Is it possible that I'm losing pressure due to flow restriction or worn pump? I would think a worn pump would be deficient in pressure, but I've eliminated a pressure problem by testing at the rail using the pump test input on the DCL and a vacuum pump to check the regulator (which has been recently replaced). Engine isn't running, of course, so I get 40 PSI at zero vacuum, and about 30 PSI at 25 inches vacuum (idle vacuum on engine is 21). Pressure with engine idling is around 32 PSI.

Now I question how fast the pressure should respond when the vacuum drops, and know that flow restriction will cause pressure drops. Obviously, I already replaced the least expensive and most easily replaceable part (fuel filter). Now I have to wonder if the sock on the pump is covered in crud. While doing my static testing (engine not running), I started the pump with the jumper on the DCL, and watched the pressure build. It seemed slow considering none of the injectors was firing. I pulled the vacuum on it, and the pressure dropped as vacuum went up (regulator doing it's thing). When I released the vacuum suddenly, it took about 5 seconds for the pressure to climb back up to the 40 PSI. If this is happening, is it possible that I'm getting a starvation problem while driving? It's a little difficult to run beside the truck at 80 mph to check the gauge, and keep it between the lines. Clutching and shifting from outside is a REAL challenge, so I need a good way to monitor the pressure while I'm driving. On level ground, light throttle, the truck runs beautifully. I found that the EGR is leaking a little, but I capped it by putting a metal plate between it and the plenum. Using the vacuum pump, I checked EVERY vacuum line on the truck except the supply line to the TAD/TAB/EGR control valves. Can't pull a vacuum on it because the EGR control valve vents when static. (Or am I wrong in my theory?) Everything I've read says the EGR is closed at idle and at WOT. Idle, I've confirmed. WOT isn't a good idea with your head in the engine compartment.

I'm relatively certain I don't have leaking injectors. The most simple of tests proved that. Push the pedal to the floor and see if it will crank. It won't. TPS is good. ECT, ACT both replaced recently. No smoke, nor fumey smell. No rotten egg smell either, but why would it if it's getting sufficient fuel for idle? KOEO codes come up 11 and continuous is 11 as well. KOER comes up 11 too. I'm getting no lean indication, but this is driving me nuts. I checked compression, and the LOW cylinder was 180PSI, and 200PSI was the high.

I did find and replace a leaking plenum gasket a few weeks ago, and that cured the rough idle and low RPM bucking (40 mph in 5th). (see my gallery)Cleaned the TB and IAC while I had the plenum off. Put new O's on the injectors, and lightly tapped them supply side down to see if anything came out of the screens after soaking them in Seafoam for a couple hours. Nothing significant, so I put 'em back in as is.

The cats are buzzing at low RPM as well, but from the tone of the exhaust (actually a nice mellow rumble for a stock exhaust), and the fact it will spit condensate from the tail pipe to objects 6 feet away, it doesn't appear to be an exhaust restriction. I was thinking about pulling the Y out and rodding the cats, but closer scrutiny told me I could get the back one, but wasn't getting to the front one unless I could find something that would go around corners and yet be solid enough to break up ceramics. Seeing that I had to drop the tranny crossmember to get it out kinda disuaded me as well. I had shorty headers installed last year after both manifolds cracked. I checked real close while under the truck today, and I don't find anything that looks like an exhaust leak caused by pressure in front of the cats. The cats don't look like they've overheated, albeit the 3" pipe between the Y cat and the second cat (where the air purge line comes in) looks like it's been pretty hot. I attribute the buzzing to the shorties.

If it gives you any idea how reliable this truck has been, I finally replaced the factory brake pads on the front today, and flushed the original brake fluid. I had to have a clutch put in it about 3 months ago when the slave cylinder let go and the master followed it. I kinda blame that on having to stay on the clutch so much when driving in city traffic with it running bad. Having 137K on it can't be the problem. (tongue firmly in cheek).

I don't think I have any intake leaks. I'm very familiar with that from experience with my '86 T-Bird with TBI or CFI on a 3.8L V6, whatever you want to call it. While I had the plenum off, I checked it very closely for anything that looked like it might be residue from a leak between the heads and the lower intake. Besides that, it's got 21 inches vacuum when idling. Got a slight bump in the idle, but I'm thinking my friendly Ford dealership may have set base idle too low. Timing is 10 BTDC (Spout off), new Bosch Platinum II's, new Motorcraft wires, cap, and rotor.

Checked the MAP tonight. 158.4hz at 0 vacuum, about 91hz at 25 inches vacuum. Had the original, and checked it too. Exactly the same results. With all the testing I've done, I feel like I've narrowed it down to 1 of 3, or possibly a combination, problems. I'm thinking gunked up injectors, dirty sock on the fuel pump, or plugged up cats.

Unfortunately (or fortunately if you look at it another way) I have a full tank at the moment, so dropping it isn't an option this weekend. I don't want that much fuel stored around my home overnight. I've got about 250 miles to drive this week, and will try to remember not to fill it up again before I get home next weekend. I think dropping the tank may be the next step. That'll let me inspect the inside of the tank too and see if I have rust problems there. If so, guess I'll order a new tank and fuel pump.

 

When you dropped the vacuum back to 0, it should have instantly jumped back up to 40psi. Sounds like you have a weak pump to me. It probably is just giving you enough fuel to run until a large demand is placed on it (up a long grade), then it can't keep up.