hello i have a 1990 f150 with a 4.9 and i have a real bad acceleration problem it bucks and sputters till 2000 rpms then smoothes out untill under 200 rpms and has to warm up for about 10 minutes to run right heres my specs;
idle : @925 rpms
battery voltage at idle :13.96 ( high output alternator)
fuel pressure : 22 psi at idle
oil pressure :42 psi @ 2000 rpms
coolant temp warmed up : 210 degrees farenheit
compression test: (psi)
1(125 2(122 3(122 4(121 5(130 6(125
no vaccum leaks ,new tps sensor ,new cap, rotor ,plugs and wires and coil and icm and fuel filter its got over sized valves all comp cams valve train and a ported head .also a cold air intake .

 

Fuel psi should be 45-50 with vacuum line connected to FPR, removed should increase 5 psi. 22 psi is to low of fuel psi.

 

+1, find out why fuel pressure is half what it should be.

 

I tried to edit my 1st post, but wanted to add this...

The symptoms you describe are Identical, with the exception of fuel psi, when my PCM failed due to leaking capacitors. The 4.9l seems to be plagued with leaking, blown capacitors versus the V8 PCMs. Just thought I would share.

 

thanks for the responses timbersteel ill check my fuel pressure at the pump it self and see what i got cause the one i gave in my previous post was at the rail and i though 22 psi was pretty low

 

Fuel pressure at the pump is going to be the same as at the rail, check the fuel pressure regulator for proper operation.

Removing the vac line to it should increase the pressure 5 to 10 psi, if you get no increase at all that'd strongly suggest the FPR as the problem.

Still might be even if you see a rise in pressure vac line removed (engine running of course)

Check for signs of fuel in the vac line too.

Suggested quick test for low pressure, pinch off rubber section of return line (not completely but enough to greatly restrict flow back to tank). If pump is up to spec pressure should rise to or even beyond spec, if so replace FPR.

If restricting return line flow to tank does not increase fuel pressure reading, the pump would be suspect instead.

 

ok sounds good ill do that and see what happens but i only have 22 psi before the regulator so how could it be the regulator just wondering

 

Fuel pressure is regulated by what is allowed to flow back to the tank, low vacuum equates engine load equates to need for more fuel, low vacuum supplied to FPR it decreases amount of fuel allowed to flow back to tank by restricting flow through the return line as it leaves the rail.

Think of it like putting your thumb over end of garden hose to increase distance you can spray it, while no more water is coming to the end of the hose by limiting what can exit it you increase the pressure.

 

i completely understand now thanks