Low fuel pressure and poor running

timpage · Sep 4, 2015

I have an '87 Ranger with the 2.9. I've spent the last couple weeks dealing with a low fuel pressure issue. Consistently running 28-29 psi fuel pressure (at idle) at the Schrader valve on the fuel rail. I have replaced the fuel pressure regulator (it was leaking at the vacuum port, and prior to my current issues. It isn't leaking now) Fuel filter, high pressure in line pump, in tank low pressure pump, checked fuel lines as best I could for kinks, verified both pumps are running, and still 28-29 psi fuel pressure. What other things should I check that'd cause low pressure? Thanks for any help!

Tim

RonD · Sep 5, 2015

Sock in the gas tank, did you clean it?

Remove gas cap and see if pressure increases, could be EVAP system issue.

Also check the battery voltage and the voltage at the high pressure pump, they should show exactly the same voltage.
I.E. if battery voltage is 13.6v with engine running, wire at pump should be 13.6v.
Corroded connectors at fuel pump relay or inertia switch could be lowering voltage.

timpage · Sep 5, 2015

The sock is new as well for the in tank pump.

I just checked the running voltage at the battery and at the HP pump... there is quite the difference (14.0 v at the batt, 13.2v at the pump).

I will check the connections at the fuel pump relay and at the inertia. Any other connections I should check out?

Thanks for the info Ron!

timpage · Sep 5, 2015

The connector at the inertia switch appears like new, no visible damage or corrosion. The connector at the fuel pump relay is pretty gummy (very old dielectric grease?) and the fuel pump relay looks to be original. I do have equal battery voltage at the hot side of the FP relay connector (12.3 v at both battery and connector). Would the old grease and/or old relay cause reduced voltage to the HP fuel pump?

enjr44 · Sep 5, 2015

Burned relay contacts can and do increase resistance in the circuit. If I remember right all those relays are the same. Swap??

RonD · Sep 5, 2015

If I remember correctly I think the dual pump system had a Reservoir bowl in the frame rail, looked like a canister type fuel filter but was just a reservoir.
It's purpose was to supply fuel for the high pressure pump on startup.

Fuel tank pump----------reservoir---------high pressure pump-------fuel filter--------rail

timpage · Sep 5, 2015

Yes, that is how mine is set up (in tank pump, reservoir w/o filter, HP pump, filter, then fuel rail)

I've spent the past few hours unwrapping loom and tape, checking resistance and voltages for the fuel pumps. Narrowed the voltage drop down to between the output side of the FP relay and the underhood harness connection for the inertia switch.

I pulled the inertia switch and opened it up.... the internals appear fine. Reassembled and remounted it. Does that necessarily eliminate it as a source of extra resistance?

Also at this point the wiring from the FP relay, to the inertia switch, and back to its underhood harness connection does not appear corroded or damaged.

timpage · Sep 5, 2015

I haven't tried swapping relays yet... that can be my next check.

RonD · Sep 5, 2015

The .8v drop is not too sever, not nothing but not sure that would cause any problems.

But that in combination with something else might.

I assume the trouble is less than 20psi when under load at higher RPMs, so engine lacks power.

You never said what symptoms you were having, just 28psi at idle.
If you hold RPMs at 2,000, for example, do you see the pressure start to drop?

timpage · Sep 5, 2015

Yes, under load pressure drops to 22-24 psi or so. The truck will sputter and sometimes dies stopped/ brought to idle. Cold idle when the truck is first started is pretty irratic but stabilizes. Warm idle is better, but still with some milder fluctuation.

timpage · Sep 22, 2015

Time for an update. I cleaned up some wiring in the fuel pump circuit, some connections that looked questionable. Also a new fuel pump relay (a little corrosion and something was rattling around under the cap... ). Voltage drop is slight now, but improved. Fuel pressure runs consistently 30-32 psi, at idle and under light load. However still having issues. At cold idle, seems relatively normal, a slight fluxuation in rpm.

Full operating temp things are different. Fuel pressure is still the same as above. Idle will run smoothly for a minute or so, then drops off almost to the point of dying. Then the engine will idle up (1100-1300 rpm), smooth out and come down to normal smooth idle. Then keeps repeating (idle down to almost dying, idles up then smooths out).

Also, on a road test, I have an intermittent stumbling/sputtering/lack of power. This will happen at various speeds, no regard to what gear I am in. the stumbling verys in length, but does smooth out and runs seemingly fine for a mile or two then will do it again. Stumbling is worst when I give it throttle...

What do I need to look for that may cause these symptoms?

Thanks

Terry · Sep 23, 2015

Sounds like you have a bad sensor. It sounds like it runs ok in "closed loop" when it is cold. I don't think the issue is the pumps or fpr. A simple test is to disconnect the o2 sensor. It should stay in full rich condition. I am thinking the maf sensor . Is you go to "broncoii.org". They have how to test all the sessions in their tech section.

Sent from my SAMSUNG-SGH-I337 using Tapatalk

timpage · Sep 23, 2015

Mine is early enough to use a map sensor vs. the maf sensor. The map sensor is new.

timpage · Sep 23, 2015

The fuel pressure regulator was also previously bad (few months ago) and wAs replaced then. Both pumps are also recently replaced.

Terry · Sep 23, 2015

It still sounds like a sensor. Have you pulled any codes?

Sent from my SAMSUNG-SGH-I337 using Tapatalk

Low fuel pressure and poor running

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