LongBlackRam

I'm working on my 97 12 valve. The truck has about 195k miles on it.

The symptoms are: Hot, the idle speed drops causing stalling while slowing to turn etc; fuel pressure is about 12 psi at idle and max's out near 25 psi but will fluctuate erratically dropping up to 10 psi.

So far, I've replaced the fuel filters and the overflow relief valve. Testing for bubbles in the fuel, I installed a clear section of hose between the pre & post filter pressure test ports on the Prime Loc filter housing. No bubbles observed in 10 minutes of running. Is this a valid test?

I suspect the transfer pump is bad.

Is low fuel pressure the cause of reduced idle speed when hot?

Thanks,

BigIron70

Read the following may be of some help to you.

FUEL TRANSFER PUMP PRESSURE/CAPACITY TEST
For operation of the fuel transfer pump (lift pump) primer button, refer to the Air Bleed Procedure in this group.

The fuel transfer pump is located on the left side of the engine and above the starter motor Fuel Transfer Pump Location

An improperly operating fuel transfer pump can cause low engine power, excessive white smoke and/or hard engine starting.

Before performing pump pressure testing, inspect the fuel supply and return lines for restrictions, kinks or leaks.

Fuel leaking from the weep hole in the pump casing indicates a leaking pump. The transfer pump must then be replaced.

Low transfer pump output can be caused by a worn eccentric (cam lobe) on the engine camshaft.

In addition to contaminants or air, restrictions in the fuel supply to the injection pump caused by a clogged or dirty fuel filter can cause problems. This may be: low power, poor starting or excessive white smoke. A restricted pre-filter screen within the fuel heater may also have the same results. Refer toLubrication and Maintenance at the front of this manual for recommended filter replacement intervals in time or mileage. Be sure to clean the pre-filter each time the fuel filter is replaced. Refer to Fuel Heater Removal/Installation for pre-filter service procedures.

OUTPUT PRESSURE TEST
Place a drain pan below injection pump.
Remove banjo bolt and gaskets from fuel inlet line fitting at side of injection pump Fuel Inlet Line at Injection Pump
Install Special Tool 6976 and tool gaskets from tool kit 6977 in place of the banjo bolt Checking Fuel Pump Pressure Tighten tool into injection pump.
Install a 0-60 or 0-100 psi fuel pressure gauge to adapter tool 6976 Checking Fuel Pump Pressure
Start and warm engine and record fuel pressure. Pressure should be 117 - 152 kPa (17 - 22 psi) at idle speed. Pressure of 172 - 207 kPa (25 - 30 psi) should be seen after raising engine speed to its rated rpm (2,500 rpm) with no load.
If fuel pressure falls below specifications, do not automatically condemn the transfer pump. The fuel filter or pre-filter may be dirty or clogged. If either of the filters are at fault, the pressure reading will rise after servicing. Recheck fuel pressure after servicing filters.
If pressure is still low, the overflow valve (pressure relief valve) Overflow Valve Location may be defective. To determine if the overflow valve is causing the low pressure condition:
Carefully pinch off fuel return hose at rear of engine Pinching Off Fuel Return Hose
If pressure goes back up to normal or above, the overflow valve must be replaced. Before condemning the valve, make sure it is located (has been installed) at the fuel return line (pump outlet) and not at the fuel supply line (pump inlet)
If pressure remains low with return line pinched, the fuel filter or pre-filter may be restricted or the fuel transfer pump may be defective.
If filters are in good condition and pressure is still low, replace the fuel transfer pump.
After testing, install banjo fitting bolt into fuel supply line and tighten to 24 N·m (18 ft. lbs.) torque.

LongBlackRam

Thanks for that BigIron. I pinched off the return line and the fuel pressure stabilized and met minimum spec at idle. i guess my new overflow relief valve is no better than the old one.