First a little back story.

When I bought the truck it didn't idle right. I replaced the IAC and the condition improved. It would still race sometimes until the truck came to a stop. That was back in June.

Now, a new problem has developed. Sometimes after driving, then parking for an hour or so, it will just crank and not start. If I cycle the key back and forth once or twice it will take right off. If I leave it sit for only a few minutes it won't do it, and I don't think it's ever done it on a cold morning start.

Aside from a persistent P0171, there are no other codes, and it runs great otherwise.

I just did a tune up, and have run several bottles of fuel system cleaner through it. I've looked and looked for vacuum leaks and have yet to find any.

What do you guys think might be the problem? On a cold start, the engine runs at 2000 RPM or so and eventually settles down to idle like it should, so the IAC is working.

 

After doing some thinking and testing, I have found this problem to be related to fuel pressure.

KOEO 35 PSI
KEOR 30-ish, even while driving down the road
KEOR FPR Vacuum line disconnected, same as above, not doing what it should.

After leaving it sit for an hour, fuel pressure was 0.

I found that after parking for a little while, if I cycle the key and wait for the pump to prime, it will start. If I just hit the key, it will sputter and eventually take off.

Going to replace the fuel filter today, if that doesn't do it, fuel pump.

 

Duplicate post

 

Fuel injection systems should hold pressure for a few months.

Ford has a safety feature that only runs the fuel pump for 2 seconds when key is turned on.
But that is EACH TIME the key is turned on.
So yes if fuel pressure is dropping down to 0 with key off then before starting you should turn the key on, count to 3, and off, 3 times, then start engine.

Fuel pressure in a 1998 should be 65psi, but no regulator.
So you have the older 1997 and earlier system, and maybe the 2.5l had that normally, it did use 30-40psi pressure

The pressure in this system is held with engine off by 4 things
1. Fuel line, but you should smell gasoline if this was leaking
2. Fuel injectors, if an injector is leaking MPG would be way down and engine would seem "flooded" on warm restart, blow out some blackish smoke on start up.
Cat converter would also be running Red Hot and making popping noises, trying to burn off the extra fuel

3. Fuel pressure regulator(FPR), if it leaks then gasoline will be sent back to the tank so no smell, the Return fuel line on the FPR is rubber with metal mesh, if you are CAREFUL you can use wide vise-grips to pinch off this Return line.
So start engine with pressure gauge attached, pinch off Return line, then shut off key/engine and see if pressure holds, if so then replace FPR

4. On the fuel pump motor is a Check Valve(back flow preventer), it is part of the motor, not a separate piece.
The check valve is basically just a flap that is pushed open when pump is on and then pushed closed by the pressure left in the system when pump is off.
If it leaks then gasoline flows back into the tank when pump is off.

If you are not "running Rich", don't smell gas, and FPR clamp off test still loses pressure, then Check Valve is most likely bad.

This doesn't effect driving, once engine is started fuel pump is on full time, so Check valve is not used.
So you can cycle key on and off for now until you can pull the bed and install new fuel pump.

 

Ron, thank you once again for the good info.

I was mistaken, what I thought was a FPR, was actually the fuel pulsation dampener, I do have a returnless fuel system with the two-line filter.

I changed the filter a little bit ago, it didn't help. Was dated 4/15 so not even that old, good to know someone took decent care of this truck.

You may have already seen in my other threads that this truck is setting a consistent P0171, and this is probably the cause of it. If an injector were leaking fuel trims would be in the negative, but they are in the +20 range. No fuel smell or anything, so I'm sure the pump has one foot out the door.

 

Pump should be pushing 65psi when engine is running in a Non-Return system.

So 35psi would cause Lean code.
Lean code is not the engine running Lean
Lean means the computer is having to open the fuel injectors too long to get the correct air:fuel mix

For fuel trims 0 = computers calculated "open time" for fuel injectors, at XXX RPMS and XXX Throttle position
"-" numbers means computer is closing injectors sooner than calculated
"+" numbers means computer is leaving injectors open longer than calculated
Numbers are adjusted from O2 sensor voltage after air:fuel is burned

-8 to +8 is usual range, codes are set below or above -15 and +15

"+" numbers usually mean dirty MAF sensor or vacuum leak, unreported air is coming in and throwing off computers calculation so more fuel needs to be added

But computer has no Fuel Pressure Gauge, it is programmed for 65psi, so low fuel pressure will also cause Lean code because not as much fuel is flowing in with 35psi as would be with 65psi

 

I replaced the pump earlier this week. Pressure is back up where it should be, fuel trims are back around zero, and the lean code went away. All good there.

The truck started idling funny again. It will lope and still race between shifts sometime.

If I unplug the IAC, nothing happens. I will check its resistance tomorrow, but it's not that old. What else should I check?

 

If you unplug the IAC Valve when engine is warmed up the idle will drop down to 500 RPM or engine may even stall, if that doesn't happen then you either have a vacuum leak or someone has "messed with" the anti-diesel screw on the throttle linkage.

Anti-diesel screw "looks like" an idle screw it isn't, fuel injected engines can't use an idle screw because there are no Jets to suck fuel out of.

The anti-diesel screw allows the throttle plate to close enough to prevent "run on"/dieseling if engine is too hot when shut off
To see if this was "messed with" unplug IAC valve with engine idling and warmed up
If idle stays at 700 or higher then adjust that screw 1/2 turn in either direction.
If idle drops or increases then it has been "messed with"
Re-adjust it to lower idle to 500.
IAC valve can't work to set stable idle unless it has full control, i.e. RPM range from 500 to 2,000, if the screw or a vacuum leak raises minimum RPM then IAC Valve can't do it's job

If idle doesn't change with the 1/2 turn then screw is OK return it to original position and look for vacuum leak.

1995 and up computers will "hold RPMs" when shifting a manual transmission.
This means if you are driving at 30MPH and push the clutch in(foot off the gas) in the RPMs will not drop, they may even go up slightly as load is removed from the engine(clutch pedal in)
This is an emissions requirement, hold RPMs high when shifting, but this should only last 5 seconds or so if you leave clutch pedal in, then RPMs should drop to around 1,000-1,200

Engine RPMs will also stay high when coasting, another emissions requirement, usually just above 1,000 RPM, until speedometer drops below 5MPH, then idle will come down to 650 RPM

Idling engines have more emissions than engines above 1,000RPM
Up and down RPMs create more emissions, which is why they added the "hold RPMs high"

If you unhook the battery the computer will "forget" it's presets, just like the radio does.
So when started up again computer will re-learn some engine parameters, IAC Valve settings is one of those, as is Throttle position sensor(TPS)
This can take 2 to 4 drive cycles
1 drive cycle is engine fully warmed up and then fully cooled down
So at first startup after battery was disconnected the idle should wander a bit as computer adjusts IAC valve to learn what "voltage" works for what RPM
Computer also has to re-learn what throttle position needs what amount of fuel added, so there could be some stumbling as it does this.