I have a manual 98 XLT 4.0 OHV with 230k on it that is having some issues. When cold the truck idles very poorly. The idle is rough and low to the point where the truck sometimes stalls when shifting gears (disengaging the clutch causes the RPM's to drop way too low) or after revving it in neutral. The truck also bucks when driving at low speeds. The idle improves a little as the truck warms up, but doesn't completely go away making it impossible to shift smoothly.

I took the IAC off last week and cleaned it which completely solved the problem... for a week. Now it's back with a vengeance.

Today I pulled the following codes:
P1131
P0300 - Random multiple misfires detected
P0171 - Bank 1 system too lean
P0174 - Bank 2 system too lean
P0301 - Cylinder 1 misfire detected
P0302 - Cylinder 2 misfire detected
P0303 - Cylinder 3 misfire detected

Any thoughts?

 

If cleaning IAC Valve solved the problem then I would look at the IAC Valves connector or replace the IAC Valve on spec.

Clean the MAF sensor as well.

Your engine should start cold this way:
Do not touch gas pedal!!!
Crank engine
It should start and RPMs should go up above 1,500
That 1,500+ RPM is the computer opening up IAC Valve all the way
RPMs should then drop to about 1,100, computer is closing IAC Valve a little to match Coolant temp(ECT sensor temp), which is cold.
As coolant/engine warms up computer will start closing IAC valve bit by bit until engine is warmed up(195degF) and "Target idle" is achieved, manual trans (650rpm), automatic (750rpm)

If engine RPMs are staying higher or not going high enough then IAC valve could be sticking(again) and cleaning it fixed it but it is now doing it again.
IAC Valve is a "controlled vacuum leak" so sticking open too far will cause Lean Codes.


O2 sensors do not work until they are above 600 degrees, which is why they are heated, so computer ignores O2 sensors until engine warms up a bit, this is called Open Loop operation, computer doesn't have feedback from O2s so there is no "loop" in the system.
When O2 sensors are online computer is in Closed Loop operation, it has feedback on fuel mix so there is a "loop" in the system.
Codes P0171 and P0174 mean the computer is having to add more fuel that it has calculated based on MAF data(incoming air) and O2 sensor data(oxygen levels in exhaust)
Engine is not actually running Lean, computer adds more fuel, the codes are there to tell the driver that computer calculations are not accurate based on its available data, MAF and O2s
These codes can not be set until O2 sensors are online, because computer has no way to "test" fuel mix, so if erased they shouldn't come back until engine has warmed up and been driven at least a few minutes.

Since O2 sensors are not used with cold engine and your engine is actually running rough I would suspect MAF sensor error, that's the only data available to computer on cold start, or ........fuel pressure could be low.

Computer expects a specific fuel pressure and has no way to measure it, I think '98 should have 40psi pressure at the injectors, if it was 20psi then not enough fuel is flowing in when injector is opened, causing rough running and Lean mix.
So engine would run rough cold and then start running a little better when O2 sensors came on line and computer could add more fuel(open injectors longer) based on O2 sensor feedback.
'98/'99 is when Ford was switching to a 70psi Returnless fuel system, so check what you have, Returnless will only have 1 fuel line coming to engine, not 2 like the older 40psi system.


To check for vacuum leak, after engine is warmed up, unplug the IAC Valve's wires, IAC valve should close all the way and engine RPMs should drop to 500 or engine may even stall, either is good it means no vacuum leak on the intake side of throttle plate.
If RPMs stay above 700 then you have a leak, PCV Valve hose often cracks or PCV valve is stuck open.

Remove MAF to intake air plenum(big tube) inspect it for cracks and make sure gaskets at both ends are good.
ALL the air going into engine MUST pass thru the MAF sensor or computers calculations will be off, so a leak in this tube WILL cause a problem, this is called a "ported" vacuum leak

 

I let the truck warm up and then unplugged the IAC valve's wires. No change.

Does this mean I definitely have a vacuum leak somewhere?

 

Yes, possible vacuum leak if RPMs were above 800 with IAC valve disconnected, or IAC valve could be stuck and not able to close all the way.

For IAC valve mechanism to work correctly it has to be able to close enough to set idle BELOW minimum warm engine idle target RPM, manual trans target is 625, auto 750.
So factory spec for IAC valve closed all the way is 500RPM, when disconnected from power IAC valve should close all the way.
You can remove the IAC valve, 2 bolts, and then turn on the key, it should open, then unplug it's wires and it should close, plug back in and it should open.


With IAC Valve in place and unplugged, warm engine running, try removing 1 vacuum line at a time from the intake, RPM should go up, then plug that vacuum port with your finger idle should go down to where it was.
If plugging a port causes engine RPMs to drop to 500 or causes engine to stall then you found the hose/system with the leak.

Gasket vacuum leaks are harder to find, you can use Quick start(ether) starting fluid, spray it around gasket areas while engine is running, you will get increased RPM or "pinging" noise if the ether is sucked in thru a vacuum leak, but you have to be careful around exhaust system and EGR tube.
Another method is a smoke test using a cigar, seal the intakes main air port/throttle port, and blow smoke into intake via PCV hose or power brake booster hose and watch for smoke coming out.

 

To clarify, you mean turn on the key but not the engine?

I took the IAC off and as far as I can tell it looked closed. When I turned on the key (without starting the motor) nothing happened, and when I unplugged the IAC's wires with the key on nothing happened.

 

Reads like IAC valve is not working.

If system is working correctly this should happen:
Do not touch gas pedal.
When you turn on the key the computer starts up and sends command to IAC Valve motor to open valve all the way, then computer checks engine temp, and waits for you to start engine, if engine is cold IAC Valve will be closed a little, if warm then IAC Valve closes alot.
So when you start the engine RPMs should go up to 1,200+(IAC Valve open all the way)
Then RPMs should drop to approx. 1,100 if engine is cold, 650-750 if engine is warmed up.
All fuel injected engines do this, it isn't Ford specific.

If this doesn't happen then IAC valve is not working or computer is not working for IAC valve.
IAC Valve not working will cause a trouble code, and CEL(check engine light) will come on, is it on again?

If above does happen then IAC Valve is working.

 

When I cold start the truck the opposite happens. RPMs stay low oscillating between almost stalling and around 700 (as if someone is continually tapping the gas pedal). Then as the truck warms up the oscillation stops and the RPMs rise.

The only codes are the ones listed in my OP.

 

IAC valve isn't working.

Get an OHM meter.

remove connector and test IAC Valve motors resistance, set meter to lowest OHM setting, motor should be between 7-13 ohms, some motors have a diode inside so test pins one way and then reverse meter probes

Switch meter to DC volts, ground meters black wire, turn on the key, test wires going to computer, one should have 12volts.

Found this, look here: | Repair Guides | Electronic Engine Controls | Idle Air Control (iac) Valve | AutoZone.com

It is unlikely the computer is the problem but not having an IAC code is puzzling.
But long shot would be ECT sensor is telling the computer the engine is always warmed up, so computer never opens IAC valve for cold start and running.