Been having a problem for a few weeks now .. cold engine morning starts the engine would stumble for about six seconds, then run ok... no check engine light, gut feeling it was fuel related, i had 20,000 miles on the Wix fuel filter it has three ports.... 2003 3.0 flex 162k... replaced it with Motorcraft FG-1061 problem solved, the Wix was plugged up for sure........... looks like we have dirty gas here in C/A.. with them miles when should the fuel pump be replaced...

 

Update..... few days later stumble came back only morning cold starts....more trouble shooting needed.. could be leaking injector overnight, or fuel pump...... i will prime the fuel pump with key on and off few times next morning start.........no problems during daytime starts only overnight 8 hrs

 

Sounds like your on the right track.
Based on it being ok for a while and then coming back... I wonder if there is debris in the fuel tank and its plugging up the filter? (that very thing happened to me on a Datsun)

Other things to consider:
- cleaning the TB & idle bypass
- Weak spark
- Dirty injectors (try running a little acetone for a few tanks at 2oz per 10 gal ratio)

 

Could be ECT sensor as well, maybe even dirty IAC Valve.

Any Cold gasoline engine needs to be Choked on startup, ECT sensor tells computer the engine temp so how much "Choke" it should use, the colder the temp the more "Choke" is needed

To Choke an engine means adding more fuel to get higher gasoline vapor % in the cylinders so it can be ignited by a spark, minimum is about 30% Vapor
On Carbs they did this with a Choke Plate, so higher % of fuel was sucked in from the jets with plate closed, via engine cranking and running vacuum

The colder the engine, and gasoline, the less vapor is present on startup, so the more fuel needed
Choke also needs a higher idle and advanced spark timing, so IAC Valve is set for higher RPM by the ECT sensor via computer control
If IAC Valve is sticking when cold it could cause stumbling as computer tries to unstick it

 

Thanks for the info guys, i replaced the IAC valve a few months ago, i replaced the gas filter 10 am i used the key to prime the fuel, started right up no problems (cold engine) rpm 1,500 then drops down as it warms up...did not drive truck it stayed in garage.. 5 pm started all ok....New plugs/wires a year ago,,, i will get back with update in a few days doing the prime first...

 

As I recall when tuning my 06 4.0L cold start A/F was not solely dependent on ECT. (engine coolant temp). The amount of fuel, timing, and IAC commanded was a mathematical function of ECT, MAF, ECT, IAT, atmosphere press, Manifold pressure, RPM, and calculated temp at exhaust valve. And as I recall A/F is NOT influenced by 02 sensors for the first few seconds (OPEN LOOP). I don't remember exactly... but its something like time since start + temp before it changes to closed loop.
It's been a while so don't hold me to it... seems like you had to get down in to -20F before it increased fuel to any appreciable amount

Also, when ECT degrade they show increased resistance. Which in turn the engines ECU sees as a value that is incorrect. So while it does have an effect on start up A/F its... only a part of the overall calculation controlling the first few seconds of engine run.
IMO the OP has either a globular fuel condition (dirty injector), or not enough fuel (both show the same condition of misfire), or a weak spark. Or maybe something else that keeping it from lighting off (like maybe coolant in the combustion chamber)

 

Sounds exactly like my ranger - 2001 3.0... starts up goes up to 1200 or so and rusn smooth for 2 sec then stumbles like the choke is left on too long on a carbed car. then smooths out and goes back to 1200 and runs smooth the rest of the day. Even start stops and sitting all day at work starts up and runs smooth when i leave - its the cooler/cold mornings when i leave for work. Ive ran countless tanks of seafoam, MMO, berymans, etc no change.

ive replaced ECT and IAC and cleaned MAF. no codes. Need to replace the fuel filter for sure tho. (rather easy i take it?)

 

So i got 2 of the 3 lines off the old fuel filter. Suspected its at least 100K on it. Blackish grey brown fuel ran out the tank side.
Front line came right off, offset rear line (green) pushed in wiggled a bit and off it came.

The bastard orange one, pushed in fully but doesnt come off. Can freely spin filter, feel it move about 1mm in and out on the line. Orange tab moves fully in as far as i can tell. Ive tried using the screwdriver how i used on the other 2 but no dice, tried needle nose, tried needle vice grips extremely lightly so i can push in and pull twist and didnt work. Sprayed with penetrating oil and spins even better but still wont friggin budge.

Any tips???

 

100K on the ff how the hell did it run lol yes them plastic connectors can be a pain ... i had same problem with the orange one, what i did was press the tab with my thumb then push in and out until it broke free...you only have to move the clip a few mm .. dont use vice grips

 

hmm alright ill try it. Cant hurt haha.

 

So this morning i did the key on and off to prime the pump......... started and the stumble was there for a few seconds,.... then ran fine...10 seconds later turned off.. so its not the check valve inside the pump unit........ i .. i think the injector is leaking overnight. I will try the FI cleaner

 

So if the injectors were getting to much fuel at cold starts would the ECT cause this ?

 

Hook up a fuel pressure gauge and leave it overnight.

 

Thats one tool i dont have .. i did start this morning no key prime and big stumble as it went up to 1500 rpm...... i will get a gauge save guessing