2nd engine Ford rebuild 100 k ago. I am @ 280k miles now.

Problem:

1999 4.0 (automatic) starts ok cold runs fine but when warmed up has a hard time starting. It stumbles and will not start 75% of the time, almost acts like fuel problem.

It will start on the 2nd or 3rd attempt but it has to act up first. Then most of the time when it is running, it is at a higher RPM 2 to 2.5 for about a minute sometimes longer, eventually it goes down to an idle RPM. Now this is after the engine is warm.

I have had the check engine light on for a bit, same old stuff, 0 2 sensor left bank. This is my 5th Ranger and I have had this code thrown before when I had a miss fire or the crank positioning sensor acting up.

I put a lot of highway miles on this truck, maintain it as needed, and I think its due for a set of plugs.

Any ideas

 

O2's are the ONLY sensors that wear out, 100k miles or 12 years, they use a chemical reaction to detect oxygen in the exhaust and simply start running out of that chemical
And as they get older they cause the engine to run Richer than needed, so MPG slowly starts to go down, by the time they would set a code you could have bought 10 O2s with the money wasted on the extra gasoline being used, maybe 15 O2s at today's gasoline prices, lol
But not part of your problem, just a heads up

All computers have Choke Mode, its so engine can start cold and stay running, with higher idle and Richer fuel mix
The computer bases Choke Mode on the temperature shown by the ECT(engine coolant temp) sensor
So with key on computer boots up and checks ECT temp, if it shows under 120degF then it applies the Choke, the lower the shown temp the more the choke
Choking a warm engine would cause symptoms you describe, it would flood out the engine, and have a higher idle

There are two different temp units on fuel injected engines
ECT sensor, 5 volt, 2 wires, used only by the computer<<<<usually grey/red and green/red wires
Temp Sender, 12 volt, 1 or 2 wires, used only by dash board temp gauge<<<< always one red/white wire
So NOT interchangeable, lol

If you have a code reader that can see "Live Data" then you can see the temp the ECT sensor is sending to the computer with Key On, before starting Cold or Warm


Other possibilities:
temp sensitive Coil Pack, coils run HOT, when you shut off the engine the fan stops circulating air in the engine bay, so everything in the upper engine bay gets "heat soaked"(even hotter), and thats where the coil pack is
If possible open the hood after you stop, then after is sits for a bit, try to start and see if the same thing happens

Fuel pressure issue, but would expect this to effect cold starts and highway speed driving not just warm starts
Is there any relation to the hard warm start and fuel level in the gas tank?

 

Thanks for the info. This start to act up when I was 250 miles from home and was able to get it back there. I have an O2 sensor that I was going to install with the plugs, wires, fuel filter, and a few other parts. I try to not just throw parts at my trucks but to do plan maintenance which has kept me from getting towed home. I have a friend that has a "live reader" and I will go talk to him after I get the parts in and go from there.
Cheers

 

You can also buy a Bluetooth OBD2 reader for $15, and a $5 APP for your phone to have your own reader
ELM327 reader
Torque Pro APP

Works on ANY 1996 and up vehicle sold in the US or Canada, not a Ford thing, OBD2 is still required by law in 2023 vehicles

 

Thanks but I have a flip phone and do zero work on it. It makes phone calls only. And I rather go fishing that diddle with an electronic device (other than a fish finder
). lol