So I've been having this cold start/hot start dilemma and I wanted to call on your the collective wisdom to nail it down. The problem is as described, on a cold start it starts up fine, idles fine and runs fine. But if driven for any period, even a couple of blocks to say refuel, and it is heated up to temp, it is very, very difficult to restart - cranks and cranks with the gear lever violently shaking. Here or somewhere else recommended replacing the coolant temp sensor - so I replaced both (as there are two) and the thermostat since I had to access the old one anyhow. Unfortunately no change. I have noted a material change in my mileage as well where I would previously get approximately 300mi./tank, now I'm getting 230-250, which I don't think is coincidental.

So what are your thoughts?

 

I think you may have a leaking injector, or maybe the Damper at the end of the fuel rail on the engine

To test for fuel leaking into the engine you need to use Flooded Engine Routine, all fuel injection computers have this
Turn on the key
Press gas pedal down to the floor all the way and hold it down
(with 0 RPMs, and Throttle sensor showing Wide OPEN, the computer will start Clear Flooded Engine routine, shuts off injectors and leaves spark on)

Crank engine over, its should NOT start or fire at all, no fuel

Do the test with cold engine, and then with warmed up engine

If you do have fuel leaking in it would evaporate by the time engine cooled off
But on warm start engine may be flooded

Damper on the end of fuel rail has a vacuum line attached, should be passenger side rear of fuel rail
Check that vacuum hose for gasoline, shouldn't be any of course

If engine is firing or starting in Clear Flooded Engine then leaking injector may be it
If engine RPMs get above 400 or you take your foot of gas pedal, computer will start injectors, so if engine fires a few time RPMs will get above 400

You can use this Clear Flooded engine thing to ID injector
If engine is firing with gas pedal to the floor then unplug 1 spark plug wire at a time, and retest
Then plug it back in and unplug the next, and so on, when it stops firing last unplugged spark plug is in the cylinder with leaking injector


I use Clear Flooded Engine every morning to start my high mile 4.0l
I crank the engine over for 5 seconds or so to get oil pumped up through the engine, then I lift my foot off the gas pedal and let it start up

And you could have a failing coil pack as well, the fuel thing is just something to test, free, lol

 

Thanks I'm going to give that procedure a try. So if I do the Clear Flooded Engine procedure on a hot motor and it starts to fire, then that means I have a leaving injector? If it doesn't do that, or performs the same as when cold (i.e, not turning over), then I should explore ignition coil, and I guess fuel pump?

 

Yes, engine shouldn't fire if there is no fuel, injectors off, if it does that would indicate fuel was in the intake, and the lower MPG could also indicate that

You could get a fuel pressure gauge and test pressure at the engine to diagnose fuel pump issues

The coil is a possibility but would usually throw codes, misfire codes, which you don't mention
But codes are not always present

There is no "that's the problem" diagnoses with your current symptoms, when there is a total failure to start it gets easier, lol

 

Well, I just did the Clear Flooded Engine routine on the cold motor: key on, throttle to the floor, then crank and it does NOT try to fire, just cranks and cranks, no attempts to 'catch' or start. I did this several times to the same result. I then did it, but after a couple cranks let up off the throttle, and about 3-5 sec. later it turned right over. I guess that is hopeful I don't have a leaky injector(s). I'm going to go drive it around and get it good and warmed up, then let it sit for 10min. or so, and try the procedure on the hot motor and will report my incremental findings here.

And yes, I'm not getting any check engine lights or such, and don't feel or observe any apparent misfires. I connected a bluetooth OBD with a code reading app on my iPhone, but the codes I got were for the 4x4 module/motor and something about a reverse or taillight issue.

 

I did the Clear Flooded Engine procedure cold and hot. In both cases it would crank and crank but not hiccup or even attempt to catch. Releasing the throttle and it started right up. I did note that when I was doing the procedure when the motor was hot the cranking was more violent - the manual gear lever shook considerably. I'm going to check the damper on the fuel rail, and I have a mechanic friend who has offered to do a fuel pressure check for me. I have concern that it is the fuel pump because it sat unstarted for about 6+ mo. 2-years ago when the passenger rear wheel got knocked off the hub sliding into a curb (don't ask, running 2H in black ice, truck spun around and there was no recovery). Fuel pump may have gotten fouled. I may go ahead and just change out the ignition coil because the part is <$100 and I can easily do it myself. Fuel pump is another matter because I'll have to remove the bed to get to it.

 

Good stuff on the testing, that takes that off the table as the cause

Damper will be fine, as leak there would have showed up on the Clear Flooded engine testing

The drop in MPG is still on the table as a symptom, could be spark related

But it would be great to get fuel pressure tested
Expected is 55PSI at idle and then hold engine RPMs steady at around 2,500 and make sure pressure is not slowly dropping
Then shut off engine and pressure should go down but stay above 25-30psi, basically for months
If pressure is dropping under that or to 0psi then that should cause harder starting cold OR warm, not just warm
In the gas tank on the fuel pump assembly is the fuel pressure regulator, and, of course, the fuel pump, BOTH of these have valves to hold pressure when key is off, either can leak so pressure drops, but you replace both when replacing the assembly so..............
But this wouldn't lower MPG since the fuel just leaks back into the gas tank, its not lost


Also I forgot to mention.............Clean the MAF sensor........always once a year on the 4.0l engine, computer hates a dirty MAF, lol, very easy to do
And this could effect MPG and warm starts