99 f150 5.4L 2wd 121k miles.

I have this strange event when its HOT outside, or it hot and has recently been sitting for a couple hours. (heat soak)
It never does this first thing in the morning...or during the entire winter.
For about the past 5 years or so, about 10 times a year maybe.
If its been sitting in the sun all day, or has been driven on a HOT day and left sitting to soak, for about 3 hours or so.
--When i start the truck, it will just spin over.
IF I hold the ignition on, let it spin, after about 5-6 seconds it will fire and run fine. (Its just a noticable delay in starting)
IF I sense it is happening in the first couple seconds. If i just wait a couple more seconds and start again, it starts and runs fine.
Once started...it can be shut down and started over and over with no issues...its just that first hit.
Once it starts it run like new...especially for a 21year old motor!

It hasn't really worried me too much because it hasn't gotten worst, but my son now drives the truck. So id like to get it figured out.
Although right now, i cant reproduce it because with COVID, its not really being driven and its just not HOT enough yet.

I thought that it might be the fuel pump, so i bought one last year, loosened the bed bolts and was going to change. After talking to a few
mechanic friends they didn't think this was the issue and as soon as it cooled off, it went away like it does every year. So i have a fuel pump waiting if it happens to go out..but have not yet replaced.

I replaced the battery last year. Also the fuel pump relay...just a shot in the dark on those two...

Truck history:
Passenger side head gasket leak...warranty.
Alternator, EGR valve, and #5 coil replaced.
Other than having all the brake lines and calipers replaced at 20years old, its all stock.

So im left with thinking that it could just be leaky injector seals...with heat and time, lets off pressure/evaporation.?
Im not ready to start throwing parts at it yet...I'd like to see if anyone has had similar issue and found a root cause.

Just reaching out for any ideas...Thanks...

 

2 things to try.

Hold the pedal open a bit. If it starts, clean/replace the IAC.

Hold it to the floor. If it starts, you have leaky injectors.

Getting into it a bit more - connect a fuel pressure gauge to the rail, turn on the key, see what pressure it primes to. Requires at least 28 psi to start. If it's low, turn the key off and on several times and see if it builds up.

 

Also, when you put a gauge on the fuel line to measure pressure, note how long it holds pressure when the pump stops.

 

Haven’t had time to check the pressure on the fuel pump yet. But for $40 and since it’s 21 years old I went ahead and proactively swapped out the IAC. A little dirty but not anymore than I expected. I did note the difference between these two. It could be a simple design change or maybe someone can tell me if the dirty one is supposed to look like this.

Felt a little bit of vibration when test driving and turned out coil number six was going bad so swapped it while I was in there. Trucks running great all I can do now is wait for the heat of summer and see if it hesitates. It’s very hard to reproduce.