'99 F150, 5.4L, 85K miles

Hello,
Fuel pump failed last year, had it replaced. Ran fine for a week or 2, then one day went into limp-home mode. Mech replace fuel pump relay, again ran fine for a month or so. Now when the ambient temp gets above 80F or so (yeah, it has already this year - I'm in Central TX), engine at full op temp, I'm experiencing some sort of heat-soak issue. Engine runs fine when cold, and at normal op temps on the road, but if it is parked for 15 - 20 minutes then restarted, the idle goes all funky - nearly dies, surges, etc. for a minute or 2, and then it seems ok.

I would go back to the same mech, but he had to close his shop due to health issues, so I'd like to see if I can resolve this.

So, it is definitely heat-triggered, and "seems" like a fuel issue, but of course it could be some electrical contact having difficulty above a certain temp.

Any thoughts on this??

Thanks

 

With a situation like this, the use of a Scanner to look at live data for being out of limit should pin point the problem.
Heat related could be the Intake Air Temp. sensor, the cylinder head temp sensor or even the Intake air controller and even a plug up that is intermittent..
Is the operating temperature within normal range of 195 to 205 degrees?
Without using these diagnostic methods it just guessing and replacing parts hoping to resolve the issue.
There was an issue with the IAT for these engines back in the day that caused a similar issue.
Good luck.

 

Thanks Bluegrass. Yes, the op temp is where it has always been; of course I can't precisely tell the temp by the dash gauge, but it never runs hot or cold.

Don't want to just throw parts at it, just thought this might be typical of a known issue at this age/mileage, etc.

 

These sensors have a huge effect on fuel injection and idle speed.
If the CHT or IAT were to be signaling the PCM, falsely, of a cold condition the amount of fuel is increased making the engine rich when it does not need to be.
This can affect idle quality and is detected by the OX sensors.
The end result is a bit of confusion as to how motor runs.
Good luck.