98 expedition fine till warms up. only misses badly when under load. have not been able to find bad vacuum line.

Vacuum lines. Some are hidden under the intake plenum.

What year is it? Your symtoms suggest oil contamination from bad fuel pressure regulator.

Look on the VECI label (Vehicle Emissions Control Information) under the hood, likely on the radiator support. The vacuum diagram should be shown there.

is it when weather damp if so get some dampseal and spray over all leads etc might just be that easy

check all vacuum hoses for leak, if there good check for vacuum leak around intake manifold or at manifold gaskets.

Check the EGR valve. Look up on the internet where on your car it is located. Find the vacuum line connected to it. Start the engine cold and as it warms up and starts to miss and stumble, pull off the vacuum line from the EGR. If the car idles fine, replace the EGR. Ignition system problems are more likely to occur at idle or lower rpm. Besides, testing the EGR is free, easy and do fail in high mileage vehicles.

a very common problem for the 5.4 fords is the coil packs going bad. normally misfires once heated up and under slight load ( in overdrive going up a grade) if the check engine light is on, it can help pinpoint which cylinder is misfiring. if plugs haven't been replaced within last 50-60k miles, plan on replacing plugs, and usually the coil pack that goes bad is one of the rear ones, but impossible to tell without a scan tool. if you don't own one, take it to a shop

I have a 1999 Ford Expedition Eddie Bower and my front heat does not work just blows cold air. The rear heart works fine any Ideas?

I also have this exact truck and I am a drivability technician.
First if you have a dead miss check for waht code is stored. Even if the light is off a code should still be stored. Once you know what cylinder is missing you can rotate the coil and and see if the miss follows by checking codes again. Do the same for the injector even tho this is alot more work. Once you determine that the code always stays on the same cylinder then you can inspect for maybe an intake vacuum leak or if the EGR is close by sometimes a EGR stuck open will miss at low RPMS. Its suppose to be open when cruising so a miss would not occur then.

Why was the engine work done. maybe you didnt fix the pryor problem if you didnt have the head sent out to be inspected and block tested. These engiinee heads are known to fail with cracks thats why it needed to be tested. Hopefully you did this

throttle position sensor may be bad or need adjusted .or oxygen sensors could be faulty does it idle ruff till it warms up?or maybe map sensor