1999 Explorer XLT 4.0L SOHC V6 Automatic overdrive, 91000 miles. New plugs, wires and fuel filter at 86000. Starts and runs very good. Problem when driving long distances on expressways. Engine will loose power for a period of time under one min. Problem clears up and it runs fine. No warning lights or flashing OD light. No problem codes found in computer. Repair shop drove 50 miles and could not duplicate. Happens most of the time after long highway speed driving. ???

THAT HAS A HUGE LIST OF CAUSES BUT BREAKS DOWN TO
THE BIG 3. (we find it with tools) otto cycle Gasoline/petrol

• bad engine (compression good?)
• bad spark ( try new cap,rotor , wires and plugs?or 6x coil pack./
• bad fueling or bad fuel. , fuel pressure at spec?

top cause is no tune ups done in 24 years,
all spark parts are bad. (no engine told)
all fuses good/?
test #1 is can you keep it running with a fast right foot actions.
i dies and repeats, or dies and needs to sit for hours to repeat this start fail act./
there were 3 engines then/
4.0 OHV v6
4.0 SOHC v6
5.0 v8 (uses twin quad lug coil packs) and hv wire sets. spark
no engine told so no why to get specific at all just ASE basics basics above.
try a tune up, at least, HV wire sets and new spark plugs gapped right.

and scan scan the PCM see if P03xx errors are stored there.
key on.

Remove mass air flow sensor Clean and re-set connector to it. See if that helps. Worked on my 94 4.0.

The 4.6 SOHC V8 was optional

make sure your fuel pump is OK first , and let me know .

White plug may indicate coolant getting into the combustion chamber, test it for a head gasket leak

Also try plug wires. If it gets worse when you press gas pedal down, could be the fuel filter.

The Engine code should be located on RH Side, Rear, on the Lower Crankcase.

Problem probably solved right here!

you might try changing the engine coolant temp sensor. could also have defective spark plugs. check the fuel filter, lines and pump.

change your battery cables and check all wires on the alternator for loose connections, expecially the grounds.