I have a 1998 Ford Expedition 5.4l, engine has an intermitent miss-fire and hesitation after reaching operating temperture. I have no CEL or stored codes. This problem is much worse when the A/C is on. I can feel the miss while stopped at a traffic signal. I have replaced the plugs, coils, fuel & air filters, TPS, cleaned the MAF sensor. Seemed to stop for a while then returned. Runs fine until engine warms up?

Does the CEL "work" intermittently, or it comes on intermittently? If it comes on intermittently, that's telling you that the PCM has identified an issue and there will be a stored diagnostic code that should point you in the correct directly to begin troubleshooting. For example, if there is a stored code of P0304, that means the PCM has determined based on sensors that cylinder #4 has a misfire. You will then need to determine if that misfire is due to a spark plug issue, a coil issue, a coil/plug wire issue, fuel injector issue, compression issue, or vacuum leak.

fuel pump? it is running lean (not enough fuel) or a leak in evap

remove that number4 plug and replace it being sure you check gap good luck

manifold air temperture or intake air temperture sensor is bad . you can check the voltage on it when cold then w/hair dryer warm sensor up an check voltage if the resistance is more than 500 ohms then it is bad.

a factory weatherstripping on the hood leaks causes the cylinders 2 &3 on the pass side to misfire. pull the coils for the cyl. .take off boot with long spring inside, thats your plug wire. they are rusty, no? clean them up, put some dielectric grease all over the spring and reassemble. theres your prob!!!!

their is a solenoid on the passenger side fender. replace it and the engine will crank and start. they are about 15.00 at any of your local auto parts stores. its called a starter relay.

it sounds like you need to tune up your vehicle. whenever the service light blinks it usually means you have a miss fire. You can take it to auto Zone to retrieve the code. The code should tell you what cylinder is miss firing.Hope this helps. Please leave comment.

a flashing CEL light means you have an engine problem and are pumping raw fuel into your catalytic converter and it will cause it to fail. big $$$$$$$$

check timing.

theres a plastic vacuum line at the back of the intake thats difficult to reach but i have found this broken many times and cause a multitude of problems long shot but check it