I have a 1999 Mercury Villager with the 3.3 engine, and a bad miss at all rpms, and it will stall at idle.
So far we have replaced:
-The distributor, brand new, with new cap and rotor, OEM exact fit,
-All new high quality wires,
-New platinum plugs gapped correctly (the old ones looked fine and were the correct brown color),
-New fuel filter,
-New intake gaskets,
-New knock sensor.
-Fuel pressure is good at 45 psi, and goes up 7-8 psi when the vacuum line is removed,
-Timing has not jumped, and was checked,
-EGR valve checked and is also good, not stuck open.
-Plugged the EGR opening in the intake, and no change.
No vacuum leaks either, checked all the hoses.
-TPS was checked and is good also.
-Battery and all electrical connections, and grounds, and PCM ground have been cleaned...
We are at our wits end, any help would be so appreciated.
The PO 300 code is a Random/multiple misfire detection. There are many conditions that can cause the engine miss.
You seem to have been on target for the most common causes of cylinder misses but there are some areas that people and mechanics miss.
The fuel injectors will cause a cylinder miss by either a bad or clogged injector or it could be an electronic problem caused by a bad connection at the injector or the PCM. This connection could be a loose or corroded causing the faulty electrical connection.
Using a Noid light at the injector can check to see if the injectors are receiving the electronic pulse from the PCM. If there is no pulse then check for a wire or connection problem. If you do have a pulse then it may be a bad injector.
Another problem that is frequently missed is the EVAP (Evaporative Emission Vapor Canister). There is a vacuum line and fuel vapor lines connected to the Canister and the intake manifold. If the purge valve malfunctions you could have a rich condition or even a lean condition cause by a broken or corroded vacuum line. This can cause a cylinder miss.
The other check to make is the good old compression check of each cylinder. If compression is low then the cause of the miss could be a burned valve, cracked head, blown head gasket between cylinders, cracked piston ring or piston.
Check these suggestions and let me know if they help. If they don't then let me know we can diagnosis the problem further.