Check Engine Light Refuses To Stay Off

I have a 1998 windstar which we've had for years. Always been a good vehicle. Lately I've had some problems with it and have not been able to fix it. I took it to AZ and was told that the number 4 plug was misfiring.I took an old spark plug and attached it to the number 4 spark plug wire and got a good spark while cranking the engine and grounding the plug against the motor. I've replaced my fuel filter, tried some RXP additive and prior to all of this replaced the spark plugs and wires. However the CEL still comes on.Is there any easy way to check the fuel injectors? I also have had a problem in the past with the vacuum line 90 fitting which comes from the top of the engine and back to the pcv valve,Is it possible that a bad pcv could cause this? Any and all help will be helpul and appreciated.

Thanks

A common cause of misfire on cylinder #4 is clogged EGR ports.
That is exactly what mine did when the EGR ports became clogged.....flashing CEL, rough running.
One way to test to see if it is the EGR ports is to TEMPORARILY remove and plug the vaccum line that goes to the top of the EGR valve and take the vehicle for a drive.
If you do not get a flashing CEL, or feel significant missing, then it is very likely that the EGR ports are clogged.
Take note that this test will light the CEL (if not already lit) with a code for insufficient EGR flow......which is due to the fact that you have disabled the EGR.
If it is clogged EGR ports, you need to clean them (or have them cleaned).
This means that the upper intake manifold needs to be removed....which will get you to the ports.
If your ports have never been cleaned, this is a likely cause of your misfire.

The EGR ports clog at the ports closest to the EGR valve first.....leaving cylinders #1 and #4 the last ones open.
When the EGR valve opens, all the EGR gasses then go to cylinder 1 and / or 4, causing a misfire.

Another thing that can happen is that crud can build up inside the "cup" over the end of each fuel injector.
I cleaned mine by removing each fuel injector and spraying a cleaner (carb cleaner) up inside the cap.....which flushed the stuff right out.
A good cleaner to run is Berryman's Chemtool B-12 that comes in a metal can.
Autozone carries it.
This is a solvent based cleaner....which is good for a periodic cleaning.
If your fuel system is significantly crudded up....you may need to run a couple of treated tanks of fuel through (1 can per tank).
I spent a lot of time chasing down a slight misfire on cylinder #2 that once in a while would light the CEL.
Once it started to light the CEL, then I knew which cylinder, and found that there was crud inside the cap as I mentioned.
I was told that this crud is from the fuel and EGR gasses and crankcase oil vapor all coming together......and NOT from the fuel.


I run a detergent based cleaner on a more regular basis to try to prevent crud from building up.

Thank you very much for your help. I've asked a few people at the auto parts and some mechanics but seemed to get no answers. What you're saying makes a lot sense. I'll see about working on this today.