Engine Help This Is Driving Me Nuts!!!!!!!!

I don't know if this helps, but my DPFE started setting codes a few months ago, but it always ran good. Changed it, and the only difference was no more codes. Sorry, I don't remember the code.

How about the fuel filter? A clogged filter will have similar symptoms.

+1 on pulling and posting all DTC codes. Confirm that the check engine light comes on during key on lamp test.

Not a bad idea to post the model year of your Mustang.

How does it run when at normal operating temperature? How's the gas mileage (now compared to the past)? Does the motor burn oil or coolant?

How long has the problem been going on? What did the old spark plugs look like?

If an injector problem is suspected, consider an injector cleaning and flow test service such as injectorrx.com.

If you pull and plug the vacuum line to the EGR value does this improve performance?

Do you have an ODB2 scanner that can read operational data?

Double check that the fuel pressure vacuum intake reference line is connected and leak free. Double check there isn't any raw gas in the line.

Would be handy to know the long term fuel trim (LTFT) values at idle and under load when cold and warm. Also would help to know the PCM reported fuel pressure, IAT, and ECT values.

Burns has you on the right track! When there is an issue that doesn't throw a code it is always more frustrating to solve. It seems like if there was a egr flow issue or a vac leak it would throw a rich or lean code, nor would it run ok at operating temps. Sounds like the one thing you have not touched on fully is fuel pressure. FPDM may not be doing its job when cold, or the pump is going bad but, in my experience at least, they usually exhibit issues at any temp if not worse hot or just don't work. I like using a mechanical guage over PCM reporting. You can buy a guage cheap and there is a Schroeder valve on the fuel rail you can connect it to to check it at cold. If you have good pressure when the car is cold, running then the rail sensor like wmburns explained needs to be test next. As far as reading the cars fuel trims you will need a code scanner that is capable of reading live data, I know my SCT handheld does so your diablo sport may or may not as I have always used SCT programmers. If so you will need to hook up the programmer and select live data (it may be under, or named something different. Might have to consult the user guide as you will likely have to select the PIDs for them) once you start the data you can write down the average LTFT for the conditions wmburns asked for. There are also PIDs for the IAT and the ECT.

Great, let us know!