Misfire with no code

[Fudge1218]

So 2003 expedition 4.6 rwd, its misfiring and I can feel it the service engine soon light came on but I was changing a few sensors so I disconnected battery not thinking and turned it back on and the code was gone, what's the best place to start troubleshooting and figuring out which cylinder is misfiring
Also it's not a constant either sometimes it's not there at idle others is it

 

[John Stephan]

If you have an OBD2 scanner that can read Mode6 data, check there first. Other than that you need to wait until its acting up and the disconnect the coils one at a time and see which one doesnt make it worse. Then you need to figure out if it's an ignition misfire or fuel misfire.

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[1955moose]

Any OBD2 scanner usually can read a spark misfire. If like John said it's fuel or a possible intake leak, then you'll need something more advanced that can read the fuel trims. Start with basics, codes and smoke the intake. A vacuum leak anywhere will cause a lean miss, and in turn give you the miss your feeling. Are you having any issues starting it? Report back.

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[Fudge1218]

I do have issues starting and I've had a minor vacuum leak for a while, my issues starting are turning the key and absolutely nothing, no crank no voltage drop absolutely nothing, the vacuum leak is only noticeable under load (my vents cutout to just floor vent)

 

[1955moose]

That's a vacuum leak in your Eatc heater/AC system. It usually doesn't affect engine performance, but who knows. Smoking through the brake booster hose should show any intake leaks regarding engine and exaust. I don't think you'll see smoke from the hoses going to your comfort controls.

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