Help! 2003 GT misfire when hot

[Modular03]

Hey guys, I need y’all’s help. I have a 2003 GT, mostly stock. Cold air and x pipe with flow masters. “ small exhaust leak driver side manifold.” I’ve been having a hard time trying to troubleshoot a intermittent misfire on cylinder 1&5. The car runs like a champ when I first drive it, but after 20 mins of running it starts to misfire. I already changed the intake manifold, coil packs, spark plugs, swapped injectors. EGR valve, one O2 sensor. Checked for vacuum leaks, and checked wiring. My buddy ran his scanner to monitor the cars vitals, and both the O2 sensors on the driver side are reading funky? Maybe causing the cylinders to misfire? I also greased up the coil packs and spark plugs with electronic lube stuff. What do y’all think? Also when the car is idling after it starts misfiring it has a extremely rough idle. Oh and I also replaced the crank sensor.

 

[gkomo]

What do you mean the drivers side O2 sensor(s) are reading funky?

 

[Modular03]

What do you mean the drivers side O2 sensor(s) are reading funky?

Yes

 

[ctandc]

Make sure the intake manifold isn't cracked JUST enough to leak a bit of coolant when hot and run down into the spark well well and cause a misfire. Seen that on more than one. Many times the heat will even cook it off and you won't see any coolant. Got a sweet smell under the hood? It's worth a check.

 

[Mustang5L5]

Pull the plugs on those cylinders, do they look clean? I wonder if you have coolant intrusion from the manifold being that it's both front cylinders. Do you smell antifreeze from the exhaust?

A compression test wouldn't hurt but what i'd do here is swap components around and see if the misfire follows. Swap the fuel injectors to a different cylinder, and swap the coils and spark plugs to another pair of cylinders.

If the issue keeps coming back to 1/5 you have a problem mechanically. Coolant can cause erratic O2 readings, but you didn't elaborate what those readings were.