Engine is throwing p304 code cylinder 4 missfire. Truck is a 99 f-150 with 130,000 miles on a 5.4, pluggs changed and new COPs at around 100,000 My question for the experts on the site is there any other reasons for this code such as IAC trouble? I was having issues with the IAC last week you know the foghorn sound so I cleaned it like Ive done before sound is now gone 3 days later started running a bit rough now throwing codes any help would be great.

 

Have you checked the heater core hoses ? That's what usually compromises #4 . It can be very hard to determine if there leaking at times. Look for signs of water/anti-freeze puddleing on top of COP #4.

Your going to have to get back in there and fix. The plug chamber will most likely be holding coolent.

If you need to replace the two lines back there - The dealership carries the good ones ..

 

Yeah I had this before and fixxed the lines but this time there is no sign of leaking antifreeze or anything cop and plug look fine. Thats why I was asking if that code would involve anything else other than missfire on cylinder 4.

 

Nope - unless it's a plug or a bad wire in the harness. Wait , there is one thing. If your wiring harness is to close to your AC accumulator it will misfire.

Moving the harness further away and taking it for a drive is worth the effort before digging into #4..

Damn , that one almost slipped my mind

In your case this is unlikely unless you have altered the location of your harness or it has come loose from it's original location and shifted enough towards the accumulator to cause problems.


You know what I mean - The MEGA fuse block has to be like this -



The Big black plug - Outside the cradle . Gray up top , tucked inside. I think yours is similar.

The accumulator is directly below and towards the fender a little from the Black Plug .

 

JBREW, I went ahead and checked the harness looked fine and inspected the hoses after thinking about it I didnt replace them last time but I didnt notice any coolant or any bad areas on the hoses. You are talking about the pressure fitted lines towards the back of the firewall correct? Any ways I pulled the plug and it didnt look bad to me Ill try to send a pic to get your opinion on it. Your pic looks alot different than my firewall (and Im not just talking about how clean yours is!) thanks for the help Ill keep ya informed

 

If the plug chamber was clear , I would replace the COP. It had to be shorted out somehow - at some point. They usually don't go 30,000 miles and just quit. New one's when purchased are either good or bad from the get go.

Who knows what happen. They would prolly short out if a bee wizzed on one - so damn vulnerable..

Well , yea - post the turn out

Good Luck with it