I need some help. I am out of town for work and got a call from my wife. She said on the way to take our daughter to school our expedition started to sputter like it was running out of gas. Then the check engine light came on. I had her take it down to the parts store to have the codes run. She said they are PO 300 (random miss fire) and PO 304 cylinder #4. She also said she is getting a bad sulfur smell. Any ideas?

 

Bad misfire in cylinder #4. The sulfur smell is from the excessive unburnt fuel getting dumped into the exhaust.

 

So maybe a bad coil pack? Any other ideas?

 

bad coil or plug on number 4...replace both while they are in there

#4 is a pita to get to....easier to pay a garage 40-50 bucks to do it for you

#4 is famous for going bad.....Ive had to replace twice...bad design due to moisture around that COP

 

Random misfire could be caused by a fuel supply issue. And if the wife says it acts like it's running outta gas, better check that out first. Was the tank running on empty?

Let me guess . . . if this truck has the 4.6L engine and has coil packs it might be an older model, like 1997 or 1998? What shape is the fuel filter in?

Misfire can also dump unburnt fuel into the cat and cause some rotten eggs.

Might want to start with attaching a fuel pressure gauge to it to see what kind of pressure its putting out when it acts up. And if the pressure is good might want to check the fuel volumn that its putting out.

 

Its a 99 with the 5.4L. The truck has just over 200,000 miles, but the engine was replaced with a brand new one about 2 yrs 30,000 miles ago. The motor was replaced by a ford dealer and I don't know what was replaced at the same time.

 

Usually not a fuel issue

Those same symptoms are mostly a COP/plug issue......most likely when they changed the engine out they used the same COPs that were in the old engine

Everybody wants to think its a fuel issue at first but w/ that 5.4 its most likely a bad COP/plug

wouldnt hurt to change the fuel filter anyway..its prob due

 

Well took it in and it was a bad COP on #4 like I thought. We had them swap the bad COP out and change all the spark plugs they also replaced the fuel filter and did a couple other tune up things.

 

One additional item to check when you get random misfiring is the small rubber fitting that sits on the left (passsenger) side of the engine for the EGR valve. This deteriates and causes a vacuum leak, hence the misfiring. Pretty common failure, especially those that 7-8 yrs old.