I been having this code for the past couple of weeks, I took it to
my mechanic a week ago he replace the sprak plug on cyl#4 cause there was coolant leaking down into the cylinder. He fixed the coolant leak also by clampping down the hose that was leaking. I pick it up last Tuesday as soon as the truck warmed up, again the CEL light came on I pulled the code samething P0304. I turned around and took it back to my mechanic, He check it again the next day and said he gonna changed the coil pack on cyl#4. I went to pick it up last Friday, It drove good, I even droved it to work & back, I work 67miles from my house. Saturday I went to drive it as soon as it warmed up and I turned my heat on the CEL come on again. I pulled over check the code, Again P0304, What else can it be. Can it be the spark plug wire since coolant did leak down into cylinder#4. I puzzle here...

 

Have you or your mechanic considered and looked into the possibility of either a bad fuel injector or a sticking valve?

 

No he havnt check that out yet, I will ask he about that, So a bad injecter or a sticky valve can cause that. What Sticking valve are you talking about?

 

An intake or exhaust valve for cylinder #4. He would have to do a compression or leak down test on that cylinder to test for a bad valve.

 

ok I got my truck back from my mechanic, CYL#4 works fine no misfiring, but yesterday I drove the truck my check engine light came on again, Ths time its showing code P0303 CYL#3. Do I need to change all the plugs & coil packs to resolve this problem.

 

No you don't need to. You can just change the ones that are giving you the problems. You might end up only replacing a few this way. OTOH, if you find it to be an inconvenience to have this problem keep popping up, then go ahead and change all of the plugs and COPs so you don't have to worry about it for some time to come. The advantage to replacing them all is that you can pick a time and place that suits you instead of it happening when you least want to to.

You can get an 8-pack of COPs off eBay for under $80 these days.

 

my mechanic put new spark plug wire & a new coil pack on cyl#4, I think
I might just change the rest coil pack to see if that solve the problem, If I change the rest coil packs should I change the spark plugs too. I just had all them change last March.

 

No I wouldn't change the spark plugs since they are fairly new. First I'd change the other 7 COPs and see if the problem clears up.

 

so replace the coil on plugs boots, not the coil pack. I thought on the 98 XLT there are 2 coil packs for both sides.

 

Well, must be you have a 4.6 then. If folks could put the model year and engine size in their sig line, alloro wouldn't have to try and guess what ignition system your '98 Expy has (although he usually guesses right ).

 

The 98 with the 4.6L motor uses coil packs. Each coil pack runs 4 cylinders and you will see spark plug wires. The 98 5.4L motor uses COPs, one per cylinder. Since you only had a misfire on one cylinder and your mechanic chose to replace the "coil pack" I made the leap that you just used the wrong terminology and do indeed have the 5.4L motor and 8 COPs.

 

So many posts, so little time...I try my best, despite what a select few might think. Backatcha!

 

Thanks Alloro, yes I do have the 98 4.6 XLT, So since my coil pack was replace with a new one, I might just ask my mechanic to replace te other one too. maye that might do the trick. but whats funny is that it only start misfiring when I turn my Heat/AC on...

 

That makes it sounds more like a vacuum leaking issue since the heater controls are run by vacuum.

 

A bottle of fuel injector cleaner in the tank might help if its the injector.