maxh

Hi, this is my first post, I hope it's in the right spot!

I have a 99 F150 with the V6, just passed 200,000, and yesterday the engine suddenly started running rough. It's misfiring on cylinder 4 at idle and low rpm, then as the rpm rises, the problem goes away and it gets smooth again.

On a cold start it'll smoke while it's running rough, but stop smoking as soon as the rpms are high enough for the misfiring to stop, and smoke again when the rpms drop and the misfiring returns. After a minute or so, all smoking stops, though. The misfiring issue seems to be erratic. I've driven it maybe 10 miles since this issue appeared, and most of the time it's there, but every once in awhile it'll disappear completely for a bit.

At first the check engine light was not illuminated, except for a brief flashing episode when it was misfiring really bad. But this morning it did turn on and stay on and the codes are:

p0304 - Cylinder #4 misfiring
p0174 - System too lean (Bank 2)

Just those two, NOT p0171 bank 1 lean.

I've changed the cylinder #4 spark plug and all spark plug wires. I've visually inspected the MAF sensor and it looks good, and looked for vacuum leaks but couldn't find any (I very well may have missed one though; I'm not very experienced in this area.)

I've attached a pic of spark plug #4 next to a new one (the plugs have maybe 20k miles on them) and though it's probably no help, a pic of the intake manifold near cyl. 4. The red box in the first pic shows where the pic is zoomed in in the other one.

Some have suggested the intake manifold (plenum?) gasket could be leaking. Or maybe fuel injector or fuel pressure issue.

Any suggestions on where to go from here?

Edit: I tried spraying carb cleaner around the intake manifold to look for leaks (engine would rev if spraying a leak.) No luck, but I may have missed a spot.

The exhaust smells strongly of unburnt fuel; if there were a fuel injector issue, it seems like that wouldn't be the case, as the misfires would be due to lack of fuel. Or am I off base with this thinking?

I replaced the coil pack, and still no improvement. What now? I'm thinking of pulling the intake manifold to check/replace the gaskets; anything to look out for on this task? Should I try anything else first (like would a compression test be worthwhile?)


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kmpconley

Check to make sure your ignition coil ain't bad. I have an 03 F-150 4.2L V6 and I would think it would be the same setup. The coil on mine was a rectangular box on the passenger side of the motor that ur plug wires pop into. Remember this is not a distributor cap, it is a solid coil pack, not like the individual coil packs on the 4.6L or 5.4L. Label your wires before you take them off so if this is not the problem, you can reinstall it the same way it came out. Mine was misfiring like crazy at low RPMs and throwing a lean code as well, and I was told by a buddy to take the coil pack out and have it tested. Make sure you disconnect your negative battery cable before starting this that way your computer should reset everything once it's reinstalled. Once I installed a new coil pack, the truck ran totally fine, not saying this is your problem, but after having the same symptoms you stating, doing this fixed my problem.

TomB275

Reach back and feel the large vacuum hose located on the passenger side near the firewall. If squishy yank it off and inspect for a slit or leaks. I replaced mine with 3/8 fuel hose.

maxh

I actually did that one about 5,000 miles ago! It was a tough one to find. I had p0171 and p0174 codes (bank 1 and 2 lean) but it wasn't running rough; I had to get the check engine light off so I could pass inspection. I searched and searched, gave up, decided to give it one last try, and luckily found that one by feel because I couldn't see it. That was the 2nd vacuum leak I had in 50k miles.

I'm going to go ahead and replace the coil pack and update this thread with the results from that.

maxh

I sprayed carb cleaner around the intake manifold, but was unable to find any spot that resulted in increased idle speed.

I also put in a new coil pack, and that didn't help.

The exhaust smells strongly of unburnt fuel; it seems like a fuel injector issue would result in too little fuel, so probably not exhaust smelling of it. Is that right?

I'm thinking about pulling the intake manifold to check/replace the gasket. How involved is this, and anything to be aware of going in to the job? Anything I should try first, like a compression test or something?

Jacob.P93

Don't replace it yet. Take the misfiring pack and swap it with cylinder 1 or the easiest cylinder to get to and see if the missfire moves. A miss is not always a plug or coil pack. And if the miss doesn't move compression check is in order. Have you listend to the injectors?

kmpconley

It's a 4.2L he doesnt have individual coils. Did u disconnect the battery to reset ur computer after changing the coil pack?

maxh

No, I forgot to disconnect the battery. How would it affect whether it worked or not?

Z7What

Maybe this?

http://www.f150online.com/forums/v6-...2l-issues.html

kmpconley

Disconnecting the battery will reset the computer. Resetting the computer should clear the check engine light, and should reset ur the electronics. Just cuz u put a new coil on, ur truck could still think it has a misfire, and not function properly. Resetting the computer could allow all the electronics to function properly.