I have a '99 F-150, 4.2L with 246,000 miles. Been a real good truck. However, I'm getting a P0303 cyl 3 misfire code. The plugs had quite a few miles on them, so I changed them out. Still have the code. I swapped spark plug wires from cyl 2 to cyl 3 and vice versa. Code stayed on cyl 3, so I don't think my wires are bad.

I've thought about a new coil pack, but wouldn't I be getting a misfire on cyl 4 as well, if the coil pack was bad?

Fuel injector? How could I tell?

Other suggestions?

Any help will be greatly appreciated.

Thanks, Hal

 

Hal,
Given the number of miles, and the fact that plugs and wires did not fix it, I'd be strongly suspecting a bad coil pack.
There have been several found bad by guys here over the years.

I think a fuel injector could be the cause, but I'd still be looking at the coil pack first.

 

Brian,

Thanks for the repsonse. A coil pack would certainly be easy to swap out and not too terribly expensive, but will a bad coil pack show a misfire on only one cylinder, or would it show up on two? Doesn't it fire cylinders 3 and 4 simultaneously?

Thanks,

Hal

 

Brian,

I swapped the coil end of wires 3 and 4 and the code continues to be on cylinder 3. If you still think it's the coil, I'll shut up and replace it. But if this additional information changes your mind, let me know.

Thanks,

Hal

 

The coil pack fires two cylinders at once, but I am ignorant of which are paired. They would obviously be on the same coil pack...
Have you checked the compression? This engine has been noted for all of a sudden locking up to due to coolant leaking into a cylinder, and then you get bent connecting rods as it tries to compress the coolant. Then you try to get Ford to cough up some moolah to help with repair.
Does it use coolant? Does the plug look like it was 'steam cleaned'? If you are feeding it coolant, you are also feeding the catalytic converter coolant, and it does not like the taste, and will quit working after a while in silent protest.
The error code is generated by the crank position sensor detecting the 'fact' that the buzz rate (rpm) generated by the sensor is lower than the others. It just compares the XX Hz for each firing position, and notes that it is a consistent problem or an intermittent. So a weak cylinder, just not as strong as the others, will possibly throw a code. That could be the case here, where all is as good as it gets, but I think there is an underlying problem.
tom

 

Poking around the web a little I found several cases where a reported misfire on a single cylinder was repaired with a replacement coil pack.
Given the fact that it fires 2 cylinders at the same time, if the problem is physically isolated to the mechanical path (inside the coil pack) of the high voltage going to cylinder 3, I can see it causing what you describe.

No gaurantees, but that is where I'd be headed if it was mine.

And please don't "shut up". We are all learning something on your dime!

Thanks and keep us posted.

 

+1 to the coil, screaming demoil CPs are slightly cheaper then stock replacement IIRC on my 3.8

 

Tom,

You bring out a very good point. This truck has never had any real problems, so I have to admit, I never check the coolant. It has never used any in the past. I changed it about a year ago. Just last month, though, my son had it run hot with him. He had to add a fair amount of coolant to bring it back up to the proper level, but it has not done that since and we've not had to add coolant since. It was a freak thing. We could never find anything wrong with it. You made me wonder though, so I did a compression check tonight. Engine was not fully warm, but not dead cold.

1 - 217
2 - 208
3 - 212
4 - 215
5 - 215
6 - 200

All are within 10% and none are low, so I don't think I have a head gasket leaking. Frankly, with the number of miles, I was quite shocked at the compression.

Additional Information:

I rechecked the codes, just for fun, before doing the compression test. In addition to the usual P0303, I had a P0302 (Misfire Cylinder #2), as well. I was really wondering about that head gasket at this point. But as you can see, that doesn't seem to be the problem. I decided to check the gap on the new plugs that the parts man told me not to check. He said they were "Pre-gapped" and I would damage the electrodes if I tried to gap them. They were gapped at .040". Sticker under hood says .052-.056. I re-gapped them all to .054.

Brian,

I'm gonna run it with the plugs gapped properly and see if it stores another code. If so, I might have to bite the bullet and try a coil.

I searched the archives and found a thread where someone said this could be caused by clogged EGR passages. They suggested you apply vacuum to the EGR valve at idle and see if it started missing or if it quit. He said if it started missing, you had clogged passages. He said the engine should quit if all the passages were open. I tried this and mine never quit. It started missing. Boy, I wouldn't be looking forward to pulling the upper manifold to clean that out. Especially, without changing a fuel injector while I was in there. Of course, I could do both, but if it fixed it, you would never know which was the culprit. How would one go about cleaning the EGR passages anyway? Wouldn't all that crap fall down into the crankcase?

Thanks,

Hal

 

Hal, you did right by setting the plugs to the proper gap.

And I had forgotten, but do remember after you mention it, that clogged EGR port can cause OBD codes. I thought it was a lean condition on #3, but maybe I am wrong?

I do think there is a thread way back abou tcleaning the EGR port. Let me know if it comes to that, and we'll see if we can dig it up.

Best of luck!

 

Update:

Going down the EGR path, I thought I would try to isolate the problem along those lines. So I pulled the vacuum line off the EGR valve and plugged it, cleared the codes, and drove it home from work. Checked the codes....It now has a P0401 EGR-insufficient flow (No surprise there). And NO misfire codes!

So, it looks like the problem may indeed be the EGR ports being plugged. But what do I do about it? Is there an easy way to clean them out? Any chemicals I could spray in there somewhere? Or do I have to pull the manifold and rod it out and scrub and....

Also, what do you guys think about that high compression? Sounded good to me at first, but the more I think about it and read about it and talk to others, I wonder if the head and piston aren't covered in carbon? I know they make something to clean that. Do you think I need to do that?

 

I wouldn't worry about the compression unless you start having bad pinging using 87 octane. If you decide you want to try something non-invasive, check this thread - post 15 in particular.
https://www.ford-trucks.com/forums/2...g-seafoam.html

As far as cleaning the EGR ports, AFAIK you have to pull the intake and use a very small brusg (I think one guy mentioned using a pipe cleaning brush) to get them cleaned.

Do a search for "clogged egr" and you'll get quite a number of threads to peruse.

 

If I were doing it, I would remove the EGR valve where it mounts to the intake manifold, splurt in some cleaner stuff, e.g. Carb cleaner, gently, as spraying into a closed volume will cause sprayback -- cover any painted surfaces --, and let it sit for a while. Then, use compressed air to blow the passages out. What happens is (i think) that the ports are ALL a little plugged, and the EGR meant for all six is concentrated at the 'mouth' of 3 & 2. Too much EGR for them to be able to fire the mix, so they don't.
Perhaps just applying air under pressure, with the throttle plate cracked open, will jar enough loose to make it work. Remember, they are only fed exhaust at 3-5 psi (or less) so a good rush of air might just make them work. Then you can add "apply hi pressure air to EGR every 10,000 miles" to your maint list.
tom

 

OK guys,

I tried the carb cleaner through the EGR hole followed by compressed air, but no luck. Guess it's time to pull the manifold. However, I am unclear on what I have to pull. This is a '99 with a 4.2L. Will just pulling the upper manifold (Plenum) do me any good? The EGR valve mounts to the lower manifold. Looks to me like I need to pull the lower manifold in order to get to these EGR ports. Am I right? Any caveats I need to watch out for while doing this job?

Thanks,

Hal

 

Hal,
Don't pull the intake apart yet. If you havn't checked your coil yet, do so first.
First take a timing light and put it on the #3 cyl. and watch the light to see if there is any breaks in the flashes. Sometimes the break is so small it is hard to pick up so watch it real close. If you see any breaks in the light your coil is bad. If you don't see a break than take the coil off and look at the under side of it and see if there is any cracks in
it. if cracked its bad.

 

Forgot to mention, I'm a fleet mechanic 10yrs for a company with 60 plus f150's with the 4.2 in them and never had an egr port plug up.