4.6L Mysterious Misfire
 

Hello all.

I have a 1998 Ford Expedition EB with the 4.6L in it.

I have replaced all the plugs, and wires, as well as the coil packs. It seemed to run a little better, but still yet have a misfire.. It's not all the time, just intermittent.

Also noticed, while driving down the back roads, doing about 55 or so, it starts to buck or surge while off the gas, or holding steady throttle.

Any help would be greatly appreciated!!

Need to pull codes.

I have, no codes after replacing the plugs, wires and coil packs.

Just comes and goes. Mainly after its up to operating temp.

Check for pending codes.

And have you checked the infamous PCV elbow at the rear of the intake manifold?

I've checked every computer option avail. We have a 7000$ snap on computer at work, and we ran it several times and cycles today at work.

I fixed the elbow about four months ago, and as far as me and the other mechanic can tell, there are no vacuum leaks.

Used carb cleaner and sprayed all around the intake, all the vac lines, etc. No change in rpm or sound.

Also FWIW The plugs are Autolite Platinum. Before that, I used Bosch Iridium, and they was the worst plugs ever for the 4.6L

If you have access to a $7k scan tool, it ought to be able to display the Mode $6 date for misfires, cylinder by cylinder. Type "A" misfire data will be misfires per 200 RPMs while the type "B" data will show misfires for each 1000 RPMs. (If its an early version scan tool the mode $6 data will be in hexadecimal, but you can use the Windows calculator to convert it to decimal. If its a fancier scan tool it will convert it to a percentage of misfires per cylinder.)

Yes sir, We used that and everything, and sadly it didn't count any misfires, but We do know its there. And Why would it surge to?

We have seen three other 4.6L's do the same thing, and no one has yet to figure them out.

Hmmmm. Lee says there is a misfire. PCM says there is no misfire. I guess we need a third opinion :D

What codes did you have before replacing the plugs/wires/packs?

Have you done a compression check on each cylinder?

Can you monitor the fuel pressure when this surge occurs?

What coils did you use as replacements? I work in a Fleet dept and we only use the Motorcraft coils because many of the aftermarket coils seem to fail or also create a misfire.

Just recently a co-worker replaced all 8 coils but he used the Duralast brand and after a week of the same symptoms you described, he had them all replaced with Motorcraft coils and the issue/symptom went away.

Back when I had my 97 Expedition I replaced all 8 COPs with ones from eBay. Those thing ran flawlessly from the day I installed them until the day I traded it in for my Navigator.

I have heard that as well from others about the various brands on Ebay, but when you are servicing Public Safety vehicles you want to eliminate any possible reason for a come back. So we only buy the Motorcraft brand of plugs and coils. They cost more, but save us on come back work.

I see. :-X22 So my next question is, if the MC plugs and coils are so good, then why did you have to replace them in the first place? :D

Nothing lasts forever!! ; )

It would seem you are missing a fuel problem. Try looking to the vacuum first by looking at all vacuum hoses and anything they attach to. Then look to your fuel pressure through out the power usage. Check for a blocked crimped fuel line. You have a mechanical problem or a bad electrical connection, maybe crank sensor. When a pump begins failure sometimes it starves the motor someand then goes away, same with the fuel filter or regulator. See if you have corrosion on the top of your PCM cover. Check the circuit board for obvious burning.