I've got a '99 expy with 190k on the clock. Lately it's developed an annoying misfire/bucking/sputtering when under load. It idles smooth and accelerates fine and handles higher then 2k rpm fine. It starts acting up once it is in over drive at 40mph or faster and I apply more then moderate throttle. No CEL. If I give it enough gas to make it down shift it runs smooth and has power. It's not a drive train issue.

Plugs were replaced around 125k with factory replacement motor craft platnum plugs and at the same time I switched to accel coils. Today I sprayed the MAF sensor off with cleaner hoping that was the cause but that wasnt it.

I thought about checking the plugs to see if any were bad but have not done so yet. I first noticed this issues when the engine was stone cold it would spit and sputter when I applied the throttle leaving my apartment but would clear itself out once I got going down the road a little. Now like I said it like to do it most at low engine speeds under load. It does not need to be on over drive to have the problem. It's just easier for me to recreate the problem in over drive.

Any and all help is greatly appreciated. It's a 99 expy Eddie bauer 5.4L 4x4 with 190k miles.

 

Sounds like a classic symptom of worn spark plugs. You might want to check a couple of the easy to get to ones, just to see what they look like. If they're okay, then next I'd test for correct fuel pressure.

 

Ok I'll check that out. I'd figure the plugs would have lasted longer then this, originals lasted 122k, but they are cheap enough and I can do it myself. It's just strange because it idles fine but any load I give the engine from idle to about 1800rpm it runs like poo but then smooths and takes off like a rocket.

 

At idle the air/fuel mixture is less dense making it easier for a spark to jump the gap. During load conditions the density of the mixture increases, making it increasingly difficult for the spark to jump the gap...misfire. A weak ignition coil will have the same result, so there is the possibility of one or more COPs acting up. But I figured that inspecting the plugs and checking fuel pressure is the less expensive starting point.

 

Yeah that does make sense.

As an update the truck is starting to misfire more at idle and higher in the rpm band. So I'm going to start with replacing the plugs first. I'm also wondering if the coil boots could be going bad.

Previous to this problem I kept getting code P0401 for insufficient egr flow. I found a broken vacuum line to be the cause of that but I ran it that way for a little while. Wondering if that played a role in possible killing my plugs.

 

Depends on long it was leaking. A large vacuum leak leans out the mixture and runs the plugs a bit hotter. The hotter the plugs run, the quicker the electrodes wear down.

 

Change the plugs today and it seems to run normal again. Thanks guys.