P0303 code (#3 cyl misfire), vacuum related?

[KaHOnas]

I feel pretty confident it's a vacuum issue. The reason is it only hesitates and bucks under light load. When driven at higher RPM or under load, it's much better, almost smooth. So I feel that it's not ignition or fuel filter related.

So, what could be causing this kind of thing in the vacuum system? Or if you've run into similar symptoms and it's not the vacuum system, what was it? I've got a new ignition system (coil pack, wires, and plugs) as well as a new ECU.

I don't want to start replacing things hoping that'll do it. Thanks all.

David
1998 4.0l 4WD 5spd ~225k miles

 

[franklin2]

That is a cylinder 3 miss-fire correct? Something like vacuum would affect the whole engine and all the cylinders? I would swap sparkplugs and see if the problem moves to another cylinder. Then swap plug wires and see if the problem moves. Then swap coils and see if the problem moves.

New parts do not necessarily mean good parts in these days and times.

 

[Curious Hound]

Good advice above except for the part about swapping coils. I'm really sure your engine just has one coil pack to service all the spark plugs. You can swap in a new coil pack. But don't mix up the order where the plug wires connect to it. That will change your firing order and make it run worse.

 

[KaHOnas]

Good advice above except for the part about swapping coils. I'm really sure your engine just has one coil pack to service all the spark plugs. You can swap in a new coil pack. But don't mix up the order where the plug wires connect to it. That will change your firing order and make it run worse.

That is a cylinder 3 miss-fire correct? Something like vacuum would affect the whole engine and all the cylinders? I would swap sparkplugs and see if the problem moves to another cylinder. Then swap plug wires and see if the problem moves. Then swap coils and see if the problem moves.

New parts do not necessarily mean good parts in these days and times.

Correct. #3 cylinder misfire.

I suppose the spark plug shuffle isn't a bad place to start.

 

[franklin2]

Good advice above except for the part about swapping coils. I'm really sure your engine just has one coil pack to service all the spark plugs. You can swap in a new coil pack. But don't mix up the order where the plug wires connect to it. That will change your firing order and make it run worse.

Thanks for the correction.