cyl #4 misfire

Background: [1999 3.1 V6 type "J." new air filter, iac valve, pcv good, fairly new fuel filter, plugs and wires are a year or two old, battery & alternator good.]

Well I ran half a can of seafoam through my pcv line. It went well although there wasn't much smoke to speak of. (maybe the dealer actually cleaned the maniflds before they put it back together with new gaskets??) Now I'm getting a #4 misfire from the pcm. Specifically, it feels like it misses at idle but not while moving. fwiw, I didn't hydro-lock or stall the motor or put any seafoam in the oil. That just sounded like a waste since the oil is 2000 miles into a 6000 mile interval.

I pulled the plug. It looked fine but I cleaned it and checked the gap anyway. (.060, just like when I put them in a year ago.) The plug wire resistance reads 1100 ohms (about 550 ohms/foot) and I'm getting about 7 kohms across the coil towers. (all good readings in other words) I can assume this is probably not electrical?

The only things left I can think of are:
1. clogged injector (possible)
2. vacuum leak (maybe)
3. egr leak (maybe)
4. uneven or crappy cylinder compression (hope not)
5. bad fuel flow, clogged filter, etc (unlikely)
6. emissions system. (she threw a smog pump code, but what does that have to do with anything.)

So I guess I'll start going down the list. If anyone has any suggestions, observations, corrections, insight etc. I'm all ears and appreciative as always.

My first guess would have been intake gasket. I would hate to think that the dealer didn't do it right. My next guess would be injector. The dealer may have damaged or got dirt inside the injector when the gasket was changed. If the gasket change just occurred I'd bring it back to the garage being that it sounds that they may have aversively caused this. If not, just go get yourself another injector.

Well, it sounds like the rocker may have broken out of the aluminum head and is slightly knocking at idle when you can actually hear it. Might take off the valve covers and check.

I had the intake gaskets done at or around 95000 miles. It has 129000 on it now. It's been "knocking" for quite a while, but it actually stopped doing that after I cleaned everything up. Just the piston slap while warming up now, so I don't think a rocker broke loose. (of course I'm not a doctor) The real issue I'm dealing with is the rough idle. The only code I get now is the "secondary air injection system failure" code. Smog pump. I don't think it's related to the rough idle as it idles rough at times when the smog pump wouldn't be on and it only really affects emissions not how the car runs anyway.

It does still feel like something misses at idle. When idling at 600 rpms the tach drops about 50 rpms or so about every 2 seconds. There's plenty of fire, so maybe a failing injector or a breathing problem. If the throttle plate is open at all the motor runs smoothly.

Is there a way to check the injectors without pulling the upper manifold and rails?

Well I've done a little more work on this and all I've come up with so far is that the thread should have the title "rough idle for no apparant reason" instead of #4 misfire.

I did listen to the fuel injectors and they're all clicking. If they're doing anything when they click I have no idea.

I did a vacuum test. 18.5 in/Hg and steady at idle (I'm about 350' above sea level here in Maryland) All the associated tests turned up nothing but a normal engine so long as 18.5 in/Hg isn't considered low at this elevation.

One of the plug wires showed significantly higher resistance so I replaced them all. They were 30,000 miles old. It could have been the problem. Of course an ohmmeter doesn't measure a spark plug wire they way a spark plug uses it, so it may have been fine to begin with, and there was no change anyway.

The more I listen to the slight knock the more I think a rocker shook its way off the head a little like bcopeland said. That would certainly knock and would change valve tolerances. Is there a way to get the valve covers off without removing multitudes of engine componants and draining the coolant? I'd hate to tear the top half of the motor off and find nothing.

Also, once I'm in there I assume I re-torque the rocker bolts (168 in-lbs plus 30 degrees) and repair threads if needed?

Thanks a lot for the help so far folks. (really)

The car was throwing a long spark at a plug designed for a shorter spark duration, so the plugs would heat up and misfire. A set of ac delcos cured the problem. Since I know the motor's not junk , on to the wheel bearing...

The car was throwing a long spark at a plug designed for a shorter spark duration, so the plugs would heat up and misfire. A set of ac delcos cured the problem. Since I know the motor's not junk , on to the wheel bearing...

What kind of plugs did you have in the motor that caused that problem? I get a rough idle once the motor gets good and hot, it's fine when warming up. I have NGK regular copper plugs in my 01 Malibu, maybe this is the cause of my rough idle when the motor heats up??

Bosch platinum 2's I think, it's been a while. This car idled like garbage cold or hot after a while though. Maybe you're in the early stages. My lesson of the day was while the ac delco parts may or may not be the best, the car's systems were designed for them and sometimes it makes a difference.

A mechanic finally got to the bottom of this by the way. I guess that's why he's the mechanic and i'm the trumpet player.

Bosch platinum 2's I think, it's been a while. This car idled like garbage cold or hot after a while though. Maybe you're in the early stages. My lesson of the day was while the ac delco parts may or may not be the best, the car's systems were designed for them and sometimes it makes a difference.

A mechanic finally got to the bottom of this by the way. I guess that's why he's the mechanic and i'm the trumpet player.

Thanks for the info, I'll put the oem 41-979 AC Delco's in and see what happens. I've found that AC Delco parts for things like oil filters and pcv valves work best too.