2004 6cyl engine misfiring

My 04 TRD Off-road (5-speed) started misfiring a few weeks ago. It threw two codes (misfire cylinder #4 P0304) and cat (P0420). I cleared the codes about 10 days ago and they haven't come back on, but the engine is still misfiring badly, but only under certain conditions. It does not misfire when revving in neutral, when coasting or applying a little gas to maintain speed. It doesn't misfire as often when I select a lower-than-comfortable gear (higher RPMs). It DOES misfire when developing power in a higher gear, say 30 MPH in 3rd or when going up a hill.

Before I start replacing good parts I'm trying to find out if this is common and what usually solves it. I have replaced the air cleaner, emptied the tank, put in a bottle of injector cleaner and filled to capacity with name-brand 91 octane 10% eth fuel. No joy.

The truck has about 138,000 miles and is occasionally driven hard off-road so I figure plugs and wires are overdue (I bought it used 3 summers ago) and a new fool filter probably couldn't hurt either.

Beyond that, though, I'm afraid of falling down the rabbit hole of MAP sensors, a new cat, new O2 sensors, new coil packs, yadda yadda yadda, if user experience says it's most likely [X] or [X and Y].

Any thoughts before I break the bank on this? A new truck isn't an option right now, but it's barely driveable on city streets like this and I sure as heck won't take it off-road in this condition. Thanks in advance.

 

First check your spark plugs. If they have never been changed.....start there.

 

I'm thinking clean the MAF sensor and/or check the EGR system is functioning correctly. Look for pinched or burned wiring around the EGR valve. Keep in mind I'm probably wrong. Good luck!

 

First off north west or north east jersey? This help is assuming that your oil level is good, anti freeze level is good and not slowly disappearing by having to add a bit every week and your engine was running well with no noises before this happened. You have to go after the P0304 code first. A miss fire in cylinder #4 will create a rich condition and the catalytic converter may not be able to reduce the emission efficiently. Ultimately creating the P0420. I would pull the number 4 spark plug and see how it looks. If it is white, it is running lean which could be an injector problem. If it is black or fuel fouled it most likely is a ignition problem. If your engine has the coil on plugs on top of the valve covers, it could be a bad coil, coil to spark plug boot or oil leaking into the spark plug well creating a miss. A coil, boot and spark plugs is not going to break the bank and easy to replace as long as the spark plugs come out easy. Oil in the plug well means the valve cover gasket and spark plug tube seals need to be changed. A bit harder to do but not impossible. Look at you tube on how to do either job. Lots of info on the site. Once the misfire goes away, your cat code will probably be gone also.

 

^^ I like your style.

 

I had a similar issue with my '98. Out of the blue one day it started running like crap. Thought it might be fuel filter, but it wasn't. Turned out to be a bad plug wire. Replaced the set and problem solved. A friend of mine had the same issue with his 3rd Gen 4Runner (also 3.4 V6). Seems to be a common problem around 120K.

 

Hey you both live in Jersey. Help each other out.

 

^^^^ That is why I asked which part of north jersey he is from. If he is not too far away maybe I can help him. Until I know where he is I can help him on this website. I am all for helping someone out. That is what this forum is all about.

 

I'm in NE NJ, near Fairfield/Willowbrook. I'm not any good with tools myself but I have a good mechanic for the usual stuff. He doesn't have any fancy diagnostic equipment but things like plugs/wires, a coil pack, etc, are well within his wheelhouse. I definitely think a set of wires and plugs should be the first step and I plan to go see him about this in the next few days when I can get an appointment. It needs new brakes badly as well (dealership told me my rotors are warped, one caliper won't release and the pads are down to 4mm) and I'm planning to get the fuel filter replaced at the same time. I'd love to be able to do this stuff myself but like I said, I'm not even close to being comfortable working on cars any more.

It threw the cat code twice and the misfire code once, but like I said I cleared the codes about 10 days ago and they haven't returned since, which probably makes diagnosing it harder. I keep a bluetooth ODB-II scanner plugged in all the time and I have the Torque software on my phone, so if you can suggest anything I should be monitoring off the ECU in real time as I'm driving maybe that could assist in diagnosing things properly?

Thanks again to all. I really appreciate it. I bought the Taco as a second family vehicle but every so often I liked to take it up to Stokes state forest to play in the mud...until the NJ gov't shut down all the decent trails last year . I was planning to take it out to Rausch Creek last month but with this going on there's no way I'm going to risk it.

 

I'm also in NE NJ, maybe I can take a look. If anything we should get a beer sometime or go wheeling

 

That's be awesome. I dropped it off at my guy this evening for brakes, plugs and wires so we'll see after that. Still nothing in terms of codes in the ECU as of 7pm today so it's been clear for about 12 days now--but still misfiring. Hoping the plugs and wires solve it, but I've asked him to keep the plugs and note the cylinder they came out of if they're unusual in any way.

Would love to go wheelin' once this is done, but other than Rausch Creek I don't know where's still open. All my favourite trails up at Stokes are permanently closed now.

 

Pmed

 

Yea closing trails is the trend these days

 

OP, did you ever get to the bottom of this?

 

Plugs and coils

 

Glad to hear you got it figure out.

 

Actually plugs and wires. Coils were just fine and it's running like a champ. HOWEVER, it's still throwing a couple of codes. P0031, P2238 and P0240. I cleared 'em out and I'll see what happens in a week or so. I'm guessing at least one O2 sensor (is that the same as the MAF) is going bad?

But like I said, it's running 1000% better now.

The other thing, does anybody know where to get the proper kind of spring to get the clutch pedal to come all the way up? I recall reading a long thread about it 2 or 3 years ago but can't find it again. I don't care about the squeak but the fact the pedal stays low after clutching drives me nuts.

 

Whoops thought it was coils too, my bad

 

Oh, I also should say that whoever did the last plug change on the truck used single electrode plugs, not the OEM dual-electrode type. All 6 looked good with no oil fouling or electrode damage. My guess is the wires were the culprit for the rough running.