Plug wires might help.

You might invest in a compression check to see if one cylinder might have a valve or ring problem.

Just guessing at this point.

 

Did a compression test already. cant remember the exact numbers for each, but they were all close to 200psi. One might have been lower than the rest, but not by a whole lot. FSM says 175psi or more, no lower than 125 I think.

So im guessing the rings and valves are ok?

 

Yep...

 

99 4Runner SR5 2WD 3.4 V6;
Cold start, all good. After truck warms, stop and restart, I get the rough idle at 650 rpm. Let it sit for a couple hours, completely cool, no problems... Codes for CEL were 300 & 301, multiple misfire and misfire cylinder 1. Today I replaced the Cylinder 1 coil, wireset and all 6 plugs, truck still relatively warm, still have the rough idle.
I have the remaining to cpils coming in tonight and I will change the fuel filter aswell, but if nothing else works, any ideas? Timing belt I guess, but ill have to take it in for that because I don't have the tools or space to do it.

 

probably injector, try some cleaner.

 

not sure it has been mentioned yet, however, have you cleaned your engine grounds and your battery connections?

now that you have the denso plugs you can cross reference them with NGK plugs next time.

you said you pulled the springs, did you seat them in the boots properly? I used a screwdriver that slid into the spring and pushed with that.

any electrical issue can help the idle be rough; also do not forget that there are two other pieces to this puzzle, one is air and the other is fuel.

be sure your air filter is good and clean and run some seafoam to clean the injectors.

If it is just slightly rough it may not get better... it is an old truck

 

Replaced the plugs, greased the coil connections. I was sure the spring was making contact....or I think I was sure. Either way, I rearanged the order the plugs and coils were installed and guess what....The alarm came back yesturday, same cylinder, no Random code this time though. First time I got the alarm I had Random as well as a cylinder 2. Now all I got were 2 alarms for cylinder 2.

SO that being the case with the plugs and coild rearanged, that means it either mechanical , valves or something, or the injector, and or fuel.

I have had degraded fuel ecconomy, is it possible my injector could be leaking more fuel than it should during an idle? I dont have a way to test this for leaks. Would a leaking injector cause poor idle, or would increased fuel cause higher idle?

 

You said you greased the coil connections, did you grease the drop tubes on the wires? the wire drop tubes need grease as well this increases the continuity.

as for your question about leaking injector and increased fuel effecting idle speed, yes they definitely can. increased fuel does increase idle, however, increased air increases fuel. O2 sensor looks at air content and adds fuel...
Injectors not spraying properly decrease efficiency due to larger fuel particle masses.

I recommend running seafoam in the tank and greasing down tubes if not done already as well as cleaning the butterfly that lets air in.

 

New injectorbin Cyl#1 fixed my P0300 & P0301!

 

I have a '96 4Runner with the 3.4L in it, sporting 242K miles right now. It started missing and chugging in the past couple months, so I replaced the plugs first as they were badly overdue (120K on the Densos I put in 6 years ago!) and that didn't fix it. So next the plug wires, also badly overdue (120k on the NGK's of 6 years ago as well). New NGK for both plugs n wires. Still no joy tho. OK, so I decide that 240K might be a LITTLE much to ask out of the OEM coil packs, so just replaced those yesterday, fully expecting the problem to be solved and good ol' Bessy to run and drive like her normal perky self. But NOPE. Still missing and chugging. I'm startin' to get aggravated here. What am I missing? MAF? Something else? She seems to run fine when stone cold, but as she warms up starts missing/chugging. Thoughts anyone?

I also tried using injector cleaner first off, just to eliminate the cheapest thing first.

 

Northstr99 are you getting CEL codes? I was getting P0300 & P0301. I did the same as you, plus pulled the throttle body, cleaned it, the Idle Air Control (IAC) valve and the MAF sensor. Cleaning those made a huge difference in performance. I also replaced the fuel filter, which added a little bit of power.
BUT, to fix the chugging and rough idle, I had to replace the fuel injector in Cylinder #1. Sounds like you may have the same problem, but I was advised to check cylinder compressions to be sure theyre getting fuel.

 

Thanks Zach. I haven't checked the codes yet, but I in tend to. Last I checked it was registering misfire on one of the plugs, don't recall which, #4 I think maybe. I don't have a code checker, so need to stop by O'Reilly I guess, they'll do it for free. It's not rocket surgery after all, right? I'll also tear into the MAF and T body and clean them up, and replace the fuel filter. Pardon my ignorance, but the MAF is located on the intake dwonstream of the air filter, yes? I've got an after-market K&N underhood cold air extension kit w a K&N filter, and what I am guessing is the MAF is located immediately after the filter.

 

Should be the same as mine, just off the air filter box. Unplug it, couple screws and be careful with it. Use a q-tip and rubbing alcohol to clean the little sensor.
Cleaning that stuff probably wont fix your problem but to get to Cyl#4, you have to take it off anyway so might as well. You'll feel the difference.

 

I have to say, Im Not a mechanic or expert, I have just done this recently as you can see from my posts above. If you take the IAC off the throttle body, be REALLY careful with the screws. 4 brass screws that strip REALLY EASILY!

 

I had the codes checked yesterday and it's coming back w P0304, Cylinder #4 Misfire Detected. I got a new fuel filter while I was there just for normal maintenance anyway, I know that won't fix it but it's due anyway. I'm suspecting the injector needs replacing, although I'm not sure why a bad injector would cause the ignition not to fire, unless it's smart enough to know the injector isn't working so it doesn't fire that cylinder? But then it would register the bad injector, would it not? It's making me nuts.

 

If the injectorbis clogged or the spring is sticking it will cause the misfire and throw the P0304. Mine did the exact same. New injector, No problems.

 

Ok well Im back with some more info on this issue. Idle is still rough. Still getting miss fire on cylinder two. I pulled the Throlltle body off and boy was that filthy. I had no idea it was that bad on the back side of the butterfly. Cleaned it up and noticed some small pin holes under the gunk. Possible they were plugging some vacume lines going to the egr. I put it all back together and cleared the codes. Ran it for a while and the code came back "Miss fire cyclinder 2" Put a new injector in cyclinder 2 and took it for a ride. Its sporatic really on the rough idle. It idles rough some times and then not otheres. Very strange. Anyways, reason I am posting again is because I did a compression test "again" because I was not sure what I was doing last time and I did it wrone. I didnt have the throttle open and I didnt wait till the neddle stopped, I stopped when it slowed down.

Anyways...
Cylinder 1 = 200psi,
Cylinder 2 = 170psi,
Cylinder 3 = 180psi,
Cylinder 4 = 200psi.

Plugs look very nice. If it were a ring issue, would the plugs be fouled from burning oild? Could this mean I have a valve issue on 2 and 3? FSM suggests not more that 14psi between cylinders ( if memory is correct). I have 20 to 30