For all you who's 4runners are getting close to 200K, I thought I'd share this and maybe some of you can benefit from my experiences... So, for the last year or so, my engine has had a miss on cyl.#3 (the CEL came on several times and it was always P0303 - never another cyl.). Every morning it stumbles and sputters and then evens out after I rev the motor a few times. My mpgs went from about 250/tank to 150/tank and my 3.4 felt almost as bad as a 22R powerwise.

Over the last year, I ruled everthing out that it could possibly be except valves and/or injector(s). I cleaned/tested MAF, TB, IAC, PCV, and intake. I replaced the front 02, coolant temp sensor, TPS, knock sensors, plugs, wires, coil boots, timing belt, water pump, and battery. I swapped coil packs to see if the miss would move (which it stayed on cyl.#3), ran seafoam twice in the intake, gas tank, and crankcase. I checked every vacuum line by eye and with propane, and I also checked and cleaned every electrical connecter I could find. And, after all that, it did nothing to help my problem (thank God I can work on my own truck because that would have been expensive for all that labor)! Even though it didn't fix my truck, I guess most of that stuff is good maintenance anyways and I probably staved off a few future problems too, so I have no regrets.

But, the problem persisted and the truck still ran like crap so I bit the bullet and took it to a local shop that has a really good diagnostician. We discussed everything that I had done prior so we could rule out what is definitely NOT causing the problem and he suggested we put a transducer on the engine when it's cold in the morning (when it misses the worst). We did and saw that the wave form on the computer screen was way off normal while it was missing and after revving the motor a few times, the wave would start to even out and look normal - right as the miss would fade away.

We could see from the wave that the valves were not closing/opening all the way until after revving the engine, and even then the wave would only get close to normal - never perfect. We also decided that if one or more injectors was be gunked up, the ECU may be overcompensating with the other injectors and causing a rich mixture which would also explain my pitiful mpgs.

So the choice at that point was an expensive valve job and an injector cleaning/flow test, or I could just play it safe and do a complete engine rebuild (especially since I found a local shop that would do it for $1500 OTD w/ a 3yr./36K P&L warranty). The tech also suggested a 3rd option which was to hook the engine up to a machine and do an extensive carbon cleaning on the valvetrain, fuel injectors, piston heads, etc.. They bypass the fuel tank and run different chemicals through at different time intervals and after about 2 hours, the engine's clean.

They said it would cost about $250 and I figured it was worth a shot since dirty valves/injectors were the culprit and this should clean them pretty good - at least enough to notice a difference. I brought it in yesterday and at the end they told me they did the cleaning 3xs longer than normal and they wanted to put it back on the computer in the morning when the engine was cold and most likely to miss so I left it there overnight.

I'm happy to say that the wave form on the computer was perfect from the first second and my truck hasn't ran this good since I bought it in '01! On the way back to work I could tell it has way more power now too! I'll have to see over the next few days if my mpgs improve, but I'm feeling pretty optimistic about it and the best part is they only charged me $167 (I think he felt bad for me that I've been dealing with this for so long)! I was ready to rip the engine out and do a full rebuild and now I may have fixed it for $167.

Sorry for the essay, but I hope this post might help some of you with higher mileage 4runners out there and if you're considering doing a valve/fuel injector/intake/combustion chamber cleaning, I can tell you it works!

Congrats.

haha ahh god my head hurts from all that reading!

That was full of good info! Im kinda on the same path now that I know the results of your engine cleaning i just might have to do this. So did you ever get hesitation off the line or an instance where you would gas it and it would go then hesitate then get on it?

Haha, I know.. It hurt my brain to write it too, but I figure if it can help someone and save them time and $$ then it's worth it. And yes, mine randomly hesitated a lot, especially when going uphill.

Not that I'm one to push this type of service but maybe a good Seafoaming may have fixed it just as well?

Did seafoam twice in 6 months.

No seafoam has nothing against a good machine based cleaning.

Its like the Diet Coke of engine cleaning.

He did seafoam it. Congrats Brian! Hope it stays running strong! Did they have a name for the service they did? Just carbon cleaning? Good luck.

You know, I'm sure they do have a name and in all the talking about it with the tech, I forgot to ask what it was called. He just explained what it did. I will find out and post the name.

Very good info. Just wondering, did you ever use fuel system, fuel injector, or intake cleaners? And are you particular about the brand of fuel you use? I ask because I'm very particular with these things and wonder if they would even matter in the long run. Plus Shell and BP like to advertise that with their fuels you can "drive your engine clean"

Is there a name for this machine engine cleaning?

My truck is getting up there in miles and could use a good de-gunk.

Besides the Seafoam, I tried some STP "pour in tank" injector cleaner months ago and noticed no improvement.

Well, I pretty much go to the local Valero, Shell, or Chevron, depending on who is cheapest at the time. None seemed to be better for my performance than the other. Techron (Chevron) is supposed to have cleaning agents in it, but apparently not enough to keep mine clean.

Alright, so I talked to the tech and he said the machine is called a Motorvac:
MotorVac

Was this a chain shop you went to that had this machine, like a Pep Boys or something? Or was it an independent mechanics shop?

It's an indy shop.