'98 3.4L, 5spd ~121k and 1 super novice wrencher with little know-how and a shallow wallet.

Okay, so after searching and reading through various threads on here, I'm inclined to think my problem lies in the ignition system, but I really can't say for sure, so I'm seeking some diag help from fellow, knowledgeable members

I recently began experiencing a stuttering acceleration under full throttle, always under 2k rpms. 2k rpms - 3k rpms is very smooth and I don't go much past 3k before shifting, anyway. If I apply very light pressure to the throttle from a complete stop, my 4r will accelerate smoothly through the entire range.

Before posting this, I've replaced (using OEM parts): plugs, wires, PCV, fuel filter, cleaned MAF (with CRC MAF cleaner, handsfree, no qtips etc.) and cleaned throttle body. All of these services definitely helped to smooth out the acceleration a bit, but the primary stuttering under 2k rpms still exists, no CEL's have been present, whatsoever since I've owned the truck (sept 09).

SO, that being said, I have a few questions and I'm looking for guidance to the "next step".

1. Is it possible to determine if one+ coil packs are bad from a visual standpoint? I noticed a dark, reddish discoloration on the the boot(?) when changing the passengers middle sparkplug. I did not see this on any of the other boots/packs and/or plugs.
pics aren't great, sorry.



also, white-ish, yellowish discoloration where the coil was pulled from.



2. Is it possible that the MAF is bad and not throwing a CEL?

3. What should my next course of action be? From what I've gathered in my reading, it could be any number of things, possibly even injectors? It seems that many instances sharing my symptoms will trigger a CEL, but I haven't thrown one yet, so I dont' know what my next step should be.

Also, big thanks to everyone here and 4runners.org for saving me lots of cash and enabling me to learn!

Beuller? Beuller? Anyone home?

I'm making progress (by process of elimination, I suppose), but I'm still unable to pinpoint the exact problem. Sputtering under 2k rpms, doesn't change if my 4R is warmed up or not. All hoses are tight, don't appear to be leaking or cracked in any location. I haven't tested them with starting fluid yet, but that might be my next step. So far... no CEL, no dice, so any input is appreciated and welcomed.

So far, I've tested the resistance on my wires, coils, CPS, MAF and TPS (I think)



actual photo below to explain my discrepancies




Sooo, What should I be looking to do next? I'm a little confused, any insight would help. I know that testing injectors may be in the near future, but I"m not sure if I I should examine anything else prior to doing that.

Is the OBDII blowing any codes?

Have you checked the catalytic converter?

Is the engine only bogging down under load, or does it complain if you hit the gas out of gear as well?


These OBDII systems are pretty sensitive. If there is something wrong, chances are the computer knows about it and is telling you... trick is to discern what the codes mean.

I would think a coil problem would be constant throughout the rpm range.

The other possability is that the computer is in "limp" mode. Characteristics of this include low power and crappy mpgs.

If there are injector problems, there will likely be a ticking noise from the top of the engine...

My bet is on the throttle position sensor or the catalytic converter.

Hope this helps,

Good luck

Is the OBDII blowing any codes?
nothing so far....

Have you checked the catalytic converter?
Not yet, but I've added it to my list for tomorrow... I'll let you know what I find.

I would think a coil problem would be constant throughout the rpm range.
With even pressure on the accelerator, it feels like it's stumbling thru the range. It'll be 100%, then 80% for a split second, then back to 100%, so on and so forth.. It doesn't feel like it's only consistently giving me 80% of the power.

The other possability is that the computer is in "limp" mode. Characteristics of this include low power and crappy mpgs. MPG's don't seem to have changed, but I'm definitely not getting the consistent power that I was. It seems like one day it was fine, and the next day it started showing these symptoms. Wouldn't the cat be something that I would notice over time, or is that something that can just "go out" on you without warning? Also, if the computer is in limp mode, would I see a CEL or no?

My bet is on the throttle position sensor or the catalytic converter.
Hopefully, I will know soon enough. Thankfully the TPS isn't terribly expensive .

I appreciate you taking the time to respond. If you're ever in the neighborhood, I'll buy you a beer or two.

I probably should have posted this in the 3rd gen section, shoulda thought of that before posting, but no harm done.

Good Evening, I am having the same problem you have described with my 2000 4 runner. This issue as well as I had an issue with my radiator. It filled with transmission fluid and the transmission was filled with radiator fluid. That was an expensive repair. Not so much for the new radiator but the transmission flushes add up when ya do 4 of them.


I am a mechanics daughter and my first though was the catalytic converter. I am planning on the complete tune up since it hasn't had one and I'm reaching 120,000 miles. I am going to try some of the things you suggested and see if that works for me.

Bikermom, Welcome aboard!

Bump for denverunner.

Any word?

This is the thread I've been looking for. Got a '98 Limited here approaching 179k, and other than the hesitation, still running STRONG!! I'm by no means anywhere even NEAR novice mechanic!

Reading through a prior thread, I found lots of discussion on the Motorvac service, and from what I gathered is a high-tech cleaning system. Just completed these two weekends ago thinking it was dirty fuel injectors. The truck still runs smooth, but day before yesterday at around 55mph, I tried accelerating to pass up a slow-poke, and guess what- she puttered.

I think at this point, the catalytic converter will be the next project, but any other suggestions/feedback is most welcomed!! Thanks, T4R!

I think I'm having the same problem with my '02

What was the verdict on yours?

Old thread, but I figured I'd offer my 2 cents for anyone else dealing with this issue. I'd be cruising down the level highway around 60-65mph in my 1999, manual Runner and the vehicle would stumble for a few seconds and then be back to normal. It seemed to happen mostly between 1800 and 2300 rpm under no specific load condition (I could be going uphill, level, downhill, or towing). I got no check engine light and the problem was getting progressively worse.

I did the typical, change plugs, wires, fuel filter, air filter and checked for any vacuum leaks, but none of that helped. Ran a bottle of seafoam through the gas tank in case I had bad gas/dirty injectors. Still did nothing to help.

I thought it could have been a faulty crankshaft position sensor which could potentially explain why I had no CEL. So, I unplugged one of the fuel injectors and started the car to make sure that the sensor would pick up the misfire. Sure enough, it caught the misfire. That ruled out any of the injectors, spark plugs, and coils in my opinion because it showed that the ECU was still capable of picking up a misfire.

I thought it could have been a fuel pressure issue, so I replaced a fuel rail banjo bolt with one that had a 1/8 NPT thread and hooked up a digital fuel pressure gauge. Went for a test drive, and the fuel pressure remained around 45psi over the whole drive, even while the vehicle would have the random stumble.

I checked resistance of fuel pump, and it seemed mostly normal (a little out of range from service manual, but not all that bad in my opinion).

I started thinking it could have been the TPS (throttle position sensor). To check, I unplugged the wire and hooked up an ohm meter to the bottom two connectors on the sensor to check the resistance (middle wire is the 5 volt supply, bottom is the signal wire, top is the ground). Then while watching the ohm meter, I found that the ohms danced around quite a bit while the throttle body was about half open. The ohms should have been linear as I rotated the throttle body. I then plugged the connector back in and back-probed the bottom two wires using some sewing needles. I then hooked alligator clips to the wires. I used a 12 foot roll of phone line cause it was only like 5$ at HD. It was 24 gauge wire, so a little on the small end. I then ran that wire under the hood and through my window and connected it to my volt meter. At idle, it measured around 4.6 volts. I then went for a test drive to watch the volts change. As the throttle body opened, the volts dropped pretty linearly (something like 2.7 volts at 65 mph). Sure enough, when the vehicle start bucking and hesitating the volts were dancing all around. That pretty much confirmed it was the TPS.

Went to the dealership to pick up a new one, installed it, and sure enough, car runs flawlessly.