My wife has a 99 4cyl 4runner. Upon start up it sometimes stumbles a bit and idles rough for a brief while before returning to normal. Seems to do this sometimes when both warm or cold. After the brief stumbling idle at start, the car otherwise runs just fine. What to check first? I was thinking it might be the O2 sensor? Would that make sense? I'd like to get a little input before I drop $120 on a part that is just a shot in the dark.

 

I wouldn't do an O2 sensor. Usually when they fail, they will throw a code.

I'd check the idle air control valve. Make sure your coolant is at proper capacity.

Is the check engine light on at all? Have you done a tune up lately? Does it happen everytime? If it happens everytime I might try looking for a leaky injector, but that is pretty uncommon.

 

No CEL. And from what I have read, an O2 sensor heading south doesn't always throw a code. As of now, it only does this intermittantly. And even when it does happen, it only does so for a couple seconds before stabilizing and running fine.

 

my 3rz does almost the same thing, it usually does it when it's cold, it hits 750 rpm's, stumbles to about 450 for about 2 seconds, then picks up to about 950-1100 rpm's. did some active tests at work (i work at a toyota Dealrship, yippie for me) and my IAC valve checked out fine. from what i was reading on TIS (Toyota Information Systems, the most awesome toyota employee info bank) there is a possibility it could be a fuel pump, but i'm going to run that idea by my supervising master tech.

 

sounds like it could be a fuel pump like 98 3rz said or maybe fuel filter. try running a can of seafoam through your fuel! and idk if your maff sensor (mass air flow) is the same as the 3.4 v6 but try cleaning it. that may help a ton !

 

I have the EXACT same problem as well. It has persisted through a top end rebuild, new 02 sensors, fuel filter, TPS, IAC, injectors, etc. These weren't aimed to fix that problem but they didnt help it either.

Id be interested to see what the tech says about the fuel pump.

 

Clean your MAF, it's free!

 

Also you can check the heater circuit of your O2 sensor with a multimeter for correct resistance.

How old is the O2 sensor, and what brand? Make sure you stick with OEM style Denso O2 sensor if you go that way. I have had O2 sensors go out but they will make the car run bad not just at startup.

 

i found the answer for me at least, make sure your running the toyota specific denso plugs. it made all the difference

 

The plugs are new NGK. I doubt that is the problem. I'm leaning towards a fuel pump or an 02 sensor, but like I said it runs perfect after the rough start. Wouldn't these issues show themselves while running?

Where is my MAF and what is the best way to clean it?

 

The plugs are NGK dual electrode correct?

I dont think a fuel pump would just act up like that in the beginning either though. Its possible obviously but I'd spend my time and energy somewhere else.

Like I said earlier, I would probably check the Idle Air Control valve first. It usually effects warm startups only, but it might be worth to check out. All you do is remove it from the throttle body and clean it. While the throttle body is off, id clean that too.

Is the starter OEM?

 

toyota's are very finikey when it comes to sparkplugs, the one's i just replaced were NGK iridium plugs that i had just put in 10,000 miles ago. go to the toyota dealership, and get some toyota spark plugs, if it doesn't work, you got anopther set for the next time you change them. oh, the 3RZ-FE doesn't use dual electrode plugs like the 3.4

 

I've said before, and I'll say it again, always use the right tool for the job. In this case the right tool is an ODBII computer. It's like taking temperature and blood pressure of someone who complains of feeling 'sick'. You do that first.

 

i couldn't agree more with that, i didn't find anything when i did that at work with a Toyota Techstream, but that doesn't mean that you won't find anything. as for the spark plugs, i'll put it this way... lets say that the stock toyota denso plugs "range" of performance on a scale of 1 to 100 is between 70 and 100, those ngk plugs of 50 to 75, so how often is that ngk plug in the range of performance of the stock denso plug? not very often, which leads to a rough idle, and poor performance. thats the way my master tech who i've been apprenticing under explained it to me. he's been working for toyota for 26 years, so i tend to listen to what he has to say. he told me to change my plugs even though i only had 10,000 miles on my NGK Iridium plugs, i did and my rough idle disappeared. the truck runs 1,000 times better, and all i did was change the plugs and put a can of seafoam in the gas tank. so just try changing your plugs to the stock denso plugs. it will be the best 20 bucks you spend on your 3RZ-FE

 

I have exactly the same problem on my 1999 Limited 4x4 w/170K (CA Emissions). I recently had the plugs and cables replaced at the dealer. I hope they used the correct parts. It ran fine after that.

It threw a P0420 a few weeks ago and I ordered a rear O2 sensor and cleared the code. I haven't installed the O2 sensor yet. Was sort of curious if the CEL would reoccur.

Since then it began occasionally running roughly at start - like it's missing. It does this whether warm or cold, probably about 1 in 4 times. A quick rev to about 3K clears it right up every time and it runs fine. Other than this, it runs great.

I'll put in the Denso O2 sensor I have since received when I get the time to see if it clears it up.

Any other thoughts?

(Since ditching the Passat for a Camry a couple weeks ago, I'm all Toyota - 1988 Hilux 4x4 V6 5spd, 1999 4Runner, 2005 Camry XLE)

 

Did you check your air filter? Mass air flow sensor dirty due to dirty air filter. Spark plug wires?

 

ok, y'all can point the finger and say i told you so, the new spark plugs didn't fix it, BUT it did improve it a little bit. i was talking to a buddy of mine at work, and even though it's not a toyota. his GMC sonoma does the same thing, he told me its one of two things. 1. a bad fuel pressure regulator, which is highly unlikely cause then it would run like crap ALL the time. 2. bad drainback valve, now give this a shot, before you start it, prime it 3 to 4 times (turn the ignition on BUT DO NOT START IT, for those who don't know what that is) then start it, it should not sputter or run rough when you start it. post back with your results cause i'm curious to see if it works for the rest of you

 

I haven't done anything to the truck (1999 Limited 5VZ-FE) since posting last. Last night it misfired long enough to start blinking the CEL.

I have a P0300 (misc misfire) and P0303 (Cyl 3 misfire). This is coupled with the P0420 that I received about a month ago (there was no misfiring detected on the trip on which it came on) that I have cleared to see if it reoccurs.

Plugs and wires were replaced last time I had it at the dealer about 7000 miles ago.

It has a K&N air filter. It's clean.

I cleaned the MAF last time I cleaned and oiled the K&N.

 

I currently have similar issues with my 91' 22RE, like SDHNTR and Bang are having. I've had this issue for years now, even after a new cylinder head from ENGBLDR last year. It has gotten worse over time, now to the point where the idle drops super low, like into the 400-500 rpm range, on the verge of dying. Revving the engine a few times usually helps to smooth things out. Also on rare occasions on a hot or warm start the engine doesn't start right away. Other than this my engine runs like it's new...and assuming everything is working like it should I have recently suspected one part that could be causing this. At least for me I'm thinking my EGR valve is not closing properly like it should (carbon build up in the valve). An EGR should be closed during start up and idle. If it's open and not seated close this will mimic an instant vacuum leak, which in turn would cause a rough idle and lower erratic rpms. I wish I had a known good one to throw on my truck to see if mine is at fault but I'll have to wait for now. I'll do a write up when I figure my problem out, EGR or not. Check your EGR and also everything in the system, modulator, BVSV or VSV, etc. It doesn't hurt to check.

http://www.autoshop101.com/forms/h61.pdf

 

I've got the same issues as you, BANG, exactly... have you found out what's wrong???