Rough idle question

I have been chasing a gremlin for a while
No mechanic but bear with me
5VZFE M/T 4x4
Cranks fine but until it warms up (closed loop I guess) sometimes (not all the time) it seems to hit a wall at about 3000RPM. Like you pinched the gas or the air off. If you shift up and let rpms drop it clears
Once it is warm I don't notice the stall or whatever you want to call it but when at idle, stoplight, line at the drive thru, etc it cycles in and out of the rough idle
Truck runs very good for 306K on the odometer
I have looked for vacuum leaks, questions fuel pump/tank filter, and all kinds of things
NO CEL
I bought one of the "fixit" gizmos that is like a OBD scanner (cheap one) and this evening when I got home I plugged it in and started looking at various readouts while truck was cycling in and out of the rough idle.
What I noticed is that when the roughness started the O2 Sensor 1 voltage started to drop until at zero and as the O2 voltage started back up the idling was cycling back to normal
My question is whether the O2 sensor is the culprit or something else is affecting the O2 level that I need to be looking for
Already looked at procedure for changing O2 #1 and it would be the complicated one, have to go in through the shifter opening to get it swapped

Opinions or advice welcome and appreciated

 

Unless we're talking about some weird variant of the 1999 v6 I've never heard of (I suppose an M/T 4x4 could be piped different I guess), there's no reason to need to go through the shifter to change an O2 sensor. They're both accessible from beneath the truck. But yes, that sounds like exactly what you need to do. Just make sure it's the right bank (there are two).

 

The connector is connected to top of transmission

If there is an easier way to get to it I am all ears

Also from what I saw there are 2 O2 sensors, one in front of catalytic converter and one past it

 

Ahhhh.... ok Ill go take a pic lol

 

Passenger side of transmission. Is yours different?

 

Yes, does not look like that

Is that pic of auto trans?

I will try and get a pic tomorrow, wire from the one I am talking about goes up over top and connector is attached to top of manual trans
The back one is accessible

 

Ya mine is auto. If manual is on top that sucks, but not the end of the world. The center console come out pretty easy

 

Your O2 sensor (bank 1 sensor 1) is an AFR sensor. On your scan tool you're looking for as close to 3.3V as possible or Equivalence Ratio = 1. If it's going to zero that means it's experiencing a RICH condition. An AFR sensor does not switch back and forth like a traditional O2. It will fluctuate a little but not like an upstream O2's switching behavior. In order to see if the AFR is lying to you and therefore defective you need to drive it lean and rich and see if it responds accordingly.

*Pull your brake booster hose and induce a LEAN condition. Watch the scan tool. AFR reading and EQR should go UP.
*Add propane into the intake to induce a RICH condition. Scan tool should show AFR voltage and EQR going DOWN. If your scan tool is quick enough, you could feather the throttle and watch the AFR response as well.

If that checks out your AFR is good. If the heater circuit was bad it would throw a code.

Given your symptoms my bet would be against a bad AFR sensor. When they die they would generally be unresponsive. I'd be interested in seeing your AFR readings when you hit the wall at 3000 rpms as well as TPS and MAF readings.

Edit: Re-reading your original post I would also check out the IAC. It could be a lot of things. You can at least rule out the AFR with the method above.

 

I can certainly try pulling the booster line
Are we talking about propane from an unlit little torch? Or how do you "introduce propane"?

You are definitely on something with the voltage/equivalence ratio. Went back and looked and the high number is 1

May pull the IAC and clean or replace it depending on cost of a new one just to see what happens

 

99 3.4L use the same O2 sensors up and downstream.
A/f sensors were introduced on 3.4L in 2000 on California emissions equipped only. Always check hood emissions sticker.
2001 and up 3.4L use A/F and O2

 

@BTLowry , I remember the saga from a while ago that involved changing injectors...could you remind us of what brand you got, and how many you replaced?

 

Oh, great info to know - thanks!

 

BTLowry, let me revise what I said in that last post since we were just told by BlackDog this year Tacoma has two traditional 02 sensors and not an AFR at the front. For the record, if you're not sure, just look at the scan tool and if you see an 'equivalance ratio' or AFR or some other terminology you have an AFR.

Your traditional O2 UPSTREAM (pre-cat) sensor should be oscillating between about .10 to .80 mv. The testing will be the same but the readings will be the opposite. Higher readings are RICH, lower readings are LEAN.

As far as how to inject propane into the vehicle - yes UNLIT torch (haha - unless you don't like your vehicle) if you have one. You could also use any flammable fuel source but propane is deemed by many to be the safer way to go. But if you use a spray of some sort I would avoid getting any on the MAF sensor and don't go crazy with it. You're just trying to induce a rich condition - you could also just feather the throttle like I said to check the response but introducing a fuel source will peg it rich for a few seconds. This is where a graphing ability on your scan tool is really nice if it has one.

 

I will have to look and see what brand but I replaced all 6 at the same time. I know it was some company in southern California because I was pissed I paid for 2 day shipping and fudex screwed me (some hub problem out there that was known)
Funny thing is the truck runs as well as could be expected other than this idling issue and pre operating temp stall/hestitation/whatever it is called. It will easily run 80mph on the highway and does not show any signs that it is lacking power

 

Motorwest Denso injectors

https://motorwestperformance.com/product/upgrade-for-23250-62040/

 

On older vehicles, If you suspect an 02 sensor is causing a problem simply unplug it and see if it helps. The engine can run without it

 

Those are highly regarded, and unlikely to be involved in the present issue.

I was going to suggest something similar, but I thought that this would just keep it in open loop, taking other things, like the MAF sensor, out of the equation as well. All this would indicate is that the truck runs better in open loop, no? Since all of these sensors rely on reference voltage, I would, as a precaution, make sure that all harness connectors, battery connections and grounds are clean of corrosion and firmly connected.

 

Does IAC do anything after engine at operating temp/ in open loop?
What does coolant running through it affect?

 

Just got done cleaning IAC and TB. Neither were what I would consider dirty enough to be culprits but about to run some errands and see.

I did find the vacuum hose that goes from top of intake to an actuator on back side of TB was fairly easy to slide off. Will replace that if idle problem still exists.

 

tough to diagnose over the internet. Check the spark plugs and cables first. if those are OK and from you description I am thinking that your fuel pump and or fuel filter are suspect. The filter is the easy part to check, take it off, drain the gas and blow through it. If the fuel pump will not flow enough volume at pressure, 44 psi ish, then you are wasting your time and money on parts.