It's NOT the O2 sensor. Hessitates, stumbles, poor gas milage, P0130, P0133, P0172

Hi Guys, longtime lurker first time poster.

I've had my 1st gen Tacoma over 15 years and I currently have 4 different 1 gen Taco's, 3 of them have the 3RZ.... so the good thing is I can, and do, swap known good working parts when I need to trouble shoot.

My ride is 1998 Tacoma 3RZ-FE. coils, manual, 4x4, xtra cab.
I swap parts with my:
1998 Tacoma 3RZ-FE, coils, manual, 4x4, base model (regular cab), and my
1995 Tacoma 3RZ-FE, distributor, manual, 4x4, regular cab.

I've got a problem I've been chasing down and it's been kicking my ass... without going into the whole long sorted story the key points are I got codes P0130, P0133, P0172. Symptoms are dog sh!t gas milage, gutless from a dead stop to the point it is dangerous and near undriveable, wide open on the freeway is fine, idle is fine, but give it gas in neutral from idle and is hessitates, stumbles and will often just die. I have to feather the throttle to get the RPM's up to take off but it's still gutless.

I've done everything, and I mean everything, read every thread online on every forum I can find, I've pulled the head, pulled the whole f*(&king engine, swapped the ECM, and everything in between. Yes, for the love of God, I replaced the O2 sensor, 3 times with known good working parts.

The good news is I have chased down the exact pinpoint problem, the bad news is I have absolutely NO IDEA what can be causing it... so here it is.

Upstream O2 sensor voltage is not fluctuating within the normal range per the FSM. Warm and at idle it usually just stays stable around 0.9v and doesn't fluctuate hardly at all... it "should" constantly fluctuate up from below 0.4v to above 0.55v in a normal wave pattern up, down and back again.

This very same sensor will act normal and give normal reading when I put it in the 98 base model, similarly 98 base model's upstream O2 sensor works great, and when I put it in to my ride, it gives the same crap voltage of ~0.9V without fluctuating.

Well, everything points to a bad O2 sensor, but it's not it, so I checked the Harness as suggested in the FSM. The ground line has great continuity, as does the HT1 line (Heat temp line) and OX1 line (sensor line). There are no shorts in the HT1 line or the OX1 line.

Back probing the HT1 pin at the ECM harness gives a normal voltage ~12.3v with key at the ON position and 0.4v at idle. FSM says "normal" at idle is under 3v. Also, I swapped the ECM with the 98 base model and the upstream O2 sensor voltage is still not fluctuating and just hovers around 0.9v

Fuel trims are-20% STFT and -24% LTFT.

When I pull the neg. battery terminal to reset the ECM, it runs great as it resets the Fuel trims to 0%, but with 30-40minutes of driving with the bad readings from the O2 sensor the fuel trims return to -20 / -24% and it starts running bad again.

One big improvement was I swapped out the fuel pressure regulator, and that damn near fixed the drivability issues, runs damn near normal, totally normal if I reset the ECM... but nothing, no matter what I do, changed the fact that the voltage readings of the upstream O2 sensor are not fluctuating (cycling) up and down normally and are just stuck way too high at ~0.9v.

So my question is... If it's NOT the O2 senors, the wire harness or the ECM... what on earth can be causing this!!!?!?!?

 

What are the voltages if you unplug the sensor?
Maybe you have bleed over somewhere along the circuit?

 

Welcome former lurker! For your coming out party this is a doozie of a post!

So... mainly wanted to welcome you, but I can add an anecdote for whatever its worth- I had an O2 sensor code in my 98 manual taco that didn't go away after replacing the sensor, and it turned out that the ECT was going bad, so new ECT (the one on the back of the block) finally solved it and at least in that engine the ECT was somehow a factor in the O2 sensor electronics and caused the bad readings.

 

Checked and replaced:
MAF,
Air filter,
PCV,
Coils,
Spark Plugs,
Spark Plug Wires,
ECT,
IAC,
TPS,
Throttle Body Assembly,
Fuel pump,
Feul pressure Regulator,
VSV,
all vaccum lines.

Checked and serviced:
Intake,
Throttle body,
intake manifold,
EGR cleaned, tested, reassembled,
Cylinder Head taken to shop and remanufactured within the last year,
Engine pulled, remanufactured and the shop and just got it back 2 weeks ago,
injectors serviced last year.

I'm seriously at my witts end... have NFI why the O2 sensor voltage is so far off in this truck.

 

The voltages are actually about the same if I pull the sensor... I found that out by accident. LOL
Bleed over somewhere along the circut? Continuity is good and there are no shorts.... none that I could find anyways, but I only tested between the sensor harness and the ECM harness.

 

Yes! good suggestion. The FSM (and others) say this is involved, but I swapped out the ECT in the front of the engine next to the coils but not the back one. I know that one is for reading temps and the other is for adjusting the fuel trim... seems weird AF that they would do it this way..... for some reason I thought the front ECT was the one that controls the trims, but I may be wrong... certainly wouldn't hurt to change that second sensor just in case.

 

Continuity is not a good check.
I'd rather load the circuit with a test light. That way we know it will carry a load.
BUT.....I'm not sure that's your issue.

It sounds like you have other issues if the voltages are the same unplugged.
We need to make a change, even if it's not "good".

Let me look at some diagrams and read about your current codes.
Let me see if I can come up with an idea.

 

Very much appreciated. I can tell you that everything I can check with the live data "seems" to be within normal limits EXCEPT FC IDL and FC TAU... they are off at idle, but the FSM says they are supposed to be ON at idle (warm idle).....

Fuel Cut at Idle and Fuel Cut at Tau (slightly open) are supposed to help stop a the mixture from being too rich, which would contribute to my fuel trims, but not sure if that would change my O2 sensor voltage.... Also, the 98 base model also says FC IDL and FC TAU are OFF so I'm not really sure if that make a difference.

 

You do make it to Closed Loop? Correct?

 

I'm still researching.
Do we have the ability to drive the O2 lean?

1. Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a lean mixture condition. Reattach the vacuum hose.
   

 

Yes... and it will eventually give a closed loop-fault.

I guess the ECT is the 2 prong plug on the back of the engine... I have not changed that. I only changed the Temp guage with 1 prong up front by the coils.

 

Roger that, I'm not home now, will stop at the store to get an ECT and when I get home I'll try this.

 

What does live data show show on the scanner for the ECT?
If the ECT is off, or bad.
It will cause a Rich condition.

 

You can get one. But I hate to see you throw more parts at it.
Live data should show the temps.

What are they cold and warm?

 

Live data shows the temps and they all look normal and respond normal it heats up and goes to a closed loop.

 

I think normal range is 174+ degrees.
CL might come in at a bit lower. But not much, IIRC.

 

Well this is very interesting. If I pull a vacuum hose a lot of times it just conks out but when I manage to keep it going it will often notice the voltage drop and certain times it'll start fluctuating up and down like normal but it's causing the fuel trim long-term to go to -38 I would have thought it would run more lean the short-term field trim does get more lean it went to about zero.

Sometimes if I pull a smaller vacuum line it doesn't notice it at all but if I induce a small leak after the throttle body I definitely get a voltage drop and then it even starts to cycle up and down like it's supposed to.

 

TnShooter, correct me if I am wrong as I cant find any documentation to the contray. The only "live" data readings are when it is in closed loop, up to that point they are all readings from the internal tables.

OP, are the replacement parts Factory toyota esp. the MAF ?

Do not hesitate to use generic OBDII as opposed to manufacturer specific (year make model) for data acquisition.

I had a buddy drop his ford truck off. 2 other shops could not figure it out. Similar situation where readings are all over the place, fuel pressures all over the place, she just didnt run "right" and ther wanst just one thing to point to. "Right" is a seat of the pants, behind the wheel type of diagnosis. I kept testing, and testing and testing having to make adpators and use 2 fuel pressure gages because it was bizarre. Is it a fuel supply issue or a fuel return issue. After I got the bed off the truck and the fuel module out it ALL became perfectly clear. My best explanation is that during high demand the fuel pump driver module would increase the pressure from the pump (this pump is capable of 100 psi) and this would seperate the pulsar/pressure buffer valve apart farther, the truck would not have power, it would slow down, spit sputter and not have any drive away power. I am just a guy with tools and a brain and I will admit that I am wrong lots of times in my inital diagnosis but I dont make the same mistake twice. The 2 other shops had guys with better scan tools and lots of patches on their shirts.

I am telling you this as an encouragment to keep checking, once you figure it out it will be obvious and I hope you will laugh at yourself.

 

The MAF and the O2 sensors are the Denso. Also the spark plugs and all the sensors associated with the throttle body are OEM.

 

Yes, you have to pull a vacuum line off AFTER the throttle body.

Something is causing a rich condition.

Are the injectors the correct ones?