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Old 08-02-2013, 07:03 PM   #1
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P0171 code

I posted this in the 1st generation forum but no responses, so I thought I'd try here:

Ahem:

I know this subject has been discussed ad nauseum, but my 2000 Taco (V6, auto, 4wd, ~170k miles) the much dreaded P0171 code.

It is always P0171, I have never seen any other code. This started about 6 weeks ago.

So far, I have changed the air filter and the fuel filter, cleaned the MAF and ran about 5 bottles of fuel injector cleaner through the system.

I was watching the voltages on the O2 sensors using Torque (pretty cool app, by the way) and the #2 O2 sensor just intermittently flat-lines. It will drop to about 0.1 volts for about 5 seconds, and then it comes alive again. While it is dead, I can see the voltage on the #1 O2 sensor start to climb. When it hits about 0.8 volts, the code is thrown.

The #2 O2 sensor is the after the Cat, correct?

I don't mind spending the $60 on a new sensor and installing it if that is really the fault, but in reading the other posts on these codes, it seems like it is more of a hit or miss sort of thing. Can anyone recommend further troubleshooting to give me more confidence that the sensor is really bad?

I'd like to pin this down quickly because in early September, the truck is due for the emission test and if they see the code during the check, then I have to have the repair performed by an "emission certified" repair facility. I don't even want to guess how much they will soak me for this.

Thank you in advance.
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Old 08-02-2013, 07:15 PM   #2
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It could be the sensor, or the sensor could be doing exactly what it is supposed to do and you might have a problem with the cat.

The O2 sensors don't actually sense O2... they are more thermocouples than anything.
Your #2 is going "cold" and the #1 is indicating an increase.
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Old 08-04-2013, 11:33 AM   #3
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Well, that is what I was thing too, and if I were in a non-emission state, I would just replace the sensor. The thing is, I can save the receipts and get a waiver for the emission check is I spend a certian amount on the fault. The repairs have to be performed by an emission certified shop (no DIY).

I forget what the actual amount is, but I do know a real Toyota Cat will easily surpass the amount.

Anyway, I can see how the sensor could become intermittent, but as far as I know, the Cat is just a metal mesh, I can see it getting weak, but not intermittent on this level. Of course, maybe I just don't know who it works either.

Is there a way to determine if it is the cat or the sensor?
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Old 08-24-2013, 12:47 PM   #4
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I have the same code coming up every once and a while. P0171 CEL will pop on whenever the truck is run at high rpms(3k and higher) or when it gets bogged down. I also notice the smell of fuel when i come to a stop. Im thinking maybe a bad cat or 02 sensor...
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Old 08-25-2013, 04:56 PM   #5
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The sensor located after the cat has one sole purpose in life. That purpose is to monitor the output of the exhaust gases exiting the catalytic converter. Nothing else. A lean code is an indication that the A/F sensor is detecting either a fault in the exhaust stream AHEAD of the cat, or that the sensor itself is having a problem. One simple way of testing the response of the A/F sensor is to watch the signal and alternate between forcing a rich condition, and forcing a lean condition.

To force a rich condition, disconnect the vacuum line from the fuel pressure regulator and plug the hose. This increases the fuel pressure at idle, which will shop up as perhaps a 2-5 point change towards the negative in your short term, and eventually your long term fuel trim numbers. By running this test you can check to see if the sensor itself is sending a correct response.

To force a lean condition disconnect one of the SMALL vacuum hoses and allow it to draw in a small amount of fresh air. Since this fresh air is NOT being measured by the MAF sensor, it is considered unregulated air, and will result in a 2-5 percent change towards the positive in your short term, and eventually long term fuel trim numbers.


This can be used to test for proper response of the A/F sensor, and proper response of the computer. If these tests both show an even response, then most likely your problem will fall into the following most common problems:
Vacuum leak
weak fuel pump
dirty fuel filter
sticking PCV valve
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Old 08-26-2013, 06:29 AM   #6
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There is no vacuum modulation of fuel pressure on a 2000 v6. The little hose on the FPR is a vent on the atmospheric side of the FPR regulator diaphragm.
Don't believe it? Follow the hose back.
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Old 08-26-2013, 09:06 AM   #7
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Quote:
Originally Posted by Dirty Pool View Post
There is no vacuum modulation of fuel pressure on a 2000 v6. The little hose on the FPR is a vent on the atmospheric side of the FPR regulator diaphragm.
Don't believe it? Follow the hose back.
Procedure below is how to test the fuel pressure regulator for proper operation. As you can see with the hose disconnected, pressure is higher, and when connected pressure is lower. Not sure where you think this vacuum hose goes to atmosphere. Connects into the intake stream if it is connected properly.
Quote:
Taken directly from Toyota Service Manual
.........
n. Disconnect the vacuum sensing hose from the fuel pressure regulator, and plug the hose end.
o. Measure the fuel pressure at idle.
Fuel pressure: 265 - 304 kPa (2.7 - 3.1 kg/Sq.cm, 38 - 44 psi)

p. Reconnect the vacuum sensing hose to the fuel pressure regulator.
q. Measure the fuel pressure at idle. Fuel
pressure: 226 - 265 kPa (2.3 - 2.7 kg/Sq.cm, 33 - 38 psi)
If pressure is not as specified, check the vacuum sensing hose and fuel pressure regulator and fuel pressure regulator.
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Old 08-26-2013, 10:43 AM   #8
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Quote:
Originally Posted by BamaToy1997 View Post
Procedure below is how to test the fuel pressure regulator for proper operation. As you can see with the hose disconnected, pressure is higher, and when connected pressure is lower. Not sure where you think this vacuum hose goes to atmosphere. Connects into the intake stream if it is connected properly.
I kinda figured you would say that . The FSM is 100% wrong (misprint). I think the page is an artifact from the very very early V6's. They had the feature to aid hot restarts. The ECM controlled a VSV that controlled vacuum to the FPR. This feature was dropped very early on.
In all the rest of the "bone stock, unmodified, just the way the factory built it" systems the "vacuum" line goes to a port on the resonator box upstream of the TB and will see no vacuum at all. Gadget recognized this as early as 99.
Since you took the time to post up the page from the FSM and apparently do not have a V6 handy to check for yourself, I will go take a pic.

Here is a Toyota diagram of the early system (red arrow).
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Old 08-26-2013, 11:07 AM   #9
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From the yellow arrow the line transitions to a steel tube, (just under the throttle cable mount) goes between the folded intake runners then transitions back to rubber and plugs into the FPR. The camera will not fit in there. EDIT, got the little camera in there.

Resonator box



Steel line under runners with rubber 90 deg bend at the end.



Same rubber 90 entering FPR.
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Old 08-26-2013, 11:12 AM   #10
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OP

On the surface, without access to all your data, you could actually be running lean for any number of reasons, from fuel supply/delivery to vacuum or a "perceived" lean condition from an exhaust leak. The latter should be easy to check.
In an actual lean condition the ECM will generate richer fuel trim to correct the issue. It looks as if your #1 sensor voltage is increasing as it should if a lean condition is present. The ECM responds to that input by generating fuel trim to the rich side. If it can not compensate enough, the lean code will be set. In other words the engine will still be running lean despite all the ECM can do by going rich.

Actual lean conditions can be any number of things, from fuel supply/delivery to a vacuum leak. Plumbing a fuel pressure gauge into the system can be a pain it the but so this is just a thought. Tacoma fuel pumps start to fail with more regularity at around 175K. Replacing that pump just as PM might not be a bad idea even it it is not the culprit. Some fail slow, others just quit. Yet another thing, running cleaner in ratios like 1 or 2 cans per tank of gas is more of a system maintenance thing as opposed to a cure for dirty injectors. It takes a much higher concentration of cleaner to do that, like >30%. I don't know how you did the cleaner, just saying don't assume they are good.

The initial diagnostic FSM page below is from an 03 but it is the same as my 99 paper one. The further testing procedures differ substantially between the 2 years.
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Old 08-26-2013, 11:37 AM   #11
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Not to thread jack, but i seem to be in the same boat...

Little history on my 96 3.4l V6

-P0171 code first came on around 187,000 miles
-Replaced Bank #1 & #2 02 sensors and MAF sensor
-Code went away
-Came back on around 200,000-210,000 miles
-Replaced fuel filter and fuel pump

It only comes on when engine is run at high rpm 3k and higher or when im at high speed 5th gear at 1,700 rpm or lower struggling up a hill. Goes away overnight.

Current miles: 250,000
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Old 08-26-2013, 11:41 AM   #12
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just a shot in the dark, you've probably already check your exhaust and gasket for leak. I had that code before but after fixing the gasket post cat and before the downstream sensor I don't get that error code anymore, its been about 6K KM now. Btw where can you get o2 sensor for 1st gen taco for 60$? you're not going to put the cheapest sensor from ebay right? When I was looking for denso or ngk it was around 270$ precat and 120$ post cat.
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Old 08-26-2013, 11:43 AM   #13
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You know i havent even thought about an exhaust leak. Whats the best way to test that? C0 tester?

My 02 sensors were NGK
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Old 08-26-2013, 12:08 PM   #14
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I didn't know my exhaust gasket was leaking either...I had to replace my muffler because it was rusted out and the guy that own the shop was good and he replaced the gasket too. He just looked at it he didn't use any tool and told me he'll need to replace the gasket too. Everything is sunshine now.
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Old 08-26-2013, 07:30 PM   #15
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Quote:
Originally Posted by Dirty Pool View Post
From the yellow arrow the line transitions to a steel tube, (just under the throttle cable mount) goes between the folded intake runners then transitions back to rubber and plugs into the FPR. The camera will not fit in there. EDIT, got the little camera in there.

Resonator box



Steel line under runners with rubber 90 deg bend at the end.



Same rubber 90 entering FPR.
I won't try to argue with you on this one, but I have checked fuel pressure on the V6 3400 engines before using this method, and the pressures change when you disconnect the vacuum line as designated. Done it several times. Venturi vacuum works.
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Old 08-27-2013, 06:42 AM   #16
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Quote:
Originally Posted by BamaToy1997 View Post
I won't try to argue with you on this one, but I have checked fuel pressure on the V6 3400 engines before using this method, and the pressures change when you disconnect the vacuum line as designated. Done it several times. Venturi vacuum works.
Interesting "claim" with interesting implications.

Any vacuum you think is present upstream of the TB would INCREASE with engine speed/throttle opening, the opposite of vacuum behavior downstream.
This would lower fuel pressure at higher speeds, hmmmm.

Toyota does have a procedure to temporarily raise fuel pressure at idle but it does not involve vacuum.

Anyone who want's proof for themselves and if you think my gauge (pic below) is not sensitive enough just touch your tongue to the hose from the resonator box. You will find zero vacuum, nada, zip.


EDIT
Here's a quote from Gadget's http://www.gadgetonline.com/Fuel.htm personal web site.

"The stock connection is to the main air induction tube which is really nothing more then a vent and gives a constant 43 PSI all the time."
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Old 08-28-2013, 06:14 AM   #17
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Quote:
Originally Posted by BamaToy1997 View Post
Procedure below is how to test the fuel pressure regulator for proper operation. As you can see with the hose disconnected, pressure is higher, and when connected pressure is lower. Not sure where you think this vacuum hose goes to atmosphere. Connects into the intake stream if it is connected properly.
Can I use the Torque readout from the ECM to measure the fuel pressure when I run this test, or do I need a separate fuel pressure sensor?

If the ECM output is ok, which one? I have a fuel system pressure and fuel rail pressure.

Thank you for your help.
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Old 08-28-2013, 06:17 AM   #18
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Quote:
Originally Posted by MapleMoose View Post
Not to thread jack, but i seem to be in the same boat...

Little history on my 96 3.4l V6

-P0171 code first came on around 187,000 miles
-Replaced Bank #1 & #2 02 sensors and MAF sensor
-Code went away
-Came back on around 200,000-210,000 miles
-Replaced fuel filter and fuel pump

It only comes on when engine is run at high rpm 3k and higher or when im at high speed 5th gear at 1,700 rpm or lower struggling up a hill. Goes away overnight.

Current miles: 250,000
I'd like to know this as well. Howdoes one check for an exhaust leak?
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Old 08-28-2013, 06:19 AM   #19
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Quote:
Originally Posted by ONALL4 View Post
just a shot in the dark, you've probably already check your exhaust and gasket for leak. I had that code before but after fixing the gasket post cat and before the downstream sensor I don't get that error code anymore, its been about 6K KM now. Btw where can you get o2 sensor for 1st gen taco for 60$? you're not going to put the cheapest sensor from ebay right? When I was looking for denso or ngk it was around 270$ precat and 120$ post cat.

I think I saw that price at RockAuto. But admittedly, they had like 60 different sensors available and I didn't do enough research to see it that one was really the correct one or not. There were several in the $100 range too.
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Old 11-03-2013, 10:38 PM   #20
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Quote:
Originally Posted by MapleMoose View Post
You know i havent even thought about an exhaust leak. Whats the best way to test that? C0 tester?

My 02 sensors were NGK
If you have a torch adjust it till you see the black smoke come out. And while it's running run the black smoke along the exhaust and the black smoke would be sucked in.
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