2005 Sequoia Significant change in Gas Mileage code P0174

I agree with cj1.

However if you are getting a reported lean condition but symptoms of a rich condition (rich exhaust, poor fuel economy) that's indicative of false lean reporting.

Lean conditions can cause severely reduced fuel economy, but it's not common in my experience. I've noticed either the same or slightly better fuel economy and poor power.

False lean reporting almost always occurs because of an oxygen sensor issue. This could be a bad sensor, bad wiring, a bad computer, etc.

Generally a vacuum leak or bad/dirty MAF will result in a true lean condition because the extra air is unmetered (unmetered air is air not detected by MAF sensor), but the oxygen sensor does. When the computer "hears" the oxygen sensor and commands more fuel from the injectors, everything works normal. The same applies to a slightly dirty or weak injector which would also be detected by the oxygen sensor.

If the oxygen sensor is reporting extra air that isn't there (false lean), the computer response will still be to command the injectors to add fuel. The extra fuel then causes a rich condition because there wasn't any unmetered air to begin with and now you just have excess fuel.

If you send me a PM, I can get you the document to correctly diagnose the issue, including how to check the computer and wiring.

Thank you both. This one is really a head scratcher for me on many levels.

Here is the P0174 Freeze Data. Apparently my scanner will not report real time data so I am going to swing by the parts store to see if they can help me get the trim data at various RPMs

ABSLT TPS 15.6%
ENG SPEED 774(RPM)
BARO PRS 98(KPA)
CALC LOAD 20.3%
MAF FLOW 7.43(GR/SEC)
COOLANT 204 DEGREES F
IAT 141 DEGREES F
IGN ADVANCE 15.0(DEG)
SECOND AIR N/A
ST FTRM1 0.7%
LT FTRM1 26.5%
ST FTRM2 0.0%
LT FTRM2 35.9%
VEH SPEED 0(MPH)
ABSLT LOAD 20.0%
FUEL SYS1 CLSD
FUEL SYS2 CLSD
REL TPS 0.0%
THROT CMD 15.6%
ABS TPS B 47.4%
ACC POS D 15.6%
ACC POS E 32.1%
CAT TEMP11 1063 DEGREES F
CAT TEMP21 1063 DEGREES F
CAT TEMP12 1002 DEGREES F
CAT TEMP22 1002 DEGREES F
EVAP PURGE 0.0%
CMD EQ RATIO 1.002
O2S12 0.720(V)
ST FTRM12 99.2%
O2S22 0.720(V)
ST FTRM22 99.2%
O2S11 3.252(V)
EQ RATIO11 0.995
EQ RATIO21 0.995
O2S21 3.235(V)
CLR DIST 124(MI)
CLR TIME 04:43(H:M)
CLR TRPS 1536
VPWR 13.222(V)
ENG RUN 04:25 (M:S)

cj1 said:

Well, both banks are at or over the LT fuel trim limit at idle. If they move down at higher loads suspect vacuum leak.
The MAF reading normal at idle.

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I agree.

Sequoia05HighMiles said:

Thank you. I will post the trims once I can get them to help validate the hypothesis. More to come.

I will go back through the the vacuum diagram. Maybe I missed something the first time. As always, any hints are appreciated.

Click to expand...

Both your banks are reading high (lean) so you likely will get a P0171 eventually. Your upstream sensors appear to be pegged lean (3.54; normally near 3.25-3.3V) and your long term trims are nearly maxed out. A lean condition in both banks is usually indicative of a MAF issue or vacuum leak that extends to both banks.

Where did you get that MAF and upstream sensor? Are they OEM or Denso? Did you keep your original MAF? Have you tried cleaning the original MAF? Where did you get the PCV valve? Is it OEM? Did the issue start suddenly or after an event (wreck, repair, etc.)? Have you inspected the MAF wiring? Do you have mice where you live? Have you ever cleaned the throttle body?

Once you can get live data, get a reading of the following while holding RPM at warm idle, 3000 RPM, and 5000 RPM:

MAF G/Sec
Short Term Fuel Trim Bank 1
Long Term Fuel Trim Bank 1
Short Term Fuel Trim Bank 2
Long Term Fuel Trim Bank 2
Oxygen Sensor Voltage Bank 1 Sensor 1
Oxygen Sensor Voltage Bank 2 Sensor 1

Depending on the results of the above, we can try narrows down the problem to either a false reading MAF, a vacuum leak near the throttle body, or a large vacuum leak affecting both banks (intake manifold, brake booster, etc.), weak injectors, or bad ECU.

For reference here are my results from the above on my 2005 Sequoia with 219K miles.

Warm idle (Don't worry about blip in sensor 2s. I had just revved it)



3000 RPM:



5000 RPM (it's a bit tough to keep it there and screenshot):



Post 5000 RPM idle:



You can compare your data to this.

As you can see my long terms goes UP ever so slightly as my revs go UP. This is indicative of either slightly dirty fuel injectors or a slightly weak fuel pump or possible normal tuning issue from the factory.

If your long term numbers go DOWN as revs go UP, that would be indicative of a vacuum leak, false MAF reading, or false oxygen sensor reading (highly unlikely in both banks).

Since my numbers stay below 10 at normal revs, I am not in any rush to fix a problem. Anything around +/- 10 at normal revs (say 4000 and below) would be cause for concern. I believe the CEL comes on at +/- 18 total trim (total of long term and short term trims) with 2 trip detection (i.e. during two run separate run cycles).

You want to diagnose the problem. Throwing parts at a problem is usually the most expensive way to fix it. I still have the documents for diagnosis of this code if you send me a PM.

Additionally, here are some good videos on lean condition diagnosis and understanding fuel trims. I highly recommend watching them until completion.

You guys are awesome. I cannot thank you enough for the help thus far. As I am sure you noticed by this point, fuel injected cars with all this fancy computer stuff aren't my strongest suit. I have the service manual for the Sequoia and went through the diags and steps for a P0174 but it didn't mention anything about fuel trims, etc. I am learning a lot and a few expensive part lessons along the way. All in all, still cheaper than a new vehicle. ;-)

Where did you get that MAF and upstream sensor? Are they OEM or Denso? Did you keep your original MAF? Have you tried cleaning the original MAF? Where did you get the PCV valve? Is it OEM? Did the issue start suddenly or after an event (wreck, repair, etc.)? Have you inspected the MAF wiring? Do you have mice where you live? Have you ever cleaned the throttle body?

Click to expand...

I picked up the MAF from AutoZone. It is a Duralast. I do have the original MAF and cleaned it today after picking up some MAF cleaner. More to come on the original MAF a littler later in this post. PCV valve from AutoZone as well, it is a Delco (iirc). Issue was well underway prior to the PCV valve replacement. MAF wiring looks ok, no mice, removed and cleaned the Throttle Body last weekend before posting here. Both the upstream and downstream O2 sensors on bank 2 were replaced with Denso sensors.

I picked up a live data OBD and have the requested information.

At warm idle: 770 rpm

MAF G/Sec - 6.955
Short Term Fuel Trim Bank 1 - 4.7% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 1 - 19.5%
Short Term Fuel Trim Bank 2 - 3.1% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 2 - 25.8%
Oxygen Sensor Voltage Bank 1 Sensor 1 - 3.261
Oxygen Sensor Voltage Bank 2 Sensor 1 - 3.260

At 3000 rpm

MAF G/Sec - 28.42
Short Term Fuel Trim Bank 1 - 3.1% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 1 - 14.8%
Short Term Fuel Trim Bank 2 - 3.1% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 2 - 19.5%
Oxygen Sensor Voltage Bank 1 Sensor 1 - 3.250
Oxygen Sensor Voltage Bank 2 Sensor 1 - 3.214

At 5000 rpm

MAF G/Sec - 70.38
Short Term Fuel Trim Bank 1 - 2.3% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 1 - 8.6%
Short Term Fuel Trim Bank 2 - 7.0% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 2 - 17.2%
Oxygen Sensor Voltage Bank 1 Sensor 1 - 3.220
Oxygen Sensor Voltage Bank 2 Sensor 1 - 3.170

As I mentioned above, I cleaned the original MAF and for grins, re-installed it to run the same tests. The truck did not like this one bit in the beginning and threw all kinds of misfires until it settled in. I drove it for about 10 minutes and then conducted the same RPM tests.....which may or may not be valid based upon all the learning stuff.

With the original MAF, here are the results.

At warm idle: 774 rpm

MAF G/Sec - 6.577
Short Term Fuel Trim Bank 1 - 1.6% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 1 - 36.7%
Short Term Fuel Trim Bank 2 - 0.8 (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 2 - 39.8%
Oxygen Sensor Voltage Bank 1 Sensor 1 - 3.269
Oxygen Sensor Voltage Bank 2 Sensor 1 - 3.247

At 3000 rpm

MAF G/Sec - 29.63
Short Term Fuel Trim Bank 1 - (-1.6%) (negative - would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 1 - 40.6%
Short Term Fuel Trim Bank 2 - (-0.8%) (negative - would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 2 - 42.2%
Oxygen Sensor Voltage Bank 1 Sensor 1 - 3.244
Oxygen Sensor Voltage Bank 2 Sensor 1 - 3.253

At 5000 rpm

MAF G/Sec - 57.152
Short Term Fuel Trim Bank 1 - (-1.6%) (negative - would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 1 - 41.4%
Short Term Fuel Trim Bank 2 - (-1.6%) (negative - would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 2 - 45.0%
Oxygen Sensor Voltage Bank 1 Sensor 1 - 3.259
Oxygen Sensor Voltage Bank 2 Sensor 1 - 3.207

I don't know if the original MAF data is valid or not since I just put it back in and there is no learned history, but it sure muddies the water for me since each MAF seems to lead a different direction based upon your advice.

With the old MAF, LTs fell with increasing RPM which suggests a vacuum leak.
With the new MAF, LTs increased with increasing RPM which points to weak fuel pump, injectors, etc.

I love a good puzzle but this one is having its way with me.

I have an appointment for a smoke test on Wednesday morning. Hopefully, I can track something down this evening or tomorrow using the spray bottle but the fall back position is in place.

I drove to work today with the OBD hooked up. It was interesting to watch the data with the truck under load instead of just idle and RPM.

idle STs at lights were 1.6 - 5+% with LT at b1 - 29% and b2 - 35%. Once I would get moving and settle at 45mph, the ST trims would be -10%+ and the LTs would come down in the teens. The downstream O2 sensors would stay constant at 0.7v. Once I decelerated for a light the O2 sensors would drop to 0.1 - 0.2v and the STs would ramp up to 5% then start to settle back to 1.6 - 3.2% and bring the LTs back up again. Just sharing since I found it interesting and it correlated with the videos you sent out. I cannot thank you enough for those links. VERY educational to me.

More to come as I find out more.

No joy for me for the second time around trying to find a vacuum leak. Hopefully the smoke test will find it on Wednesday.

More anecdotal information and kinda weird in my opinion:

I was analyzing more live data this evening as I tried to find the leak and I noticed something that didn't seem right to me. I was playing with different RPMs and I noticed that between 1700 and 1760 RPM things behaved differently than other RPMs. For all other RPMs everything seemed to move similarly between bank 1 and 2 and evidenced above.

However, between 1700 and 1760 RPM, the banks were different. Notice the ST on bank 2 versus bank 1. The bank 2 ST stays between 6 and 11 anytime I am in the RPM range. Above or below and it settles back in line with the ST on bank 1.

MAF G/Sec - 14.893
Short Term Fuel Trim Bank 1 - 1.6% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 1 - 24.2%
Short Term Fuel Trim Bank 2 - 10.9% (would cycle in +/- .8 increments)
Long Term Fuel Trim Bank 2 - 32.8%
Oxygen Sensor Voltage Bank 1 Sensor 1 - 3.255
Oxygen Sensor Voltage Bank 2 Sensor 1 - 3.246
O2 Sensor Bank 1 Sensor 2 - 0.72
O2 Sensor Bank 2 Sensor 2 - 0.66

Maybe this is nothing, but since the truck tends to cruise at 1700 RPM or so, I found it interested and could also help explain the difference in the LTs between the banks.