'96 Toyota Tacoma P0171 Lean Code

My '96 Tacoma, 3.4 L V6 w/ MT w/ 265k set a P0171 code last week.
Freeze frame data shows: Eng load: 9.8%, Coolant Temp: 190F, STFT: 18.75%, LTFT: 17.97%
RPM: 703, Speed: 0 mph, Intake air: 69.8 F

After it's warmed up at idle, I get ST: 19.5, LT: 23.4
@2500 rpm, I get: ST: 0, LT: 9.5

O2 sensors appear to have normal activity.

I checked arond for a vac leak, by pinching off vac hoses and spraying some carb cleaner around the intake manifold gaskets (as best I could).

I did find an emissions VSV hose that was disconnected, but reconnecting didn't make a significant difference.

Thinking it was a vac hose, I pinched off the fuel return line and the total fuel trim corrected to ~5% pretty quickly.

Would that point to a fuel pressure or fuel regulator issue?

Suggestions?

Thanks!

Paul

Typically with a fuel delivery/ regulator problem fuel trim will go more positive at high rpm (lean) if fuel pump is not putting out enough or regulator is stuck / not functioning correctly. I would graph o2 sensors, maf/map sensor, ect sensor, iat sensor, with rpm to see what might be causing it that way. other thing to do would be to check for exhaust leaks around manifold, smoke test intake as that will show spots you cant spray with carb cleaner.

That looks like classic vacuum leak data. I imagine that you have that lean condition at idle, and when you pinched off the return line, you increased fuel pressure with nowhere for the return fuel to go, and enriched the mixture a bit, which helped your lean condition.

Are you watching stft while looking for the leaks while using propane or brake cleaner? I'd go after that smoke test that hardtopdr recommended.

Sent from my moto g fast using Tapatalk

Thanks for the suggestions. As you may have guessed, I'm a DIYer but am working on my YouTube Certification
Has anyone used the ~$70 Smoke Machines advertised on ebay? The look like they're made out of a 1 gal paint can and a Weber BBQ propane regulator.

Those would work or you can make one with a old sock, mineral oil, a popcorn can, some vinyl tubing and a wood burning soldering iron (in a wood burning art kit the real low wattage kind) with some sealant. Poke three holes in top of lid feed iron through middle hole and tubing in other two holes 6 feet length each is best. Seal around tubing on both side of lid and seal around iron making sure sealant does not touch metal part of iron. Let dry. Then take can and seal around seams of can and let dry. Put old sock folded up to where it will touch iron then soak sock with mineral oil. Put on lid and hook one hose to intake sealing off ducting on engine side of maf sensor separating maf sensor from smoke tested portion of ducting. Use other tube and blow into it slowly several exhalation puffs then while blowing into tubing check for smoke using a white led flashlight. And wallah u have a smoke tester. Some other smoke testers have a leak gauge like redline does. But for a diyer this gets the job done. Then when finished drain unused oil into a container and wring out the oil in sock into container and save for use later.

maybe try a compression test or leak down test or something like that

Really appreciate the replies. Attached are a couple of pics of B1S1 Voltage waveforms (there are just 2 sensors, one upstream and one dowstream of the cat). Since the O2 connector is difficult to reach, I had to backprobe at the ECM and got a ground from a powerprobe adapter connected to the cigarette lighter. I have a 20 year old MODIS, so can only display images on my monitor and grab screenshots.

The first pic is fully warmed up, at ~2500 rpm. As you can see the range is ~0 to 550 mV. For that portion of the scan, total fuel trim was ~5%. At idle, I'm flat-lined on voltage and total trim is 20-40%.

I've been looking for vac leaks. When I first started investigating, I found one small vac hose on the back of the intake that was disconnected. Probably has been disconnected since I replaced valve cover gaskets a couple years ago. Reconnecting it didn't make much difference. I wasn't able to check heater voltage and gnd at the connector, but was able to ohm the heater: ~13.2 ohms.

Truck has no drive-ability issues. The O2 sensors are original, w/ 262K miles. Section 5 of SD's book indicates healthy O2 sensors should oscillate between ~200~800 mV, w/ peaks <200 and >800.

Is my O2 sensor just worn out, or should I continue to go down the vacuum leak path?

You have no 02v at idle B1S1? No oscillating at all? But you have 0-55mv at 2500 rpm. The heater in the 02 is out. At 2500 rpm the exhaust is heating the 02 to make it do a little. Even so that mv should go to 800mv. You should be seeing an open loop at idle on scan tool. You're at 37 total trim at idle. ECM is pouring in fuel.

Thanks for the reply. I ordered two Denso sensors from Rock Auto earlier today. I'll be sure to post some after waveforms with the new sensors installed.

Update: I replaced both O2 sensors w/ Denso units. Waveforms look better at 2500 rpm, but fuel trims are pretty much unchanged. Sensors become active within a minute or so, so I have to believe the heaters are working. I should have let my snapshots run longer. Sorry about that.

Here's Idle (696 rpm, ECT: 86C, AF: 2.38 g/s, STFT: 19.5, LTFT: 27.3)

Here's 2500 rpm (2569 rpm, ECT: 89C, AF: 12.03 g/s, STFT: 3.1, LTFT: 11.7)

Here's transition back to idle


Yellow is Sensor 1, green is Sensor 2.

I've tried, water, Carb cleaner and pinching off hoses to look for a vacuum leak to no avail. When I first looked for vac leaks, I did find a fuel pressure regulator VSV hose that was disconnected from the intake (it had probably been disconnected since I replaced V/C gaskets a couple years ago. Reconnecting it made very little difference. The config of the 3.4 L engine intake manifold makes it difficult to get to the joint between the upper and lower manifolds. I used new OEM gaskets when those were replaced.

Looking for direction, should I continue down the vac leak path and get a smoke machine or investigate fuel pressure or MAF?

You pinch off the brake booster hose? And purge valve? Also I’d try a capacitor discharge on it to reset the computer. Unhook both battery cables connect a jumper wire between them for 10 minutes. Not the terminals on the battery but the cables. Then retest. On them older Toyota’s like that I’ve had them not correct after a repair

That wave form at idle is bad... You should have 100-800mv oscillation just like the 2500 rpm not flatline 0. Are they going into open loop at idle?

To answer your question about fuel pressure. Yes, a bad pressure regulator on a return line fuel system can bleed off too much pressure and cause a lean condition. Check fuel pressure with the regulator vacuum line on and off. With vacuum line off pressure should be 8-10lbs more than factory idle specs with vacuum line on.

MGNut wrote: Here's Idle (696 rpm, ECT: 86C, AF: 2.38 g/s, STFT: 19.5, LTFT: 27.3)

Looking for direction, should I continue down the vac leak path and get a smoke machine or investigate fuel pressure or MAF?

Your idle g/s is way low. Somewhere around 3.5 to 4 g/s would be about right for that motor. Since the situation improves at higher rpm it's more likely to be a vacuum leak than a misreporting MAF.

What I'm seeing is no O2 activity at idle then when rpm is increased to 2500 02's respond then go back to no activity when engine returns to idle.

Heater circuit is not working as I said prior. Not the 02's, the wiring circuit. Check power and ground and check for blown fuse on heater circuit. If fuse is good you have an open on power or ground.

The reason you get activity at 2500 rpm is that exhaust is heating 02's.

scannerjohn wrote: What I'm seeing is no O2 activity at idle then when rpm is increased to 2500 02's respond then go back to no activity when engine returns to idle.

Is that no activity or pegged lean? Whatever it is happened pretty quick in the "transition back to idle" trace. And happened to the upstream first.

Cheryl wrote: You pinch off the brake booster hose? And purge valve? Also I’d try a capacitor discharge on it to reset the computer. Unhook both battery cables connect a jumper wire between them for 10 minutes. Not the terminals on the battery but the cables. Then retest. On them older Toyota’s like that I’ve had them not correct after a repair

I have pinched off the hoses mentioned and more. After replacing the O2s I did disconnect the battery for ~10 minutes. Fuel trims started at zero and it stayed in open loop for awhile. After ~15 minutes or so of driving it went into closed loop and trims went back to where they were.

scannerjohn wrote: What I'm seeing is no O2 activity at idle then when rpm is increased to 2500 02's respond then go back to no activity when engine returns to idle.

Heater circuit is not working as I said prior. Not the 02's, the wiring circuit. Check power and ground and check for blown fuse on heater circuit. If fuse is good you have an open on power or ground.

The reason you get activity at 2500 rpm is that exhaust is heating 02's.

When I first start the engine at idle, the O2s do oscillate pretty quickly. As it warms up they oscillate slower and finally flatline. It does stay in closed loop when flatlined.

With the old O2 sensor, I have the makings for a test harness. I'll put light bulb on the end of the heater wires and report back.

Appreciate your comments.

Paul

Paul

You have a 3.4l V6 engine. You will have three 02's. Two upstream, one each just below the exhaust headers on each side of the engine, and one post cat. Your scan tool should show B1S1, B2S1 and either B1S2 or B2S2 for the post cat 02.

The upstream 02 heaters are controlled by the pcm. At startup the they are in open loop so the heater circuit are active until they go into closed loop at which time the pcm monitors signals from other sensors ECT, MAF, IAT, MAP to tell it to pulse ground the heater circuit according to engine conditions and 02 heater temp to keep the 02 at 700+ degrees. Above idle the 02 will remain hot from exhaust gases but will be pulse grounded as they drop below 700 degrees.

When you use your 02 light adapter, the light should be steady at startup for only a few sec or so then pulse on off.

Your engine temp is low. Should be 205-210 and the MAF g/s should be 3.4-4 at idle. I'd you have not done already, clean the MAF and IAT. Take care in cleaning they are delicate. So not touch them only use a spray MAF cleaner.

The voltage on the post cat 02 should be steady at 500-700mv. It should not oscillate.

Look forward to hearing your next post.

Thanks - I wouldn’t be surprised if Toyota added bank specific O2s in later years, but my ‘96 only has one O2 directly upstream and downstream of the single cat.