P0300 P0303 P0171 - diagnosing

My money is still on the aftermarket intake screwing with the MAF readings, recently had someone in the 2nd gen section have lean fuel trims and MAF codes after installing an AFE intake. Aftermarket intakes generally just don't work well on MAF engines without tuning to correct the MAF.

 

According to this article the downstream sensor helps to determine the long term fuel trims

"Long-term fuel trim relies on sensors further downstream in the exhaust system, past the catalytic converter."

So it could very well be that one? At this point hopefully that's what it's been narrowed down to

 

I can't get on board with this. These trucks are 20 years old; K&N has had their product on the market equally as long. If aftermarket intakes were a huge issue it would have been well documented at this point. K&N have sold multiutdes of intakes, so that sample size is large. The largest issue with K&N is that they end up pulling air from the engine compartment, and are not "cold" air intakes- and that their filters are oiled, and if over oiled, can coat the MAF and skew the readings.

Sensor manufacture is more important. I'm still unclear if the OP is using an OEM MAF or aftermarket MAF.

 

He had an aftermarket MAF but recently put in an OEM one, saw some improvement (code went away) but long term fuel trims are still lean/too high according to OBDII readings. His O2 sensors are aftermarket as well but he said he replaced the front one with Denso but the rear is still aftermarket. I think the rear should be replaced as well and go from there

 

Sounds like vacuum leaks are mostly ruled out from all the testing so it's gotta be that aftermarket o2 sensor, unless it's a pesky hidden vacuum leak in the evap system or something

 

or brake booster- but I think I already mentioned that in this thread...

 

If I can locate a stock air intake, I will definitely give it a try. Can't hurt. But I'm not convinced this could be the case as the diameter of the K&N tube is the same as the diameter of the stock air box elbow/bellows.

Interesting. This gives me some serious hope, especially since I already spent the money for the downstream Denso sensor. The current one is just rusted on. I have been trying to avoid cutting off the factory flange at that sensor, but it looks like I'm going to have to in order to get the rusted nuts off. I soaked them with PB, but even if I get the nuts off, the bolts seem to be completely coming apart in rusty bits. The entire bung may need to be replaced.

The upstream wasn't rusted at all. In fact, I was able to get the nuts off with a simple turn of my ratchet and a 12mm socket, and there was no major corrosion. I can't help but wonder if the flange on the downstream O2 sensor was serving to capture snow and hold it there which caused this downstream one to rust so badly. I do drive through some deep snow.

I have a Denso MAF now. It cut the fuel trims by 10% and made the CEL go off. Trims are still about 26% combined tho. Also purchased Denso O2 sensors (both) and installed upstream, but not the downstream yet. Will be doing soon.

Love all the input guys, you're keeping my hopes up. I can't believe how much I'm wasting in fuel budget right now. The cost of these parts is nothing by comparison. I'm averaging 13 mpg on this truck with the shitty fuel trims and it should be much better than that.

 

Can you take pictures of your intake?

 

Yeah - when I get back from work.

i got the back sensor off this morning. It was a bitch. The nuts had started off as 12mm nuts and were so rusted that I could get my 9mm socket on there. I couldn’t ratchet them off. Had to cut them. Even after I got the nuts off, the sensor wouldn’t come off. More PB blaster and a lot of hammering and then I got it off.

The bung itself is so rusted that there is not a good platform for the new gasket to seal. It’s gonna have to be cut out and a new one welded in.

I got the new sensor wired in and took the truck to an exhaust shop. They’re welding in a new bung.



The shop guy says the exhaust pinhole leaks at the cat welds just next to the downstream sensor would cause bad readings because they will suck in additional air and throw off the readings, causing the ECU to dump more fuel into the mix.

So they’re gonna try to seal up those bad welds too. Should be done today.

 

All right ... for those of you still hanging with me (this party is getting old fast), I got the new downstream Denso O2 sensor in. The muffler shop cut out that pipe section and put in a brand new bung. They also repaired several small air leaks around the cat just upstream from the downstream sensor, which could have affected performance (they thought).

I pulled the negative battery cable for 10 min just for fun. Fired the truck up and ran some sensor readings... basically there is no change in the LTFT after about 30 min of driving in various conditions. Stop and go, highway, up and down hills, etc.

So after both new O2 sensors, the LTFT is still at 19.5% (this is no new change). STFT fluctuates above and below zero, but is typically very low or negative.

Can anyone tell me what LTFT and combined fuel trims I should be expecting with a 3.4L?

LETS RECAP!

Started out this journey with LTFT at 39.5% and CEL with P0171 code

Found and repaired 4 vacuum leaks in and around intake area. LTFT dropped to 33.6%

New Denso MAF sensor dropped the LTFT to 19.5%
>> LTFT dropping below 30% also cleared the P0171 code and CEL

Currently the LTFT is 19.5%. I will put the new readings from today below.


Things I have replaced so far (bold items made a difference in LTFT):

• Motor West upgraded injectors
• fuel pressure regulator
• fuel filter
• vacuum hoses (all of them)
• Denso MAF sensor
• spark plugs (~18 mo ago)
• ignition wires and coils (~18 mo ago)
• Denso upstream Air/Fuel O2 sensor
• Denso downstream O2 sensor
• Repair leaks in exhaust

Tests conducted so far:

• fuel pressure (44psi at idle and 2500RPM)
• vacuum leaks via smoke test (found several, fixed them)
• vacuum leaks using propane (nothing found)
• vacuum gauge reading at idle 16 in hg
• vacuum gauge reading with brake booster disconnected 16 in hg
• vacuum gauge reading with EVAP disconnected 16 in hg
• looked at PCV, it seems in order
• EGR port on back of supercharger is blocked off/sealed

Other things that have been mentioned as potential causes:

• EVAP (I'm not sure yet how to further test this)
• Brake booster (not sure yet how to further test this)
• supercharger - I have mentioned that I am not convinced that it is producing net positive boost and not just drag on the engine
• determining a performance baseline using a stock air intake (remove K&N intake, replace with stock intake)

New fuel trim readings as of today, in PARK at IDLE, 1500 RPM and 2500 RPM


Fuel trim readings from today at 25 MPH, 45 MPH and 65 MPH


Downstream sensors (old and new)


New downstream bung and repair

 

Out of curiosity- do you have anyone nearby who would let you hook up that ODBII reader to their truck? I mean, I see you have other toyotas you could hook it up to... but I'd be inclined to change the least number of variables. The point would be to check the reader, and possibly the app... Maybe get yourself a scan gauge II. Just want to make sure you're not chasing a problem that doesn't actually exist.

 

I almost don't want to say this but... My fuel trims, both LT and ST hover around 0%

 

yeah from everything I've read, "normal" functioning trims both long and short should stay between -10 and 10, closer to zero the better

 

I am not familiar with whatever app he's using with the reader, but it's odd to me that the LTFT reading is constant - it should fluctuate. I know with the SCII you have to set up hex codes for STFT and LTFT, and I'm wondering if maybe the reader is pulling another property. That was why I am suggesting either plug that reader and app into another Tacoma to see if it's something wrong there or get an SCII / second opinion on the LTFT.

 

That's just the screenshot. They're constantly fluctuating. If I choose the graph view, you can see it plotted on a line graph over time. LTFT tends to stay pretty stable at 19.5%, whereas the STFT is fluctuating between -9% and 9% constantly, depending on throttle.

But you guys make some good points. I can run the tool on my other cars, I have two other toyotas, and I can get some feel for their fuel trims as a comparison.

 

I'm not so much concerned about the other vehicles' fuel trims as much as I am about validating the reader is telling the truth. Switching directly to another Tacoma (presumably with the same hex code to pull the LTFT) would keep things apples to apples. If you hook up to another vehicle type, it may use a different hex code - apples to oranges.

 

I get it, I just don’t own another 1st gen Tacoma or know anyone near me with one.

I think would be easier to borrow a different scan tool and compare readings than find another Tacoma to test mine on.

 

Well now we know it’s not my scanner.
Borrowed one from O’Reilly and it shows LTFT at 19.5% also.

:/

 

Another thing you could try is get some pliers and start pinching all of the hoses coming off the intake one by one and seeing if any of them change your fuel trims at all. you'll either see the LTFT go down or the STFT shoot up.

 

Bummer that it's the same, but at least it rules that out.