P0300 P0303 P0171 - diagnosing

Welp. Snagged a MAF. Ouch. My ass hurts.

Headed home for install and testing.

 

New Toyota MAF sensor installed.

Warm idle in driveway in 78° ambient temps it’s running combined fuel trims at about 26.5%.

At ~2500 RPM the fuel trims increase to a combined 33%.

So that’s a slight improvement but still way too high. I believe combined fuel trims should be around 10% for a healthy running engine, yes?

I also tested vacuum again. Vacuum at idle is about 15-16 in hg. With evap or brake booster disconnected the vacuum is the same.

I think I need to test fuel pressure again but do it under load. When I tested it before I had 44 psi. But I didn’t check the pressure under load. Can I test load just by revving to 2500 RPM? Or do I need to be driving around with a gauge hooked up?

Could a dirty throttle body or IAC contribute to high fuel trims / lean condition?

Dunno. Not sure where to look next. It could maybe be the fuel pump. That’s about the only component I haven’t replaced. I don't think the brake booster is culprit - it didn't change vacuum when I disconnected it, neither did the EVAP.

Unless the supercharger is somehow causing a problem ??? Maybe it's not producing boost and so therefore is actually creating drag? Just a theory.

I am searching eBay for a complete 5VZ intake plenum, air intake box and elbow.

I'm also going to try another smoke test. Maybe I missed something.

Replaced so far:

• injectors
• fuel filter
• fuel pressure regulator
• MAF sensor
• plugs (2021) but I didn't do the duals...
• vacuum lines everywhere

 

Been out driving it. The code cleared itself.

fuel trims at highway speeds around 26%

 

....... where's your 7th injector?

 

Doesn't have one

 

Is the egr port on the back of the supercharger capped? If not it will cause it to run lean and throw the p0171 code

 

the EGR port is blocked off with the factory plate. It has never been removed.

 

I'm not an expert on this particular subject, but I've heard that there needs to be a 7th injector for any 6 cyl with a SC. Does anyone have more than bare minimum knowledge on having a 7th injector?

 

Its not required, but a lot of 3.4l's do have pinging issues without additional fuel mods like the 7th injector or other upgrades to better control fuel and timing...lots of guys on here are starting to upgrade to Haltech ECUs so they can customize all that stuff.

Even if you were having pinging issues without fuel mods, I don't think it would cause it to throw a lean code...I ran without a 7th injector for a couple years and had bad pinging problems under load, but it never caused a lean code to pop up.

 

so it’s running better, but still dumping in too much fuel. You can smell the gas. And when I accelerate hard, the performance isn’t there.

I feel like the trims combined should be closer to 10%, not 20-25 like they are now. Although it is an improvement and the lean code has cleared itself. So I still think there is a contributing problem somewhere.

I’m not opposed to investing in something to further tune it, like a 7th injector, and/or methanol injection. The haltech is crazy expensive!

 

yeah the Haltech is expensive but it blows the pants off every other option really. The 7th injector helps but it is decades old technology and if the standard box tune doesn't cut it, you need an old Windows PC/laptop to reconfigure the tune. URD support isn't any help from what I've heard. For me though it worked ok out of the box.

Yeah those LTFT being in the upper 20s is no good, I'm surprised the code went away when it's still running lean according to the trims. I don't really know what else to recommend considering it sounds like you've done everything so far (replaced sensors, verified no vacuum leaks etc). Though vacuum leaks can be a bitch to track down because there are so many places that a small leak can trip a lean condition

 

I scanned the initial post and didn't see anything about replacing the O2 sensors, have you tried that? Usually if it's not vacuum leak related then its MAF or O2 sensor related from what I've read, and I know your MAF is a brand new OEM one.

as with the MAF sensor though you want to make sure the O2 are Denso/OEM as well to avoid trouble with aftermarket ones

I know there are some procedures to check the O2 sensors as well

 

You would think if it's in the 20s that would still throw the lean code. But that video I linked (P0171 Diagnosing) at 22:22 he mentions that Toyotas throw the lean code when combined fuel trims get into the 30s. You can still have a lean condition in the 20s but not throw a code. And that's what I'm seeing now.

I haven't heard good things about URD support either. If I could get a Haltech for $1k I'd jump on it.

My upstream O2 sensor is <18 months old, but I don't know that it's a Denso/OEM. Its probably a Napa special...

I think you're right. I should focus some attention on testing the O2 sensors. This has really been a good learning experience for me.

 

yeah I would definitely zero in on that aftermarket O2 sensor, Toyotas in general don't get along well with aftermarket sensors. The parameters have to be perfect or the ECU detects a problem that sets off a code, and with aftermarket that's a gamble

there are some aftermarket parts I would spring for over OEM, but when it comes to these trucks...sensors and ball joints = OEM only is my motto lol.

 

Guys, am I correct in assuming we have 2 upstream O2 sensors (one for each side of the V6 engine) and one downstream O2 sensor after the cat?

 

one in front of the cats, one downstream

 

I just figured WTF and ordered both O2 sensors, Denso, from Rock Auto. Should be here in a few days. I tried looking at my existing sensors and I can't tell of they are OE or not. So this is basically paying for some peace of mind.

For my 2001 4WD 3.4L, I checked Denso's site and the parts are

$146 Denso 234-9001 Upstream Air Fuel Ratio Oxygen Sensor with 10.6” 4-Wire Harness, for Auto Trans Equipped Vehicles

Denso 234-9001 upstream and

$61 Denso 234-4189 Oxygen Sensor

Denso 234-4189 downstream. Rock Auto was $200.44 including tax and shipping.

I did check Amazon, but the photos of the "Denso" ones showed "Made in China" and I frankly don't trust Amazon or anything that says "Made in China". Amazon's prices were not really much better. That would suck to pay the price for Denso from Amazon and get a counterfeit part made in China. Denso products are made in Japan. Further, genuine Denso sensors would come with the gasket. Some of the reviews said there was no gasket included. ?? Amazon's basically a Chinese store anyway, IMO.


I stopped by the stealer and picked up new nuts for the bolts at the O2 sensor bungs. I checked and they're rusted solid, so I'll be needing to replace them after I get them off.

While I wait for the O2 sensors to arrive, I decided I'll replace all my drive train fluids, including a full trans flush. I read a few threads on here, including this one, and this one. This would be a good time to add an auxiliary trans cooler... I don't tow very often, and when I do it's just 3-4 dirt bikes in a light trailer. But we do use this Taco off road all the time, and at 250k miles it certainly won't hurt to add a cooler to keep trans oil temps down. Can't be too careful. Utah desert adventures are far away from any service stations...

For all my drive train fluids - front and rear diff, transfer case, and trans, plus adding a trans cooler, replacement trans filter and gasket, I'm looking at $275 after tax. Going with Valvoline MaxLife ATF (3 gal) and some LubeGard for the trans and Lucas

$29 75W-90 Synthetic Gear Lube - 1 Quart (Pack of 2)

75W-90 (1 gal) on everything else.

 

If that doesn't do it then there's gotta be some pesky vacuum leak somewhere, chasing down well hidden vacuum leaks isn't fun...hopefully the new O2 sensors do the trick

 

Yesh, hopefully they do. I have tried my mechanic stethoscope, I've tried propane, and I've tried smoke tests... I sealed up all the vacuum leaks I found (there were 2-3) but now the only thing I see are pinhole leads in the exhaust pipe after the upstream sensor, right at the welds for the cat or resonator. I can't see any other air leaks in the engine area.

One thing I considered is cleaning the throttle body and IAC. I don't think that a dirty IAC or throttle body would affect my mixture, but I also don't think it's been done on this truck and I'm sure it needs doing.

I could look further into testing EVAP...

 

Got the new upstream O2 sensor in easy peezy. The back one has rusted bung bolts and the nuts crumbled. I can’t get it off so I may have to cut that shroud to get at it.

but I tested after installing the upstream sensor and my combined fuel trims are still at about 26%.

I’m pretty sure that the downstream sensor has nothing to do with air fuel ratio and is for monitoring emission levels. I will install it regardless.

Ran another propane test in the engine bay. I was careful to go slowly along the vacuum lines, connections, PCV valve and FPR, valve covers, and I tried to stick the nozzle into where the fuel injectors are just in case one of them was allowing a vacuum leak.

No change in throttle.

stumped.

Wondering if it could be a bad computer?

brake booster or evap? I tested them by disconnecting vacuum lines, and didn’t see a change but…