Hello everyone,

Hoping to tap the collective knowledge to figure out this enigma. I appreciate any advice.

June 26th, 2019 I got my first P0175 and P0172 codes, at the same time. Rich in left and right bank. Clearly caused by something fuel control/emissions related that effects both banks of this '05 V8.

I have the doug thorley shorties and running all 4x magnaflow high-flow cats (installed in 2017). Original O2 sensors... all of which continue to pass all sensor tests, and always have (first device we wanted to blame). I have the SAIS bypass kit as well. I contacted the manufacturer and they provided a set of instructions to test the unit to see if it may be causing the code. Unit passed testing, and connected or not, makes no difference.

At the time I likely had around 300,000km

I had the intermittent stalling problem around that time, eventually figured out it was the throttle body, and replaced that. Stalling issues disappeared but not the P0175 and P0172 codes showing up.

Back in the summer of '19, I replaced the spark plugs, cleaned the (original/broken) throttle body, new OEM MAF sensor, new air filter; all of which had no effect on the CEL.

Both codes would appear and disappear intermittently, problems undetectable with smoke tests, on-board systems diagnostics, and external testing. Whatever was causing it wasn't easily found. At one point my fuel trims were out over 30%, but still no direct cause could be found. All systems registered and measured as normal, all devices would operate within normal perimeters during testing & test driving. I spent some big money having the guys drive it around with equipment connected trying to figure it out...

I got fed up in Dec 2020 and decided to throw parts at it. The shop's Toyota Master tech said that the three major things that could cause it are:

• Thermostat
• Coolant Temp sensor
• Fuel Pump/Filter
• (for good measure) injector service

So we did all those... and it's run great since. Until Friday, May 21st. I had my summer tires installed and the CEL came on after about 30 minutes of driving. Of course it's my old friends, P0175 and P0172. I don't understand how that can be related, seems to me to be a pure coincidence. My summer set (BFGs) are significantly heavier than my winter set, likely around 30% heavier.

Anyway, just as strangely, the CEL has disappeared once again, indicating it is running within acceptable parameters, but it'll just come back later.

I was hoping others have had this issue and may have some insight into what could be causing it. When it runs good, it runs perfectly great. Smooth as butter and pulls all the gears hard.

Thanks in advance.

-Don

Could also be exhaust leak. That’s what I had. Needed 2 new gaskets between cat and Y pipe, 1 on each side.

Derek V

Keros, have you figured out anything since your post? I don't want to send you down my rabbit hole just yet, but wondering if you had figured anything out to fix yours. I have been dealing with those 2 codes for the past 3 years (i rarely drive it anymore). Replaced multiple parts multiple times, to no avail... all seem like a temporary fix.

Negative. Just driving it with the codes on.

Not really sure what to do with it at this point.

The computer tells me my milage isn't bad at 15.5L/100km, at 45 km/hr average speed, on the calculator... but it does feel like there's a hole in my gas tank and the truck runs like hot garbage.

It'll probably be a few months until I feel like putting more money into it. There's two options to move forward:

A) Find a new shop that has someone with specific experience with this vintage of Toyota, give them $1000 to diagnose something, fix their top 2 best guesses.

B) Buy URD's supercharger kit. Replace all OEM sensors that it uses to operate the engine.

It took 4 months for the problem to re-appear after the last fix attempt, so it's not really easy to even know if whatever I've attempted to fix it... actually worked.

A stealership will likely blame my extensive list of modifications as the problem. Regardless, they're partsreplacementspecialists anyway.

I contemplate how much of the truck would need to be ripped out to run a standalone setup. No one anywhere has ever documented doing a full standalone ECU on these trucks (why would they).

The URD kit is a parallel system, but I'm sure it utilizes a specific set of sensor systems directly to run (based on my conversations with URD) and then feeds information into the OEM ECU to make it happy about providing information to other subsystems around the truck.

The truck's likely worth more as parts than a whole with CEL problems unknown. I can't unload it onto the next person without being upfront about the problems. With the price of 4Runners climbing, it'd still make more sense to spend $15k CAD on any other solution

So it's Fix it, dump it, or mod it even more heavily.

I have a pretty similar setup, including DT headers and the SAIS bypass. I had these codes and it was because my air filter was ever so slightly not fully seated in the air box, causing it to not be able to close fully. All 4 clamps on the air box were able to clamp, but there was a tiny gap since the air box was pinching the air filter. So maybe check that? Luckily for me it was that simple.

I'm in the same boat too. I don't want to give up on it, and don't want to pass it along without being upfront on the issues. It would make more sense to fix it and keep it, but I can't find the @#$@$ source of the problem...

Just out of curiosity, pull your ECU connections and see if there is oil on them. This is the rabbit hole I'm currently going down, albeit slowly. I have kind of given up for a bit on replacing things like you have, lol... I have 2 threads that I am started or following but there doesn't seem to be a 1 exact fix. I've thrown a lot of parts at it and hoping someone has a sure fire fix soon!

oil wicking up my o2 sensor wire harness?

P0174, Rough Idle & Strong Exhaust Smell

I do have transmission oil (not much but some) in my ECU harness connections, which could explain all of the strange and intermittent issues it has. Just recently I've been driving it more and it is pretty much constantly in a failed state and running way off LTFT on both banks. Sometimes the car hesitates, but just seems to be labored all of the time. I have replaced MAF multiple times with aftermarket and true Toyota, I have replaced upstream AF sensors multiple times (which masks the problem for 6 months to a year). so something is still underlying issue that's causing the car to do this every so often...

I'm currently at the following theories:

1. Bad ecu either just because or because of the transmission fluid issue
2. Injectors need to be replaced
3. the car just hates me and there is nothing really wrong.

Will definitely check this to be sure, but have done multiple tests with carb cleaner to see if there was air getting in anywhere and haven't found it yet. Also replaced a K&N drop-in filter with a paper filter just to be sure that wasn't causing it, and replacing the MAF multiple times, lol... super frustrating, but glad yours was an easier fix! I'm sure in the end I'll figure out that it was something stupid and easy that i could have fixed a long long time ago...

Hooooooolllllllllleeeeeeeeeeeeyyyyyyyyyy crap

Yeah, it could be that.

Maybe I have a crack in the airbox.

I'll pull it right out of the truck for inspection this weekend.

F*ck that's a good one...

You could get the injectors serviced. Lots of places have the equipment to clean and flow test them.

I'll have to give a re-read and a think about the rest.

Hey, I really appreciate you guys reaching out with these ideas!

aFe intake ordered.

I didn't think it would be possible to adequately inspect the OEM intake that had 330000km and 16 years inside a hot engine bay.

If it has a crack anywhere in it, it would be the cause of the problem.

I'll report back in a while

Any reason to upgrade stuff, right?! Haha, definitely let me know if this solves it. I've gotten so much help from reading/posting on here, love it when I'm able to pay it forward.

I'm late to the party but wanted to offer this suggestion: check your fuel pressure. Running too rich - less air and more fuel. The FPR controls thr amount of fuel and the injectors control the duration (duty cycle) of the opening. So perhaps start there?

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Checked over mine really good this weekend and couldn't see any obvious cracks or gaps. Let us know if that intake solves it for you @Keros , I may have to go that route as well

Definitely considered this as well. I ran a fuel pressure gauge for a few weeks and even when it was messing up, the fuel pressure didn't seem to flinch. Also when turning the car off, the pressure went down normally not quickly which would indicate a leaky injector. May have to try the FPR anyway just to be safe though. I'm not sure if I was measuring before or after that in the system.

Supposedly this was diagnosed by the shop, prior to replacing the fuel pump and various other bits.

Emphasis on supposedly.

It seems to make more sense at the moment for it to be a physical issue. The next part I'll be looking at is the intake manifold itself. Perhaps a crack has developed somewhere in it.