RuffRider

My 4Runner starting throwing this code about a week ago. I cleaned the MAF which improved drivability and performance but the code keeps showing up.

Last night I hooked up my neighbor's Auterra Dyno-Scan and drove around. The MAF is showing volumes ranging from 0.50 to 11.00 and both of the EGO sensors look good. Do the Gen 3 V6s use heated EGOs? I didn't check the heater circuit...

Is there anything else I should look at? I took some captures which I can post if anyone is interested.

midiwall

A search popped up a couple of ideas...

https://www.yotatech.com/forums/f2/p0171-s-c-7th-other-stuff-66459/
https://www.yotatech.com/forums/f16/p0171-wont-go-away-help-56618/

Short form... Something's making the ECU think that you need less fuel than you really do. So.. MAF (you did that), air filter, O2 sensor... had you done the ISR mod? if so, look for something obstructing the air flow in the tube.

Did you reset the ECU after cleaning the MAF? You'll need to in order to clear the code, or you can use the Auterra to do it.

RuffRider

I researched the problem here and elsewhere for the past week (including the threads you mention). I haven't been able to find anything to indicate I'm overlooking the obvious.

I didn't reset the ECU but I did clear the codes using my scan tool. I wondered if resetting the ECU might clear any funky entries in the adaptive learning tables.

No ISR mod and I haven't replaced the OS sensor. The output and response looked pretty good on the Auterra. That's the next step, I guess, if I can't find anything else.

Do the Toyota MAFs and OS sensors usually fail completely (e.g. no voltage) or do they simply become less sensitive? I know the VW MAFs usually fail completely when they go...

midiwall

It wouldn't hurt... The tables aren't necessarily used to "build up" to a fault code, but since you've cleaned the MAF and such, you should give the ECU a chance to re-learn.


Yeah, but it's an expensive attempt if it doesn't fix the problem. How many miles are on the present sensor? Is it the original?

If you do end up swapping it out, then the best price we've all found is from URD USA (aka "Gadget"):

http://www.urdusa.com/product_info.p...s_id=430006000


I've only known the MAF to completely fail, but the O2 sensor will get "weak" and start becoming sluggish to respond. They should be rotated out every-so-often. I'm not sure what the real maintenance suggestion is, but I've been through a couple in 140k.

RuffRider

Understood. I was thinking of the air/fuel ratio adaptive tables. I used to write ECU software but not for Toyota so I'm making (perhaps wild) assumptions that they do that... Code P0171 is set when (ST Trim + LT Trim) > 35. Some manufacturers use the adaptive tables as modifiers for long term trim.

Yep. 100k miles.


Too bad he doesn't offer MAFs :-( Anybody know what the difference is between the CA OS sensors and the other 49 (besides $30)? I purchased this vehicle in CA but it is registered in CO.

midiwall

Ahh, very cool... I know that in tuning mine post S/C and injectors I pushed past that threshold and never tossed a P0171. Of course, that doesn't mean that the threshold isn't there somewhere or that this ECU may want to see that threshold crossed for n samples.

Either way... yeah, reset the ECU.

dogtrainer

The 171 code is a blown MAF. The blown MAF is tricking the computer and causing the computer to run the engine lean. If you sit on the problem for too long your going to overwork your knock sensors and they will also blow.

Try to find someone to swap the MAF sensor to confirm the above.

MAF sensors on the internet are about $100.

RuffRider

How do you overwork a sensor? Its not like they aren't working in the absence of knock. I'm not an expert on knock sensors tho...

One is on the way (thanks NorCalVP).

midiwall

Not absolutely... It's been shown to be caused by a number of things, but in RR's case, he's working down to this being the last possibility.


Dunno about that.. The knock sensor is a piezo microphone which has no moving parts to wear out. What can happen is that the piezo crystal can crack, wiring can get whacked, but "overwork"? nah.

What can happen though is that excessive heat on the block from running REAL lean could affect the crystal... but you'd have other problems at that point - like a hole in a piston.

Ironmike4x4

Please excuse my ignorance here, but what is an EGOs? It sounds to me like you are talking about an O2 sensor? Or is it something else?

dogtrainer

The 171 code says (correctly) that you have a lean running engine. Lean running engines cause engine knock. The knock sensor will detect the knock and send a signal to the ECU (computer) to retard the timing so as to avoid engine knock.

All that work (extra) that the knock sensors are doing is at no cost. Electronic and mechanical components do wear out. Those that are working overtime will wear out sooner.

Put a timing light on the engine and compare it to the specs. in the manual. You will probably notice that it is not correct. You may even notice it fluctuating. Also note that you cannot change the timing yourself. The computer adjusts the timing. (The above would apply to latter model Toyota engines; you didn't say what year or engine model you have.)

midiwall

Right, and then the knock goes away (the ECU can easily correct for the level of knock that would happen in this case). So the knock sensor is no longer subjected to the implied horrible bashing that's going to "wear it out".


Not really... not in this case. The specific device that we're talking about will not wear out from usage. It CAN break from physical damage or defect, but a piezo crystal is used in this application because of it's ability to withstand heat and repeated shock.

YES a knock sensor can go out, but it's not going to happen from engine knock. If the knock got THAT bad (i.e. causing a severe enough shock that it would crack the piezo crystal), then you'd have a piston flying through the block.


Good idea... or, RR has said that he has access to a good OBD II reader (the Auterra) which will show you the timing shift that the ECU's applying.



I used piezo mics all the time for Avant Garde' music work - some people call it Music Concrete'. I used to beat the daylights out of piezo elements with hammers, cement blocks, steel beams - striking forces WAY beyond the shock of pre-detonation. Sure, I had failures... but we're talking hitting a mic with a 10lb sledge in full-swing.

The knock sensor is the SAME technology. Practically the same device. In fact, a knock sensor is even more durable than what I used to beat on.

Repeated engine knock will not cause a knock sensor to fail.

RuffRider

Sorry Iron Mike, that's what we called them when I was at Ford and now the term is permanently embedded in my brain. O2 sensor is what I should have said.

EGO = Exhaust Gas Oxygen (sensor)
HEGO = Heated Exhaust Gas Oxygen (sensor)

RuffRider

dogtrainer makes a good point about knock though. Earlier this week I had knock problems for a day or so. Under slight load it would knock like crazy. Once I would tip-in further then it would go away. I strongly suspected it was related but I still couldn't narrow it down to either the MAF or the O2s (and I had just refueled at an unfamilair gas station). It was around then that I got a code for TPS circuit failure which could also cause knock. The code never returned once I cleared it and neither did the knock.

I'm not sure what Toyota's control algorithm looks like but Ford and VW both use values from MAF _and_ O2s to determine the correct air/fuel mixture (the Stoichiometric ratio). So, in the case of a failed MAF, or one O2 in a bank of two, it is still possible to determine the trim required to get back to stoich. If both fail then the control system will run in open loop and things can get ugly (holes in pistons can develop, cats ruined, etc).

I'll keep everyone posted. I'd like to develop a test to differentiate between the two sensors without having to swap then out.

bob200587

Ds2810

RuffRider

Anybody know what the "normal" volume (in lbs) range for the MAF is on the V6?

midiwall

RR.. you've got a PM

MTL_4runner

I have seen cases where people clean the MAF sensor and it still does not respond as expected from the ECU and thus it continues to throw CELs.

Have you tried just going to the junkyard and grabbing one from a 99-02 3rd gen?

MTL_4runner

If the ECU no longer gets a signal from the MAF then it will revert back to the TPS and O2 sensors (or A/F sensor in 99+) to correct the problem as best it can (there are no MAP sensors on a 4runner). If you lose more parts of the system (ie the O2s), it will get uglier just as you mention and begin to run open loop.

RuffRider

For those who haven't been following the thread (or are just lazy) - my MAF appears to be very responsive and functioning. I also purchased a replacement and awaiting its arrival.

I figure it would be nice to know the normal range of operation for future reference. A former co-worker is in town this week at the SAE J1962 (OBDII) conference in Pasadena so I'll bend his ear over breakfast tomorrow.

Also, I am curious as to what your favorite scan tools are. I've been using Auterra due to the rave reviews but I find the interface very klunky. Maybe I should just write my own...