question is how did that get there? was the grommit on the engine bay passenger side loose?

I cleaned up the connectors as best I could. and will keep an eye on them.

I had Adv. Auto reset the CEL and read the codes
P0136
P2238
(both 02 sensors on bank1)

I've been dealing with the rear parking brake show assembly at the moment, so I haven't had a chance to look that deep into the engine bay harness / grommets.

But the dealer thinks the fluid is coming up from the transmission wiring harness…

Capillary action, its really interesting actually. Like I mentioned earlier in the thread its common on some of the European makes like BMW where their coolant sensors rot through and then coolant is transferred THROUGH the wiring harness in the engine bay all the way into the ECM in the cabin.

The thing is that they should be able to tell you which sensor/switch/connector is suspect. So far it just seems they stopped trying after finding fluid, which is kind of crappy if they were already under the car looking at stuff?

BlackWorksInc

how do the european models fix the issue? if its capillary action, shouldn't a splice in the offending wire stop it?

here are a few more pics of the passenger side O2 sensors

lower





this plug was next to the lower passenger O2 and appeared to be completely dry.


Upper O2

The BMWs, Hondas, GMs, ect. that have these types of issues have updated sensors/switches that are not as prone to rotting out as the previous design.

As for a splice, that may or may not work. From my understanding, the fluid actually travels inside of the physical wires. There's microscopic space between the wire strands when their made into electrical wires (since most electrical wiring is stranded and not solid copper), so the rubber/plastic outer layer actually encourages capillary action. I would suspect that a splice would slow it down some, but would probably not eliminate the issue as it still has small holes/spaces inside of the splice connector and between the wire strands.

So what was the conclusion to this story?

I have confirmed all of what you mentioned.
-ATF in the o2 sensor connector
-ATF connector as the source wicking fluids
-ECM connector (2nd form top) oily.

Is it worth changing all the harness, ECM and o2 sensors?

Alright, so I have been chasing down 02 codes and lean codes (p0174, p0171, p0136) for over a year. Replaced both downstream 02 sensors, a/f relay, and all general tune up stuff (plugs, cleaned maf, pcv, tb clean). The codes would always come back....sometimes not all 3 at once.

A deep Google search last night and I came across this oil in wire harness phenomenon. A light bulb clicked and I remembered last year while replacing one of the 02 sensors, there was moisture in the plug. So sure enough today, I pulled the top 3 plugs from the ecm and there was red-brown oil on the second and third plug.

@BlackWorksInc , any idea which sensor may be wicking moisture?

Can someone give me a diagram of this harness?

@OP if you're still around, what was your solution?

Here is my original thread; 02 Sensor Check Engine Lights

Attached Images

   

Looks like it could be transmission fluid? I am not sure... haven't seen this on a 4Runner before.

Ya, that is what I was thinking.

This link shows the sensor in the transmisison pan may be the culprit https://www.justanswer.com/toyota/6x...r-chronic.html

I think I am just going to dry everything out with compressed air and see how long it takes to re-saturate, and the check engine light to come back on.

Just thinking out loud here, but what if I cut the connection from the transmission sensor as the OP was going to do. Is there a type of connection to re-join the wires that would be less susceptible to capillary action (spade, butt, solder etc.)? What about splicing in solid core wire? I know solid core wire and vehicle vibrations don't mix too well though....

Going to add the end of the link so everyone knows what's going on. Thanks for posting all this it's very mysterious and I wish you the best getting through it so you can drive your 4Runner again with confidence.

I am very interested in how this turns out. I have had similar intermittent issues driving me crazy. I have replaced the A/F sensors and O2 sensors, just to have them go bad in a few months. The car intermittently will run like crap, and then will run fine for a few days/weeks/months, etc... and then run bad again... I'm thinking this may be a very possible culprit to the root of all my problems... Thanks for bringing it to my attention dta1984! Please keep us posted on the progress and ideas!

Thanks for posting that! It does clear up what I was referring to.

Hey no problem! Glad to see it may be the root of your issues as well. Our symptoms are very similar.

Most likely it will be this coming weekend i'll get back around to working on it. My intention is to first blow out all of the connectors and ECM, and see how long that lasts me. Next step will probably be to splice the wire somewhere to see if I can stop the capillary action.

I'm still find it hard to believe that the transmission fluid is getting pushed through the wiring harness, especially the O2 harness, and into the ECM. I don't recall ever seeing that on any of the vehicles I've worked on. I would think you would see one of the connectors between the transmission and ECM start to leak first. Plus, if the connectors are dry inside, there's no way that's the issue.

It's funny that this thread popped up because a friend of mine was dealing with some lean codes on his super clean, pampered FJ after the dealer forgot to put the air filter housing together. He put the air filter housing back together himself but ended up taking the FJ back after the codes re-appeared. The dealer's findings were that the transmission over-pressured due to a plugged transmission vent and pumped ATF through the harness to the ECM. They recommended replacing the ECM, entire engine/trans harness to the staggering cost of $5k, and that was just to start. It didn't sound right to my buddy because they never mentioned anything about clearing or replacing the trans vent or to find out why the transmission was over-pressurizing so he had them put it back together and he drove it home. Not to mention that the FJ is driving and running perfectly fine other than the CEL was on. The dealer supposedly showed him oil on the ECM connectors but the oil, or whatever it was, had no color, it was clear. When my buddy inquired about that the dealer stated that the ATF in the FJ was clear, not red (more red flags).

I went and checked the FJ out and pulled apart several connectors (O2 and some others) and found them to be dry. I next went the MAF, since the whole problem started with the dealer leaving the air filter lid off. I cleaned the MAF and so far the CEL is off and the FJ is running great. I feel that the dealer was completely trying to take advantage of my buddy. I never dug into the ECM to see if oil was on those connectors but I really doubt oil is getting to them anyway. Like I said, it just seems highly unlikely that such a thing could even happen, much less to a low mile, very pampered FJ.