Oil wicking into ECM - random A/F sensor codes P0171/P0174/P2195/P2197 (1 Viewer)

[white_lx]

(edit: Source of the oil confirmed: leaking transmission connector. See post #10. I hope the information below saves someone time and a lot of trouble one day.)

I've been trying to find the root cause of frequent A/F sensor codes on my 2008 GX470 with about 145k miles. P0171/P0174/P2195/P2197 would randomly appear and disappear. Initially, I replaced the A/F sensors as the A/F active fuel injection test showed that the sensors were indeed lying. The code would then go away for about 5000k / one year. But recently the frequency of the codes has increased.

After a lot of testing, I'm pretty sure I found the root cause of these problems as there is oil present all throughout the engine harness. It actually pools in the ECM and seems concentrated on connector E6 (2nd one from the top). I was expecting that of a Mercedes. But certainly not of a Toyota! Here are some pictures.

Left: ECM with top 3 connectors removed. Right: detailed view of socket E6.

The oil is wicking through the wiring harness into the ECM (edit: wicking as a result capillary action between the individual strands of a wire or wires). This particular connector E6 has the A/F signal wires and so this all seems related. I have also found the same oil in the connectors of the A/F sensors. The picture below shows the oil-covered A/F sensor connector locking tab after removing it from the connector, with about 4000 miles since replacing the sensor. The first time I noticed this, I didn't think too much of it as I suspected I had been too aggressive with Fluid Film and it wicked into the connector. However, I'm now 100% sure this is not oil from undercoating.

A/F sensor connector locking tab.

With that information, I since found numerous examples online about ATF wicking into the ECU on 4Runners and FJ's (see for example here). The cause is a leaking 'transmission wire' aka the transmission wiring harness resulting in ATF making it all the way into the ECU and then on to the oxygen/air-fuel sensors. It seems to be limited to A750(F?) transmissions.

In my case, it would certainly make sense for the fluid to be ATF, as the transmission's solenoid and temperature sensors also end up in connector E6 (highlighted in red below). Changing out the transmission wire would not be a real difficult job in itself, however in the rust belt the M6 bolt that holds the wire into the transmission is known tot seize and snap off requiring removal of the transmission for drilling / tapping.

Connector E6: Transmission related terminals highlighted in red. A/F sensor signal are on terminals: 22, 23, 30, 31.

What has me puzzled is that in all online posts the fluid was clearly red. In my case the fluid looks like engine oil. The oil that I found has absolutely no red color to it. I used a white tissue and dripped ATF, engine oil, gear oil and brake fluid onto it. The 'migration' behavior of the oil that I found seems to match the engine oil characteristics. It also has the visual (yellowish) appearance of engine oil rather than ATF.

Looking at all the other connections on connector E6, there are none that have any relation with engine oil (such CAM sensors, VVTi solenoids etc). So I am really puzzled by my findings. I used an 2005 GX EWD, so I'm not 100% sure that some of the wiring changed. Does anyone have an EWD for an '08?

Some other observations:

1. Symptoms seem to get worse in the summer and/or with humidity. I suspect this is because of hygroscopicity of the oil. The A/F sensor signals are very low currents and can easily be biased by even high resistance 'leaks'. I measured the resistance of the oil on tissue paper to be in the order of ~ 1MOhm. So not a perfect insulator.

2. Cleaned the ECM connectors (M&F) with contact cleaner and re-inspected after a few weeks of driving. Oil did not return, but codes did. I have not opened the ECM yet to clean the circuit board.

3. The A/F sensor signals turn lean only, not rich. For the problem to occur the engine needs to be pretty warm, at least 10 minutes of driving. Letting the engine idle for a minute or two after that, the signal will return to normal (stoichiometric). For example: the graph below shows logged data of total fuel trim on bank 2. At 600s the anomaly occurs and all of a sudden the A/F sensor signal starts to give incorrect readings, as expressed by the total fuel trim. Idling the vehicle for a few minutes will return the signal to normal.

 

[FrankTorres]

Very unique situation you have there.
An interesting correlation with the oil and your fuel trims.

I would defiantly be very interested in the source of that oil.
Could it be a blown/ leaking capacitor in the ECM/ ECU? but your showing too much for a mini capacitor.
Could it be refrigerant PAG oil? Spillage? ...does it smell? is it sweet?

Ether way you should open up and have a look, it should be cleaned anyway.

Good luck

 

[aztoytec]

This is an interesting one. If you can check the oil sender unit. Unplug it and see if the connector has oil in it.

 

[white_lx]

This is an interesting one. If you can check the oil sender unit. Unplug it and see if the connector has oil in it.

That was one of my first thoughts too! However, I was surprised to find that there is no connection between the engine ECU and the oil sender. The oil pressure switch signal is only used as a warning indicator on the cluster (see relevant section of EWD below). The engine will happily continue running even when the oil pressure switch is open. Similarly, I checked the VVTi selenoids, and they are dry. I have not checked the VVTi cam sensors yet, but I expect the same thing and they also do not end up in connector E6 of the ECM.

Very unique situation you have there.
An interesting correlation with the oil and your fuel trims.

I would defiantly be very interested in the source of that oil.
Could it be a blown/ leaking capacitor in the ECM/ ECU? but your showing too much for a mini capacitor.
Could it be refrigerant PAG oil? Spillage? ...does it smell? is it sweet?

Ether way you should open up and have a look, it should be cleaned anyway.

Thanks for your suggestions. Pretty sure it is not a capacitor failure as I'm finding the oil in the AF sensor connectors which are about 4 ft away from the ECU. Toyota's capacitor problems were solved by around 1998, IIRC.

I have not used the A/C (nor is it operational as this is my 'winter beater' to keep the LX rust free). Pretty sure it is not PAG as the ECU is not located underneath the HVAC box, but beside it. Also, the oil that I found is not really present on the outside of anything. Just inside the connector sockets. It doesn't seem to have a particular smell to it.

Spillage? Very unlikely for the same reason. Also, I have owned this vehicle now for 5 years and has maintained beyond the book (by myself, so no questionable rushed mechanics involved). The PO always had it serviced at a Lexus dealership.

I agree with you that I need to clean the PCB, but I was hoping to stop the flood of oil first. Otherwise I would need to clean it again and I'm a concerned about disconnecting and connecting the ECM over and over because I'm trying to prevent the pins from bending/breaking.

 

[Rednexus]

Trans fluid is usually only red when it's new/new-ish (like less than 10K on it). Toyota WS in particular is known for turning a pretty dark color quickly. So the color might not exclude transmission fluid as the culprit.

I just popped the hood on my GX and didn't note any obvious ways where oil could get into the ECU harness that leads into the cabin. It's well above the valve covers and other areas that might leak. Nevertheless, if you want to pinpoint if further, I'd take a razor blade and slice open the harness sheathing and see if the wires are oil-covered or not. If they are dry under the hood, than it's coming from somewhere else. If they are wet perhaps you can continue to probe around until you find the branch of the harness where the oil is coming in at.

 

[white_lx]

Trans fluid is usually only red when it's new/new-ish (like less than 10K on it). Toyota WS in particular is known for turning a pretty dark color quickly. So the color might not exclude transmission fluid as the culprit.

I replaced the ATF when I purchased the vehicle, about 5 years ago. Since then I put on about 22k miles. Recently I had to replace the radiator and the ATF that came out of cooler built in the rad was a nice red color. The fluid that I'm seeing now in the ECU may have had a long time to travel through the wires. So, I could be seeing fluid from before my replacement. It is just strange that there is absolutely no trace of the red left and that is was behaving so much different from the WS sample on my tissue paper. It would explain the electrical problems as Toyota WS is hygroscopic as per their Service Bulletin "if exposed to the atmosphere, ATF-WS may absorb moisture and potentially cause shift concerns if used in operation".

I just popped the hood on my GX and didn't note any obvious ways where oil could get into the ECU harness that leads into the cabin. It's well above the valve covers and other areas that might leak. Nevertheless, if you want to pinpoint if further, I'd take a razor blade and slice open the harness sheathing and see if the wires are oil-covered or not. If they are dry under the hood, than it's coming from somewhere else. If they are wet perhaps you can continue to probe around until you find the branch of the harness where the oil is coming in at.

I should have been a bit more clear in my first post. The fluid is traveling via capillary action between the individual wire strands inside the wires. The outside of the wires and the harness have no evidence of any oil at all. That's why I was looking for a breached internal seal at one of the sensors in contact with the engine oil (CAM, oil pressure etc).

Over at the 4Runner forums, it has been reported that soldering in solid wires at the A/F sensors can prevent symptoms coming back after replacing the sensor. The capillary action disappears when using solid core wire.

 

[white_lx]

Trans fluid is usually only red when it's new/new-ish (like less than 10K on it).

Your comment had me revisiting my earlier kitchen towel test (below). It shows the samples of the different fluids and a sample obtained from the connectors (top left). I started with just a few drops of each fluid. As they have spread out, they have become very pale. The red from the ATF is still just visible.

The fluid samples were collected about two weeks ago. But now after all that time, a yellow ring has appeared around the WS fluid! The yellow could be a match to what I found on the connectors - as indicated by the blue line. (The kitchen towel was able to separate the ATF WS into different fractions, very similar to paper chromatography.)

Since the oil moves by capillary action through the wires, it is certainly possible that it is separated into the different oil components and only the yellow makes it all the way through the wires into the ECU connector. The red dye was insoluble in that oil fraction of the ATF.

The next logical step would be to remove the connector from the transmission and see if it is filled with ATF.

 

[Rednexus]

You could also try a capillary break in the trans wire if that is the culprit That could be a simple as a butt splice heat shrink connector which will have larger "pores" and hopefully stop further migration/wicking of the fluid between the strans. Or splicing in a section of solid wire which would be a total break.

I really don't think there is good mechanism for it to get in via the engine bay - the harness is all above the top of the valve covers.

 

[white_lx]

You could also try a capillary break in the trans wire if that is the culprit That could be a simple as a butt splice heat shrink connector which will have larger "pores" and hopefully stop further migration/wicking of the fluid between the strans. Or splicing in a section of solid wire which would be a total break.

My concern when you use the heat shrink ones is that the oil will pool inside the splice and then keep migrating to the next section of wire. There has to be a real break. The oil even crossed between the different terminals of the ECU connector. This happened because the capillary action pumped the oil into the connector socket. From there on this pool of oil was carried to the A/F sensors. The knock sensors are on the same connector. It wouldn't surprise me if they are affected too.

The best way to stop the migration, would be to install an hermetic glass/metal seal and solder the wires on both sides. Or a DIY version using potted copper pins and solder to both sides. One complication is that I tried to solder a soaked wire and was only able to get the solder to stick by cleaning the strands with flux intended for pipe first. It made a huge mess and there is not a lot of space to work behind the glove box. Therefore crimping is probably a better solution than soldering. Removing the wiring through the firewall grommet into the engine bay would provide more working space.

Some other alternative repair options I have been thinking of:

Replace transmission wire and connector. This would be the real fix and prevent any wicking. The part is inexpensive and has been updated (new part #).The pan bolts and especially that connector bolt have me concerned in our climate here. If not for the corrosive salt action, that would be my preferred option.

Another repair option would be a short jumper cable (F->M) for connector E6 using solid-core wire. I found some connectors, but they have a different part# from the ones that Toyota lists in the EWD. On my list of things to check. Fixing it this way could be somewhat clean fix without needing to cut into the harness. It would also be totally reversible.

 

[white_lx]

Source of the oil leak confirmed: internal transmission wiring harness. ATF is leaking past the pins of the transmission connector. The connector fills up with ATF and capillary action wicks it into the ECU. From the ECU, the ATF made it all the way into the A/F sensors. Pretty crazy that that is even possible.

The ATF pooling in the transmission connector has the reddish ATF color. It seems to get separated by the capillary action between the fine strands of the wires and comes out yellow over at the ECU end.

 

[FrankTorres]

Congratulations is in order!

I assume one would need to drop the transmission pan for a simple R&R of the harness?
Now will the fuel trims normalize after you clean the ECU?

 

[BoostinZX]

Thank you for digging more into this! I have been suffering with these problems as you saw on the 4runner forums for years now and was close to giving up. What is your next course of action you think in order to fix the issue permanently? I would love to get my 4runner running well again!

 

[white_lx]

I assume one would need to drop the transmission pan for a simple R&R of the harness?

That would be the preferred fix. However, 'simple' might not be the appropriate description. I have gotten some cold feet about this after watching a video were things go south quickly. There is a single bolt holding the connector that is often difficult to remove because of corrosion (aluminum housing + steel bolt + road salt = a real problem). You can see it on the picture in post #10. There is also not a lot of space to work. At the minimum the propeller shaft to the front differential needs to be removed to have access to that connector bolt.

Other options would be to install solid wire or some other blockage in the 13 wires running to the transmission of connector E6. Or a patch cable for between E6 and the harness., using 13 solid strand wires for the transmission signals. 3rd party part# for connector E6 looks to be 1674558-1. A quick search shows that the female + male + pins can be purchased online. This would also be a very easy low cost DIY fix for anyone that has the same problem.

Have not decided yet on what approach to take. Suggestions are welcome. I would like to have the vehicle on the road in the next few weeks. Winter is coming.

Now will the fuel trims normalize after you clean the ECU?

I think so, but I may need to replace the A/F sensors as well. After the wire repair, the plan is to clean the ECU and go from there. It is very difficult to figure out what is going exactly on with the randomness I was seeing lately.

Also it looks like my B1S2 oxygen sensor has gotten very slow to respond. It is also on the same connector. I'm not sure if the sensor is damaged or it is also affected by the contaminated ECU. The B2S2 oxygen sensor is on the next connector of the ECM and seems to be working fine.

I'm hoping complete engine wiring harness replacement is not required and that over time the oil will work itself out of the wires.

Thank you for digging more into this! I have been suffering with these problems as you saw on the 4runner forums for years now and was close to giving up. What is your next course of action you think in order to fix the issue permanently? I would love to get my 4runner running well again!

Sorry to read that your problems have gotten so bad. Did you ever take the vehicle to a shop? Did you check the connector of the ECU for oil? After I cleaned it, it didn't seem to come back very quickly. So I am wondering if that would be enough to get it back on the road.

Do you have access to Techstream? You can run the engine in 'open loop' mode and test that the engine runs fine with A/F sensor feedback. That would exclude other underlying problems.

 

[BoostinZX]

Sorry to read that your problems have gotten so bad. Did you ever take the vehicle to a shop? Did you check the connector of the ECU for oil? After I cleaned it, it didn't seem to come back very quickly. So I am wondering if that would be enough to get it back on the road.

Do you have access to Techstream? You can run the engine in 'open loop' mode and test that the engine runs fine with A/F sensor feedback. That would exclude other underlying problems.

I took it to a reputable local Toyotas shop a long time ago, and they saw my secondary air bypass and hung up on only wanting to fix that first and then attempting to troubleshoot past that if there were still issues. I showed them the oil in the ecu and the front A/F sensors and they just kind of shrugged it off.

My ECU has the same oil in the same connector, and in the A/F Sensor Connectors as well, just haven't checked the source (Transmission Harness Wire). Mine isn't rusted, so I may try to get that wire and get it up on a lift to see what I can do to replace that, but still won't stop it from happening in the future i guess? I'm also assuming I need to clean out the ECU and all the other connectors associated with that part of the wiring harness.

Such a weird and frustrating issue for sure. Definitely keep us posted on what you end up deciding to do and I will do the same! Mine isn't my primary car, so it just sits most of the time, but when I need/want to drive it for mountain bike trips etc, it sure would be nice to have it reliable and not shutting off on me

 

[white_lx]

Thought a bit more about the approach to fix this problem. I am going to attempt to fix the source of the leak, rather than patching the wiring. Ordered the following parts:

8212560651 WIRE TRANSMISSION
3517830010 GASKET (FOR TRANSFER OIL PAN) (2x, for drain and overflow plug)
3516860010 GASKET, AUTOMATIC TRANSMISSION OIL PAN
9030131014 RING, O (FOR OIL STRAINER)

I took it to a reputable local Toyotas shop a long time ago, and they saw my secondary air bypass and hung up on only wanting to fix that first and then attempting to troubleshoot past that if there were still issues. I showed them the oil in the ecu and the front A/F sensors and they just kind of shrugged it off.

Oil in the ECU is certainly not related to the secondary air bypass! What were they thinking? Note that the engine stalling could be explained by an intermittent connection of the air bypass device (loss of MAF signal).

Are you able to read the codes? It is very helpful to get Techstream up and running (or have a scanner with active test capability). That way you could also retrieve the freeze frame data and perform A/F active testing.

so I may try to get that wire and get it up on a lift to see what I can do to replace that, but still won't stop it from happening in the future i guess?

The part# has been updated, so I hope there was an improvement? If not, what would be the probability of winning the 'lottery' twice?

The problem may be more common than we think, as it only manifests itself with vehicles with primary A/F sensors. The primary oxygen sensor signals of the earlier model years are probably only minimally affected by the oil contamination.

 

[BoostinZX]

Thought a bit more about the approach to fix this problem. I am going to attempt to fix the source of the leak, rather than patching the wiring. Ordered the following parts:

8212560651 WIRE TRANSMISSION
3517830010 GASKET (FOR TRANSFER OIL PAN) (2x, for drain and overflow plug)
3516860010 GASKET, AUTOMATIC TRANSMISSION OIL PAN
9030131014 RING, O (FOR OIL STRAINER)


Oil in the ECU is certainly not related to the secondary air bypass! What were they thinking? Note that the engine stalling could be explained by an intermittent connection of the air bypass device (loss of MAF signal).

Yeah I don't know... I wasn't super impressed with them, but everyone around here swears by them... They also lost my keys, lol..

Are you able to read the codes? It is very helpful to get Techstream up and running (or have a scanner with active test capability). That way you could also retrieve the freeze frame data and perform A/F active testing.

I have a cheap obd2 reader and running torque pro app on a tablet. I can constantly see high fuel trims on both banks. Sometimes the car runs perfect and the codes clear, but then they usually come right back. I am now pretty constantly pulling bank1 and bank2 too rich codes (P0172 & P0175). It originally started as P0174 (too lean) and then went lean on the other bank... then manifested itself into too rich, probably from over compensating for so long... or it just truly doesn't know what's going on because of the inconsistent readings of the sensors. I'll PM you the link to my thread with all the details of my issues. I think we are on the same path... I may have to just order the same stuff you did and then find some time to work on it... I'll probably have to replace my front A/F sensors again (for the 4th time). And also clean out the ECU really well just to make sure. We have an E parts cleaner at work that i may throw it in and give it a good bath.

The part# has been updated, so I hope there was an improvement? If not, what would be the probability of winning the 'lottery' twice?

The problem may be more common than we think, as it only manifests itself with vehicles with primary A/F sensors. The primary oxygen sensor signals of the earlier model years are probably only minimally affected by the oil contamination.

I sure hope it would fix the issue, from watching the video links you sent in an earlier thread i really hope that pan comes off clean, that looks like a much more involved job if you end up having to pull the entire trans, etc.

 

[BoostinZX]

Also found some interesting wiring options if one chose to solder in a "dam" in the transmission wiring to prevent it from traveling further up the wiring harness.

Anticapillary wire - HUBER+SUHNER

www.hubersuhner.com

StackPath

No clue on pricing, but was googling and found it interesting... I'm assuming most of the newly manufactured harnesses are made out of this or some type of it. I wonder if the new part number for the replacement harness on our rigs has something similar.

 

[white_lx]

I sure hope it would fix the issue, from watching the video links you sent in an earlier thread i really hope that pan comes off clean, that looks like a much more involved job if you end up having to pull the entire trans, etc.

Step 1 completed! I was able to replace the transmission wire without any casualties. The transmission connector bolt was actually not that tight. I hit it with penetrating fluid for a few days in a row. I also sprayed penetrating fluid into the open end of the through-hole. You may need to bend the straw a little, but I was able to soak it from both ends that way. For the pan bolts, I applied the penetrating fluid from the top side where they are exposed. I used a small dropper bottle because it is really difficult to access them with the spray can without making a huge mess. I was able to re-use all 20 pan bolts and the single fastener for the connector.

Some other hints:

* Take a picture of the routing of the wire and the connections to the solenoids before removing the harness. It will help with the installation of the new wire.

* When feeding the wire harness through the opening of the top of the transmission, the wires and internal connectors will pick up dirt as they drag around the outside case of the tranmission. I covered the new transmission wire in a newspaper bag and taped it tight with a few pieces of masking tape. After feeding this 'sausage' through the hole, I removed the bag and ended up with clean wires and connectors.

* I used a 10mm flexible ratcheting wrench bent 90 degrees as a 'crowfoot' to remove the connector bolt. There is just enough access between the transmission and the forward propeller shaft. No need to remove the shaft.

* Transmission was refilled with about 5-1/2 quarts of WS ATF. About 1/2 quart then came out of the overflow plug at temperature.

* Here are all the torque values required for this job:

Connector bolt	48 in.lbf
Oil pan bolts	39 in.lbf
Strainer bolts	7 ft.lbf
ATF temperature sensor, 36mm fastener	8 ft.lbf
ATF temperature sensor, 12mm fastener	7 ft.lbf
Drain plug	21 ft.lbf
Overflow plug	15 ft.lbf
Fill plug	29 ft.lbf

I'll PM you the link to my thread with all the details of my issues.

I found the thread. Your oil in the ECU is clearly yellow as well. I wish I had seen that earlier. The color of the fluid really sent me off in a tangent.

Also found some interesting wiring options if one chose to solder in a "dam" in the transmission wiring to prevent it from traveling further up the wiring harness.

I'm assuming that the Toyota wires for the transmission are made of the anti-wick wire. The insulation definitely has a different look to it, but that could also be because they designed to be oil-soaked?

There is actually very little potting in the connector. I may open it up to see what is going on. Not sure if the potting is designed to prevent the oil from coming out, or if the wires are non-wicking (or both). It is a very flexible rubbery potting material. Not like the hard epoxy typically used for electronics. The potting level in the new connector seemed a little higher.

On a Mercedes forum I read about removing the insulation of the wires for 2-inches and then soldering them solid to prevent wicking. That would be another method.

On to step 2 now: getting the A/F signals back to normal. This will be delayed some by (Thanksgiving) this weekend and a few other maintenance items for the winter season while the vehicle is up on jack stands.

 

[white_lx]

Decided to cut one of the failed A/F sensors open and found clear evidence of the ATF inside the sensor.

Denso 234-9051 A/F sensor

Inside the sensor body are several ceramic parts and the sensor element. The largest ceramic part at the rear of the sensor houses a 4-pin socket that is very similar to those used for wedge light bulbs. The sensing element has PCB-like traces that make contact with the spring loaded terminals of the socket. The burned ATF is very obvious from the pictures below. After seeing this, I think that sensor replacement is going to be a necessary part of the repair.

For completeness, this is what the element looks like.

 

[white_lx]

Repair step 2: The A/F sensors.

I started with unplugging the A/F sensors on both banks and cleaned the connectors with contact cleaner. The connectors were left unplugged for several days and I did not see a single drop of the fluid return.

After seeing the burnt oil inside the A/F sensor that was cut open (previous post), I wanted to start with new sensors, otherwise I may be chasing a ghost. However, the B1 sensor was just replaced (<300 miles) and Techstream testing showed it was behaving ok. So for now I decided to replace B2 sensor only (they are a lot more expensive than regular oxygen sensors).

Initial testing with Techstream is very promising. For now I was unable to re-create the original symptoms of random lean fuel trims. Even when getting the engine fully warmed up, the fuel trims stayed below a few %. I also ran the fuel trim active test and both sensors were behaving perfectly. Once I put the vehicle back on the road, I can start putting on some miles and see what happens with the oil that is left in all the wiring.

---

Footnote: The location of the AF harness connector between the firewall and the engine is really tight. Maybe my hands are too big, or my arms are too short (or both). I'm always struggling with those connectors and got pretty annoyed with them with all the sensor replacements over the years. To make manipulation of the sensor connector a little easier, I printed a little holder that slips on the sensor's connector so it can be held firmly with pliers. With 12" long needle-nose pliers the connector is back in place in 10 seconds. After the connector clicks in place, the little plastic piece just slips off the connector and it can be re-used.