Expecting the Unexpected - GTO Wild FT Numbers
This vehicle came to us with a very poor run condition, MIL on, with the customer reporting that all four 02 sensors had been replaced multiple times recently.
A scan tool was connected and we found that B1S1 was unresponsive allowing Total FT to reach +74%.
We then went KOEO while watching both 02 sensors return to their expected values. After starting the vehicle for a short period, we then shut the engine off and went KOEO and opened the B1S1 connector, this is where we saw the first sign of trouble. There was quite a bit of oil on the connector, which seemed odd but we moved on without giving it too much thought initially. This red flag should have prompted us to look deeper and we failed.
After replacement of both from 02 sensors and numerous circuit checks while consulting the wiring diagram, we unplugged C2 at the ECM and found the connector full of oil.
What we found is that the oil pressure sensor was allowing oil to be pumped into the wiring harness traveling all the way to the PCM and then back out to both front o2 sensors. A followup scope check on both 02's with propane and an induced vacuum leaks produced expected results.
Now, this was not a straight forward operation and it took a while to get to the bottom of this. We used compressed air to push as much oil out of the harness as we possibly could and advised the customer on potential harness repair in the future.
Great post Scott! It's amazing the oil made it that far. I've seen oil pushed up into a module but never back out. I wonder if the heat of the 02's caused them to draw the oil out of the module.
Wow what a fantastic find on this. So do you think the oil was causing somewhat of a short? That is crazy to think of all that oil wicking like it did and where else it may be hiding. Great job getting this solved its definitely in the realm of not seen everyday. Thanks for the share and great captures.
Hi Michael, I suspect that the oil was providing a path to ground at the ECM In C2.
This is a good one Scott. I was actually going to do a quick tip on this exact thing. A good way to combat future wicking is to de-pin the connector, clean it, and put a little solder on the wire. This will completely encapsulate the wiring and not let the oil wick any further. You also have the added bonus of that wire not succumbing to the green crusties.
Interesting idea, however after thinking about this further, I wonder how the insulation will hold up with all that oi?
I would suspect as well as a harness in a valve body. I don't know if the insulation is a different material or not. It would seem cost effective to have it made of the same stuff. The weather pack would expand, but that's about it in my opinion.
Great find Scott. Haven't seen any GM's doing this. Remember seeing older Dodge Durango's, with the power steering fluid wicking up the wires. Would also skew the oxygen sensor signals.
I suspect that this was caused by the fact that the o2 sensor was not able to reference to atmosphere. What are your thoughts Scott?
Not sure Brin but I suppose that’s possible. Note the KOEO O2 voltages returning to reference but as soon as oil pressure is applied the voltage is pushed low. I need to research SI further and get back to you. One thing is for sure, there’s no machine out there one could have connected this to and spit out the the instructions for repair...
I think that we often think of engine oils, transmission fluids etc. as being conductive but through a little research, (Thanks Tanner) it seems that they have very low conductivity. I assumed that with automatic transmission fluids considering how many AT circuits are submerged in ATF. I suspect that engine oils do have a greater negative affect on insulation and weather seals. I'm not
I think you are right on the money Brin. Location of the vent to atmosphere can either be on the sensor itself or through the wiring conduit/insulation. Here is a portion of an article I just read. "While most automotive sensors are submersible, zirconia-based sensors require a very small supply of reference air from the atmosphere. In theory, the sensor wire harness and connector are sealed…
Excellent and thanks for sharing. I’ll see if I can dig up a closer look at the voltage change right after engine start up for any interesting behavior.
Another part of the same article.
Do you have a link to that article? We should credit the source.
Absolutely! en.m.wikipedia.org/wiki/Oxygen_se…
Its a bugger when a ECM starts pumping oil :) I'm glad I'm not the only one that gets tripped up along the problem analysis trail.. Nice find!
This one didn’t kick our butts too bad but it was an interesting find though. One I felt that worthy of sharing with fellow professionals one day! Btw, I spoke with an NTK SME a while back about this and he said that they’ve seen quite a few cases like this.
Hi Everyone, I am a technician in the UK and have been recieving posts from DN via Autologic very recently so am new to the family. We are Mercedes Benz specialists and have been coming across this situation of capillary transfer of oil through a wiring harness on certain engines (and transmissions) for quite a while and the one thing we always find is that oil will naturally travel to the
Hi Terry, Welcome aboard and thanks for sharing your experiences, this is what this platform was designed for. In regards to your question about vehicle selection, I suggest that you consider posting a new message to DN.