Okay everyone, I need some help. My truck had been running great after I had to install a new upstream O2 sensor on Bank 1 back in August. At that time I decided to use dielectric grease for the first time. I did so by squirting the grease directly into the terminal recesses in the sensor-side connectors. (I did the same with the Bank 2 connector thinking I was doing something that would prevent problems down the road.)

When I learned earlier this month that that is the wrong way to use dielctric grease I decided to disconnect both sensors, clean them with electrical contact cleaner and reattach them. When I did that late last week I got a rude surprise. When I drove the truck short-term fuel trim shot up to +25 on both banks (showing closed-loop operation) and then within a mile or so the system went into open-loop and showed short-term trim of 0.00 on both banks and threw the P0130 and P0150 codes (front sensor LH/RH bank stoichiometric ratio(s) outside operating band). That's no surprise, given the +25% fuel trims.

I tried cleaning the connectors twice more, including removing the sensors from the truck and cleaning the connectors as carefully as possible on the bench. Same results. I then tried disconnecting the battery overnight in case that might clear the ECM. No help.

Does anyone have any other ideas why I'm getting these results? If not I guess I'll pick up two new O2s and try installing them but I'm skeptical it is a problem with the sensors and not just the connectors which I have cleaned carefully.

 

Nothing wrong with your method of using the DG except for there are some exceptions when/where/how to be used.

No personal experience using DG on O2 sensors. Did read an article that said the sensors are vented to atmosphere through the connectors and grease or sealant should not be used on them.

It's possible the combination of the DG and CC may have entered via vents and damaged the 02 sensors when heated back up.
......

 

Thanks for the suggestion. I recall hearing before that the sensors are vented through the connectors. I doubted that was the case (perhaps it is true for earlier or other O2s), but I checked a new sensor this morning and confirmed that there are only the four wires going from the sensor to the connector. I still don't know how the sensors are vented to be able to sample ambient oxygen levels (which I assume varies primarily with altitude). The only hint is in this language from the RAVE.

"The HO2S uses zirconium contained in a galvanic cell surrounded by a gas permeable ceramic, this produces an output voltage proportional to the ratio difference between the oxygen in the exhaust gases and to the ambient oxygen."

Is the ambient oxygen read via the gas permeable ceramic? I don't know how ambient air would get to the gas permeable ceramic since the sensor appears to be sealed except at the tip where the exhaust gas enters (Bosch product info - "Double laser-welded stainless steel body protects against contamination"). I don't know.

What I do know is that the ECM is not seeing any signal from the sensors (zero volts). That fools the ECM into thinking the mixture is very lean, beyond the range the
sensor can report. After a few minutes of that the ECM defaults to the fixedmapped data contained within its memory and UltraGauge shows fuel trim of 0.00 because the ECM is no longer adjusting fuel trims.

I'll just go ahead and replace the O2s and see what happens.

 

Well, here you go. This article explains O2 sensors very well, including how they are vented.

http://www.autotap.com/techlibrary/u...en_sensors.asp

I have two new sensors waiting to be installed but now that I have read this article I will try cleaning the connectors from the back and see if that works.

The ironic thing is that the O2 sensors were the things I wanted most to protect with dielectric grease because of how finicky they can be if the connectors get wet but the article points out that grease should never be used on 02 connectors.

 

The ambient oxygen is read at the MAF, is it not? Or does that only read total air flow and temperature?

 

Interesting point. The O2 percentage in the ambient air could be read elsewhere, perhaps at the MAF. The engine management system section of the RAVE states simply that the oxygen content of the exhaust gas is compared with that of the ambient air, or words to that effect, but it does not say anything about where the ambient air reading is obtained. Maybe I will go back and read the MAF section to see if it sheds any light on this. (Note: I reviewed The MAF section of the RAVE and found no indication that ambient air oxygen content is measured there. Air flow and intake air temp only. Perhaps ambient air oxygen percentage is assumed at a fixed value, but that would be surprising given that modern systems are supposed to be able to operate adaptively at elevations from sea level to high mountain.)

 

I'm pretty sure that modern, 4-wire sensors measure and compare the O2 both inside and outside of the pipe. Obviously at the perf tip, but also at the metal portion on the wire side. Anti-seize on the actual threads is fine but if you cover other areas, it could cause problems.

The MAF looks at volume and temp.

 

"... newer style O2 sensors "breathe" through their wire connectors and have no vent hole. It's hard to believe, but the tiny amount of space between the insulation and wire provides enough room for air to seep into the sensor (for this reason, grease should never be used on O2 sensor connectors because it can block the flow of air).

Well, this is apparently true. I installed the new O2s and went for a test drive. Short-term fuel trims returned to normal levels almost immediately once the sensors were heated and the system went into closed-loop operation. So that confirms it for me --- NEVER use dielectric grease on the O2 connectors!

 

Thanks for the update and good to know your experience information. Glad to hear LR back to purring sans codes/lights.

 

Looks like I spoke too soon. Everything was great on my mile and a half test drive, but when I left a little later to meet a friend for lunch the problem reappeared. I thought I'd cleaned the harness-side connector carefully, but there must have been just enough dielectric grease left in it or on it to cause the problem. Arghhh!!!