jgrant721

I recently put a set of Doug Thorley headers on the 3VZE in my '91 4Runner along with a Magnaflow high-flow cat and muffler (the engine had the head gasket done as a warranty repair during the original recall). At the time the headers were installed I also replaced the original O2 sensor (120K miles) with the recommeded NTK unit. This first NTK O2 sensor lasted a month. I knew it was dead because when the truck gradually became almost undriveable (balking, stumbling from 1500 to 3000 rpm) I went thru every other possible engine diagnostic, finally scoping the O2 sensor and seeing that it was flatlined. I took the sensor back and got a warranty replacement, installed it, and everything was fine, engine running great. Now a month later I'm back to exactly the same state.

I have searched extensively for info on the causes for O2 sensor failure that might apply here but I'm not having much luck. The only source of contamination that I can think of is the gasket sealant I used on the flange where the sensor mounts. I used a small amount of Loctite Red, Sensor Safe High Temp Sealant but it specifically says safe for oxygen sensors.

If anyone has any relevant information on this problem I would greatly appreciate it!

Thanks,

Jim

Swansen

hmm, yeah, i'm having something like this problem to, i didn't even think about it, but i used some high temp RTV to help seal up the exhaust. My thoughts are leaning a little towards electrical wiring, that being 20+ years old, thats kinda what i am thinking. Other than that, if your truck is burning oil (like mine) that will decrease the life of the senors.

abecedarian

sigh....
I'm thinking Swansen hit the proverbial nail on the head, sort of.... "...if your truck is burning oil... that will decrease the life of the senors {sensors}."
but I might add that an exhaust leak or non-functional EGR valve can affect the O2 sensor life span as well.

and I don't know of anyone using RTV on a gasket in the exhaust. anti-seize compound, yes... rtv, no.

Targetnut

Is it possible that your sealnt is leaking?

Air introduced to the exhaust stream will affect the sensor output.

abecedarian

yep. an exhaust leak does let some of the exhaust out, but also sucks air in. the extra air will cause the sensor to read a 'lean' condition and signal the ecu to enrich the mixture. and a too rich mixture will burn an O2 sensor out.

jgrant721

Actually an air leak on the sensor flange is the reason I was using the sealant in the first place! After I finished the exhaust work described above, I couldn't pass emmissions. I tested every one of the engine/emission control systems (EGR, AS, Thermostat, TPS, timing etc) and found them all 100% good. Finally I went along the entire exhaust system with a mechanics stethoscope. I found and corrected some small leaks at the pipe clamps but then found a leak right on the O2 sensor flange. It turns out the flange was really badly made, with a huge concave area that let a lot of air under the gasket. I sealed it up with the Loctite Red and it passed emissions perfectly. As I said in my original post, that only lasted a month.

By the way, yesterday I disassembled the exhaust pipe and took out the bit with the O2 sensor mount. I built up the concave area of the flange with my MIG welder and ground it flat so I get a good seal without needing any Loctite Red. I put in yet another new sensor (#3, under warranty thanks to the great guys at Lordco Auto Parts) and the truck once again runs perfectly. We'll see if it lasts more than a month this time!

I did speak to the Tech Support guy at NGK Canada and he also thought that having even "sensor safe" silicon that close to the O2 sensor could be causing the problem.

Also, to answer the other posts, there is no oil being burned by the engine as confirmed by my regular maintenance and the successful emissions test.

I will follow up on this post after a couple of months to report any useful information.

Jim

Jay351

Hey Jgrant, where you from?

jgrant721

Jay,

I'm not too far, just across the river in Cloverdale.

Jim

Jay351

Cool, perhaps I will see you around sometime....

jgrant721

Just to wrap up this thread, the cause of death of the two brand new O2 sensors was definitely the use of Loctite Red High Temp Sensor Safe Sealant on the O2 sensor flange. The truck has been running fine for several weeks with the 3rd new sensor which was installed without using the sealant on the flange (see previous post for details). I guess even though this sealant is OK to use on header gaskets (as indicated on the label) the contamination caused by having it in close proximity to the O2 sensor will kill the sensor within a couple of weeks.

So, although I will continue to use Loctite products, which typically do a great job, as advertised, I caution anyone against using any silicon based sealant, even if it's "sensor safe", right next to the O2 sensor itself.

Keep on truckin'!

Jim