I've got a very odd problem and I'm not sure what to do with it. Doesn't seem like anyone's had this issue before.

I've been chasing a code 25 for the last two months on and off and came across the problem: bad gasket on the 02 sensor. I bought a new one and put it on (after tightening it pretty well) but there was a gap about 1 mm wide in the middle when I could stick my fingernail through. Obviously, this wasn't going to work. I took the metal rim off the old o2 gasket and narrowed it on the ends (to thicken the center) and put both back on. I checked it again and it seems to be holding for now.

Obviously, this is not a long-term solution. I expect the old gasket will break down pretty soon. Does anyone have any ideas on how to fill the gaps in between the gasket and the pipe? The exhaust pipe gets so hot, probably up to 300-400 degrees so I'm not sure if there's a sealant out there that can stay stiff enough to keep a seal. I'm open to any and all suggestions.

 

Did a gasket come with the O2 sensor? What brand is it? If you have that kind of issue, I'd guess it is some of the old metal gasket still left on there.

 

Yes, the o2 sensor did come with a gasket but I did not install it, had a shop do it a year ago. I filed down the exhaust pipe where it comes in contact with the gasket thinking there might have been leftover gasket or sealant on the pipe. It helped a tiny bit but there wasn't anything substantial.

I was milling about and found these:

Exhuast System Joint and Crack Sealer
K & W Copper Coat Gasket Compound

Do you guys think either will work? I don't want an adhesive, just a sealant.

 

Make sure that the O2 sensor flange is dead straight/flat.

Should look like this...


What usaully happens is the flange gets bent when someone (over)tightens the nuts on the studs holding the sensor to the exhaust pipe. Since there's a metal ring in the middle of the gasket, which doesn't compress when the nuts are tightened. And it's very easy to tighten the nuts enough(rather too much) so that the gasket squeezes out under the ends of the O2 sensor flange. Causing the flange to be supported by the metal ring in the middle, while the nuts force the ends of the flange to be bent downward(towards the exhaust pipe).

If that's the case, you need to straighten the O2 sensor flange. Or buy a new one. Then reinstall it with a new gasket.

Gasket showing ring in the middle...


Nope. I can pretty much guarantee you they won't.

Don't ask me how I know...

 

Kinda off topic/thread jack...but it's in the ballpark.

Is the stock exhaust tubing 2 inch OD? And is the O2 sensor suppose to be dead center in the exhaust pipe to get a good reading?

Back on topic...sorry for the thread jack.

 

I have a feeling something is not right. I can't really see the top of the exhaust pipe but when I run my finger along the top I can feel it's not level. It seems to dip in the middle just slightly, enough for air to escape. But I think the exhaust pipe has always been like that, I can't imagine excaping exhaust gases for 7 months on a 21 year old truck would corrode it all away that fast.

I came across a post, can anyone confirm that the Toyota o2 sensors are indeed bent to fit better? Or, could I bent the flanges up ever so slightly to make the one I have fit better?

 

Yes it's 2 in. stock, I had 2 1/4 put on for the cat-back replacement. The o2 is just north of the cat so it's still on the original piping. It's set right in the middle on the top of the pipe.

 

In relation to the suggestions made by MudHippy, I would also check the flange on the exhaust itself. Being an older vehicle things become more brittle overtime. If the O2 sensor has been changed out and over tightened more than once it could have slightly bent up the ears on the flange itself. I've had to replace that on my 4Runner. Standard Motor Company makes a weld on flange (comes with gasket) to replace the one on the exhaust pipe. You should be able to get it any local parts store it ran me around $10 dollars. Problem solved, no more O2 leakage.

 

I don't have any welding tools or access to any, so not sure if that's an option for me. Good idea though, I would do it if I could.

 

The pic above is an OEM Denso O2 sensor(exactly what Toyota installed at the factory).

Here it is again. Now look carefully. Does it look bent to you?


Looks straight as a fricken arrow to me...

Let's take another look at the OEM-type gasket too..

This is the TOP of the gasket(faces the sensor mounting flange).


This is the BOTTOM of the gasket(faces the bung on the exhaust pipe).



O.k., now let me try putting this another way. The actual seal made by the gasket(so you don't get any more codes) is established by the metal ring in the middle of the gasket. The rest of the gasket is secondary(and may as well not even be there once the metal ring is firmly in place). So there needs to be maximum pressure on the metal ring in the middle of the gasket for it to seal properly on both surfaces. But, how could that work if the flanges weren't perfectly flat? Think about it...or don't bother. And just take my word for it...because you can.

P.S., I could be an expert on this subject(as in ALL THINGS 3VZ-E). And you could be a little too new here to realize that yet. Just some things to consider...

 

I'm not questioning you personally, it's just the nature of forums ya know? Some guy could make an account and post his 2 cents and not even own a Toyota. Then someone like yourself who actually knows the issue could post. The OP then has to decipher which advice is good and which is not and act accordingly. So when I see one person saying they are flat, and another saying they are slightly bent, what is one to do? Who knows, maybe someone just took an old o2 and could verify if they are indeed perfectly flat or not. Nothing personal.

The o2 I have on there is perfectly flat. The gasket I have is also, perfectly flat. I tightened it good and there's still an air leak, right in the middle. The old gasket, also perfectly flat, has carbon deposits coming from the center ring because it did not have a good seal. What I'm looking for is a better way to create a seal in the middle, since tightening it any more will break the bolts off (been there, done that).