P0420 Exhaust leak

[GrassFed]

Howdy guys, '00 California Emissions Dual-Catalytic Conveter

New Rear O2 Sensor as well as Upstream A/F Sensor.

Here Goes-

I tripped a P0420, it seems to come on when I drive at low speeds for a long duration. I have read that others with this code experience it showing up in similar situations.

I connected a vacuum to my tailpipe and started spraying dish soap all over the joints and O2 Sensor bungs. I have a leak at the driver side manifold to cross-pipe flange. I also have a larger leak at the rear O2 sensor bung. Not the actual bung itself, but possibly out of the stud and or gasket area.

Better yet, and a huge question I can't believe I haven't seen discussed yet is the flange just downstream of the rear O2, a couple of inches away.

My question is, this leak being so close to the rear O2, even though it is downstream of the O2 as the exhaust pulses, could it not pull some fresh air and throw off the reading of the rear O2? I can see my rear O2 voltage switching from normal about .7, to very lean, about .2. This switching from good to lean could be caused by the exhaust sucking in outside air right next to the rear O2 correct? Ultimately telling the system that the Catalytic Converter is not doing its job.

The leak at the cross-pipe is before the A/F Sensor and should not pose too much of an immediate problem for my current code.

Especially because my fuel trims are not bad at all. Usually +- ~5% NET at all times. Meaning, I add Short term and long term together. Many times this comes from a Long Term Fuel Trim of 0% and a short term that jumps around around 5%.

If a video of the leaks would at all help y'all visualize what I'm talking about please let me know.

Thanks in advance,

Matt

 

[baja1]

I don't understand anything you're talking about, but tell me about this soap test and do hear an obvious exhaust leak? I have a tick that starts after its warmed up but I can't pinpoint the noise...hoping its an exhaust leak (started after a recent engine swap. thanks!

 

[GrassFed]

I put a shop-vac oriented to push air into the tailpipe, then mix some dish soap and water in a spray bottle. Spray all of the flanges and the exhaust manifold at each cylinder and look for where more bubbles are being created.

I can definitely hear a leak. Usually I would guess that an exhaust leak would be worst when cold and best when full warmed up. As the metal expands it'll help seal the leak.

 

[19963.4lsr5]

The fuel trims are not added together. Is more like a decimal point between the two. LTFT.STFT is how it acts. Every time the STFT maxes out at either 30 or -30 it adds or subtracts a 1 to the LTFT and then zeros itself.


Sent from my iPhone using Tapatalk

 

[Miatafanatic77]

So I got the exhaust leak all taken care of. Dang, its like the car grew two extra cylinders I thought it was fast before

 

[GrassFed]

Miata, if you're looking for your 15 post minimum please do it elsewhere.
That is good news about your exhaust leak though.

What do you gurus that have dealt with P0420 think about all of this?

 

[calimobber]

What do you gurus that have dealt with P0420 think about all of this?

I Just recently have been wondering that since I lost my exgaust gasket a while ago and never even thought it could be my 420 issue until recently. I cant get the bolts off so I think today im going to cut them off and replace the gasket and bolts. Hopefully this fixes it.


Did you ever find a fix for your p0420 issue?

 

[calimobber]

Quick update after I installed a exgaust gasket, this seems to have fixed my p0420 check engine light.I drove about 8 hours over the last few days and the light never came back on. I guess even leaks after the o2 sensor can set it off if too close to the sensor. Definetly happy about finally getting that annoying code fixed.