The Great EJ251 Exhaust Issue

[aal219]

This is a bit of a long story, but any insight would be appreciated.

 

I have an EJ251 (Non-CARB) engine mated to the 4EAT that will not stop throwing P0420 despite having a new exhaust, two new cats, two new oxygen sensors, and an exhaust O2 sensor spacer.

 

In the summer of 2018, an exhaust shop in VA scrapped the whole exhaust (was rusted beyond use) and used an aftermarket UEL header (similar configuration to this https://i.ytimg.com/vi/QxCr6KroLU4/hqdefault.jpg) with two new catalytic converters, two new oxygen sensors, and a 2.5" stainless catback. I then moved to NC. This exhaust worked for 1,000 miles before it threw P0420. I leak checked it and found it leaked at the header flange to the cat. Needed head gaskets done at the time, so I ignored it and the car went down for 6 months while I performed various engine rebuild things.

 

In summer 2019, the car was running and I took it to a shop for A/C and exhaust fixing. In "fixing" it, they also put in 1 new O2 sensor and I don't know why. When they gave it back to me, they said it had rod knock and that the exhaust still leaked. It sounded a lot like rod knock (which it didn't have before I dropped it off), but ended up not being it. The car sat until January 2020 before I felt like dealing with it again and it turned out that the second catalytic converter (which had roughly 1,500 miles on it) had failed (matrix was sideways in the housing). Had a shop fix it and leak check it, still threw P0420. Took it back to them and they found that the FIRST CAT was completely missing (also had 1,500 miles on it: no clue where or when it went, but it probably is what destroyed the second cat). The shop put a LEV certified cat in as the new first cat, said it didn't leak and it STILL threw P0420.

 

They troubleshot the voltages at idle and it was reading happy. Cleared the code and drove it 50 miles (at 50 MPH) and no codes. Threw a check engine light the next day almost immediately after accelerating to 60 MPH on the highway. Took it back and they put an O2 spacer in. I then drove it 50 miles (at 60 MPH) with no check engine lights. It then threw P0420 on the drive to work the next morning.

 

My next thought was maybe the aftermarket UEL header was getting too cold at highway speed, so I put the stock skid plate back on the car to help hold the heat in. The car STILL throws P0420 on the highway.

 

I did perform my own leak check and there is a small leak in the header on a weld the shop in VA performed. I do not believe it is big enough to cause P0420 on a brand new cat. My IAC valve was cleaned 1,500 miles ago and the code does not show up at idle. I am not throwing ANY other codes. I stethoscoped my injectors and they are all ticking over correctly.

 

The shop recommends replacing the O2 sensors with Denso sensors (I have no clue what is in there now), but before I spend another cent on this thing, I want some outside advice. Thank you for taking the time to read this, feel free to ask any other questions I may have overlooked.

[08SpecB_DE]

What are your fuel trims at idle and while cruising down the road around 50mph, light throttle?

[doublechaz]

SpecB's comment drives toward my thought. I had one running like crap that turned out to be a bad gasket header to head. It lets in some air after each exhaust pulse and the fuel trims go way up. Then there is so much fuel in the exhaust that the cat's can't keep up and you get 0420 and burned up cats. Start by making everything upstream of the first O2 sensor totally leak free.

[aal219]

Trims once I got to operating temp were about 2.3% Sensor 1, 0% sensor 2 at idle. Seemed to be fluctuating, but anywhere from -3.2% to 2.3% sensor 1 and around 6.2% sensor 2 at 50 mph.

 

I'll actually log my data for my drive home and post the results in about 10 hours.

 

Thank you for all of the help.

[aal219]

Here's a link to my data logging. This is for a 15 mile drive starting in a town, going to a 55mph highway, then back to another town.

 

https://docs.google.com/spreadsheets/d/1c6uzU-YGNfR0YrU58EQJd_dMqjOTavjdUQ1kBZorZys/edit?usp=sharing

 

I'm putting the car away for the weekend and having the leak in the header welded on Monday.

 

I am pretty sure it was the app I used (Torque Light) but I wasn't able to get O2 voltages from sensor 1. Maybe it can't do STFT Volts at the same time...

 

Let me know if you need anymore information.

[doublechaz]

We're looking for long term fuel trims. I don't know if torque lite means long term when it doesn't say short term.

 

The short term fuel trim should pretty much only move away from zero during hard transitions in throttle like when there is a guy in the other seat telling you about the upcoming road conditions.

 

On my 95 I have bank 1 sensor 1 (front) and bank 1 sensor 2 (rear). For those two sensors I have STFT and LTFT. But I'm running the pro version and also 95 is OBDII-ish, so I don't know exactly how it would show up for you.

[RumbleRumble]

If your burning oil you can trigger the 420 code

[aal219]

Had the header welded and leak checked and it still throws P0420. Here is more cleaned up data from my drive.

https://docs.google.com/spreadsheets/d/1dVOuriq0j1Vp7WHvzwULwxQXKVv19Y09iWMv-PwRYxg/edit?usp=sharing

The first block is rolling down the highway around 55 mph, the second block is idle, the third block is the highway around 55 mph. I still can't get voltage readings from sensor 1 through torque, but the shop was able to with a diagnostic tool when they tested it at idle.

 

The shop bet me that it's non-Bosch oxygen sensors (allegedly there is a Denso and a brand with no name in there from the first install) and is going to order them, change them out, then try to replicate my results. They say they'll only charge if it stops the problem.

 

I'm starting to think that it may be the UEL header and that the stock tune isn't set up to interpret the change in pulses. The only thing that makes question this is the original over 1,000 miles I got out of it before it through a light. If anybody has any thoughts one way or the other, let me know.

 

@RumbleRumble, the heads were totally rebuilt and the compression is good. I do not believe I am burning oil.

 

Thanks again for all of the help.