Catless Downpipe Tuning

Hey everyone! I'm looking for some advice. It's for my 2017 WRX.

Back when I first got it, it began throwing P0420. The engine was blown and replaced before I got it, and the DP was never changed, which I suspect to be the source of the issue. Wanting to jump right into repairing, before I gained the knowledge I have now, I erroneously bought a catted mid pipe which did not solve my issue as the cats are on the downpipe.

My question is that since I am a welder fabricator, I am wanting to weld my own pipe. However due to placement of the front and rear O2 I assume it will continue to throw P0420, as my previous tuner explained when I asked about it. I tune with SubaruEdit which can be confusing. I recently threw my pro tune back on because I was having issues with the fueling being too rich and every time I nailed down the trims it would change again. Even on the pro tune which originally had no trim at all, the car seems to be acting strangely with consistent trims. This is a separate issue but another one that I would like some advice on, as I don't want to keep fighting with it. I do think it is a mechanical issue, but I can't for the life of me find the source.

Basically I want to know how to make sure that when the new catless pipe is on, that the emissions codes won't keep a constant CEL on my dash (I know i can turn the code off in SubaruEdit which i have done, but I would like to know as well if there are any tables that will need adjusting due to it being catless and also going from 2.5inch to 3 inch pipe).

Additionally, I have a GrimmSpeed 3-port EBCS with the additional coolant line for Canada. I would like to install that as well since I'll be in the area and would like to know what tables will need adjusting with that change as well.

And finally, I'd like some insight into why my trims are so wild, even on the pro tune. As well, having taken the tuning course I know nailing the fueling is the first crucial step. However my engine loves to knock untuned, and that was another issue I was having while trying to tune the fueling. I would get close or bang on, then a week later it would run rich. The primary area being at idle with as much as -25% trim. It's possible I'm missing a temperature compensation table here?

Sorry for the long winded post, unfortunately there are a couple things on my pile that I haven't been able to figure out no matter what. So before I force myself to have to run on my tune by putting on parts that aren't compatible with Cobb tunes anymore, just wanted to get some advice to try figuring this out.

I cant give you any solid answers, but i can point you in a direction to research and learn about. This is regards to your maps changing.. Are you aware of the Learning Tables and the Value Ranges A, B, C and D? If not. Have fun hit the usual forums... I also have ecuedit... I dont think any of your issues are software related.

Hmm that's a good point. I'm going to check that out again. Thanks! Still gotta figure out what needs to be changed for the new exhaust though

The mods you've mentioned are downpipe, EBCS. Neither should cause -25% idle fuel trim, so it sounds like there's a mechanical fault with the vehicle if that happens when the MAF scaling is left stock.

Do you perhaps have an aftermarket intake, or maybe an air leak?

I have a GS intake, and I did scale the MAF properly. It was happy for some time, but then would progressively worsten again. I'm thinking a leak somewhere. I'm fabricating the downpipe and I've checked the turbo inlet to intake ram connection and it appears to be solid, same with filter to ram. So I may have to look elsewhere.

Faulty MAF extension harnesses are a common source of issues on those cars, so I'd give that a check as well.

I never knew that. Thanks so much for that information, I am totally looking into that.

You're welcome. A number of designs allow water intrusion which leads to minor corrosion on pins that can cause these bizarre shifts in fuel trim due to shifting MAF voltage. Let us know what you find on your car though, this is just one of multiple possible causes.