N/A lean issue stock ecu
 

Naturally aspirated/stock ecu and intake with a 2.25 inch goodwin racing header/cat/catback and experiencing 14ish AFR at WOT.
Strangely, I see more normal AFR (about 13) if I'm giving the car 70% throttle, then tipping to 100% I can watch the AFR jump up to 14s and don't experience any stronger pull. Read that the stock ecu runs VERY rich and I don't believe simply adding an exhaust to the car would cause it to go that lean.

Had a turbo on the car for about 2 years and it was able to fuel for 200whp, so that indicates its not a fuel pump/FPR/filter/supply issue?
Checked everywhere for vacuum/intake leaks.
Used obd2 tool to check maf airflow rate (shows reading increase when going from 70-100% throttle, so ecu is seeing more air...)

I also live at about 4500ft, so a little leaner makes sense... but again why am I richer/correct at 70 percent throttle but not 100?

Looking for some help...

In my experience dynoing stock ecus, this sounds normal. Also it’s never just 14, it’s 14 below a certain RPM, then dips to 11s by redline. What’s it do throughout the entire rpm range?

At WOT it runs ~14.7 until about 4k rpms, then drops down around 14, then will drop only slightly more to mid 13s by redline.

What's got me boggled is that it runs normally at ~70% throttle... 14.7 until about 4k, mid/low 13s above than and down into high 12s near redline.

Yeah that’s completely normal for a stock ECU.

OEMs generally tune to 1.0 lambda essentially, so you’re right on there. Im assuming it goes richer towards redline cause it stops using closed loop correction. Doesn’t sound like anything is wrong.

Also generally altitude will make a car run richer—less oxygen for a volume of air—but MAFs are generally really good at compensating for altitude changes as they’re reading the mass of the air not just calculating volume and extrapolating density like an Alpha-N or VE tune might.

Stock ecu at WOT:
1-4k rpm: 14.7ish
4-7k rpm: 14 down to 13.7ish by redline
(The other night randomly this was more like 13.5 down to 13ish)

At 50-70% throttle:
1-4k rpm: 14.7ish
4-5k rpm: 13.5ish
5-6k rpm: 13ish
6-7k rpm: high 12s

So far I've replaced: injectors, fuel filter, FPR, fuel pump, checked tps, cleaned MAF.
I am totally stumped and don't know what to do next. Based on how rich I've read these cars run bone stock, I wouldn't think JUST a header, high-flow cat and muffler would cause it to run in high 13s at WOT which is dangerous territory. I have not touched the intake in any way.

Could it be an issue with the wideband itself?

That is definitely not dangerous for a stock, n/a motor. I really think you’re chasing a non-issue here.

Isn't it 100% indicative of some problem that the car leans out as more throttle is applied? More air should mean more fuel...

Well, the difference in flow between ~75% throttle and 100% throttle is actually pretty small. Most n/a fuel tables level off around 70-80% throttle anyway.

What year is your car? The NA maf-based ECUs are pretty simplistic if my memory serves, and I think they have the equivalent of like 7-8 cell tables for fueling. So it’s possible that even though you are seeing slightly more air, the ECU might already be at it’s maximum calculated fuel load. I forget, does your ECU show you long term fuel trims?

More importantly, how does the car drive? Is there some functionality issue you’re trying to solve? If it drives properly I’d leave it alone.

The LTFT is +4-7%.

Since I installed the headers/exhaust and wideband, the car was 5mph slower down the front straight at my local track, my corner exit on the corner before this straight was significantly higher. The car seems to be down on power.

I am also planning on installing an MS2PNP to take advantage of the exhaust as well as a better intake and 91 octane. BUT before I put the MS2 on the car, I obviously want to ensure that the afr it displays is correct so that my fuel tuning will be correct. Based on everything I've read, my car should be much richer than 14-13.8 at WOT, which tells me there's an issue with the reading.

I'm going to remove the header and check for leaks at the header flange as well as cracks/leaks in the header itself. If that doesn't identify the problem I'll be buying a new wideband sensor.

Possible leak at the header skewing the reading. If it is reading incorrect, it will trick the ecu and add the wrong amount of fuel. I would check the spark plugs to see how they look. The AFR that you are seeing is not too far out of range, but if the O2 sensor is seeing a large false reading, it will try to compensate by adding a lot of fuel to get it back to where it is happy. You can confirm this by checking your spark plugs to see if the engine is actually running rich. You probably will smell it in the exhaust as well.

If you increase the pumping efficiency of the engine it will move more air. If the fueling is a constant then the net will be a higher afr. A low compression small bore engine can run 13.5 no problem. The ideal range would be 12.5-13.5 for NA power. I don’t tune below 12.5 unless the compression is over 10:1. Although for NA it is better to trade afr for timing. Read your plugs, look for peppering. Look at the piston crowns. If you’re concerned about detonation for hp driving add a little race gas. A 25% of 100oct to 91-92 will be more than safe.

I agree with you on most of what you commented. I was wondering why the fuel would remain constant? Normally the computer would compensate the extra air by adding extra fuel to achieve the desired afr.

Narrow band o2 sensor and 90’s tech ecu is like a carburetor. As mentioned previously they don’t run in closed loop at full throttle.

If only.

The stock computer really doesn't measure the air/fuel ratio often or well. Narrowband o2 sensors are pretty much on/off switches and are mostly used to say "yes it's in the prescribed range" at cruise. They don't do what widebands do which is communicate whether it's high or low and by how much.

ok. I was thrown off when he said he was using a wideband o2. Thank you.