Ferp

So my car will hit my target afrs most of the time (11.5-11.9 in the powerband) but it will randomly go a full unit leaner on some pulls, so I will be in that 12.5-13.0 range, which is way too lean. I can't figure out what causes it. I have a log of it happening if anyone would like that provided.

So I went out and did some pulls in the country side this morning (I didn't have my laptop so no logs of this) and I did about 25 3rd gear pulls and the afrs were perfect. I headed back into town, stopping at a red light. I then did a little 3rd gear pull on the ramp and it leaned out. It stayed leaned out every time after that so I stopped trying. I then did another 3rd gear pull on the highway about ten minutes later and it was fine again. I stopped at a red light, kept going on, and it was lean again.

No vacuum/boost leaks (100% sure on this, even had a local shop check it out for me). Spark plugs/ignition system are all good, fuel pump is working fine. I have tried three different MAFs, all off Evos that were confirmed working.

I did a little experiment where I dropped my afrs about a full unit so I would hit 10.5-10.9 consistently. It would then randomly start hitting 11.5-11.9 and then go back to the target afrs after a few stops/attempts.

Mods: Intake, COP, DW200 FP, Wideband/Truboost, making ~21 PSI from the stock 14, I have a few other mods too but they wouldn't have any effect on this kind of stuff.

Thanks guys.

4b11slayer

Maybe your pump is on its way out? Or inconsistent boost levels.

RightSaid fred

You've changed the pump but you haven't measured fuel pressure.

Ferp

Pressure was right around 40-45 when I checked it.

RightSaid fred

Spec is 33 at idle with vac line connected.
Not 38, not 40, not 45.

Ferp

I did it with the vac line disconnected. Should be 43.5, which is right around where I am. Should have mentioned that, sorry.

Anyway I was able to fully replicate the issue tonight. So it will hit my target afrs perfectly, 100% of the time (did twenty third gear pulls, gotta love country roads at night), as long as I enter wot around that 3000-3500 rpm range. So the lean issue happened every single time if I put it in neutral, slow down, and go wot in third around 2000-2500 rpm. So say if I came to a stop and then floored it in third right after. It would do this every single time if I kicked the clutch in, let the rpms drop to idle, then rev match to my speed, and then floor it. If I let off the gas for about 1-2 seconds then tried flooring it again it would then hit the target afrs, instead of being a full point off. If I quickly shifted from 2nd into 3rd, 3rd into 4th, etc. when it was running lean it would stay lean, unless I let off the gas for like two seconds.

I'm stumped, would appreciate any kind of lead.

Ferp

I found the problem for anyone who may come across this thread.

So hardwiring the fuel pump fixed my issue. The Evo 4/5 (apparently the 6-9 have remedied this) have a relay in the fuel pump line that switches the voltage, but has trouble switching from low to high voltage with an uprated pump. So flooring it once the car is on the lower voltage would prevent it from switching to high voltage. Letting off the gas for a moment then flooring it again would kind of set the pump straight. Anyway, works fine now that it's hardwired. Thanks everyone.