2G AFR lean (12.5-13.0) at WOT basically stock

[XIT]

Just finished installing an AEM X Series wideband and an Autometer boost gauge in my car. It is a 98 GSX with ~84k miles. The only other mods it has are an unidentified BOV from 2004, BCS mod, Apexi N1 turbo back, FP intake tube, and K&N filter. All else is completely stock.

I just finished doing basic maintenance to the car (including fuel filter). I've tried researching the issue and most responses are fuel filter being the cause. The one I put in is a Mitsu replacement. I don't think the car would be running out of pump at this level. I guess I could be wrong?

Anyway, the car idles around 15:1, cruises at 14.5-15:1, and under WOT in 1st and 2nd gear it'll go from ~14.5 straight to ~12.5 with boost spiking to ~17psi and settling down to ~13 until redline. In 3rd gear, afr goes from ~14.5 to 13.0 then tapering down to 12.5 with boost spiking to ~17psi and settling at 14 until redline.

From researching, I know the boost spiking issue is the exhaust. Part throttle in 3rd gear ~3000rpm causes boost to spike to 25+psi. Yikes.

The only thing I can think of that would cause the lean condition is a leak between the MAS and the turbo but those connections are good. Any ideas?

 

[habitatguy187]

With those mods I would at least re-wire the fuel pump. Under wot your voltage is probably dropping (as many cars will do) and the pump is getting less voltage than it requires. Just an educated guess of course but either way, it's time for some fuel mods.

 

[gofer]

I would suggest checking out the vacuum lines going to fuel pressure regulator. Fuel pressure should increase 1:1 with boost pressure so if you have a cracked/leaking vacuum line running to it then the fuel pressure isn't going to increase like it should and result in a lean AFR. It's not uncommon to see people run their boost gauge pressure source to this line with a tee fitting either, if that's the case with your setup then you could have a leak in your boost gauge hose/fitting at the gauge causing it - especially if it's the crappy clear plastic stuff that the gauge comes with. To do a thorough check, you should be able to find any leaks doing a BLT.

If you've ruled out the fpr then it could be dirty injector(s) or a clogged fuel filter. Looking at your mods, I don't think you'd be running out of fuel pump at this point with a perfectly functioning fuel system but it's never a bad investment of time/money to do a fuel pump rewire.

 

[XIT]

I rewired the fuel pump today. No change. The vacuum lines to the FPR are untouched. Wouldn’t dirty injectors impact the AFR at idle and cruising?

 

[gofer]

I rewired the fuel pump today. The vacuum lines to the fpr are untouched.

Hopefully you see the results you're hoping for after doing a pull after your rewire. If the vacuum line to the fpr is untouched but is still the original 24 year old line it would still be a good idea to perform a thorough inspection of it, dry rubber cracks. It could also be the fpr itself causing your lean issue (not rising fp 1:1 with boost) which can be tested but it's a bit trickier to do w/o the proper tools to do it.

Wouldn’t dirty injectors impact the afr at idle and cruising?

Yes and no depending on how dirty they are. Since the ECU is typically in closed loop while the car's idling/cruising the ECU can maintain the proper AFR (14.7:1) because it's using the voltage input it's receiving from the front o2 sensor. When you're WOT the ECU is in open loop and is in some sense flying blind so a dirty injector(s) that are masked at idle/cruise could appear as a lean AFR at WOT because the ECU can't account for the loss in fuel being injected.

I'm not telling you to throw money at a problem and hope for the outcome you want but getting injectors cleaned/tested is also good routine maintenance. FIC cleans/flow tests injectors for $15/ea ($60 + shipping). I understand some of us still use our DSM's for daily driving so not having it for 2 weeks isn't practical but if you can, I'd recommend it if you've never had it done before. FIC Injector Clean/Flow Test If you do send them off it would be a good idea to replace the fuel filter while the car's down. If the fuel filter's dirty I would hate for you to get the injectors cleaned and then clog them up again because of a bad fuel filter.

Just a few things to keep in mind as you try to troubleshoot your WOT lean issue.

 

[XIT]

No change with the rewire unfortunately. The fuel filter was replaced recently (less than 10 miles on it). I have some 660cc injectors waiting to go in along with a handful of other mods. I was planning on having them cleaned and tested so I’d hate to do it twice but I’d also hate to modify the car if it isn’t running at its best.

Any chance the problem is the fuel pressure solenoid? Would running the line from the fpr straight to the intake manifold test this?

Thanks for the responses.

 

[gofer]

Ideally you'd get a handle on this first, as I'm sure you know you can't just install 660's w/o installing ECMLink or another tuning solution to account for the additional fuel.

Its been so long since I've seen a 2g with emissions still intact I forgot that solenoid even existed, plumbing a line from the IM directly to the FPR is a good idea to test it.

 

[XIT]

Yes I’ll be tuning the black box ecu and have all the stuff needed to do it. But yes, for the most part the car is completely stock and untouched. I probably won’t have time to test it until this coming weekend but I’ll hook up a vacuum line directly to it and see if the solenoid is the issue.