Wild afr fluctuations at WOT

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Posts: 7
Joined: Sun Sep 15, 2019 4:10 pm
Location: St. George, UT

Wild afr fluctuations at WOT

Post by Boosted90Hardbody »

Over the last 2 years I've been fighting an uphill battle trying to maintain a stable afr at wot. I've read every post on every forum on the subject, or related subjects, but the (not quite) infinite knowledge of the internet has not rendered a solution. I've replaced my wideband gauge and sensor three times now, adjusted every setting in TS, checked high speed tooth/trigger loggers, checked every single damn ground, rewired everything I could think of that might help, nothing has made a noticeable change. Figured it's about time to start a thread to try and get some help. Here's my setup:

1990 nissan d21
Sohc ka24e
8:1 wiseco pistons
Eagle rods
Mspnp2 ecu running sequential injection
Deatschwerks 800cc injectors
O&j stage 1 hydro cam
Pioneer valve springs
Martin industries adjustable cam gear, 6 deg advance
Wahlbro fuel pump(not sure which one, boss gave it to me)
China special t3/t4 5456 .63 ar turbo
Aem 30-4110 wideband sensor
Hks ssqv bov

Things I've ruled out:
Injector latency
Coil dwell( unless it's supposed to be over 5.5ms, didn't test any higher than that)
Oil contamination in intake through pcv( had some, none since new catch installed)
Fuel pressure seems steady(3.5 bar at idle 1:1 rising rate fpr)
Duty cycle and pulse with are steady, max 65% duty at 20psi boost
Fuel and spark tables are smooth af
Signal from cas is clean af
Injector timing(could be a factor at such low duty cycle, I've tried changing quite a bit, no improvement)
I have developed a slight exhaust leak pre turbo, afr from previous data logs show the same fluctuations, I doubt this is the problem.

Here's a snap of a run I did on the dyno last weekend
I'm straight out of ideas, has anyone ever seen afr fluctuations like this?
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Re: Wild afr fluctuations at WOT

Post by R34SR »

So the sensor responsible for measuring the AFR content of your exhaust gas is fluctuating.....but you have ruled out the exhaust gas leak pre turbo/pre afr sensor as the culprit....

Where is Tiny T when I need him....

Anyways let me not be "that guy" that turns this place into zilvia.net.....please fix your exhaust leak.
BW S257 SX-E .78AR
ID1050x Injectors
Full Race twin scroll manifold
Mazworx z32 tranny swap
STANCE LX+ Coilovers
AME FS01 WHEELS 18X9 +25

TinyT wrote:for the love of god, post your setup, do you really think you can get an answer after saying HI ME CAR HAS TURBO NOW BUT I CANT BWAAA PSHH WITH IT WHATS WRONG
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Re: Wild afr fluctuations at WOT

Post by p00t »

If AFR is unsteady at WOT only that is not an indication that your AFR sensor is having an issue.

It is related to load and is likely due to combustion variation. Try gapping down your plugs real close and read them to see if only one cylinder is going leaner than the others (in case an injector is hanging up).

Also are you sure you don't have knock at WOT?

And like R34SR said you should fix the exhaust leak. It may have been leaking before and you weren't aware (but I would think you would see it at lower load too).
Current Experiment: APS SR56, Methanol Injection, & Built Block
Posts: 7
Joined: Sun Sep 15, 2019 4:10 pm
Location: St. George, UT

Re: Wild afr fluctuations at WOT

Post by Boosted90Hardbody »

Alright, I've had the tuck apart for almost a month, I figure it's about time for an update. (Buckle up, its gonna be a long one)
So the sensor responsible for measuring the AFR content of your exhaust gas is fluctuating.....but you have ruled out the exhaust gas leak pre turbo/pre afr sensor as the culprit....
Data logs show the exact same fluctuations from the beginning and haven't changed after 2 turbo swaps where I replaced all exhaust gaskets forward of the o2 sensor. I figured I can safely assume that wasn't the cause, but you're right, it is an issue that needs to be addressed.(there was a 1/2" section of the metal T3 gasket that was completely burnt up/missing.)

p00t, after a faulty aem wideband gauge the first go around and a bad (brand new) sensor the second time, I have trust issues with my wideband, lol.
I had it on the dyno at work the week before posting and afrs were steady through the tailpipe o2 sensor which made me question the aem unit even more. I stayed pretty conservative with the ignition timing so I doubt it was knock related.(managed to put down 334whp@5400 and 401ft/lb@3700 @20psi, clearly I need to get rid of the truck intake to move that torque up a bit in the rev range)

So, where does it sit now? Well, about a week after my initial post, it started burning a little oil while warming up, which is fairly normal on a 10 degree morning, usually clears up by the time I make it out of the driveway. But this time it didn't and I smoked out 3 blocks of my neighborhood. I figured it was an oil seal in that cheap turbo that finally gave up on me, which turned out to be the case. This time around there was no excessive shaft play, leading me to believe that the oil drain wasn't draining correctly, probably due to high(ish) crank case pressures. I pulled the head to check my piston rings thinking they might be shot, as soon as I pulled my #1 piston knew I was boned.

The piston skirts were pretty hammered, primarily on the thrust side but the ring lands were completely untouched, which indicates severe piston slap from excessive piston to wall clearance. Sure as ****, I measured my clearance of the #4 cylinder(it was in the best shape of the four) and ended up with .0085" which is more than double the .003" I very specifically asked for when I had the engine bored out for the 89.5mm Wiseco pistons. Looks like the good ol machine shop screwed me pretty hard.

Long story short, I'm almost certain that's the root cause of my afr issue. Moving forward with the truck, I have some 90mm pistons on order from wiseco, a new borg s257sxe, a sohc 240 intake from a friend, and I'm right in the middle of port matching and polishing the combustion chamber of my head since I'm waiting for parts anyway. As far as machine works, looks like the block is getting bored out again(correctly this time I hope), rotating assembly balanced again, and a 3 angle valve job cause these valves and seats are pretty pitted. Its gonna be about another month of waiting for those pistons and another month after that to get it all back together unfortunately. Hopefully mid to late May?

At least its tax return season, so its all pretty much paid for. :dunno:

p.s. I got pics if y'all wanna see pics
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