Mysterious Fuel Cut - RPM based
 

Long time lurker, first time poster.

My tuner and I are stumped. Its probably not MS related at this point but we're not sure where else to look.

Symptoms: Threw car on the dyno. At 2,850 RPMs she goes completely lean. We've thrown tons of fuel at it, so much that the scaling results in pig rich right before and after the RPM range where this is happening. Nothing on the MS is on (launch control, fuel cut, etc.) Injector PW looks good. No loss of sync in the logs. MAF readings look good. Trigger wheel looks great in the logs. When the car is started cold it actually runs through the RPM range without a problem and then, once it gets warm, its does it again. I should note that its not just the reading on the wideband / logs; the car bogs just like fuel cut.

The Car: 1991 chassis, BP4W swap, e85 running cam/crank sensors.

We've Tried:
Changing injectors
Changing trigger wheels
Changing MAF pickup
Using a different MS box
Swapping to a different microsquirt board

Analog fuel pressure gauge looks great when its happening.

Its like the MS is telling the injectors to fire but they're ignoring the signal at this one RPM point. Interestingly, we changed squirts per revolution and it moved where this is happening to a different RPM range. Its also not load based - its tied to a specific RPM across the entire load range.

Noise in the signal somewhere? Any ideas?

Small Update: Tuner tried from 2 to 4 squirts; car hits the lean spot at 1/2 the RPMs. We bumped back down to 1 squirt and the dead spot jumped to 5k RPM.

Grounds maybe?

Would you be able to post a log of the event happening? Also including the current tune may help.

Maybe, but it won't be until tomorrow as I accidentally left my laptop in the car and its over 2 hours away. I'll need to have my tuner email it to me.

The Update: This might be in the wrong place, now...
My tuner re-wired the entire injector harness, then indicated it was "slightly better but still the same issue." His follow up comment set off all the bells and whistles I needed:
"It's like there's some type of mechanical resonance or pulsation happening in the Flyin Miata fuel rail."

BINGO.

Tonight I get to drive 5 hours (round trip) after work to deliver him a stock fuel pressure damper. My original setup was in a different car. I swapped the motor to another chassis and, at the time, put in the FM big fuel setup (rail, stainless lines, etc.) in place of the normal '99 rail + 1.6 FPR. A call to Flyin Miatas technical support had indicated that a fuel damper wasn't needed unless it was a turbo car pushing 300+ hp. Its not - its a n/a car on corn pushing 140-150.

Having chased issues this long in the tune (and everything appearing "right") I wanted to share the update for anyone searching with a similar issue.

Your knowledge and update is appreciated.

Let us know how it goes after the visit.

Final(ish) Update!
With RX-8 injectors and the FM "Big Fuel Kit," despite what their tech support says you definitely need to add a fuel damper to the rail. The mystery lean condition was resolved late last night.

In other news, we got one pass with a base map and the Fab9 36-2 trigger wheel (stock crank) sheared itself in 2 at 7,200 RPMs.

Not sure if we're allowed to plug vendors that aren't affiliated, but Ryan at NorthCoast Speedworks (in Lorain) has been nothing short of awesome to work with. I dropped the damper off last night around 8:00 PM. He couldn't sleep knowing he had the possible fix and went to the shop late at night just to find out. He's going to swap in a stock trigger wheel today and get the car tuned so I can pick it up on the way to my first race in the car this weekend at the Pittsburgh Match Tour.

Great news! Also thanks for the solution to a possible issue that others may face.