DOTKR

MS Problem? Engine will stops accelerating at 5500rpm and sputters/hesitates.

Car: 1999 NB, 5 speed, Naturally aspirated. Fuel; E85 or 91oc. Fuel System is return less and aftermarket Sock to injectors, CAI with honed Square Top and Skunk TB. Exhaust, RB Header and custom 2.25 exhaust with flow through mufflers. Megasquirt PNPPro 99-00. 949 Twin disc clutch/Light Fly wheel (Revs fast). 8000rpm Red Line. Motor is super strong. Never had an issue. I ran this set up and fuel map for years without issue.

When conducting some wiring over a year ago, we blew one of the two MS Fuses. Replaced the fuses and removed the bad wiring. I had many engine performance issues since but have systematically solved and remedied all of them except this one (Though I cannot say for sure this is due to the event of blown MS Fuses, it was there but mashed in with all the other issues):

The last PITA problem. I have been chasing this one for over a year, but have been tackling it pretty hard the last 6 months. The symptom is similar to hitting a rev limiter or engine retarding effect at ~5500rpm, while accelerating…. but rough, hesitating, and a little jerky…like the engine is trying to decide what to do. It is also very consistent: always. It doesn’t matter if LC is on or off, and or set if LC is set to a lower RPM.

E85 or 91oc same issue. Flex Fuel sensor connected or not, enabled or Disabled. Hot or cold engine same issue. Accelerate engine rpm via the Pedal or the TB, no change.

If in neutral and idling, applying the Throttle at a slow or medium rate of application yields no change. If idling and you punch it to WOT the engine it will often have a very brief hesitation and push past 6000rpm to redline.

The Data Log was conducted under load in a straight line on 91oc Fuel. You will see instances of the engine hesitating to pass the 5500 rpm mark, the AFRs go completely lean as if fuel cut is occurring, but no data shows injectors not spraying. This behavior is apparent in neutral, under load as well when at AutoX.

There are also some changes to the parameters "engine in cruise state", "engine accelerating slowly," and "engine" during the ramp up to 5500rpm, and over the period of hesitation and until Throttle is released.

13s. I show my Launch Control set around 3500rpm and relieve the clutch in neutral, no launch to serve as a comparison to MS induced “Rev limiting.”

50s. I try medium rate of throttle up to the symptom at 5200rpm, then stab the throttle, release completely and then WOT, then the engine stumbles and struggles its way to 7000rpm (56s) where I have the same issue, same symptoms.

77-84s. I try quick on/off stabs of the throttle to push past it, kind of helped, but didn’t feel healthy.

The Rest of the Log shows readings stabilize and return to idle.

The fuel map and tune hasn’t really changed in two years…

Problem solving to date:

• Vacuum system has been fully tested.
• WBo2 sensor calibrated and matching MS read out.
• Leak now test, no more than 2% across 1-4 J
• Compression Test 180/185/180/180psi
• Upgraded to FM 36-2 trigger wheel for more fidelity, and ATI Harmonic Dampener. No Change.
• Timing has been verified. Timing light, MS Trigger wizard, and spark map all in sync up until event.
• CAI and filter cleaned. GM IAT connection good.
• TPS sensor checked and calibrated. Wiring good.
• Coolant temps match stepper Gauge and MS ENG CLT.
• New Plugs, gap confirmed. Ran up to temp, recreated symptoms, checked plugs: bit of coffee and cream all 4.
• Fuel system from sock to injectors cleaned, checked, tested. All wiring good to PCM. Pressure at rail confirmed @60psi. Injector wiring good to PCM. Injectors firing.
• Scoped cylinders, all look normal.
• Oil clean and smells beautiful.
• New Plug Wires. Swapped wires. No change. Iridium or copper, no change. Confirmed Spark 1-4.
• Tested and swapped two different coil packs, wiring good to PCM, no change. Swapped cylinder wires 1 and 4, 2 and 3, no change.
• ALL vehicle grounds checked and good.
• All Engine harness wiring checked and connections cleaned.
• New Crank Angle Sensor, gap confirmed. Wiring good. Tested Gaps from 0.5mm to 1.5mm, no change.
• New Cam Angle Sensor. Wiring good. No change.
• Tried older TS tunes, no change.
• MS Firmware up to date
• LC Disabled. No change.
• Flex Fuel Sensor Disabled. no change
• MS Noise filtering on or off, no change
• MS Overrun; on or off, no change
• PCM opened and looked at, looks brand new and no visual or sign of fried transistors.
• New PCM fuses tested, swapped, no change
• Tried adding more fuel to the map a in the 5000-6000rpm range, very slight improvement in “smoothing of symptom” but still can’t accelerate past.
• I Stuck with Falling edge as it seemed to run better.
• · Cam sensor connected or disconnected. No change.
  
  · Tried changing Trigger wheel to Toothed Wheel settings, 36-2, got it running nice. Same problem, no change.
  
  Updated trouble shooting:
  
  · Back to Miata 36-2 trigger wheel settings…
  
  · Gapped CKP to 0.010” no change.
  
  · Started new project, manually inputted all settings, imported spark/VE tables. No change.
  
  · Full sweep of Trigger settings for Miata 36-2 settings, no change.
  
  · Dropped Miata 36-2 for “Toothed wheel mode,” full sweep of settings, played with variances in settings, ran well….no change to problem, back to Miata 36-2 wheel mode.
  
  · Tried gradual and course increases to fuel from 5000-6000rpm, no change.
  
  · Tuned knock sensor, no change.
  
  
  

DOTKR

Is there another forum section I should try to place this in?

SpartanSV

Misfires will always cause a lean AFR regardless of the cause.

Your VE tables look pretty awful but I don't think it's your main issue. I'm not seeing an obvious cause for your problem.

Have you monitored fuel pressure?

gooflophaze

yeah, no sync loss, everything looks alright, but going lean and > 4-ish timing errors (I see 16-ish swings) indicates a misfire. Which coil packs? You've got 3.0 dwell which is fine for toyota/d585 coils, but low for NA/NB1 coils - don't know about the NB2 coils though.

DOTKR

Yes. I have pressure gauges after the pump and before the fuel rail. Pressure is steady up the RPM.

Thank you

DOTKR

I have the FM Coil packs for 99-00. I also have OEM. Swapped them back and forth, no change.

Ill have a look at the Dwell (3.0)...Ill try a stepped approach to 5.0

Thank you

DOTKR

Played with Dwell which was 3.0.
2.5 will not run. or would die.
At 5.0 the issue is still there at 6000rpm ....BUT what was never there before was the exact same symptom now also at 3000 rpm....
Back to 3.0...
There might be something with this...