This is a secondary waveform from a 2 cylinder Aprilia motorcycle. It is a 3 wire cop coil. I am not sure what to make of this. Has misfire on that cylinder. Short term fuel trim shows negative 30 on that cylinder. Thoughts. I have included the Primary waveform as well.

---

Negative 30% fuel trim gives me the impression the ECM knows that cylinder is rich and is taking away 30% of the fuel for some reason. Is it possible the oxygen sensor is wrong and falsely telling the ECM its rich so the ECM incorrectly pulls fuel causing problems?

Cyl 1 is negative 30, cylinder 2 is positive 30. Checked injectors - fine, disable o2 sensors, cyl 1 trims go near zero, cyl 2 stays at positive 30. No air leaks, only one map sensor which is common to both cylinders, no MAF.

Is it possible the mechanical timing is off?

On cars when we have opposite fuel trims, many times its because of an airflow imbalance from bank to bank.

Things like VVT that is stuck or jumped timing chains can cause this.

No, also did an in cyl compression test and the intake vacuum on both cylinders if fairly close. I still would like to see an answer for why the secondary looks like that..

I see a couple things.

The first is that the gap is small and/OR the compression is low. ~6kV peak seems low to me.

I also see low resistance in the secondary circuit.

---

I agree on the low resistance, but plug gaps are good and plugs are not fouled. Cyl 2 looks totally normal - see waveform. The secondary voltage is not uncommon from the bikes I have tested. I have included an in cyl comp test on idle for cyl 1, seems normal to me except the intake vacuum is slightly lower than what I'm use to, usually closer to 10psi but it is Fly by Wire so that may make a difference.

---

Your secondary captures have 350-1200V differences in the burn line.

Can you swap the plugs to see if the issue moves?

Also your primary capture seems like both cylinders have events where the burn line is too smooth to be real combustion.

---

I did swap plugs and the cop coils as well. The issue did not move. Really has me stumped.

So just to recap.

Misfire on 1 of 2 cylinders

Huge fuel trim imbalance cylinder to cylinder

Swapping the ignition coil and spark plug did not change the issue.

The secondary ignition pattern from the faulty cylinder appears to have substantially lower spark line voltage than the "good" cylinder.



Is the misfire a dead hole or is the issue fault less frequent?

Does this machine have individual exhaust oxygen sensors?

What is the exhaust system like?

What kind of fuel system does this machine have?

Does it hold pressure when the machine is off?

If you were to disable fuel trim correction how does the engine run? Unplug the O2 sensor(s).

Misfire on 1 of 2 cylinders - YES

Huge fuel trim imbalance cylinder to cylinder - YES

Swapping the ignition coil and spark plug did not change the issue. - YES

The secondary ignition pattern from the faulty cylinder appears to have substantially lower spark line voltage than the "good" cylinder. - YES



Is the misfire a dead hole or is the issue fault less frequent? - INTERMITTENT, MOSTLY WHEN ENGINE WARM

Does this machine have individual exhaust oxygen sensors? - YES

What is the exhaust system like? - STOCK 2 INTO 1 - LINK PIPE BETWEEN PIPES CLOSE TO EXHAUST PORTS

What kind of fuel system does this machine have? - 2 THROTTLE BODIES LINKED TOGETHER (THEY ARE SYNCED) - FLY BY WIRE CONTROL - SEQUENTIAL INJECTOR FIRING

Does it hold pressure when the machine is off? - YES, TESTED INJECTORS FOR FLOW AND LEAKAGE

If you were to disable fuel trim correction how does the engine run? Unplug the O2 sensor(s). - UNPLUGGED O2 SENSORS. CYL 1 THAT WAS NEGATIVE 30 GOES TO BASICALLY ZERO, CYL 2 THAT WAS PLUS 30 STAYS AT PLUS 30

More confusion
Leakdown Test 8% Cyl 1
                     14% Cyl 2

Waveforms - Cranking Cyl 1 lower than Cyl 2
                   Running - Cylinder 2 lower than Cyl 1

---

James H wrote on Oct 2^nd, 2026 at 9:16am:

More confusion Leakdown Test 8% Cyl 1                      14% Cyl 2

Were you able to determine where that additional leakage was going?

How are the valve clearances managed on this engine? I'm sure by now you've checked them but we haven't discussed it.

Is your cranking WOT picture using two WPS transducers or is it the combination of two different captures?

One thing that worries me when comparing in-cylinder captures on multi cylinder bikes is the effects of uneven firing strokes. According to the web this on uses a cross plane crank with a 270° firing pattern. This will cause larger crank speed variations which WILL have an effect on cylinder pressures when measured by the WPS.



James H wrote on Oct 2^nd, 2026 at 6:20am:

If you were to disable fuel trim correction how does the engine run? Unplug the O2 sensor(s). - UNPLUGGED O2 SENSORS. CYL 1 THAT WAS NEGATIVE 30 GOES TO BASICALLY ZERO, CYL 2 THAT WAS PLUS 30 STAYS AT PLUS 30

My purpose with this one was to see how the engine ran on fixed values. By unplugging the O2 sensors the feedback loop for the ECM is broken which SHOULD prevent the fuel trim from changing. I don't know if you would have to clear the learned fuel trim to put the ECM back into a predetermined values.

Have you scoped or otherwise measured the O2 sensors output?

Have you scoped the fuel injectors?

Were you able to determine where that additional leakage was going? - SEEMED TO BE MOSTLY RINGS

How are the valve clearances managed on this engine? I'm sure by now you've checked them but we haven't discussed it. - YES, CHECKED AND OKAY

Is your cranking WOT picture using two WPS transducers or is it the combination of two different captures? - 2 DIFFERENT CAPTURES

One thing that worries me when comparing in-cylinder captures on multi cylinder bikes is the effects of uneven firing strokes. According to the web this on uses a cross plane crank with a 270° firing pattern. This will cause larger crank speed variations which WILL have an effect on cylinder pressures when measured by the WPS - AGREED

Have you scoped or otherwise measured the O2 sensors output? - NOT YET, BUT DISCONNECTED SO IN OPEN LOOP MODE CYL 1 GOES TO 0% TRIM AND CYL 2 STAYS AT PLUS 30%

Have you scoped the fuel injectors? - YES VOLTAGE AND CURRENT - LOOKED FINE

Embarrassed that I missed this but just realized that my cranking compression valve of 160 psi and running compression value are close to the same which obviously isn't right. I originally checked for air leaks and there was none that I could see. I am going to have to look again or for another possibility...