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S2 Crank but no start. Low d09

Reading this thread
https://www.600riders.com/threads/fz6-no-enciende-caliente.60598/
I realized another possible contributing factor to my problem. Loss of compression. On February I went the bike to the mechanic to address a revving mild hesitation. Changed spark plugs (the one I recently checked was barely tightened), changed fuel pump motor and filter, cleaned injectors, synced throttle body and also did a valve adjustment. I remember he commented that my valves where strangely tight instead of loose (or the other way around, I m not sure). And my problem appeared two days later. Will a compression test show a bad valve adjustment?
 
To determine a specific reason for a loss of compression you will need to perform a leak-down test (differential compression test). If the valves are the cause of compression loss, air will escape from the exhaust, throttle bodies, or both.

Before doing the leak-down test, conduct a total compression test to see if your values are within spec. I only do a leak-down test if the values are out of spec.
 
leak down test = pouring some oil in the cylinder and measure compression again ?

i ve seen that after mesuring a low compression they pour oil to seal the piston rings and see if the compression gets better on the next measurement.

I ve also seen a quick way for a first indication of leak, by measuring the voltage drop during cranking (having the efi fuse removed), expecting to see identical waveforms on four cylinders (compression creates mechanical resistance on the starter, which increases the drawn current, which lowers the voltage which can be seen in the oscilloscope). I might try that but i m not sure if a small leak will be visible. A large leak (spark plug removed for example) is clearly visible. I ll have to wait for the mechanic to get back from vacation for a proper measurement.

Not sure what to prioritize, up to now i have indications of
1) lack of spark (sometimes when refusing to start there is no spark at all, other times there is rare spark, other times there is spark which seems almost consistent)
2) vapor lock (pressure loss was verified by the mechanic), i am not sure if this is a real problem or if the fz6 is immune to this. Fuel pump assembly was replaced with a used one but it was never verified with repeated measurements that the pressure is now always maintained after the engine is turned off.
3) compression due to valves misalignment or spark plugs not tightened (nothing verified, but valves where adjusted just before the issue came up)

I often have error 12, but the signal seems good on the scope. When the bike starts, it works just fine. Usually the problem appears when cold after it was hot enough on the previous ride. The last days i have it parked and just turn the engine on for a minute or two to monitor the cps signal. All the times it started at once except once that i think i had the engine get hot the previous check. I let it get hot today too to verify on the next check if is this is a pattern.

Edit: pattern not verified, started normally hours later.
I also measured the voltage during cranking (started ok). There is a theory that the ecu resets below a threshold (which I failed finding a documented value).
Screenshot_20260819_163113_BKmonitor.jpg
 
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A leak down test is NOT putting oil into the cylinder…it is putting a specific amount of air into the cylinder (typically 80psi) and seeing on the gauge how much of that 80psi the cylinder is able to maintain. If it holds 90% or more (72psi) the cylinder passes the test. Less than 90% and you track down the source of the lost air.

Thats an enormous voltage drop at the 10 second mark.
 
Thank you for your reply. Sorry for the huge picture (I was using my phone). I had a rule of thumb changing the battery when the instruments illumination start turning off during cranking, or before the battery's 4th winter, and the time has come I suppose.

Can anyone figure out what is the second voltage drop after the engine has started? I always had the impression that there was a voltage drop then (lights slightly dimmed and something like an actuator felt, i also have this at my Alfa Romeo and I am pretty sure its the ABS auto activation - so it never gets stuck, but I doubt if this is the case here. I already had my ABS stuck in the past.)

The only good thing having this problem is that I have learned a lot of things i did not have a clue about.
 
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Is it possible to do a compression test by loosening ( or removing) the coolant radiator? I feel more confident doing this instead of removing the tank, air filter, battery mount and the underneath cover. I understand I will have to remove the efi fuse and coil primary connections too.
 
Any decent compression tester will have an independent hose that you screw into
the head. Then, the actual gauge will plug into that.

So yes, if you can change a spark plug W/O removing the tank, you can screw in a
compression tester.

And yes, the plugs can be swapped leaving in the radiator, etc. Tight, but do-able

What I use and it's an excellent tool...

Compression Tester Kit | OTC Tools

www.otctools.com www.otctools.com


 
I installed a fuel pressure gauge for a few days. Running pressure is stable at 2.7 bar (no accuracy rating marked on the gauge). Engine off drops at 2.3 bar. An hour or two later it drops at about 1 or 1.5 bar and rests at 0.6 - 0.9 bar hours later. The falling rate is not stable, one day may drop somehow faster than the other. I also had the chance to record the cps signal while the mechanical lock happened. The engine was at 98°C when turned off the previous day ( but this has happened other times and the bike started ok the next time, so this is not consistent). There is a clear signal while cracking but no starting with an absent or intermittent spark. On the first video I captured the cps, when realized that it won't start I stopped the recording, installed the spark tester which is on the second video. The oscilloscope trigger is manual which means the frozen waveform you see before cranking on the second spark test video is from the previous starting attempt. It is also clear on the first video the waveform freezes (crankshaft stops turning, the signal disappears, waveform freezes). There are many times that the bike won't start with no mechanical block happening.


 
I don't know if you've seen this video, but this guy was having a problem with an intermittent misfire.

He had already checked the coils and they had tested fine, but then he found that actually one was going bad:

 
Yes i ve seen this video and thank you for pointing this out. The truth is that i am at a point where i have not spotted an obvious root cause so I should start replacing parts until i hit the right one. A yamaha mechanic had said to me that "perhaps" the coils are going bad but I could not correlate it with refusing to start, i always thought that coils just cause misfires or momentary cutouts. Are Aliexpress coils any good or should i try used yamaha ones
 
I still don't understand why you have total loss of power when the starter cuts out. Maybe it is the power loss that is causing the starter to 'bang', and not the other way around.
Even if the starter completely locks, I don't think that should kill the LCD.
 
It is felt like something hits somewhere. Huge mechanical load to an electric motor tends to increase the current to a theoretical infinite, dropping the voltage nearly zero. It is not felt like an electric short which stops the mechanical movement due to voltage drop, it seems like the opposite, but then again I could be mistaken.

Today I had the chance to :
- Measure the coils primary winding resistance ( both at 1.9 ohms)
- Measure resistance between the high voltage output terminals ( 33KΩ and 33.4KΩ)
- Correct the connection on one coil ( the negative trigger was at the + terminal )
- Clean various connectors around the front area
- Measure the compression ( cylinder 1 at 205psi, cylinder 2 at 199psi, cylinder 3 at 200psi cylinder 4 at 201psi )
- test successfully using diagnostic mode d30, and d31 all four coil outputs for spark.
- fix a leaking thin rubber hose coming out of the front mid section of the throttle body ( no idea what it is, there was a cut just before its fitting end, I cutted the last part and refit it)

I wanted to put a split wire at the negative trigger of each coil so I can hook the oscilloscope probe whenever it refuses to start and monitor the waveform. The connectors i built were not proved quite solid during fitting everything on its final position so I did not fit the extra cables today. I wanted to see what is happening on the primary winding side during the misfires. I am not sure if there is another connector between the ecu and the coils to back-probe there, is it?

The most difficult part for me, as stupid as it might seem, was to place back in its correct position the rubber protective sheet underneath the battery compartment. After fitting everything back it started ok as many times as I tried. I m not sure if I fixed anything crucial, but at least i removed out of the equation the compression factor.


Edit: I just read on the service manual that
Secondary coil resistance should be 12.0–18.0 kΩ while I measured 33KΩ.
Edit 2 : It seems there is an extra resistor of 10k in each plug cap, so I must be in spec.
 
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Edit number 2 is correct. The HT lead adds resistance as well. You do appear to be in spec.

I don’t think coils are the issue. You can check them hot…but I believe you problem occurs whether the engine is hot or cold…correct?
 
jomor147 said:
- fix a leaking thin rubber hose coming out of the front mid section of the throttle body ( no idea what it is, there was a cut just before its fitting end, I cutted the last part and refit it)
Click to expand...
I can't help you with the rest of this, but that sounds like one of the vacuum take-off tubes which you use to balance the throttle bodies. There should be four of them, one for each cylinder, coming back into the under-seat area with small plastic plugs in the ends.

As you say, they're only thin tubes and can get a lot of vibration, so they can easily wear through.

They're not critical, if you have a leak in one, your idle may be a bit ragged, but I had one with a hole in it for ages and the bike was fine.
 
Gary in NJ said:
I don’t think coils are the issue. You can check them hot…but I believe you problem occurs whether the engine is hot or cold…correct?
Click to expand...
Usually I have a cold start difficulty. Only once it happened when hot.

AI claims that in wasted spark configuration system a reversed polarity can cause starting problems because:
  • reversing primary polarity causes the spark plug's central electrode to fire with positive polarity instead of negative, it requires roughly 20% to 30% higher voltage to jump the spark plug gap due to reduced thermionic emission.
  • Cold Cylinder Demands: Starting a cold engine requires a significantly higher voltage to form a spark because cold fuel-air mixtures are harder to ignite
  • Cranking Voltage Drop: When cranking the engine, the starter motor draws heavy current, causing the battery voltage to drop (often down to 9–10 volts). Because the input voltage to the coil is reduced while the required spark voltage is increased, the coil may fail to generate enough secondary voltage to jump the gap, leading to hard starting or long cranking times.
I guess i will know the next days if these are just a theoretical approach or do matter in real.
 
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