DF70 cranks, no start, spark ok, fuel injection ok

[borjeran]

My DF70TL, 2010 model failed to run. Changed plugs, worked for a few days and failed to start again. Tested sparks--> nothing. Replaced coils--> got sparks, but no start. Tried resistorless plugs which gave a better spark, but still no run. Tested fuel injectors by removing block--> sprays fuel pulses nicely. Coughs a bang occasionally but won't run. Tested CKP and CMP timing sensors with oscilloscope, seem to work. Bought USB test cable, which shows no problem.
What's wrong????

[redlowrey]

From what you are describing it sounds like the transistor turn on time is not long enough on the primary windings in the coils, either from an internal fault or low primary voltage. Did you perform a voltage drop test on the supply and grounds and made sure the supply voltage and grounds are good. Very important low primary voltage means low secondary voltage.
Because it has coil on plug you won't be able to check a primary voltage waveform because of the transistor ass in the coil, but you should be able to do a current waveform check with an amp clamp on the supply side. On the igt five volt signal you might be able to see how long the coils are being switched on and off for, My guess is the coil burn time is not long enough that is why when you changed the plugs to non resistor plugs it started, because the they need less secondary voltage to fire.

For the coils to fire properly they need at least one to one point five milliseconds of burn time to function properly.

[borjeran]

OK,
I will check the timing an the voltages tomorrow. However, it still gives a good spark, so it should work with the non-resistor plugs. It does not.
What other causes are possible:
If the fuel sprays in open air, I assume that fuel pressure is sufficient; The h.p fuel line seems stiff, and venting the injector block from the top bolt causes a healthy fuel spray. I'll try to find a pressure gauge.
I'll inspect the positions of the crank and cam shafts visually;
As the exhaust bangs occasionally; II assume that the exhaust is not blocked; I assume that it is OK to open the hatch on the exhaust block to make sure.
I did not understand " because of the transistor ass in the coil". According to the tech manual, the transistor is in the ECM, and the primary coil has a resistance of a few ohms (as verified by meter).

[redlowrey]

I thought you meant the spark was weak and when you put non Resistor plugs in it ran but back fired I have misunderstood what you meant. The ecm is turning the transistor ass inside your coil off and on that is why it has three wires the IGT five volts is sent out from the ecu and will turn the transistor on and off depending on how much dwell and timing it needs. It has got three wires going to the coil hasn't it.

Did this stop on you while you were out in the boat, or it just would not start when you tried from a cold start. I am pretty sure your coils have thee wires and you should be able to check the IGT signal with your scope it will give you a five volt square wave and measure the time it is on and off.

What are you calling a good spark are you using a spark tester I would expect the spark to jump nearly half an inch out side the combustion chamber. If your coils have three terminals you cannot check the primary or secondary windings.

[borjeran]

Enclosed is the timing diagram (2008 suzuki DF70). The pull of the open collector transistor in the ECM is 60ms, with cylinders 1-4 alternating with 2-3 at about 7Hz when cranking.
When fed with motor starter spray into the throat of the air intake, the engine occasionally give a brief cough, but will not start. This probably means that fuel is not the problem (as also suggested by the fact that the injectors produce spray).
Does this help in diagnosing the problem?
Regards,
Borje


Timing:
The lower trace is the 12V rail, which stays reasonably clean. The 1.5ms inductive kick is about 30V in the coil primary.

SuzukiDF70Timing.jpg