Nissan CDI failure mode...part 2

km1125

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Well, my first thread got closed before the issue was wrapped up, so I'll pick up from here.

In the first thread, I was having problems where the engine would die after 15 minutes of full throttle operation, or slightly longer at less than full throttle. Not a fuel issue, but something electrical. I found I could pour some water over the CDI module and get the engine started right away, or I could wait 10-15 minutes and it would run again. I would first notice the engine start to stumble at high throttle settings, then as I slowed down the engine would continue to stumble. If you got down to idle it would die and you could not restart it.

After weeks of repeating the failure mode and recovery, I was convinced the CDI module had a heat-related failure. The other thing I had noticed with it -but much before any failure symptoms- was that it was on a fixed 25 degree advance. The specs call for it to be about 5 degrees at idle and increase to 25 at around 3000 RPM (don't remember the exact spec). It had run fine for several years. One day, after putting on a tach (the kind that wraps around the plug wire), I started to get the "heat related" failures.

So I bought a new CDI thinking that was the culprit. Well, I think I was wrong. I still have a similar failure now, but the symptoms are slightly different. Now I don't notice anything when I'm at full speed, but when I drop down to lower RPMs (less that 2000 or so), it becomes intermittant. A quick blip of the throttle MAY save it from stalling, but if it stalls you can't restart until you "cool the CDI'. I put that in quotes because now I don't know if I'm actually doing something by "cooling the CDI" or if it's just the time elapsing. I haven't done it enough or just waited a couple minutes to see if would would restart without "cooling the CDI". Haven't had to long enough on the water to experiment. Trying to figure out how to duplicate the failure mode in a tank so I can troubleshoot at home.
 

pvanv

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I think (if I recall) that this was on an NSF4/5/6A2, right? You may have a fuel issue, or a thermally-intermittent exciter or pulser coil.
 

km1125

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Yes, it was originally an NSF5A2 and I put the larger carb on it. I brought it home so I can pull it apart. I will look at the bowl to see if there's anything suspect in there but the symptoms are much quicker than a fuel issue when it cuts off and recovers.

Any idea how the pulser coil actually works? Does it just pulse a shorter pulse but similar to the exciter coil or is it more like a Hall effect sensor?
 

pvanv

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The exciter charges the CD; the pulser triggers the spark. Uusally checked with an analog ohm meter, but you can see the pulse on an oscilloscope, or latch it with a DVA adaped voltmeter.
 

km1125

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Update. It's been a couple weeks since I could get on the water. Went out last weekend and still had the same issues, but it's actually harder to start now, especially when warmed up. Even cold, it takes several pulls whereas before it would always start on the first pull (it might stall, but it would restart easy). I brought it home and it seemed like the Pulsar Coil had much lower peak voltage than the exciter coil. I was using an oscilloscope instead of a DVA meter, so I wasn't sure exactly what voltages to read as "minimums". The Pulsar coil was only $28, so I got a new one.

Brought it back out to the lake, and I could NOT get it started. It would misfire (sometimes back through the carb, sometimes late through the exhaust). After MANY pulls, I decided to swap the old CDI back in. Started on 2nd pull. Seemed to run OK that day, but I didn't have a lot of time on the water to see if it would quit after ~20 minutes as it had before.

More details to come...
 

km1125

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Ok, so this weekend got out to the lake. Have the old CDI on the motor, with the new Pulsar coil. Engine started fine, but stalled and had to restart twice. No biggie, but definitely started much better before this whole saga began. Took the boat for a cruise but couldn't get up on plane because it was too rough. Still ran all the way down to the cove, and then putted around the cove looking for an anchor spot. Previously, would sputter before we made it to the cove, then die when we got down to idle. Returning from cove was similar experience.. ran fine but couldn't push it as hard due to lake conditions.

Went out this morning and ran it wide open up and own the coast. Slowed down a few times for wake or turns but otherwise ran very strong and consistent. The only thing I noticed was a stumble if you opened the throttle really quick from idle. After running around for a while, I stopped and put the new CDI back in. No start. Pulled plenty of times and it would try every now and then to fire, but would not run. Put the old CDI in and it started on first pull. I really think something is wrong with the new CDI.

Late in evening we went to a restaurant with the dingy. It's just a short run, but it started and ran fine. Leaving the restaurant was another story. Would not start. Would fire a few times in succession every now and then, but not enough to get running. I must have pulled 50 times or more. Finally got a standing ovation when it did fire and we made our way down the fairway.
 

pvanv

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When dead cold, you need choke. When warm, no choke at all, but possibly a little throttle. Look for a bad ground...
 

km1125

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Of course I use choke when cold, and after it starts I can normally go to 1/2 choke for a few seconds and then choke off and it runs fine. Now it will typically die once or twice and I have to restart again.

Took it out on the lake again today. Ran flawlessly (after the initial problems of getting it running from cold start). Ran various speed and then quite a stint at full throttle. Then, it misfired and stumbled. Came off plane and babied the throttle. Checked primer bulb and it wasn't soft (like you'd get with a plugged tank vent) and would firm up pressure quickly. From that point until back at the dock, I could not run more than 1/2 throttle or it would misfire and start to die. Sometimes I could recover it in neutral, sometimes it would stall and it would take several pulls to get started again. Even at no-wake speed this happened twice.

Grounds all go to the same bolt on the ignition coil. They are clean and tight.

Either I have two CD units with the same failure mode, or something else is actually the culprit. I just haven't found a 'something else' that would make sense with the symptoms.

Something else I thought of that makes this a bit harder to diagnose with the 'misfire' symptom, is that these engines are 'waste spark' design. That means it's also sparking between when the exhaust and intake valves are opening and closing instead of just during compression. A backfire through the carb or a "backfire" through the exhaust may not be as unusual as in engines that only spark on the compression stroke. (still should not do that, but it just means there is unburnt fuel at that point)
 

pvanv

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This may sound odd, but I personally would go through the fuel system, from tank to intake. Dip the fully disassembled carb in real carb dip overnight... spray out every orfice with acetone-type generic carb spray... verify float height. You may actually have a fuel issue that was mimicing an ignition problem.
 

w2much

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Great advice Paul. Had the same motor, similar issues. Ignition components all tested good, but I thought I was losing spark. Was testing in sunlight , now I wait till dark or set up a shade. I was was certain I had the carb cleaned . Go back through the fuel system fron fresh fuel , tank pickup, connectors, hoses, fuel pump and a meticulous carb cleaning. Recheck your work, every bit of it.Do it with a manual or parts diagram open.
 

km1125

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So... a follow up to this:

I was still having the same issue. On a whim, I replaced the SPARK PLUG and it seems to be better. Have had it out for a few runs and hasn't shown the problem yet. Old plug had NO visible signs of an issue. If the problem was more intermittent or the symptoms changed over the last year or so, I might have suspected it earlier. Several had suggested to change it early on, but I dismissed it as it didn't seem "logical" compared to the symptoms. Being a super-inexpensive part and super easy to change (and a maintenance item anyways), I should have changed it very early in the troubleshooting process. So far I've just been taking short runs, but they are longer than the previous runs that would definitely show the problem,but early indications look promising. I'd really like to take it ~12 miles across the lake and back to really say "it's fixed!"
 

pvanv

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I have had brand new plugs that did not work properly. I have been using the Iridium plugs (equivalent to the DCPR6E and intended for the MFS 40/50 motors), and find that marginal issues are now better due to a slightly hotter spark (even though the combustion chamber temperature is the same). In the US, they are only available from Tohatsu, and they are much more expensive. But they work well.
 
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