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Discussion Starter #1 (Edited)
EDIT 9/12/17: Problem turned out to be a sticking throttle position sensor. When the sensor warms up, it doesn't return to the idle position and causing the engine to stall. Read below to see all the tests performed and for how to build a peak voltage detector circuit. Part ordered and replaced.

I have been chasing a stalling issue on my 2008 Suzuki King Quad 750AXi for a while now. I have done a lot of reading and research, so I thought I would put together a thread that combined all that info in hopes it will help someone else. There is a lot of good info on here and I want to thank everyone for their contributions. They have kept me searching.

Details on the machine:
2008 Suzuki King Quad 750 AXi
HMF Performance Slip-On
HMF Fuel Optimizer (Set to factory settings)
NGK CR6EIA-9 Iridium Spark Plug
Factory Air Cleaner
3154 Miles, 409 Hours
Coolant flushed and changed, Oil Changed, Front and Rear Diff Fluids Changed
Warn 2500lb winch, LED Pod Light Bulb, Heat Demon Grips, 126W Light Bar, 2x 27W Work Lights

So this issue started 2 years ago. Quad cut out and wouldn't restart until it cooled. I thought it was the spark plug at first. So I replaced it with the iridium plug: http://www.suzukiatvforums.com/forums/repair-maintenance/6320-2008-750-king-quad-spark-plug-upgrade.html

This did not fix the problem. I thought then I didn't seat the boot well and water was getting in the plug. I used dielectric grease and sealed everything up. It still did it.

After reading, I turned to the fuel system. I replaced the fuel filter and did the screen mod on the pressure regulator to bump the pressure a bit: http://www.suzukiatvforums.com/forums/suzuki-kingquad/9461-2008-kq-750axi-fuel-filter-pump-repair.html

I also cleaned the fuel injector. It began to idle much smoother.

It worked fine from here on out. Fast forward to last fall/winter. After coming to a stop, it would just kick off and then it would fire right back up. This was annoying but not really detrimental. And when I used my light bar and work lights they would flicker to the RPM which was annoying. It still had the original battery, so I put a new one in. No change. After some more reading I came to the ground regulator mod: http://www.suzukiatvforums.com/forums/performance-modifications/17537-ground-regulator-mod.html

It is also related to this issue: http://www.suzukiatvforums.com/forums/suzuki-kingquad/19658-08-kq-750-idle-issue.html

This boosted my idle voltage to over 14 volts. See the ground regulator mod for my contribution. This solved the random idle stalling issue. While the machine was apart I decided to change coolant, oil, and diff fluids. With all my panels off, this worked great. The quad idled perfectly. I put everything back together and let it idle in my driveway. It idled fine. When I revved the engine, it revved and instead of returning to idle it misfired and died. I've been able to repeat this result several times.

I consulted the factory service manual and some threads on here: http://www.suzukiatvforums.com/forums/repair-maintenance/4310-750-king-quad-shuts-off-after-about-10-min.html
https://www.quadcrazy.com/atvforum/topic/7684-2008-suzuki-king-quad-shutdown-after-10-mins/

So I started hunting. TPS was in range and I did not have any service codes.

Easiest suspect was that the coil pack was overheating because it sits above the exhaust. I remove it, cleaned all the connections up and replaced with no change. I measured the resistances and they were all in check... 0.1-0.6 primary ohms and 12k-19k secondary ohms. These were measured when the coil was cool. I ordered a new one from Caltric: Suzuki LT-A750X KINGQUAD 750AXI - 2008 - Ignition Coil

It was cheap, but complete. This should at least tell me if the problem was the coil. No one had one in stock nearby.

I installed it and there was no change in the issue. The new coil measure correctly on the primary but I only got 3k ohms on the secondary. I may not have made a good connection but I figured i'd try it. It fired right up and drive great. I let it idle in my driveway and it shut off. I was frustrated to say the least. Time for more reading.

I was following 1H-3 to 1H-6 in the service manual and the troubleshooting codes for C12 (SKP sensor) and C24 (ignition coil) neither of which were displaying since I did not get a check engine light. I checked the resistance of the CKP sensor and it was 220 ohms, right within spec of 150-250 ohms. I did this test while the engine was hot just after cutting out.

Continued.....
 

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Discussion Starter #2
The operation is very simple at this point.... CKP sensor tells computer where the crank shaft is. Computer says time to fire spark plug and sends signal to the primary side of the coil. So in my mind, either the CKP sensor is cutting out, or the computer is malfunctioning. In order to do further testing, I need the accessory called "peak voltage detector." Annoyingly, these devices are kind of expensive and of course you can' find the Suzuki on easily. So you can build one: https://thumpertalk.com/forums/topic/1034259-build-your-own-peak-voltage-adapter/

I ordered the parts this morning. I got enough to build 3 of them if anyone needs one. PM and we can work something out.

Once I get the parts and build it I will test the CKP sensor cold and see if I get the required 5V or more from the manual. Then I will check the voltage for the primary side of the coil. It should be 80V or more. Then I will run it till it fails and take the readings again. My gut feeling at this point is that my computer is failing. If that is the case hello VDI upgrade :devil

Please note that I have also researched this on the now crappy NYROC site: https://www.nyrocatv.com/tech-tips. See "Stalling" and "Start".
 

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Discussion Starter #3
Here are the coil photos:
 

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Discussion Starter #4 (Edited)
I built the peak test circuit tonight. I am waiting on the universal set of probes I ordered so it can be plug and play.




I bought enough to make 2 more just like you see here. I just need to order more probes. If you interested, PM me and we can work out the details. I should be able to test this one later in the week.


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Discussion Starter #5 (Edited)
Probes arrived today. I had a little bit of time so I finished up the adapter. I tested it on 120V wall outlet and it seems to work perfectly.





Just to be clear, in the US 120V is measured in Root Mean Square or RMS. I won't bother going into details, but the actual sine wave is actually more like 166V at peak voltage and that is exactly what this circuit demonstrated.

I have to mow today but hopefully I'll get a chance to test it on the quad later today.


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Discussion Starter #6
CKP cold and warm both put out a peak of 6.5V. I did not test it at the failing condition yet. I checked the primary coil voltage with the engine running. It should be at least 80V. With the new coil The highest I saw was 25V at idle. I need to swap coils back to the original and try again. I also need to pull the spark plug and just crank the engine as the manual describes. Also, revving the engine increased the voltage to ~30V. This puts them seriously out of spec.



More to follow....


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Discussion Starter #7
Didn't have enough time tonight to do much more testing. I have learned that the 22 uF capacitor is a bit big for testing the spark as it takes a long time to charge up. It may need switched to a smaller capacity so it does not take so long to read the coil primary voltage.

Other than that, I need to swap the original coil pack back in and run the test again tomorrow. I think at this point the problem is the CDI. Hopefully I'll know more tomorrow.
 

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Discussion Starter #8
I contacted VDI with some questions and Lyle replied back later in the day. Here is the reply I received:

It’s possible that your peak detector cannot detect the pulses fast enough. The factory ECU will give one 5 degree spark every 720 degrees of rotation (so quite short in the overall scheme of things). If the capacitor in your peak detector is too large, it will average out the reading, showing a much lower reading. Our ECU will give a constant 170V peak to peak regardless of RPM/cranking.

A couple things that could be issues though:
  • Bad rectifier. If you unplug the rectifier, will it run fine and not stall (it won’t charge at this time, so the battery voltage will continue to drop).
  • Low fuel pressure. The ECU expects 42 to 43PSI of fuel pressure at the injector with key on, or running.
  • Bad ground to the engine block. There should be zero ohms resistance between one of the coil terminals and the negative battery terminal, as well as the engine block to the negative battery terminal.
  • Bad thermostat. When it gets hot, is the radiator hot? If it’s still cool then water isn’t leaving the engine and the fan won’t operate correctly.
It is possible that the ECU is failing (as it gets hotter, it would be more prone to losing spark). Immediately after it stalls, can you measure voltage on the coil? If there is no voltage (or much lower than running), I’d suspect the ECU is bad.
I didn't suspect the rectifier causing a problem... but it is possible. I'll have to check it when heated up. my fuel pressure was good, but I can recheck it to make sure. I think the engine ground is good since I just redid it, but I can use the multi-meter and check resistance. I think my thermostat is OK since my fan does turn on and off.

The ECU failing in the heat still seems most likely to me, but a little more testing should confirm it.

In regards to the highlight section about the factory spark... The engine idles at about 1350 rpm. Divide this by 60 seconds to get 22.5 rotations per second. If you take the inverse of this (1/22.5) you get 0.044 seconds per rotation. We know that the engine only sparks for 5 degrees out of 360 degrees (1.389%) in the rotation, which means out of the 0.044 seconds in a rotation, spark occurs for .044 * (5/360) = 0.000611 seconds... So each spark takes 0.000611 seconds and if you extrapolate that up to RPM you get 0.000611 seconds * 1350 rpm / 2 (because it is a 4 stroke engine) = 0.412425 seconds of spark in a 1 minute period.

I did some iCircuit analysis to see if the designed tester would work and it did on paper. Knowing this now, I will need to crunch some more numbers and see if it can detect the spark accurately.
 

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Discussion Starter #9
Did some more reading...
http://www.electronics-tutorials.ws/rc/rc_1.html
Capacitor charge and discharge calculator | MustCalculate

I decided to order new capacitors and bleed resistors. Changing from 22uF to 0.47uF and 1Mohm to 47Mohm.

Time Constant as-is (R=300 ohms and C = 22uF) = 0.0066 seconds. Multiply by 5 for fully charged capacitor = 0.033 seconds.
Current spark duration = 0.000611 seconds


As you can see, the time constant is magnitudes of order larger than the spark at idle. A small capacitor is needed to get a more accurate and quicker value.

New Time Constant (R=300 ohms and C = 0.47uF) = 0.000141 seconds. Multiply by 5 for fully charged capacitor = 0.000705 seconds.
Current spark duration = 0.000611 seconds


As for the bleed resistor, it would take approximately 15 seconds for voltage to halve itself at 22uF and 1Mohm. To keep with that same rate with the new 0.47uF capacitor, the resistor has been upped to 47Mohm.

This new configuration should give a much better reading when testing spark peak voltage. We shall see when the parts arrive next week.
 

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Discussion Starter #10
Got the engine to the stalling point after swapping the factory coil back in. Idle voltage on factory coil was reading about 39V and went down just a tiny bit at the stalling point. I'm hoping to get better reading with the new circuit parts.

Checked the CKP and still got good readings when the quad wouldn't start.

I temped the voltage regulator. I had no idea it got to 200 degrees. I pulled the plug and tried to start the engine with no change. And battery was reading 14V when it stalled so I am fairly certain it's not the regulator.

Check grounds and they all looked good including the ground to the coil.

The CDI/ECM was cool except for a warm spot on the back which was about 100 degrees. I thought it would be warmer but it stays pretty cool since the fuel tank is underneath it.


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Discussion Starter #11
Just a quick update. The math paid off. Thanks to Lyle for the info. I had a moment to fire it up and test the new capacitor and resister I installed in the detector last night. Worked like a charm. At idle, I am getting an expected voltage over 80V.


When I have a bit more time I can let it idle for a while and see what happens when it stalls.


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The TPS can get sticky when the sensor gets to a certain temperature. It will over fuel and stall when the throttle is released. The machine will not start until the sensor has returned to the closed position
 

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Discussion Starter #13
The TPS can get sticky when the sensor gets to a certain temperature. It will over fuel and stall when the throttle is released. The machine will not start until the sensor has returned to the closed position
That's interesting... Can the sensor be cleaned or does it need replaced? Will this show up as the position being off in diagnostic mode?
 

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The sensor needs to be replaced. The easiest way to determine this is have a dealer hook it up to the computer to see what the tps is doing.

If you do not want to pay a dealer you can use a jumper wire on the diagnostic plug for setting the tps.

Do you need instructions how to do that?
 

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Discussion Starter #15
I know how to get it into dealer mode and check the symbol by the C for placement. I'm trying to think if I checked that when it was hot. I know it was in spec cold. Hmmm....


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Get the machine hot. rev the machine up and return to idle slowly. If faulty the dash beside the c will stay at the top.
 

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Discussion Starter #17
Will do. Thank you!


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Discussion Starter #19
I will. Not sure which day I'll have a chance to try this weekend. I don't want to take it to a dealer in this area. I've been to all but one and they are slimy. I'm not giving them my hard earned money.


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