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Best test for under load misfire

  • vb4me
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07 May 2019 19:30 #29376 by vb4me
Best test for under load misfire was created by vb4me
Hi guys. Have a 2009 Ve Commodore coming to me for an under load only single cylinder misfire. The engine is a 3.6 LYF same as used in the Pontiac G6 and some similar year model Cadillacs. It has been to several mechanics over the last year and no one has diagnosed it properly. It was running LPG and petrol and two fixes have been to replace the offending cylinders injector and then to disable the LPG system. I would like to diagnose this as efficiently as possible as this guy is getting fed up and am wondering what peoples go to test would be for ignition breakdown under load? I am thinking of either stressing the coil with an adjustable spark tester or hooking up the scope to the possible offending cylinder and driving it to the same conditions that set the codes which are P0300 and P0305. Codes are from his last mechanic's invoice and confirmed with a cheap code reader the customer has recently purchased.
Thoughts anyone?

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07 May 2019 20:01 #29379 by PDM
Replied by PDM on topic Best test for under load misfire
Does it miss under WOT or only with moderate to heavy load? Is it constant or an intermittent miss?

I’d start with swapping plugs with a good cylinder and see if the miss follows the plug. If no change, swap plug wires and see if the miss follows. Do the same with the coil pack.

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07 May 2019 20:09 #29380 by Chad
Replied by Chad on topic Best test for under load misfire
If you have a scope, I would look at voltage and current waveforms of the Coil and Injector.

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."

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07 May 2019 20:16 #29381 by vb4me
Replied by vb4me on topic Best test for under load misfire
Have been told (I'm assuming) moderate to heavy load. Overtaking on the highway but has happened to his wife accelerating from 30 or 40 km/h to 60km/h. These are an older couple so not hoons. Apparently happens every time. Engine light illuminates then he can feel the miss. But if he stops the car and restarts the light goes out and car runs fine until put under load again. It is a cop design ignition so no wires to worry about. Was going to try to diagnose through scoping and electrical testing only but it may be quicker to do as suggested and swap coils and or plugs to see if the miss follows.

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07 May 2019 22:03 #29385 by vb4me
Replied by vb4me on topic Best test for under load misfire
That was one of my options. Just seeing if anyone had a quicker or better method. Just trying to do the best for this customer as he has been mucked around enough and spent money I believe wasn't needed.

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09 May 2019 01:38 #29437 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Best test for under load misfire
The symptoms really point straight to.an ignition secondary misfire due the spark jumping to ground.. The car has been running on LPG. LPG puts very high demands on ignition secondary voltage the KV goes through the roof when the engine is under load. Once the coil insulation had broken down a path is burned in the insulator and the spark going to jump to ground every time a certain KV level is exceeded. New coil pack required test is difficult because of the coil pack cassette design which gives no easy access to the ignition secondary for scope testing unless you have a full set of plug wire extensions. I would also fit new plugs On engines that run on LPG the spark.plug gap should be narrowed by 10% as a minimum. Personally I set plugs on LPG car to 0 8mm 0.032 inch.

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09 May 2019 02:02 #29438 by vb4me
Replied by vb4me on topic Best test for under load misfire
Cheers Andy. Totally agree. Just hoping to be able to give the customer something to make him feel better about having to spend more money and alleviate his concerns of will this time be the right solution? Lol

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16 May 2019 16:23 #29726 by vb4me
Replied by vb4me on topic Best test for under load misfire
Update.
I was able to use the Pico cop probe and capture the waveform on the known offending cylinder and a known good at both idle and whilst brake torqueing the engine, which made it easier to show the customer that he needed a new coil.
I talked to the bloke that services this car and he explained that for some reason this car chews through coils. He has personally replaced two sets in the 6 years he has serviced this vehicle. The coil I replaced also had a non-matching coil to the other cylinders. The customer has in the last 3 weeks had the LPG system removed as the tank needed retesting and he wasn't prepared to spend the money on it. Hopefully, with the LPG removal, this vehicle may not go through so many coils.

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