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Seeking advice. Engine failure. Non-GMC [message #352236] Mon, 17 February 2020 23:28 Go to previous message
NextGenGMC is currently offline  NextGenGMC   United States
Messages: 146
Registered: December 2017
Location: Washington State
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With so many people on this forum having experience in engine rebuilding I thought I may be able to find some answers.

I recently experienced a severe set back with my daily driver. One cylinders had failed in a spectacular way - a piece broke off along the edge above the top compression ring. I'm trying to figure out what may have caused the failure and I'm not having any luck so far.

The engine is a Mitsubishi V6 (6G74 version) from an SUV with 250k miles on it. I owned the vehicle since we bought it new. It was always maintained on schedule, never overheated, never ran out of oil. All parts (except few external things) are still OEM from assembly in Japan. The failure occurred at highway speed when I was passing another car. The engine started to shudder hard and check engine light came on. I stopped and pulled the codes - misfire on #5 cylinder (the one that failed). After some testing I determined that there was no compression on the said cylinder and the leak was going through the piston and into crankcase. Timing was on the spot correct, so no issue with the timing belt causing it.

I pulled the engine and found out that #5 piston had a hole in it. Pistons #1-4 have no wear marks at all, just normal discoloration from use. Cylinder walls on #1-4 still show good crosshatch marks. The failure of the #5 resulted in some scoring of the cylinder wall and aluminum deposits on the bottom of the head, but not much else. It seem that the failure occurred after some abnormal wear on the piston. Surprisingly to me, the #6 piston is showing what I presume an early stage of similar wear pattern to the #5. The wear pattern on #6 piston is very strange. The is no wear at all on the piston skirt (where you'd expect it most), but instead there are score marks on four sides of the piston at about 45 degrees from the piston pin. Rings are not stuck on pistons #1-4 and on #6. On #5 the rings are stuck due to damage. None of the pins are sticking (not even on th3 #5), the main bearings are within spec and show just normal and very even wear pattern on all four mains. Same goes for all the connecting rod bearings - good even wear, no scoring, no binding. Connecting rods on affected cylinders are not bent.

So the question is - does anyone know what may have caused such weird wear pattern on the affected pistons and what could cause piston to fail in such way?
Appreciate all of your input on this issue.

I'm going to try to attach links to photos. I hope it works.

Piston failure, top view, bank 1:
https://photos.app.goo.gl/FcmgXyMKJEDETsZ97

#5 failed piston, damage side view:
https://photos.app.goo.gl/kQCn1FXLwz67feYy6

#6 piston, no wear on skirt:
https://photos.app.goo.gl/k9L9pkG9JbdEdS5z7

#6 piston wear marks on 45 degree to pin:
https://photos.app.goo.gl/r7HG6FXXVj7v9Mnx7

Vadim Jitkov '76 Glenbrook 26' Pullman, WA

[Updated on: Wed, 19 February 2020 12:43]

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