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Cylinder 8 misfire

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Old 02-25-2017, 06:29 PM   #1
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Default Cylinder 8 misfire

I have been trying to figure out why my 8th cylinder is missing under WOT. I have done a leak down of the cylinder 80-90% compression I have swapped out plugs,coils,injectors and nothing has made it stop. Any one that has VCM scanner I have the .hpl file of my last run, any advice would be greatly appreciated.
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File Type: hpl sss.hpl (74.3 KB, 14 views)
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Old 02-25-2017, 07:27 PM   #2
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Check the valve springs for cracks.
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Old 02-25-2017, 08:18 PM   #3
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Could I hear if their was a cracked valve spring?
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Old 02-25-2017, 09:42 PM   #4
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Quote:
Originally Posted by RockinWs6 View Post
Check the valve springs for cracks.
I was checking my run at 4000 rpm my Manifold pressure is 98 kpa. that seems really high. Is it?

Last edited by aechild; 02-25-2017 at 09:48 PM.
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Old 02-25-2017, 11:03 PM   #5
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I took off the valve covers I didnt notice any cracks but I can move the rockers side to side slightly and I can spin the pushrod on cylinder 8.Heres a video

Last edited by aechild; 02-25-2017 at 11:32 PM.
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Old 02-26-2017, 11:18 AM   #6
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Next thing I would do is turn the engine over by hand and watch the valve action. Make sure number 8 valves are opening and closing fully. If you have access to a remove camera I would pull the spark plug and scope the cylinder. Are you using engine oil? Number 7&8 are know for broken rings and ring lands from overheating. Can you feel the misfire or are you just getting a misfire code?

The sideway motion is on the rockers is normal. If you are only getting a misfire at WOT I'd still suspect the valve springs-lifters-pushrods on that cylinder. Have you checked fuel pressure at WOT?

Last edited by RockinWs6; 02-26-2017 at 12:09 PM.
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Old 02-26-2017, 08:16 PM   #7
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So after messing with it today, if you wait long enough cylinder the engine will (for lack of a better word) hiccup. And it will log 1 misfire for both cylinder 7&8. I know those 2 have a tendency to run lean. My fuel supply line is steady at 50 psi, is that 2 low?
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Old 02-26-2017, 08:37 PM   #8
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I know its a completely different animal,, but my BMW with Coil on plug will misfired when the battery got tired and when the alternator wore out. Check your charging voltage when its having the problem. On mine the miss shows up on the 5&6 end first.
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Old 02-26-2017, 08:41 PM   #9
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Yes way too low, hopefully you haven't toasted a ring and broken a ring land. Is it tossing a code for a misfire? I would scope the cylinders before doing anything else. If the low fuel pressure already broke a piston you don't want to run it till the engine fails. You could try replacing the fuel filter and see if it helps the fuel pressure. I wouldn't go WOT unless you are in the 55 or higher on the fuel pressure. Those rear cylinders break very easily and QUICK. How do the plugs look on those cylinders?

What do the fuel trims look like?

Last edited by RockinWs6; 02-26-2017 at 09:01 PM.
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Old 03-07-2017, 11:02 PM   #10
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Have you changed the fuel filter? I changed one in a `99 Suburban 350 yesterday and it went from 50 to 60 psi.
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