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Misfire on VG30I


ahardb0dy
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Trying to help someone on another forum. He has a Hardbody with the VG30I (88), seems like it has a misfire on number 5 cylinder. He has replaced the plugs, wires, cap and rotor, the number 5 plug has spark when it is removed and placed near the engine. Engine has good compression and timing is correct.

 

When he removes the number 5 wire while running the RPM's do not change.

 

Any thoughts on this? Thanks

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Hmm. TBI, so it's not a dead injector. Assuming all tests so far are accurate, and all parts replaced are good, all I can think of is a restriction in the intake or exhaust runner for #5--like the air cleaner was off and a mouse crawled into it or something like that. (How's that for a long shot?) Could try positioning the engine with the intake valve open, spark plug out, and jam an air nozzle in the hole to see if anything blows out.

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Is he sure the intake valve is opening? I have never heard of issues with Nissan cams, but Chevy was notorious for cam lobes rounding off.

 

The only thing that is missing is fuel, since he has spark, compression and correct timing.

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I don't know if he checked for codes, it runs, but runs rough, he said it was backfiring on deceleration but than adjusted the timing and that stopped, His mechanic was saying it might be a stuck intake valve

 

More info, engine has about 225K miles, has good power and doesn't burn oil. He said when he shuts it off it runs for a few seconds (dieseling??)

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I think he is missing fuel. So the plug would still be sparking away but no detonation. That's why I asked if the valve was opening, because that is about the only way to get a no fuel condition to a single cylinder on a tbi, a stuck closed valve.

 

Especially considering that the compression is good so we know that the exhaust valve is closing completely.

 

Does the plug look like it is lean?

Edited by Citron
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The other possibility is the new plug wire is leaking and when it is in the plug well it is arcing over to the head instead of sparking the plug. I have never heard of it with a new wire though. Is there any whitish discoloration anywhere on the wire to evidence arcing?

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Dumb I know, but is the plug wire seated on firmly? I swear every time I do plugs on my pathfinder I fire it up and it has a miss. Every time there was one wire that was not on securely.

 

Easy to tell if its getting fuel and not firing, the plug should be soaked with gas. I say "should" as that has been my experience before.

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he said he doesn't remember what the old plug looked like.

 

He also posted a video of doing the compression test on #5 cylinder. I told him to get a new gauge because there is no way the reading he is getting is correct, he is showing 210 PSI, The service manual shows a new engine should be at 173 PSI.

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Crappy tools are always fun. But the plug that he's been trying to run it on should show fuel pretty much immediately if it's getting fuel but not burning it.

 

I'd be inclined to stick a boreoscope down that hole and see if anything jumps out. I don't know how much carbon buildup you'd need to make a misfire (shrouding the plug?) but it's something else to check, anyway.

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