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Over 50,000 missfires

Shaners

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Shaners Senior Enthusiast

Shaners

Posted
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My buddy with the 2500 Avalanche and the 8.1L motor was noticing a little stumble and lackluster performance form his 7K truck.  He took it to the dealer and they plugged it in to the computer.  It told them that the #8 cylinder had recorded over 50,000 missfires.  There initial responce is to replace the #8 spark plug wire.  I wonder if that is the true fix to this problem   ???
MarkD Senior Enthusiast

MarkD

Posted
Posted

Geez!  I wonder if they checked the plug?  I would definately replace the plugs as well (cheap insurance).  Does he have any mods?

 

Ignorant question alert...Does the 8.1L have the coil-per-cyl. setup?  I don't think so, but I am not sure.

 

The wire could have a loose connection or corroded terminal.

Shaners Senior Enthusiast

Shaners

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I believe they have a capacitor for each cyl. but I'm not 100% on that.

 

They are going to replace all 8 wires.  I told him to have them replace the plugs, and check the capacitor and fuel injector too.

Guest

Guest

Posted
Posted
Ignorant question alert...Does the 8.1L have the coil-per-cyl. setup?  I don't think so, but I am not sure.

Im almost certain GM uses it on all new engines...  :thumb:

CMNTMXR81 Senior Enthusiast

CMNTMXR81

Posted
Posted

Yes, it has a coil for every cylinder.  50k misfires sounds more to me than a plug wire.  But I wouldn't over look the basics.  I'd look into a bad coil.  Swap 8 & 6 and if the misfires move to #6, then you know it's a bad coil.

 

Once ignition is isolated, move to the fuel side.  Have them do an injector cycling test with the TechII and a fuel pressure gauge hooked up to the fuel rail.

 

With the TechII, then cycle each injector.  When cycling each injector, the pressure in the fuel rail should drop 2-5psi (more depending on how pressurized the rail is).  If you get to #8 and no pressure drop or minimal pressure drop is detected, BINGO....you just found a bad injector.  It probably also wouldn't hurt to check to make the injector electrical connectors are all connected.

Shaners Senior Enthusiast

Shaners

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New wires and plugs were installed and coils were swapped and some sort of cleaner was run through the engine.  Now it is miss-firing on 7 and 8.

 

He said the service mgr showed him the plugs and there was a tan build-up on them that could be flaked off with a fingernail.

 

There is 8K miles on this 8.1L motor....  and the truck is going back in next week after the discussion takes place with the service mgr and field rep.

CMNTMXR81 Senior Enthusiast

CMNTMXR81

Posted
Posted

I really think it's fuel injector related.  #8 is still misfiring and now #7 is.  How often is #7 misfiring?  Did I ask what gas he uses?

 

What's happening is the plug is firing, but there's nothing it's igniting in the combustion chamber due to lack of fuel delivery.  Although, I'd think it'd be ghost white in this circumstance, I can see the "flakiness" being caused from heat.

 

If it were an ignition related misfire, his SES light on the dash would be flashing because more likely than not, you'd get absurd O2 readings as well as you'd probably generate a secondary air injection code as the air/fuel is being unburned due to lack of ignition and going right out the header and into the cats.  The Air injection system then goes to try and re-route it back into the intake so it can be reburned but it isn't enough generating the code which would be a "below threshold" code.

 

This is why I think it's fuel related.   :D

Shaners Senior Enthusiast

Shaners

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You've been a great help so far CMNTMXR81, and I too think it is fuel related (injector/computer).  The flakey tan burn on the plugs bothers me...

 

I think from time to time he runs "Quickfill" gas, which I don't really care for except in the higher octanes.  His service mgr did ask him when he looked at the plugs, if there were ever any additivees run through the motor and the answer is "NO".  Just straight regular unleaded....

CMNTMXR81 Senior Enthusiast

CMNTMXR81

Posted
Posted

No problem, my life is a never ending battle against the dreaded misfire.  :cheers:

 

As to gas, I know places like Quickfill, Speedways, and other quickies may be cheaper than Amoco, Mobil, etc, but you get what you pay for.  Essentially these places get the bottom of the barrel gas after the big boys take the good stuff.

 

A friend in Green Bay trucks gas and Amoco, Mobil, Shell's take the prime stuff on top and then they deliver the bottom to the "quickies".  Also stay away from Citgo for reasons I mentioned in the General section.

Shaners Senior Enthusiast

Shaners

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Shane, what is "quickfill" gas?

It's a chain of gas stations in this area.  I think it is actually called Kwikfill.

Shaners Senior Enthusiast

Shaners

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Once ignition is isolated, move to the fuel side.  Have them do an injector cycling test with the TechII and a fuel pressure gauge hooked up to the fuel rail.

 

With the TechII, then cycle each injector.  When cycling each injector, the pressure in the fuel rail should drop 2-5psi (more depending on how pressurized the rail is).  If you get to #8 and no pressure drop or minimal pressure drop is detected, BINGO....you just found a bad injector.  It probably also wouldn't hurt to check to make the injector electrical connectors are all connected.

He shoots and scores!!!

 

It was a bad injector.  He was to pick his Av up this afternoon so I'll talk to him at work and see if he notices it running better....

 

I still don't understand the flaky tan on the plugs though, unless the computer was trying to compensate for the bad #8 injector causing strange fuel mixtures in the other cylinders.

CMNTMXR81 Senior Enthusiast

CMNTMXR81

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Posted

Patting myself on the back right now...being careful not to hurt myself too much...J/K!!!

 

The flakiness on the plug was probably due a small amount of fuel it was actually able to deliver, then the plug firing and having virtally nothing to fire therefore no "burn" in the combustion chamber.  Essentially the plug was delivering, but with nothing to "burn" is was simply burning itself up.

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