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3&7 cyl. missfire

sg27

By sg27
in Ask A GM Technician

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sg27 New Member

sg27

Posted
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i have a 97 suburban that has a3&7 cyl. missfire only trouble code is P0300 random missfire have changed dist.,crank sens.,computer and after further checking i found there is no injector drive on these two cyls. . i checked at computer plug so no broke wire any suggestions

sg27 New Member

sg27

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after further checking i found there is no injector drive on these two cyls. .

 

How did you check this?

 

 

 

 

i chased the wires from plug on intake to computer plug then with a noid and two test leads i probed the injector ground and the pulse wire at computer and all have drive but 3&7 cyls.

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted
i chased the wires from plug on intake to computer plug then with a noid and two test leads i probed the injector ground and the pulse wire at computer  and all have drive but 3&7 cyls.

 

Sounds like a PCM problem. Uncommon, but it happens. Keep in mind that the PCM has an 8/80 warranty, though it appears you're probably over the 8 year mark if not the 80,000 miles.

sg27 New Member

sg27

Posted
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i chased the wires from plug on intake to computer plug then with a noid and two test leads i probed the injector ground and the pulse wire at computer  and all have drive but 3&7 cyls.

 

Sounds like a PCM problem. Uncommon, but it happens. Keep in mind that the PCM has an 8/80 warranty, though it appears you're probably over the 8 year mark if not the 80,000 miles.

 

 

 

 

i have changed the pcm twice still same problem

71GMC_3/4T Enthusiast

71GMC_3/4T

Posted
Posted

If your 3 and 7 cylinder injectors are shorted, you won't see any drive voltage, it will read zero all the time.

Ohm out a good one vs. a bad one.

snoman Senior Enthusiast

snoman

Posted
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If your 3 and 7 cylinder injectors are shorted, you won't see any drive voltage, it will read zero all the time.

Ohm out a good one vs. a bad one.

 

 

 

 

I would check the wiring to injectors too because the chance of two injectors failing at once is slim.

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted
i have changed the pcm twice still same problem

 

Are you testing with the injectors connected at the time or disconnected? If connected, they could be pulling the circuit down but if disconnected and you still have no pulse from the PCM, I would think a connection problem if you've replaced the PCM twice.

 

The only other thing I can think of would be a loss of a reference signal only at the moment that the #3 and #7 cylinders were due to fire. That would be very odd though. Never seen anything like that.

sg27 New Member

sg27

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i have changed the pcm twice still same problem

 

Are you testing with the injectors connected at the time or disconnected? If connected, they could be pulling the circuit down but if disconnected and you still have no pulse from the PCM, I would think a connection problem if you've replaced the PCM twice.

 

The only other thing I can think of would be a loss of a reference signal only at the moment that the #3 and #7 cylinders were due to fire. That would be very odd though. Never seen anything like that.

 

 

 

 

i have tested with pcm plug in and out same result no injector drive on these two cyls.

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted
i have tested with pcm plug in and out same result no injector drive on these two cyls.

 

Personally, I wouldn't test with the PCM connector totally disconnected. No telling what other circuits you're disabling, confusing the results. I suspect if you've tested other cylinders with this connector off and they work, then you're probably okay but I wouldn't do it that way.

 

Dunno what else to tell you at this point. Assuming the injector circuits are not dead as a result of having the entire connector disconnected, then it seems that it can only be what's telling the PCM to fire those two injectors.

snoman Senior Enthusiast

snoman

Posted
Posted
i have tested with pcm plug in and out same result no injector drive on these two cyls.

 

Personally, I wouldn't test with the PCM connector totally disconnected. No telling what other circuits you're disabling, confusing the results. I suspect if you've tested other cylinders with this connector off and they work, then you're probably okay but I wouldn't do it that way.

 

Dunno what else to tell you at this point. Assuming the injector circuits are not dead as a result of having the entire connector disconnected, then it seems that it can only be what's telling the PCM to fire those two injectors.

 

 

 

 

 

 

Do those two injectors share a common ground in loom as they are across for one another cylinder wise?

snoman Senior Enthusiast

snoman

Posted
Posted

 

Curiuos how they have 1,3 and 4 "paired" in one junction and the other five in another junction. He should maybe try swapping injectors because for some reason I just am not convinced that it is the injectors though I may be wrong.

GM-Tech Senior Enthusiast

GM-Tech

Posted
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Curiuos how they have 1,3 and 4 "paired" in one junction and the other five in another junction.

 

Who knows their reasoning. Also note that 1-7 are in one of the PCM connectors while #8 is all by its lonesome in another connector. Doesn't make a lot of sense, but there is probably a reason for it.

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