No spark #7

[Rukee]

Hi everyone, first post here hoping you guys can help me out.

I have no spark on # 7. have put new plugs, wires and a coil on that cyl but still no spark when testing with a timing light and I get a random misfire code. Is there a common problem that would cause this issue? I see no problem with the wiring. Thanks for any responses.

[bossier62guy]

You didn't even state what type of truck, engine, etc. If you want help you need to provide the basics at least. List the code as well....

[Rukee]

Sorry, it's an `07 sierra with the 6.0 L. The only code is p0300 random misfire.

[gmtech4]

why are you focused on #7 then ,?

 

you have access to a scanner ?

[scooby5121]

Random Misfire: 2 or more cylinders are bad

low fuel pressure

vacuum leak

Bad battery

Bad spark plugs/wires

So now you know a little more on random misfires.... it is not just one cylinder so you have more to look for !!!!!!

[scooby5121]

Random Misfire: 2 or more cylinders are bad

[Rukee]

why are you focused on #7 then ,?

 

you have access to a scanner ?

#7 has little to no spark checking with a timing light. All others are firing fine. When I rev the truck up the light might flash 1-6 times but then at an idle it drops off to nothing. Ohm testing the wire and it matches the other wires. Swapping the coil and wire with another cyl and the miss stays at #7. Also after rev'ing it just starts smoking out both tail pipes almost like a blown head gasket but the smoke doesn't smell sweet like antifreeze and is white to light blue. I should rescan, I cleared the codes and after starting up the cel came back on and I just assumed it was for the random misfire.

[Rukee]

Fuel pressure is at a constant 60psi. Still struggling to find out why the P0300 and the all but dead # 7 coil.

[MikeInCtown]

If you swap out coils and get no spark then you most likely have a wiring problem. I'd start tracing the wires through the connectors and make sure you don't have a wire somewhere that got rubbed, pinched, or grounded by accident. I can't believe the computer is not firing one cylinder, so I am making the assumption that your issue is in the wire harness/connectors.

[Rukee]

Okay thanks, I see the coil harness plugs into the main harness between the center coils. Think I'll disconnect that plug and check for continuity between that plug and the # 7 coil. That should tell me if it's in the coil harness or the engine harness.

[Rukee]

That was easy enough, the coil harness checks out. Does anyone has a wiring schematics for the ECM? There are three main plugs, it would be very helpful to know which plug and pins I should be looking at. Thanks in advance.

[MikeInCtown]

While you are disconnecting the various connectors, look at them closely for any signs of corrosion or a bent pin or something out of place. With the truck running, wiggle the wires at various connectors along the line and see if you start getting the spark.

 

I know you checked with your tester, but as you have things unplugged, you may get an ok reading. However, once reassembled, the pin may move slightly causing an intermittent connection.

 

This is how I have found several electrical problems on a motorcycle I own. You can trace the wiring and if it all seems to check out then you start wiggling things and looking to see if there is any change. Found connector issues. I've also done this with an old S-10 I had. Was constantly getting EGR codes even with new sensors and EGR. Eventually the truck was running and we started moving the harness around. the problem went away and we found the harness had rubbed against a manifold and caused one wire to ground against the cylinder head. Anyway, check every bit of wire you can.

[Rukee]

Yeah I did inspect the plug-ins, and have been tugging on the engine harness and connectors with the engine running but nothing changes. Next to the PCM the wrap around the wires is all coming apart/crumbling, but looking underneath I don't see any scuffed wires.