Misfire - Cylinder 7

2

2QuickZ's

Club Member
The car is a 2002 Z28 M6 w/ 16k miles. Possible causes?

- Bad plug
- Bad wire
- Bad coil
- Bad injector

How do I trouble shoot any of these? I'm not worried about the troubleshooting the plugs. I bought a new set and will throw them on tomorrow. If that doesn't fix it, how do I check the wire? Visually it looks fine. How do you test a coil? An injector?

Here's what happened: On my way into work today, the SES light flashed at me a dozen or so times then stayed on steady. I reset the code after I checked it. It flashed at me on two different occasions on the way home from work but didn't stay on either time. The first time it flashed a couple dozen times then went off when I changed speed. The second time it only flashed a half dozen times before I changed speeds and it went off. The weird thing is that all 3 times it flashed at me today was when I was doing a steady 75 mph in 6th gear. Never in any other gear or during accel/decel. Any thoughts?

No headers. Still has the stock manifolds on it that have never been off. The valvecovers were off once before to put in chromoly push rods in prep for a cam install that I still haven't done but the plug wire was firmly connected and shows no signs of arcing. I took the plugs out of cyl 3 & 7 and they look almost identical. :confused: The threads of the plug on cyl 7 had oil on them that I have no idea where it came from since the valve cover isn't leaking but the plug itself shows no sign of oil in the cylinder. Here's pics. The plug from #3 is on the left and #7 is on the right. Factory gap was 0.057.

plugs003.jpg

plugs002.jpg
 
you can swap the wire with like #5 but if you don't have a scanner you won't be able to tell if the problem moves.
stock springs and better rods are a big NO NO. The weak rods bent instead of the vavle but when you put in better rods then the vavle becomes the issue
 
1BADAIR said:
you can swap the wire with like #5 but if you don't have a scanner you won't be able to tell if the problem moves.
stock springs and better rods are a big NO NO. The weak rods bent instead of the vavle but when you put in better rods then the vavle becomes the issue
Click to expand...
you can swap the coil as well

if the plugs don't fix it, I'd swap the wire with one cylinder, the coil with a different one, and see where/if the miss moves(if it follows the wire/coil, ect.)

if it stays on #7, look into compression, fuel injector(pluged?), ect.
 
I don't know why I didn't think to just move stuff around and see if the symptoms follow. It better not be a compression issue. If it is, I'm going to be pissed. I haven't overrevved the thing so there is really no reason for a bent valve. I'll keep my fingers crossed it is something simple.
 
Mine did the same thing before I had it tuned. I am not saying its your problem but crusing at about 75ish in 6th it would flash.
 
I had a mis-fire last year. It actually turned out to be a broken valve spring.

Remove both valve covers and look very carefully at the valve train. One of my intake springs was broken at the 2nd coil from the top, difficult to see with just a quick look.
 
Doc99SS said:
I had a mis-fire last year. It actually turned out to be a broken valve spring.

Remove both valve covers and look very carefully at the valve train. One of my intake springs was broken at the 2nd coil from the top, difficult to see with just a quick look.
Click to expand...

See when my valve spring broken it sounded like a knock. Plus the car didn't want to stay running.
 
Previous to locating the broken valve spring (which was on the opposite bank!), i had switched plug wires, switched coils, checked out the fuel injectors. When none of this got rid of my mis-fire, I removed the valve covers & found a broken spring. The valve would open but not all the way, and maybe I dinged that piston. But immediately after changing the valve spring, the engine purred like a kitten and run balls out.
 
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