For several years now I have been experiencing an intermittent, cold idle misfire in cylinder #5. I ignored the problem because a) I wasn’t able to solve it easily and b) the car wasn’t up for emissions inspection so procrastination.

Fast forward, I had a differential leak, had a bunch of lights out on my IP, and my cat was shot, so I needed to start addressing deferred maintenance and decided to refresh a whole bunch of things. Got myself a new cat setup, figured I’d replace the O2 sensors while I was under the car, decided to bust a bunch bolts so I had to pull the right side head, tore the dash out, so on and so forth. Great project, got everything I wanted to do done but now I still have this misfire that won’t solve.

More specifically, the car starts and idles smoothly for maybe 5 seconds, then #5 will misfire intermittently while the engine warms up. If I give it gas, the misfire goes away, and it will rarely misfire at idle once warm. Occasionally the stumble is severe enough to trigger a misfire count on the next cylinder in the firing order (#4), but otherwise the other cylinders are firing perfectly. Current misfire counts vary from zero to sometimes 10-20, and the historical count will climb up to a 200-300 during a warm up cycle. This will trigger a P0300.

I have performed the following tests.

I have confirmed good spark.

• Ignition coil shows proper resistance
• I swapped #5 ignition coil with a non-misfiring cylinder’s coil
• Swapped plug wires
• Swapped plugs
• Replaced plugs and wires anyway because the old ones were pretty old
• Tested #5 ignition coil ground to chassis ground and got a good result
• Cleaned the ground connectors.
• Checked resistance across the injector wiring harness and everything was good.
• Tested continuity across the main injector connector and got a good result.
• Replaced the injector harness anyway because it was showing some age.
• Put a timing light on #5 and am getting a reliable spark

Confirmed good fuel.

• Swapped fuel injector #5 with a non-misfiring injector.
• Changed the fuel injector gaskets
• Tested fuel pressure (61 psi, in range)
• Performed Fuel Injector Coil balance test, passed with flying colors

Reasonably confident there are no vacuum issues

• Visually inspected hoses, all good.
• Tested with some propane and a home made smoker and did not find any leaks.
• Cleaned MAF.
• I had to pulled the intake to replace the air check valve, so I replaced the intake seals and visually inspected intake for cracks.
• No exhaust leaks.

Mechanical

• Performed a compression test, passed
• Visually inspected valve springs, no broken spring.
• Checked harmonic balancer for any obvious failures.

After all of this, I still have the same misfire behavior. The misfire is not horrible from an operational standpoint, but the second O2 sensor on the bank is indicating a lot of activity so I know I am going wreck the new cat if I don’t solve this. The only idea I have left is a bad valve or valve seal. The only other odd thing I see are some hairline cracks in the plate covering the engine valley, but I think if this was creating a vacuum issue it would not be isolated to one cylinder.

Anyone have any suggestions?

oh, stock 2001 Z06

 

I'd check valve springs, you may have one broken, common issue on the Z's.

 

checked that, no broken valve springs

 

Check compression/leakdown?

 

yep

 

Does it have some sort of a custom tune on it?

 

bone stock

 

Guess your just possessed by gremlins. Maybe a lifter not pumping up, how's the valve train sound? Seen a few GM engines over the years that a lifter started hanging up and rounded a cam lobe.

 

Interesting...should have checked that when I had the intake off. I haven't noticed any unexpected noises but I'll get the stethoscope out.

 

Have you compared the Injector pulse widths between cylinders especially when it misfires?

I would also find a way to check the dwell on 5 compared to other working cylinders. Find an old points dwell meter and slip a wire in the pin for the coil firing signal and see what it is during normal operation and misfire operation.

Sounds like you checked/swapped out all mechanical stuff. Maybe you have a bad PCM driver for the fuel or coil.

Dont know if the dwell meter would work on the injector but, it cant hurt to try.

I knew that those old 1970s tools would STILL be handy one day in the future!!!

If all else fails,, get a used PCM and swap that in.

Bill

 

I pulled injector pulse width off of #5 (misfiring) and number #1 (good). Pulled #5 first, from a cold start, idling. At first pulse width came up at about 3.0 mS, but after a few seconds dropped and held steady at 2.7 mS. Switched to #1 with the engine warm but not at operating temperature, and it pulled a steady 2.7 mS pulse width at idle. I was expecting about 3 ms, but this is probably close enough.

Never tried pulling dwell off an electronic ignition, but tried what you suggested and it worked. Put a pin up the purple wire on #1 and clamped my meter red to it, put black to chassis ground. Dwell came ups 43.6. Did the same thing on #5 (green wire this time), also came up with 43.6. Not sure if this is expected value, but that they agree is encouraging.

The misfire is intermittent but frequent at idle, so these values were pulled while the engine was misfiring and not.

Bill, you had mentioned swapping a PCM as another option. Before I reload the parts cannon, can you think of any other diagnostic I can try?

 

Now that you know how to slip pin test the back of the connectors ,, try power/voltage on a good and the bad coil. Check the VOLTAGE on that dwell signal and compare voltage to other coils. Also look very closely at the female pins on the offending coil connector and the connector for that bank. Look for loose spread pins. Wiggle, and bend the wires in the connectors and see if you get a change. The power voltage should be very close to FULL battery voltage. Make sure that you are reading it when it misfires.

Measure the voltage to engine ground and then chassis ground on the test slots on top of the fuses for the IGNITION FUSES. Fuses #16, 22, & 18









Bill


If you are POSITIVE that you swapped all the misfiring cylinder individual parts with a good cylinders parts,,, After that, its got to be mechanical.

 

Something is DIFFERENT on the cylinder with a misfire. We just have to pick it apart and narrow it down to FULE, AIR, SPARK, COMPRESSION and figure out what one if suffering.

Can you spray come brake parts cleaner around the intake runner and vacuum points for Cyl #5 and see if you have a small vacuum leak?

Obtain an inferred thermometer and measure all the exhaust runners at the points just out of each cylinder and also measure the INTAKE runner at the point it enters each cylinder. The exhaust temps should be pretty consistent.

If you have a bad intake valve spring, the weak valve spring will allow some exhaust gasses to pass back up the intake runner and that runner will be higher in temp.

Bill

 

How different will the runner temps be if I have exhaust blowing back? I did a quick check on all four on bank one, and they were all slightly different but nothing remarkable.

 

Depends on how much back flow you have from the improper sealing. Ive seen 30-40 deg difference.

A leak down and compression test will tell you more data. Thats just a quick check.

BC

 

Crankshaft variation relearn was the answer.

 

Hmmm. I was going to bring it up because I went through the same issue. But, in my case, the car was running perfectly all the time. It was just inaccurate misfire reporting causing P0300 code and blinking CEL. A CKP relearn process solved my issues. But, you state that you can feel the misfire (stumble). If that is the case, a CKP relearn won't fix it. A false misfire count from a CKP sensor does not affect vehicle operation.

 

 

Sorry I think I posted to wrong thread, my bad

 

Couple days ago my C5 after I got on it, backed off quickly started to run rough, idle rough, backfire but got her home. Now after researching the P0305 code I am in the process of removing the ignition coil, ok, no problem so far, then pulled wire off coil to spark plug, tried to remove wire from spark plug and yes I broke it, ok, will buy another, the problem is I can not get the wire off of the spark plug (very tight for hands to operate) or able to remove the metal can cover protecting the spark plug. I would appreciate any help/advice on how to remove the plug and wire, from cylinder #5? Thank you, a new member.