707vette

2005 base c6 a/t with 205k on the clock

Getting misfires on cylinder 4 at idle primarily. misfire counter decreases dramatically off idle and no misfires at all past about 1200 rpm. this is all in park.
When put in drive and sitting at idle, misfiring, but if i put a load on it the misfires go away(all cyl 4.) Car runs great down road with no misfire counters at all.

Swapped plug---miss did not follow
swapped wire---did not follow
swapped coil---did not follow
swapped injector---did not follow
Suspected possible intake gaskets----checked and they were almost brand new
performed crank variation relearn---no change
checked compression comparing to 6 and 8.---nothing significant
leak down test passed/ same as number 8
valve spring, pushrod and rocker arm look ok
cleaned maf sensor as well as ran unplugged---no change
cleaned throttle body--- no change


only code is p0300, no other codes....the thing thats throwing me off is that it doesnt do it under load...making me suspect some sort of vacuum issue....
Any Ideas on what to check next.? ...thanks for reading

Dave S

Considering the high mileage on your C6, I would guess that the problem might be due to a vacuum leak.....rubber vacuum lines that have deteriorated or even a leak around the base of the intake manifold due to deterioration of the manifold material, which is some sort of plastic. If you haven't already done so, there's also a ton of info on the 'net....just do a search for "PO300 code". Good luck.

rpmextra

excessive vibration from another source could trigger a misfire fault code.

how's the harmonic balancer look?

also try spraying the coils with water and look closely for arcing

KB9GKC

Hello,

How did the O ring in the intake look when you removed #4 injector?

Douglas in Green Bay

707vette

Thanks for replies...
Injector o ring looked good top and bottom...also ran with belt off with no change. balancer has a little wobble, but no more than most c6's i see in shop...

I decided to pull intake again and inspect for any sign of vacuum leaks.
Got everything back together and misfire is gone...but...... now theres noise in the lower end, mainly at idle, and im suspecting a bearing from the way it sounds.

I'm still a little stumped as to where misfires were coming from, but with 205k on engine, im not too surprised at lower end noise. At least I know now that the electronic side of things are working properly.

liv2race

I hope you don't but If you need a good longblock I have one out of my 05 C6 for sale. It is in the for sale section under LS2 longblock.

BLACKBELT2

Car ran great last weekend today went to use it and started it up and wow idle was very rough where I wouldn't dare drive it , also noticed at the tail pipes the exhaust on one side was warming up and the other side stayed cold . Its dark now and started it up and didn't see and arc from wires , can anyone help with any information ?

BLACKBELT2

[QUOTE=BLACKBELT2;1594016718]Car ran great last weekend today went to use it and started it up and wow idle was very rough where I wouldn't dare drive it , also noticed at the tail pipes the exhaust on one side was warming up and the other side stayed cold . Its dark now and started it up and didn't see and arc from wires , can anyone help with any information ? C5 1998

ringo11

bad fluid filled motor mount?

BLACKBELT2

Not sure what that means , bad fluid filled otor mount?

ringo11

Alot of modern motor mounts are fluid filled, if one ruptures and leaks it's fluid out, it will transmit vibrations into the car. It can be misdiagnosed as a misfire.

1hander

check the looms that feed the coil packs and t he individual wires and connectors that feed each coil pack.. mine only has 62k on it.. and the wires to the coil packs, the main one and the individual ones get super hard and crack....when you move them or wiggle them...the plastic cracks wide open...

my number 4 had cracked open and gotten water and corrosion min it to the point that the wire was just hanging on by a thread... at idle the wiggling of the motor would cause the wire to connect and disconnect intermittently ...at higher rpms the engine would smooth out...

finally number four died all together... i found it by accident when i bump the wire harness and all of a sudden the engine jumped a bit and smoothed out.

i replaced both sides coil pack main looms and the loom for the coil packs themselves.

BLACKBELT2

thank all of you for your suggestions i did put new wires on new plugs , then checked all wires going to injecteors nothing worked , then i decided to check fuses and saw in book there is a fuse for injector 1 and another fuse for injector 2 so i pulled fuse for injector 1 and sure enough it was blown my engine was running on 4 cylinders now with changing wires and plugs and fuse its running like a fine tuned machine ,,, thanx again everyone!!

1hander

Holy chit.. I never woulda checked that.. But it's good to know there are separate fuses for each bank or odds or even cylinders...

ringo11

Interesting that with an entire bank out, it ran ok, but only had a miss at idle, seems very odd.

1hander

I was thinking the same thing... But when mine had dead cylinder I could barely hear the misfire... It probably has to do with the fireing order