darrenc

Ok I bought a 98 firebird with the ls1 m6..it is all stock with 285,000 miles on it ..I know it a lot of miles but the original owner was a sales man and drive it all over the place and always kept up.on all maintenance ..so I bought it and found that it has a misfire on cylinder 7 and it feels like it's only at idle ..it will idle like it has a big cam in it but I will never stall or anything but as soon as you start to drive I don't fell a miss or anything . The ses light will come on and flash 3 time and remain light and that only happens when I'm on the high way doing a steady speed for awhile ..sometimes at idle it will be normal but that's hardly ever ...things I have done new coil on 7 ..8 new injectors brand new y pipe with new cats and 02..pulled the intake off just tell see if there where some vacuumed leaks and there was I fixed all them and put new gasket on the intake ..egr delete ..cleaned maf...the original owner said he changed plugs and wires and the iac and that did not help either ..so any help would be great so.I can get it running good ..the car seems to pull hard like it should so I'm just thinking it's at idle only

HappySalesman

iTrader: (3)

Have you run a compression test on the cylinders?

What (if any) codes are being thrown? Just the Mis-fire on Cylinder 7?

If it's just at idle I would take a look at the PCV system. I used to have a slight miss when idling, until I installed a catch-can and it cleared it right up.

darrenc

I'm doing a compression test the weekend ..but the only code it throughs is the cylinder 7 misfire ..it is always at idle and some times you can feel it till you get over 1200 rpm

darrenc

I did replace all the vac lines as we'll for the pcv system ..I also took.off the valve covers and looked for broken springs and stuff but everything looked good

HappySalesman

iTrader: (3)

Replacing the vac lines wouldn't solve the issue. At 285k you're probably getting quite a bit of blow-by, which means more vapors are running through the PCV system, and thus, more oil.

However, if this misfire is happening consistently on one cylinder, and it happens constantly at idle (not just a random miss here or there) I wouldn't suspect PCV.
Oil can get in and cause a miss, but not that consistently.

So, if you're getting good fuel and spark, then all signs point to compression. Run the test, and let us know what you get.