engine misfire on 350 vortec

[Doug Domm]

I have a 98 z71 1500 350 vortec that shows a misfire on #4 cylinder with a Snap-on scanner. This misfire occasionally goes away but is there most all the time. It will show as #4 cyl. and once in a blue moon will swap as #3 cyl. Shows only at idle and will go away if you turn on the air conditioning or if you just break the throttle of idle. It runs fine on the road and has as much power as it ever had before. Sometimes at idle it will get so rough it'll almost shake your fillings out. Replaced plugs, wires, cap, rotor and has good compression. Help!!!! Thanks Doug

[FED 387]

got a 99 Tahoe 105,000 miles---gonna replace the injector on #5 did everything ya did same symptoms nothing else to do---mine doesnt jump around tho same cylinder all the time

[buzzinhalfdozen]

Change the dist. cap, #4 &#3 are next to each other in the cap you'll see what I mean when you flip the cap over and trace the terminals. I see this quite often and can almost never SEE an issue with the cap, I suspect it's burning thru internally. While you're at it replace the rotor also. Though I can't guarantee it I'm quite sure this will resolve your issue. Joe

[Mark Ruset]

Doug you didn't mention if this miss began after work was done on the motor such as having the intake manifold off. If so, check the injector lines for correct order. It is possible to swap two injectors and end up with a miss. GM has posted a service bullitan about this condition after having the manifold removed and replaced

[buzzinhalfdozen]

Sorry didn't see you had replaced cap. Have you looked to see what your camshaft retard is? This is the degrees the cam sensor triggers at, if it's off you can move the dist. just like setting timing and set to 0 degrees... you must rev the engine to over 1000 rpms after adjustment to read the new setting. Could be the rotor phase is off and you're getting spark cross in the cap. Kind of a long shot but easy to check and adjust, just seems odd that it's those 2 cyls having miss fires since there not in direct relation to each other. Mark is correct that installing the nozzles in the wrong hole will cause a miss though hard to believe they could have crossed nozzles to other bank. Have you performed an injector balance test yet, if not it may tell you something, not sure if the scanner you're using has this as a special function or not. Typically "harsh" misfires are Ignition related where "subtle" rough idle are more fuel related in my experience. Joe

[Doug Domm]

Yes the intake was off some time ago but this problem only started recently. Twice this week it performed flawless then the next time driven acted up again. It will run perfectly cold and only act up after operating temp. reached. Soft miss vs harsh miss, well it is a soft miss most of the time but occasionally goes to HARSH !! and I mean shake your fillings out harsh. I've had replace the spyder suggested but most have seen that item usually affects all cylinders?