'97 L29 ---Annoying Idle

[BeXtreme]

My standard procedure for a new vehicle that isn't running or isn't running well is to pull all the plugs first, then run compression test, then install NEW correct plugs, verify and install plug wires in correct firing order, then try to start the engine and verify/set timing. If it still won't start after all that I move on to fuel and then carburetor(if its that old). The newer stuff is nicer for the later parts of that procedure as you can just look at the scan tool for misfire counts, timing, and fuel pressure. But the first part is still critical since it covers the mechanical health of the unknown engine. Any failure to start after checking all that MUST be either fuel or computer related.

 

[RedBurb]

My standard procedure for a new vehicle that isn't running or isn't running well is to pull all the plugs first, then run compression test, then install NEW correct plugs, verify and install plug wires in correct firing order, then try to start the engine and verify/set timing. If it still won't start after all that I move on to fuel and then carburetor(if its that old). The newer stuff is nicer for the later parts of that procedure as you can just look at the scan tool for misfire counts, timing, and fuel pressure. But the first part is still critical since it covers the mechanical health of the unknown engine. Any failure to start after checking all that MUST be either fuel or computer related.

I changed the plugs and wires right after buying the truck and performing a compression test. What I never suspected was that I'd force one of 045 gaps closed during the install. The engine had several other 'significant" issues that "clouded over/drowned out" this slight misfire until these were addressed. After killling them all off I began to notice this lingering and annoying "slight idle miss"

My scanner read occasional misfires on almost every cylinder (somewhere along the way I've read its not unusual fo L29s to have 10-15 misfires per cylinder). In this particular case even my scanner didn't show #7 being any worse/better than the rest. The plug must have been firing but at a much reduced spark power level.

At the end of the day I had to go back to the basics you've outlined above to find the cause. The lesson learned here is is never assume gapped plugs remain gapped while installing them. In other words at times S-happens and nothing short of raw persistence will overcome it.

 

[454cid]

My scanner read occasional misfires on almost every cylinder (somewhere along the way I've read its not unusual fo L29s to have 10-15 misfires per cylinder). So even my scanner didn't show #7 being any worse/better than the rest. The plug must have been firing but at a much reduced spark power level.

I've always felt like mine has a mis-fire, but I don't think I've ever seen one on my scanner, when I've checked. It could be just that the plugs are gapped too wide. I think spec is .065", but everyone recommends .045", and mine are right around .055". I just had my plugs out the other day, and I wish I would have regapped them to .045".

I was a bit concerned I bumped one of my plugs, so I checked it again..... I think that was actually on my Buick, Though.

 

[RedBurb]

I've always felt like mine has a mis-fire, but I don't think I've ever seen one on my scanner, when I've checked. It could be just that the plugs are gapped too wide. I think spec is .065", but everyone recommends .045", and mine are right around .055". I just had my plugs out the other day, and I wish I would have regapped them to .045".

I was a bit concerned I bumped one of my plugs, so I checked it again..... I think that was actually on my Buick, Though.

For what it's worth ---This is my first experience with a L29. So part of my problem was not having a reference point for what a "perfectly running" L29 should feel like. Adding to that uncertainty is the fact the engine has 180K miles on it. But heck even with the faint miss the L29 idled much smoother than my old school carb'd Mark IV 454 street fighter. Now that I've corrected the collapsed plug I'd characterize the idle as "constantly smooth" (a notch or two down from "silky smooth"). When in seat I barely feel the engine idling and the rpm hovers @ 690rpm +_ 15. Before the plug fix the range was jumping +- 35 rpm and I felt occasional light shudders.