Ashley P

LS misfire only at idle

19 posts in this topic

5.3 w/DoD.  Engine has misfire only at idle, it runs smooth w/any more throttle.  No codes.  Wires and plugs are brand new.  I've swapped wire, plug, coil, and injector w/a known good cylinder.  Cranking compression is 100 psi on the first stroke, 150ish the 3rd, maxes at 175 w/several revolutions (and 3 tests back to back were identical).  MIsfire counter shows misfire only on #8, only at idle.  I've sprayed "carb cleaner" around that intake runner and the entire intake, no change.

What have I over looked?  It's the simplest engine on the lot....:shrug:

 

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my 07 Tahoe had this issue. I would change the plug and it would stop for short while then come back. I finally put a new throttle body on it and a tune and it cleared up long enough for me to pawn it off. good luck

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1 hour ago, mstrpth said:

my 07 Tahoe had this issue. I would change the plug and it would stop for short while then come back. I finally put a new throttle body on it and a tune and it cleared up long enough for me to pawn it off. good luck

Seems possible.  If a late model GM won't idle at the set rpm, instead of opening or closing the throttle body, it will add or remove timing.  When I had the 90mm tb on my V without the proper values in the tune, it would feel like it had a miss at low rpm.

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Mike, did it actually have a misfire that the ECU "saw"?

 

Other pieces of the puzzle:  The ECU monitors Ignition and Injector circuits, it never shows a fault in either one.  Wiggling wiring makes no difference either.

Edited by Ashley P

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1 hour ago, Ashley P said:

Mike, did it actually have a misfire that the ECU "saw"?

 

Other pieces of the puzzle:  The ECU monitors Ignition and Injector circuits, it never shows a fault in either one.  Wiggling wiring makes no difference either.

A lot of my misfire stuff is dumbed down in the tune because of the large cam.  It could have them and never set a code.

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2 hours ago, Ashley P said:

Mike, did it actually have a misfire that the ECU "saw"?

 

Other pieces of the puzzle:  The ECU monitors Ignition and Injector circuits, it never shows a fault in either one.  Wiggling wiring makes no difference either.

mine would occasionally, and then other times nothing.   

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I've got the customer taking it out of my way to bring it back in a few weeks.  Maybe a fix will dawn on me by then... :coffee:

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This problem reminds me of the old Ranger that a customer brought in for "head gaskets".  Come to find out, almost every exhaust valve had sunk way into the seat and I put reman heads on it.  The truck ran decent but acted lean and the ECM was nearly maxing out fuel adapts to richen it.  It took me some time to figure out that someone had put the wrong injectors in the truck over a year prior.  

Things like that a pain.

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I again look at this truck yesterday.  The customer had put about 1k miles on it.  I verified it was still missing on #8 only, verified spark, I checked compression....only 80 first stroke, rising to low 100s after 3 strokes.  I backed off the rockers and pressurized the cylinder, no leakage.    So...the cam is improperly actuating the valves.  Do I have pitting on a roller/lobe that causes lash that the hydraulic lifter takes out, only to hold the valve open too long because of it?    Told customer I'd have to look at lifter/lobe to know how bad the damage is; I'm waiting for him to commit to exploratory or Jasper replacement.

 

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Well, maybe so, but I have not heard any noise (lifter tap).  And, at idle, the rockers appeared to travel normally.  I also let my fingers ride on the rockers to feel for any jerking that play would cause, but rocker action felt smooth.  :shrug:

I should have stethoscoped the rocker bolts....

Edited by Ashley P

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My "leak down test" was simply backing off rockers and putting shop air in the cylinder and not hearing any air out of the intake or exhaust pipe.

I've concluded (and await any correction) that since #8 has low compression, yet rings/valves seal, that the valves are not being closed properly.  They are being actuated incorrectly, I don't know how.  I'm grasping at straws to try to blame pitting for allowing lash to be taken out of the "pit" which might hold the valve open. 

You got any theories?  Have I completely lost my mind?  :panic:

Edit:  Disney has a notable idea, that low oil pressure is allowing a lifter to collapse.  My observation of "good" valve travel at idle and not hearing or feeling any lash (lifter tap) makes me want to rule out that very good suggestion.

Edited by Ashley P

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Ive seen on a couple of occasions that a roller had locked and worn flat in that spot resulting in the valve not opening properly and another where a cam lobe wore flat

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If I’m reading right you have a lifter that’s not collapsing at all, like it’s locked with the plunger all the way up.  Narrow it down to intake or exhaust.  Then let that valve sit at full lift overnight, that would let it “pump down”.  If it doesn’t, dead lifter.  8 isn’t a dod cylinder, so it should have normal (cheaper) lifters.

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28 minutes ago, KNIGHT347 said:

Ive seen on a couple of occasions that a roller had locked and worn flat in that spot resulting in the valve not opening properly and another where a cam lobe wore flat

I've seen that too, and every time I heard more internal noise than I do now.  I did stethoscope the valve cover and heard a faint double click, similar rhythm to a heartbeat.  I actually compared my pulse to the noise to see if I was hearing myself, and it wasn't me.  

15 minutes ago, Mike said:

If I’m reading right you have a lifter that’s not collapsing at all, like it’s locked with the plunger all the way up.  Narrow it down to intake or exhaust.  Then let that valve sit at full lift overnight, that would let it “pump down”.  If it doesn’t, dead lifter.  8 isn’t a dod cylinder, so it should have normal (cheaper) lifters.

Well, would/could a stuck "up" lifter run right at all speeds/loads above idle?   

 

Have I "proved" that valve control is the issue?  It's a non DoD cylinder, so it's a lobe or lifter, right?

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20 minutes ago, Ashley P said:

Well, would/could a stuck "up" lifter run right at all speeds/loads above idle?  

Yes.  If if it’s an exhaust valve

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54 minutes ago, KNIGHT347 said:

lobe, lifter possibly faulty injector, i mean no fuel will cause a misfire too

 

 

Injector swapped last time, passed NOID light test.

38 minutes ago, Mike said:

Yes.  If if it’s an exhaust valve

Maybe so....hmmmm....low battery on my 'puter, both the laptop and muh brain.

Thanks for the tips.

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