'97 L29 ---Annoying Idle

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RedBurb

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Not happy with the engine idle. Cold engine starts up flawlessly, initally runs great during warm up (@1100 rpm), but once warmed up the idle rpm drops and begins to constantly hop between 620rpm to 710 rpm (according to DashCommand). The short fuel values also constantly hop between -0.5 to 2.3. Otherwise the engine runs strong and flawlessly off idle. But next stop light---same thing ---idle constantly hopping between 620 - 710 rpm. It feels like a very slight misfire. Looking at the front of the engine the serpentine belt tensioner also hops up and down (in sync with the rpm fluctuations).

Is this rpm flux normal for a L29? It's subtle but annoying. First L29 I've owned so I don't have another point of reference for comparison (please advise).

Here's what I've done thus far ---New Distributor, new cap, new wires, new plugs, new CATs (Magnaflows)+ new O2 sensors (all four of them, NTKs), new air filter, new intake manifold gaskets.

No codes.

What am I missing? I'm considering changing the MAF and IAT sensors next. But I'm honestly shooting in the dark - open to other ideas ---thanks.
 
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Carlaisle

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That is not normal for an L29. In good repair it should idle pretty much smooth as glass. I would suggest more diagnosis before more parts. You can unplug the MAF and IAT sensors separately and determine if any change is observed. If not, they are not the culprit. Are the correct plug wires on the correct plugs? Do you have a vacuum leak? How is your fuel pressure? Are the injectors OEM or have they been replaced? Have you looked at the IAC? Is the throttle body clean? Was a new gear installed with the new distributor? How many miles are on the vehicle? Does DashCommand show timing? If so, how does it look?
 

81nascar

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Years ago when I first got mine it did a similar thing. Warm idle at light would have this suttle miss. Did everything you did. Even had some shops give it a go and nobody could figure it out. Had the injectors flow tested ,they seemed good. They were cleaned and checked out ok.. So just let it go for a while .. Had to rebuild the motor (long story) put everything back on except I put in some new BOSCH injectors. Idle problem gone. In my situation it was the only constant.. I only did it as reading older posts that others have complained about idle issues with the stock injectors once they get some millage and age.. and were never that good to begin with so upgrading to the BOSCH seemed to fix it. Not saying that is your problem ,but may give you a direction to go in..
 

JOHNGAAA1

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Start with removing and cleaning the throttle body. Start with the simple things first. vacuum lines, and etc.
 

RedBurb

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That is not normal for an L29. In good repair it should idle pretty much smooth as glass. I would suggest more diagnosis before more parts. You can unplug the MAF and IAT sensors separately and determine if any change is observed. If not, they are not the culprit. Are the correct plug wires on the correct plugs? Do you have a vacuum leak? How is your fuel pressure? Are the injectors OEM or have they been replaced? Have you looked at the IAC? Is the throttle body clean? Was a new gear installed with the new distributor? How many miles are on the vehicle? Does DashCommand show timing? If so, how does it look?
I assumed the L29 idle quality should be better ---thanks for confirming. The engine has 181K miles of which I've run 3K (bought truck a few months ago).

The timing at idle is 24-25. The distributor and gear are brand new (aluminum billet Accel) as are the wires and plugs. All correct part numbers and placements. I have no idea on age of injectors ---might be originals so definitely suspect. Haven't checked the MAF, IAT, fuel pressure etc., but will...

Question - what are the steps for op checking the IAC?
 
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RedBurb

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Years ago when I first got mine it did a similar thing. Warm idle at light would have this suttle miss. Did everything you did. Even had some shops give it a go and nobody could figure it out. Had the injectors flow tested ,they seemed good. They were cleaned and checked out ok.. So just let it go for a while .. Had to rebuild the motor (long story) put everything back on except I put in some new BOSCH injectors. Idle problem gone. In my situation it was the only constant.. I only did it as reading older posts that others have complained about idle issues with the stock injectors once they get some millage and age.. and were never that good to begin with so upgrading to the BOSCH seemed to fix it. Not saying that is your problem ,but may give you a direction to go in..
Interesting and does sound similar ----it's a subtle stumbling but definitely screwing with my OCD. LOL. The injectors might indeed be the issue but I'll tackle a couple of the other "simpler" culprits mentioned thus far before diving into this replacement job.
 

Schurkey

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Not happy with the engine idle... ...once warmed up the idle rpm drops and begins to constantly hop between 620rpm to 710 rpm (according to DashCommand). The short fuel values also constantly hop between -0.5 to 2.3... ...It feels like a very slight misfire.
Connect a scan tool, look at cylinder-by-cylinder misfire counts at idle.

Verify all the sensors, and see if you can command and hold various idle speeds while you have the tool connected. I think a '97 has the option to select an idle speed on the scan tool, so that the IAC responds and holds the selected speed. I know my '03 Trailblazer will do that.

Here's what I've done thus far ---New Distributor,
"Cam Offset" set properly?

No codes.
Codes can be useful. More-important is the data stream and bi-directional control system.
 

454cid

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Mine idles a little wonky and has for a very very long time. I've never seen a misfire with the scanner, and I don't think the the rpm changes as much as you're reporting. Injectors are an interesting thought.
 

RedBurb

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Connect a scan tool, look at cylinder-by-cylinder misfire counts at idle.

Verify all the sensors, and see if you can command and hold various idle speeds while you have the tool connected. I think a '97 has the option to select an idle speed on the scan tool, so that the IAC responds and holds the selected speed. I know my '03 Trailblazer will do that.


"Cam Offset" set properly?


Codes can be useful. More-important is the data stream and bi-directional control system.
Unfortunately I don't own a scanner (yet). Currently DashCommand is my only view into the digital innards. And even Dashcommend is limited for the '97 L29 i.e., "Advanced PIDS" are not available. What scanner would you gents recommend for a cheapskate DIY hack? Is there a unit available in the + - $300 price range equipped with a decent set of troubleshooting functions/features? I might be willing to up the ante to $400 but my internal whining voice usually kicks-in at this expenditure amount (nauseatingly whiny when related to tax payments).
 
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