Smooth idle for 5 secs, then garbage

[brianko]

Got an '02 Camaro with bored 6.0L (to 410), FAST heads, 102mm TB, mild cam (232/227 or something close to that, 580 lift), long tube headers. This thing will simply not idle right. Even with those mods, I should be able to get a rough self-sustaining idle with stock idle tables.

I've read and tried just about every sticky and suggestion here, and am about to declare a mechanical issue is at fault. I've attached a log. Of interest is the first 10 seconds or so, then the part after the long 2000RPM run. This was run using the sticky that Bill posted here.

Sorry, no AFR because it's too erratic to register on my dog-slow LM-1.

[Sherminator]

Attach your tune as well.

[brianko]

I'm not using the HP editor on this one, just the HP scanner (the tune was originally done with another editor). The only modifications from stock for this run are the idle parameters in Bill's post, plus an adjustment for cylinder displacement and injector scaling.

After doing a couple of dyno pulls, I've discovered some rather periodic fluctuations in the fuel trims at speeds above idle (sawtooth waveform, about 4 second periods). WB reads very lean at times (about 17), but absolutely no KR, with strong fuel smell. The plugs were pulled earlier after some driving on the initial tune and were pretty fouled. It's almost as if the engine is running rich, but the computer sees a lean condition.

On the attached, you can see the FT weirdness starting at about the 2:12 mark.

Maybe one or both O2 sensors have gone south?

[smithers]

I'm not using the HP editor on this one, just the HP scanner (the tune was originally done with another editor).

???
You should still be able to READ and save the .hpt file for us to view.

[brianko]

???
You should still be able to READ and save the .hpt file for us to view.

I'm not looking for advice on the tune. I was just asking for some interpretive thoughts on the logs. Think of this as more of a technical discussion...

[Sherminator]

Ok then....you aren't going to fix things if the tune is messed up. You're running a 350cu.in tune on a 410cu.in engine with a bigger cam and intake setup, everything is out to lunch airflow and fueling wise. Scanning it is only telling you that the tune really is wrong.

[brianko]

Ok then....you aren't going to fix things if the tune is messed up. You're running a 350cu.in tune on a 410cu.in engine with a bigger cam and intake setup, everything is out to lunch airflow and fueling wise. Scanning it is only telling you that the tune really is wrong.

Actually, I'm not...I've already said I adjusted the tune for cylinder displacement and have scaled the injectors for the idle test, plus the idle air changes noted. Despite my low post count, I'm not new to the forums or tuning, so you're preaching to the choir. I'd rather not clutter up the discussion with tuning techniques.

At any rate, let's just focus the discussion on the idle logs.

[Ls1c5vette]

(I cant pull your log im at work) but does it run any better in OL vs CL? I know you prob dont want to hear this but you have to get a good, trustworthly wideband to get the idle fueling right or its going to act up. After you start it (when engine is warm) switch it to CL and see if the idle gets better (give it a ~10 seconds), this will, for now, let you know if the O2's are adding/subtracting fuel like they are supposed to to get the fuel somewhat correct.

[brianko]

(I cant pull your log im at work) but does it run any better in OL vs CL? I know you prob dont want to hear this but you have to get a good, trustworthly wideband to get the idle fueling right or its going to act up.

Idles great in OL...it's the transition to CL where everything goes to hell.

And yes, I can certainly agree on the "good, trustworthy" part of your comment! The LM-1 is crap...looking at a PLX to replace it with.

For the sake of argument, let's assume that fuel/spark/air are set such that a sustainable idle should be achievable.

What mechanical issues might cause the inability to achieve stable idle? I'm thinking either O2 sensors or fuel pressure issues (regulator or pump)...anything obvious I'm missing?

[robbyredneck]

exhaust leak.

[BlackZJ]

exhaust leak.

^^ x2

[Ls1c5vette]

X3, highly possible, this will throw off the O2's, I think thats your issue as it runs good in OL.

[axekick]

It would, in fact, throw off your the O2's to cause exactly the condition you described. When the fresh air get in the exhaust, the O2's see it as unburned O2 and therefore reads lean even if its running exactly right or even rich. so the computer thinks the engine is running lean and adds fuel to attempt to get to stoich. Now you're running rich but because of the fresh air getting in you're still seeing a lean condition in your log that is false.