Rough idle VY Commodore

[Biggsy]

I'm an auto technician by trade(but not a tuner!). I'll give this a shot. You already know what is happening. It's obvious you have a lean stumble and if you fatten it up the miss goes away. You might try to take away a tiny bit of timing right at that idle RPM. Before I even try that I would try to isolate the cylinder(s) that are causing the problem. Can you change {C5621} misfire idle per cylinder mode and try to get the thing to code by making it very sensitive?
Most GMs that have a misfire problem take an act of congress to set a code, then most VW/Audi products can misfire like twice (really) and set a MIL light....VERY SENSITIVE!
Once you see which hole(s) are acting up then it might be easier to diagnose. Also we use a cylinder leakdown tester in conjunction with a compression tester to verify cylinder leakage (much more accurate). Also a mechanics stethoscope will sense even the tiniest of vacuum leaks near the intake manifold for cheap money!
Your engine should idle fine at stoich, you already know that. A wide band might even "pulse" enough to show a lean cylinder. Might be a tough one but you are already on the right track.


Food for thought, John

Food for thought indeed!

I'm loving your idea about increasing the sensitivity of missfire detection! Something I would normally think of but didn't

I'll have a play this week.

Thanx for the idea John

I'll keep everyone informed.

Cheers,

[macca_779]

How do I idle at 12.5 but keep in closed loop for other areas?
It should idle at stoich, every other car does!

Simple, you use Custom OS3 or 5 and command it. The Semi Open loop nature of OS3/5 means that when you command 14.63 its closed loop.. Anything else and its open loop. Pretty much every cammed car I've tuned with a tightish LSA and over 224 duration I command around 14.2-13.6 as that's what they tend to like in my experience.

[joecar]

What do you mean EVAP, EGR, AIR "getting in the way"?

If these are leaking then you would see a stumble at idle...

Each of those has a connection to the intake manifold (the AIR solenoid allows vacuum to open the AIR router)...

Oh, but if those were leaking then you would see it in the wideband AFR, which you don't... I was thinking out loud.

[Biggsy]

Simple, you use Custom OS3 or 5 and command it. The Semi Open loop nature of OS3/5 means that when you command 14.63 its closed loop.. Anything else and its open loop. Pretty much every cammed car I've tuned with a tightish LSA and over 224 duration I command around 14.2-13.6 as that's what they tend to like in my experience.

Thanx for that, I didn't know!

Cheers,

[Sikhabib]

what kind of spark plugs are you running and what did you set the plug gaps?
On my car i run tr6 plugs cause i have turbos. i was having trouble with spark blow out at higher rpm so to fix it i closed the plug gaps down to 0.26 which fixed the higher rpm problem but it then developed an idle miss similar to what your having. i opened my gaps slightly and it idled beautifully again.

maybe something to try??

[Biggsy]

what kind of spark plugs are you running and what did you set the plug gaps?
On my car i run tr6 plugs cause i have turbos. i was having trouble with spark blow out at higher rpm so to fix it i closed the plug gaps down to 0.26 which fixed the higher rpm problem but it then developed an idle miss similar to what your having. i opened my gaps slightly and it idled beautifully again.

maybe something to try??

I run the genuine plugs (Delco I think), can't remember the gap off hand but it was what the OEM Manual said.

I'll try what others have said first because this is the second set of plugs that I have tried. Incidently, all plugs looked healthy and the same when I pulled them out (ie could not see the 1 or 2 cylinders faulty like I am suspecting). Maybe if I let it idle for 1 hour then pull the plugs, they might tell me something.

If I can't find anything else, I will have a try of what you have suggested. Just got to finish work on time 1 day this week so I can try people's suggestions

Cheers,

[joecar]

Dave,

GM have revised the plug gap from 0.060" (1.52mm) to 0.040" (1.01mm).

Ignition System - New Spark Plugs/Gapping

Bulletin No.: 03-06-04-060A

Date: February 12, 2007

INFORMATION

Subject: Information on New Spark Plugs and Gapping

Models:
2004 Buick Rainier
2002-2004 Cadillac Escalade, Escalade EXT
2003-2004 Cadillac Escalade ESV
2004 Cadillac CTS-V
1997-2004 Chevrolet Corvette
1998-2002 Chevrolet Camaro
1999-2004 Chevrolet Silverado
2000-2004 Chevrolet Suburban, Tahoe
2002-2004 Chevrolet Avalanche
2003-2004 Chevrolet Express, TrailBlazer
1999-2004 GMC Sierra
2000-2004 GMC Yukon, Yukon XL
2001-2004 GMC Yukon Denali, Yukon XL Denali
2002-2004 GMC Sierra Denali
2003-2004 GMC Envoy XL
1998-2002 Pontiac Firebird
2004 Pontiac GTO
2003-2004 HUMMER H2

with 4.8L, 5.3L, 5.7L or 6.0L V-8 Engine (VINs V, P, T, Z, G, S, N, U - RPOs LR4, LM4, LM7, L59, LS1, L56, LQ9, LQ4)

Attention: Please disregard the Set Gap Per Vehicle Specification statement on the side of an ACDelco(R) Iridium Spark Plug Box. The statement will be removed from the boxes in the future.

Supercede:

This bulletin is being revised to bring attention to the Set Gap Per Vehicle Specification on the ACDelco(R) Iridium Spark Plug Boxes. Please discard Corporate Bulletin Number 03-06-04-060 (Section 06 - Engine/Propulsion System).

A new spark plug has been released for use in the above vehicles. The new spark plug has an Iridium tip instead of the current Platinum tip. Due to the different tip design, the gap of the spark plug has also changed.

The new spark plug, P/N 12571164 with AC Delco P/N 41-985, is gapped to 1.01 mm (0.040 in) when the spark plug is made.

The spark plug gap is set during manufacturing and should not be changed or damage to the spark plug may result. Any new spark plug found to not be properly gapped should not be used.

[Biggsy]

OK, I set {c5621} values to 1 and still couldn't get it to log a missfire, even when I unplugged an injector! It does not work.

I have uploaded COS3 to run semi-OL, that has given me a temp fix, but can still feel it (but know where near as bad)

The stumble is worse when the ambient outside temp is colder.

NB O2's are switching fine and are similar in both banks.

Cleaned throttle body.

Changed spark plugs again, gapped them as per Joecar's post.

Listened for air leaks with stethascope, could not find one.

Begining to think its just a dud motor and has always been like this from new.

[joecar]

Oh, I just remembered that failing the MAF prevents the misfire monitor from running...

sorry Dave, I should have thought of that...

[Delco]

The first thing is to clean the injectors and do a cylinder leakage test - no amount of electionic trickery will cure a mechanical problem like a low flo injector or a leaking valve , guides wear pretty badly on a LS1 , I see a lot of engines that have intake valve leakage when doing cam upgrades