Rough idle VY Commodore

[JezzaB]

have you checked the Idle Air Control (IAC)

So there are no vacuum leaks i.e the PCV valve near throttle body.

Try a reset - PCM/Idle learn here cost you nothing

Good point but if your Desired Airflow is good, a good old PCM reset wont do jack. Sorry Les :p

[vxchev8]

come on Jez worked back in 2002 , Biggsy give your IAC a clean see how that goes.

[joecar]

Dave,

Do you have access to an automotive style oscilloscope (e.g. Pico scope)...?

If you do, you could examine these waveforms:
- crank sensor voltage,
- injector voltage/current,
- coil secondary voltage [capacitive probe],
- fuel pump current.

Any misfire/stumble has to show up on at least one of those.

If you always had this problem, then is must be due to something in the "next layer down".

Post up some logs.

[Biggsy]

come on Jez worked back in 2002 , Biggsy give your IAC a clean see how that goes.

I have already done that when I resealed the manifold, thanks anyway.

12.5 AFR makes it run fine, this is the biggest clue I have.

[Biggsy]

Dave,

Do you have access to an automotive style oscilloscope (e.g. Pico scope)...?

If you do, you could examine these waveforms:
- crank sensor voltage,
- injector voltage/current,
- coil secondary voltage [capacitive probe],
- fuel pump current.

Any misfire/stumble has to show up on at least one of those.

If you always had this problem, then is must be due to something in the "next layer down".

Post up some logs.

No oscilloscope unfortunately Joe

Logs do not show anything.
I have a log that I did that shows a straight WBO2 line, straight timing line, straight MAP line and a RPM line which keeps within +/- 15rpm yet I can feel the stumbling and hear it poping in the exhaust.

"AFR 12.5 makes it run fine" keeps repeating in my head. I'm thinking I have one or two cylinders running lean. I have been tuning with this condition so to get the right AFR in the exhaust.

I would have increased the overall fueling to make the AFR average to be stoich @ the O2 sensors.
Now I have 6 or 7 cylinders running richer than stoich and 1 or2 cylinders running leaner that stoich which averages
The motor will run quite fine richer that stoich but doesnt lik running leaner than stoich.
So, when I command 12.5 @ idle, I get 1 or 2 cylinders running @ or above stoich and 6 or 7 well above stoich (>12.5 AFR) which allows it to run smooth.

Having said all that, I have to find the cause of this leaning!
Heads, gaskets, worn cam/rockers/lifters could also be an issue.
Of course, I could be totally wrong.

Might have just justified myself getting a 6lt ls2 or l98 crate motor (getting cheap as now)

"Darling, me and the EFILIVE boys have worked out the motor in my car is worn out and I need to get another one!"

How'd you reckon I'd go?

Cheers,

[joecar]

Dave,

lol... "and I can save money by buying a new motor"...

Move the wideband to the other bank and see what it shows.

Do your logs show a variation in rpm...?

Is EVAP, EGR, AIR getting in the way...?

[macca_779]

What are the NB's doing Biggsy. Force it closed loop and log both sides. If you don't trust the sensors you could swap them over too. On the other hand, and I know you don't really want to do it but if its likes 12.5 at idle, give it 12.5.

[ssvolvo]

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

[Biggsy]

Dave,

lol... "and I can save money by buying a new motor"...

Move the wideband to the other bank and see what it shows.

Do your logs show a variation in rpm...?

Is EVAP, EGR, AIR getting in the way...?

Logs show very little variation

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

[Biggsy]

.......... On the other hand, and I know you don't really want to do it but if its likes 12.5 at idle, give it 12.5.

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