JimMueller

iTrader: (9)

Why does my wideband indicate that I'm hovering around stoich, but the exhaust still smells rich at idle? I also have an issue where a large plume of smoke comes out the exhaust a few seconds after cold-starting the car, but I haven't been able to see the same plume when the engine is warm.

I did tune the VE under colder conditions, but under different ambient temps it appears to be thrown off. I have Bosch 13111 (rear C5 O2 sensors) in my front O2 bungs, and they're brand new.

Cal

iTrader: (1)

It's because the overlap in that cam is allowing some unburned oxygen to blow right through the engine and into the exhuast. The O2 sensor picks this up, and the PCM is saying Ah hah, mixture is lean, better throw in a bunch more fuel. And of course that makes the engine burn rich as hell, because it wasn't really lean to start with. Your wideband is seeing the same deceptive situation that the car's O2 sensors are. Just keep tuning the VE, but you may never be able to totally eliminate the rich condition because if you back off the VE too much, the software accelerator pump will be too lean. O2 sensors just aren't the greatest on a race engine, but hey the good fuel ecconomy is sure nice for a DD.

JimMueller

iTrader: (9)

So would the O2 rich/lean tables fix this, by altering the swing points which constitute 'too rich' or 'too lean'?

What about the smoke?

white2001s10

iTrader: (4)

Nope,
he's right, the problem is this "228/228 cam".
The smoke sounds like worn guides
also caused by this "228/228 cam"

Side effects of overcamming a street driven car.

Another_User

The reason this happens is because your wideband is wrong. Cammed cars need to be tuned using an exhaust gas analyzer that measures CO in your exhaust. I have done a lot of research on this. Your O2 sensors are not made to deal with a lot of overlap. As far as anyone tuning an LS1 this way, it has not been done yet to my knowledge. It would be a real PITA. I was thinking about it...

jimmyblue

iTrader: (11)

Also wideband reports an average (my LM-1 has a user
adjustable smoothing, even) but it's going to be exhaust-
tube-averaged as well. But the PCM is swinging the mixture
rich, lean, rich all the time in closed loop and the wider it
swings the more stank you get. Let alone, if the O2s are all
comatose from cold.

What you smell as "rich" may just be hydrocarbons, raw
gas that shot through the overlap. Can't tune that away.
Cats might crack some of it, if they were there. But
not likely all of it. No such thing as 100.00% effective.

JimMueller

iTrader: (9)

Hmm, I still have my cats.

So long story short, there's nothing I can easily do to fix the rich smell.

Should I run a leakdown test to see if I have guide or seal problems causing the smoke at startup, or is that something to do with the overlap also?

jimmyblue

iTrader: (11)

If you bias the idle leaner you may find the smell
reduces to a point (at least, until lean starts to
elevate HC emissions from mis-burning) as the
excess air can be used to burn down the shoot-through
in the cats & pipe. But, closed-loop will try and defeat
you.

How do you like what you see for the O2s' activity at idle?
Do they look like busy, happy little campers or are they
taking a nice long nap and waking up when the lunch bell
sounds? The slower they lurch about, the worse / longer
your degree of excursion off true stoich will be.

If you disabled closed loop and messed around the OLFA
table at your MAP idle point, I think you could find by
experiment the place of least aroma. If it's well better
than you get closed loop, and stoich (open loop) don't
stank nearly as much, then I would suspect the mixture
dithering as an actor (the mixture swing coughing out some
nasty, at the endpoints) and you might play with those
odd O2/airflow mode, time constant / linear type thingies.
I forget the names and am not at the tuning computer.

Leakdown might show some things but, in the good old
small block Chebby, startup-only smoke was often the
exhaust valve gudes / seals letting a little oil weep down
from the head into the exhaust port, to be boiled / burnt
off next time. That bypasses the combustion chamber
entirely and isn't something a leakdown test shows.

Another_User

In closed loop your PCM does whatever it wants to maintain the O2 sensor readings it is programmed to aim for. The only way to adjust AF ratio in closed loop is to change those parameters. Unfortunately it is:
1) not easy
2) narrowband O2 sensors are only accurate at what they perceive to be stoich, so adjusting to a real stoich that conflicts won't really work.

jimmyblue

iTrader: (11)

Thing about the closed loop operation (on this
system) is, it works by (as I understand it) the
time-averaged O2 voltage, in a switching mode.
It makes no attempt to keep it linear and at a
constant stoich point. In fast it purposely swings
(dithers) mixture to ensure a sufficient rate of
crossing events to make time-averaging sensible.

But, things that are not as-stock can mess this up.
Slow-reading or seldom-awake O2s are one. What
I've suggested is meant as a diagnostic, not a cure
(though, in an emissions-hassle-free environment
running open loop idle or just open loop full time,
might not be so terrible if it works). In fact, if the
sensors you need for feedback are just wrong (for
reasons such as gross overlap -> excess exhaust O2,
headers -> no output, etc.) then ignoring them
entirely may make more sense than acting on bad
data.

JM, what type of O2s are you running in the front
positions?

SmokingWS6

iTrader: (1)

He's running the Bosch 13111's. So I don't think thats the cause, I agree its the valve overlap.

The best solutions i've read is to scale the ve table for 1000rpm and below by 60-80%. This corrects for the additional fuel the pcm thinks it has to dump in at idle. Another is to enable PE at 0% tps for 1000rpm and below at set at 14.7 a/f.

Depending on idle speed you may want to raise the ve or pe settings to 1200rpm.

gojo

iTrader: (12)

Did your problem start or get worse with the new Bosch sensors? A well known tuner claims the stock rear 02's are the best at reducing this problem with LT's that have the sensor position farther back.

JimMueller

iTrader: (9)

Sometimes when I was tuning in SD mode, the exhaust would smoke REAL bad at startup, warm or cold. It might just start smoking real bad sitting at a stoplight - I'm talking Spy Hunter smoke screen here. Switching back to closed-loop made this intermittent smoking disappear, except at cold start.

I ran two bottles of GM Top-End cleaner through it, replaced my valve cover gaskets (passenger side was leaking), and added thread sealant to the rocker arm bolts (not done a couple of months ago when I replaced lifters). Once I changed the oil after the top-end cleaner, I installed the 13111's. That was about two weeks ago.

I realy haven't monitored the O2's or fuel trims since installing the 13111's. I do run a semi-permanent LM-1, and the AFR seems to deviate less from stoich than with the old O2's. So far as the smoke goes, the 13111's don't seem to have made a difference.

I've got my idle speed at 900RPM... it was at 850RPM until I installed the lightweight flywheel. I guess I'm less concerned about the odor than the visual cue of the smoke indicating I need to dig into the engine.

white2001s10

iTrader: (4)

holy smoke,
maybe stage 1 heads don't come with any valve seals

Another_User

Ha ha ha. Nice.

Closer_2001

iTrader: (6)

G5X3 here - Same problem and driving me crazy.

It smells like I am towing rotten eggs behind me at drive throughs, the bank, etc.

I tried the PE enable and it didn't help, as the LTFTs still seemed to influence the PE mode.

I think I am going to scale my idle VE cells down to almost 20%, as I expect the o2s can't add that much correction.

I will report back.

Billiumss

iTrader: (11)

Once you have a cam that has 0 or positive overlap, you will have smell in closed loop mode.

There is a trade off for high HP... Having that awesome lope at the expense of exhaust smell.

SmokingWS6

iTrader: (1)

The 13111 bosch sensors are their vette rear.

gojo

iTrader: (12)

Are you sure they are not just OE style replacements? I've been told the stock Vette rear sensors are performing better as front sensors, than the Bosch, with LT's that have the sensors moved farther back.

SmokingWS6

iTrader: (1)

100% positive the 13111's are the OE replacement for vette rear. they have the 65w heating element. I've not seen anyfeedback on acdelco rears in LT's but there are many threads praising the 13111's in Long tube applications. I'm sure the acdelco o2 work great too but my guess is that they cost over twice as much.