Range of AFR variation at idle?
 

How far should it be varying? At idle and at low throttle, mine swings between about 14.5 and 15.5 (read off an Innovate wideband O2 with analog gauge). It seems to steady out under moderate to high throttle. It's also running pretty rough at idle and light load but also seems to smooth out under more throttle.
Would a vaccum leak explain it?
I wasn't sure about this because I have 72lb injectors and high impedance box on there with a PCMforless tune, and thought that without feedback data, he might just not have been able to program it to run too smoothly.
And does anyone know how lean an engine has to get before it will start knocking or pre-igniting--whichever one of those it would be there--?

-Dave

Re: Range of AFR variation at idle?
 

When you are in closed loop at idle or motoring down the avenue, it switches back and forth from lean to rich, so it bounces around a little bit, and depending what is in your fuel it will tend higher or lower than 14.7 average, but when you are under heavy throttle, it goes into enrichment and the fueling comes from last BLM and rpm/temp tables so it is more steady. A little more complicated than that in reality, but you get the idea.

Re: Range of AFR variation at idle?
 

Thanks. That may be the answer, but it still feels rough enough at idle that something has to be fixed. I'll update if I find out what the problem is.

I've watched the O2 voltages vary through a scanner on a mostly stock vehicle before, but I've never seen an actual AFR variation to see how steady it should really be. I've also asked a couple mechanics and they didn't know either.

And if anyone knows how lean a car has to go for there to be pre-igniton or valves burning, I'm interested to know. The internet doesn't have any answers coming up through my searches.

Re: Range of AFR variation at idle?
 

Are the injectors high or low impedance - not clear from the "....and high impedance box" comment? What brand/type injectors?

Running a very large injector on the stock PCM brings the issue of how low the PCM can go on injector pulse width. Drop the pulse width down low enough to idle with 72# injectors, and the PCM injector drivers may not have the fine resolution required to control the fuel precisely, and the injectors themselves may have enough variation in response times to make the idle A/F ratio unstable.

One of the features of the MoTeC ECU that I run is the ability to drive low-impedance injectors (running 78 #/HR Bosch) with a pulse width resolution of 0.00001 second.

The PCM does toggle the mixture back and forth from rich to lean in closed loop, but the variation is very small.... +/- 0.1 A/F ratio units (14.6 - 14.8:1).

14.7:1 typically does not produce the best fuel economy.... somewhere in the 15's might. But the catalytic converter is designed to operate at 14.7:1 where the combined output of CO, HC and NOx is minimized. Go richer, CO and HC spike. Go leaner and NOx spikes.

A lean mixture at idle or low throttle is not typically going to cause detonation..... combustion temperatures and pressures are low. Where you might see a problem would be the transition from part throttle to full throttle. Where you go lean and get into trouble will be highly dependent on the camshaft events, cylinder/coolant temperatures, compression ratio, inlet air temperature, ignition timing, fuel octane, boost, etc. There is no "one size fits all" answer.

Drive it with data logging scan software and look for signs of knock retard vs. A/F ratio.

Re: Range of AFR variation at idle?
 

Thanks Injuneer. They're Siemens 72's, low impedance, with an Acceleronics Impedance Box.
I appreciate the information on detonation at idle. Nothing on the internet discussed it. And you're the only one of four people I've asked that could answer how steady the AFR should be at idle.
It turns out that the roughness was probably due to the damper. I got a cheap one for $120 that came with a keyed hub (Professional Products), and that's about when it got really rough. It wasn't conclusive to me, though, just because I had also put on a stock thermostat at the same time and took out the 160 degree one. I put the stock damper back in, and it has smoothed out a lot. It doesn't feel like I need to turn around and get it off the road any more, but it is still rougher than it should be at idle.

I put the SSI-4 in a few days ago and hooked up the TPS, tach signal (with you guys' help), and the intake air temp. It's running really rich, which anyone with a nose could tell without the need for an O2 display, and I got 15 mpg over 170 miles, with about 100 of them on a highway. But Brian Herter should be able to sort it out with the feedback data.
This thing is almost finally driveable on the street with a reasonable cam, and a real muffler. If I do any more to it, it will be to get some stock springs and take out the Eiback Pro's. It looks great lowered and handles like a race car, but it's just unbelievably rough riding.