thegstring

So i put a wide band AEM air/fuel gauge on my car because i was afraid i was running lean. The problem is now I'm not quite sure what lean really is.

At idle, the A/F is off the chart, 18:1 at least, because that's all the gauge will read.
Cruising along at 2K it reads about 15.5-16.
WOT it's in the 12.7-13.5 range (it jumps around a lot)

Now i don't need a definite answer, I'd just like a few opinions about where your guys' cars are running. Mostly, will i grenade my engine if i drive it like this for a week or so?
Keep in mind my car is an 88 5.0 5 speed with the speed density system. I put a 75mm throttle body on it, and i have the 76 mm MAF ready to go on, just waiting for new injectors which will be in next week.

mattdel

14:1 is the target AFR for any internal combustion engine, variables aside. You're not receiving enough fuel except WOT, where you're a bit high. Now, its never good to be too picky here, because it will take alot of work to get a consistent 14:1 no matter what, but clearly something isn't up to par. You're lean, so the culprits are: FPR, FP, vacuum leak, intake leak, bad MAF, bad MAP, EGR. Probably some other ones but I've had a few too many beers at this point to recall everything.

mgmuscari

14.7:1 is actually the optimal stoichiometric mixture. Lean is higher than that - under light load and cruise, modern fuel injected cars lean the mixture out to save on emissions and gas. Under WOT conditions, these cars will run slightly richer than stoich, in the 13:1 range. Slightly rich is unlikely to have any adverse affects, and can actually help to keep engine temperatures down.

18:1 is waaaaay too lean at idle. I target 14.7:1 at idle in my car. 15.5-16:1 under *light* cruising conditions is fine. Most cars target this from the factory these days. 12.7-13.5:1 is perfectly acceptable under WOT, IMO. Some people will talk about the possibility of fuel washdown and premature ring wear under rich mixtures, but I don't think that's rich enough, especially at WOT, to lead to washdown. Keep in mind that until the engine hits normal operating temp, the ECU will be commanding more fuel from the injectors, and so it will run very rich when cold.

I'm a little confused as to why you're talking about a MAF sensor, when you've stated that you have a Speed Density computer. Speed Density calculations don't take MAF readings into account. They perform fuel calculations based on the MAP sensor, the MAT and CLT sensors (for air density and engine temperature), the throttle position sensor (for acceleration enrichment), and some hard-coded VE tables. Are you planning to swap out the ECU and engine harness when you get that MAF?

Also, out of curiosity, where did you place the O2 sensor for the AEM gauge? Did you install a new bung on your midpipe? You aren't going to get accurate numbers for AFR from a narrowband sensor like the stock one, you need a wideband for that. The stock computer is only set up to accept input from a narrowband, too...

EDIT: Oh, when's the last time you did a base idle reset? An SD computer can *probably* handle a mild change in the throttle body, but the idle may be affected by the TPS being out of range with the throttle closed. If it is the computer should probably throw a code for it. I don't remember and can't check, since I don't have the stock computer anymore.

thegstring

I have the entire kit for mass air conversion, computer, harness, meter, everything except the injectors (which are on order).

The O2 sensor was interesting to place, since the install directions stated that it must be placed pre-cats. Unfortunately my H-pipe is after the cats so i had to put it on the passenger side about 16 inches after my header collector, before the catalytic converters, but also before the h pipe.
Yes, i welded a new bung on, and yes i used the new wideband O2 sensor.


Im not sure how to do a baseline idle reset, the bbk throttle body has no idle air hole in it so i had to adjust my idle by cracking the throttle open with an allen key.

88 orangepeel notch

I just put the same wideband in my car and I'm getting the exact same #s that your getting. Except my idle only goes lean once in awhile but only for a second or two. It will idle at 16.5 to 17.5, I read that these are good #s to shoot for 13.2 (WOT), 14.7 (CRUISE), 15.5/16.5 (IDLE). And yeah mine does sweep higher/lower when running, usually .2/.3 either way.

mgmuscari

FWIW I'm also using AEM sensors and controllers, one set on each bank... but mine are connected to a MegaSquirt and used for EGO correction and tuning purposes.

I'm not sure what you mean about the idle air hole... I'm assuming that your TB does have a provision for an Idle Air Control valve, and that it's hooked up. If it doesn't, you're going to get a much better idle if you get a TB that does have one... You want to open the throttle blade only far enough so that the car will just barely idle when warm, less than 800rpm, with the IAC disconnected, then pull the negative battery cable for 30 seconds or so, and reconnect it to reset the computer. The computer will sample the TPS value at that throttle position, and will open the IAC as much as it needs to do hit its target idle speeds (depending on temperature). If your idle screw is open too far, and the car idles without the IAC higher than the target idle speed, the computer may not be able to control the idle well at all.