chevy1500z71

the motor is a 355 with forged flat tops and 76cc heads, the cam has a 112 lsa with an advertised duratin of 262/268, with a whipple supercharger and tbi on top. so heres the deal, either i can get it to surge up and down at idle at about 11 or 12:1 air fuel ratio or as i try to lean it out it will sit at about 14 for a while but if there is any minute changes in the idle it will start getting leaner and leaner. it sort of runs away from me so to speak, but the rpms dont change a whole lot, it will start to lope very hard(not in a bad way, sounds pretty good actually) and before i knwo it it is sitting there loping very consistently fluctuatong from 15-17:1. if i let it sit there and idle lean it loves it, it will idle great liek that, stays consistant, lopes good, rpms dont fluctuate any... its just too lean... or is it? i try and tone it down it doesnt want to stay consistant, it wants to fluctuate and eventualy starts surging which really makes it impossible to hold a consistant air fuel ratio, which is why i always end up throwing in the towel and leting it idle rich at about 12:1, surgine between 11.5 and 13 at times.

it seems like it starts to lope harder and get leaner the longer i let it idel too, like as the cylinder temps get hotter it idles better.

i dont know, what do you guys think. is it ok to let it idle so lean? a buddy of mine has got a cam in his ls1 truck and it just sits there and lopes consistantly and smoothly, just like mine will do if i let it go lean, is that igniture loping sound only hapening if your idleing lean? is it just how its supposed to be? i have a feeling no.

another thing is that i have big block 80# injectors, which makes things more difficult.

iv never messed with any 02 swing points or anything. im thinking maybe the overlap effect throws raw fuel in the exhaust, raw fuel gives a false lean reading right? probably not the case, but just a thought. i dont care about having a smooth idle, i want a rough idle, i want a nice choppy idle but i want one that is consistent, not one that idles 5 different ways depending on the day, how warm the motor is, what mood its in, ect lol

calebzman

iTrader: (3)

What is your spark advance at idle?

chevy1500z71

well its at about 15 degrees in my closed throttle table. and as i went to look i noticed i have a bit of a flaw, its at about 22 degrees in the open throttle table, i must have played with it before and when i changed it back i forgot to in the closed throttle table. more timing smooths it out it seems. im going to go out there in play with it in a moment.

chevy1500z71

bumped it up to about 22* at idle(it was at about 15) and that really fights the surge, making it idle a million times beter. in fact, id pretty much call ti acceptable now, stays between 13.5 and 15 at idle, lopes pretty good and stays somewhat consistent. but it still seems like tis trying to surge, i can watch the vacuum guage fluctuate and the rpms do a little too. it really wants to surge but the added timign is fighting it. i though surging was from being to rich and blower just amplifies it... but now its not too rich so whats the deal?

calebzman

iTrader: (3)

I'm glad it's getting better. Add another 10° to your idle advance and see what that does. Also, what is the rpm you are trying to idle at and what does your vacuum gauge show?

chevy1500z71

im trying to get it around 600 in drive and park/neutral just because like i said earlier im going for a chopy idle, and the higher the rpm, the smoother the idle gets typically.

the vacuum pretty much fluctuates between 10 and 13, for the most part staying close to 10 but as it surges or the idle changes minutely it will dip down a fuzz, sometimes going as low as 13 or 14. the cam has 41* of overlap, my vacuum isnt much lower then the stock motor did, from my fading memory of the stock motor at least.

chevy1500z71

another issue i just though of is i have the mechanical idle setting all the way down, the throttle blades are completely shut at idle which is probably using up the iac up for all tis worth. although, i probably did this because it would not idle as low as i commanded it to, i tried screwing with the the iac counts i think... this was very long ago, like when i still had the blower on top of a stock motor... probably need to go through that again now that im more experienced and understand it all. it just hit me, i never put enough thought in to remember crap like this until i get a thread going lol.

305sbc

iTrader: (3)

Bottom line is that there is no load at idle, so it's not too lean. Let it idle lean. The AFR you're seeing is in the exhaust. The AFR in the chamber may be just what it wants. If it was too lean, then it simply wouldn't run, or the idle would be very weak and inconsistent.

chevy1500z71

ok, so thats basically the answer i was wanting to hear. let it idle how it likes to, no right or wrong. the thing just loves to idle around 15.5, it just idles perfect there. still needs some more timing to smooth it over and kill the surge. the only problem with a lean idle is my stock type water pump doesnt keep up, it will get hot quick idleing lean, hoter then it would ever get driving around(i have a 160 stat so its not like im overheating it, by hot i mean like 190*)

junkcltr

iTrader: (1)

If there was too much unburned air & fuel into the exhaust then the O2 sensor would read too much O2 and the ECM would try to add more fuel. If there was too much EGR going on then the O2 sensor would read rich and the ECM would pull fuel to lean out. It sounds like you have EGR going on. My engines with cams tend to do the same. You can modify the O2 switch points, but then it messes up your cruise AFR which doesn't have the EGR problem. I end up making the ECM run open loop at idle and closed loop at cruise. It seems to work well. You could also create a new O2 switch point table to be used for idle. The open loop at idle works great with NB02 sensors. The second O2 table would only be useful if using a WBO2 for your sensor feedback. You need the linear range for doing the new O2 table.

EDIT: I wouldn't run lean at idle like you have it. It is hard on exhaust parts and gives a lopey and sensitive idle.

305sbc

iTrader: (3)

It sounds normal to me. If you have a cam with overlap + headers, then you're probably getting a good bit of oxygen pulled into the exhaust via scavenging. Your O2 sensor is reading unburned O2 in the exhaust. Even a completely stock engine has extra O2 in the exhaust at idle. That's why there are calibrations for O2 idle error. There are also calibrations to correct O2 error when AIR is delivered into the manifold ports.

I assume your ECM isn't trying to correct the lean condition, or that it is correcting to include the correction for AIR to the ports. I'm also assuming that you have a narrow-band O2 and a WBO2 installed into your exhaust at different points. Are you running open-loop idle or full time O/L?

15.5:1 is definately not too lean. Since there is no load at idle, and very little cylinder pressure, there is much less heat being generated. It's nowhere near enough to damage anything. The worst you will get from a lean idle is misfires if it's too lean for the conditions. This is assuming you are not running certain emissions equipment.


Edit: For your heating up at idle you may want to try blocking off your radiator/coolant bypass and adding some holes to the outer ring of your thermostat. The bypass is the small hole where the waterpump attaches to the block on the passenger side of the engine. It allows coolant to flow from the passenger-side head to the pump, where it is then pumped back into the block without having seen the radiator. The effect is that the engine warms up faster, and is more pronounced during idle because it is the easiest path of flow back to the pump.

chevy1500z71

i use the wide band for feedback to the ecm, its the only sensor installed in my exhaust system. it is in closed loop all the time, and that is mroe then likely my problem. i bumped the timing up to about 29 degrees and that helped, i also set up the mechanical idle adjustment at 2%, that also helped. but it still wants to surge and stumble when im in gear. the surge is coming from closed loop trying to correct it i guess.

how do i set it up to idle in open loop, i assume itl be a flag/switch?

EDIT: minimum rpm to enable blm will probably work

chevy1500z71

i can not find any functions to make it switch to open loop at idle, any body know?

Ronny

iTrader: (1)

My mask has a flag to check for OLIdle.

305sbc

iTrader: (3)

What mask & ECM are you running?

Ronny

iTrader: (1)

EBL-ECU + EBLxdf(mask).

I also have a constant to hold OL until I attain X MPH. Mine is set cuurently at I believe 15 mph.

chevy1500z71

iv got a 7427 ecm. my mask id is 0D. i see nothing in my xdf for any functions like that. iv got a maximum rpm for closed loop, but no minimum setting.

chevy1500z71

is ther any way i can make a new table? i know i can create a new xdf item, i just dont know how to do it and make it function and what not.

theres definitely no functions for me to get it in open loop at idle and i really need it to.

EDIT: i did find a flag/switch thats labeled "disable closed loop for idle for manual transmissions", i think ill give this a shot even tho im worried it may do soemthing with my automatic transmission tables, could just be the way its labeled tho, i hope so.

305sbc

iTrader: (3)

Sorry, I don't know about that one. It might be possible to patch a line or two of code to add another qualifier for C/L. It depends on how much of your code you have access to, and if you have a really good hack/source file.

chevy1500z71

i tried checking the "disable closed loop for idle for manual transmissions" box and i dont think it did a thing, still wont hold steady vacuum at idle. fluctuates between 10 and 15. bumped the idle up to about 700, barely helped, only way to make it smooth is to make it pig rich. im getting really sick of trying to get this thing to idle, i spend hours on it and it only gets worse.