Hello! I am trying to lean my idle up in closed loop. I am running speed density mode and would like to know what table I need to be in? I tried going into the VE vs. RPM vs. MAP table and reducing the number first by 1 then by 2 in the 800 to 1000 RPM and 55 to 60 MAP range because that is where I need to richen it and loaded it to my car. I logged before and after and there was no change to my BLM's. Could someone tell me if I am doing this right? I am working on a 95.
Thanks

 

Does anyone know why I'm not seeing a change? Am I changing the right table? Lt BLM's are low so I am changing the number in The VE table in that location to a lower number and I am getting no resaults. Should I drop the number more?

 

If you are in closed loop, changing either the VE tables or the MAF calibration tables for idle will have little impact on the actual fuel mixture because the PCM will (try to) correct it based on the measured AFR at the O2 sensors to about 14.7:1. I have read that if you go outside the LTFT bounds, it will go into open loop at set a fuel system too rich/lean code, at which point you can impact the mixture, but I have never tested this.

This is a big problem with cammed cars because cams with lots of overlap can cause the O2 sensors to report erroneous data at low airflow conditions (e.g. idle up to 1500 or 2000 RPM). Generally, EFI engines with cams will idle too rich.

I was experimenting on a new technique yesterday to address this, however - and it seems to work. My Impala SS has an LT4 HOT cam in it, and idles rich, too. If you set the TPS threshold for PE mode to 0 in the RPM range that the engine has problems with fuel mixture, it will go into PE mode, which is open loop. You can then set your PE tables to the desired AFR. I simply set my to command a 14.7 ratio, which is what it would target for closed loop anyway. With the stall converter my car has, it flashes above the upper RPM range where I set the 14.7 ratio so it enrichens properly. I don't think actual AFR was 14.7 as the O2 sensors were in the 900mv range, but the point is I now have a way to command AFR in the RPM range where the cam causes problems and I do not have to give up closed loop operation where it will function properly.

Once I confirm this works under all circumstances, I'll do a write-up on this.

 

Thank you Vince. Learning this stuff is a constant struggle for me. Although I like doing it, it is frustrating sometimes and I don't get the time that I want to spend on it. Anyway, thanks. You have helped me out a couple of times now and I appreciate it.

 

Vince, I've done the same with mine. The STFT's will still increment even in PE mode. You'll probably want to limit them from adding too much fuel.

The only problem is that you will have an SES light at idle if you limit the STFTs. It'll shut off when you're driving.

Definitely need to do something like this with a big cam.

 

Yes, I noticed that STFTs were showing non-zero values from time to time, even when it was supposed to be in PE mode. I wondered if the PE TPS hysteresis was coming into play... Clearly more research is required - the weekend is here. ;-)

 

How about a MAF car? Can you tune the MAF table for idle or would that have little or no effect? Once I tune my VE tables I plan to go back to MAF.

 

I was seeing STFTs adding 40+% but that is with a fairly large cam. With the hotcam, I didn't typically see any LTFTs or STFTs.

 

Depends on your cam. Your o2 sensors will try to bring you back to 14.7AFR if all you change is the MAF table.

 

I don't understand. If datamaster is reporting a BLM of 120 which isn't 14.7AFR because 128 is 14.7AFR then I would be just helping it get to 14.7AFR by changeing the tables. It has to be useing one of the tables to do this. Right? Do you know what mv the o2's are when it's 14.7AFR?

 

A BLM of 128 means that the PCM is not adding or subtracting fuel to keep at 14.7AFR. Your BLM of 120 says that the PCM is removing fuel to keep it at 14.7AFR. In that case, you probably would want to lower the MAF table to compensate for it.
If you have a fairly stockish cam, try tuning the MAF to give you a BLM of 128 at idle and partial throttle.

What I was referring to was with a larger cam, there is unburnt air passing through which the o2s pick up and compensate for. This will work out to 14.7AFR, but is much too rich at idle.

Your o2 mv will be ~450-550 at 14.7AFR, but in closed loop, they will be cycling rich/lean which won't really tell you much.

 

OK, I see. I am running a 230/236 cam with a 112 lobe separation. Maybe thats to big of a cam to tune it out.

 

You have enough overlap on that cam that the PCM will be adding fuel at idle due to the o2's. You could tune the MAF to bring the BLMs to 128, but it will run very rich at idle.

Setting the PCM to PE mode at 0 throttle would be a start, but the fuel trims are still active. Locking the BLMs to 128 @ WOT prevents them from moving at idle, but the INTs still accumulate. Lock those as well so they don't add fuel. This does cause a CEL, but it will turn off as soon as you step on the throttle.

Make sure that you have your partial throttle MAF table tuned right before locking the INTs.

If anyone has any better ideas for leaning out idle in Closed Loop, post it up.

 

You'll need to change the VE tables more than 1 or 2 points if your BLMs are way off (<116). Same goes for MAF tables.

I've gotten my 236/242/114 cam to run nicely at idle just by normal tuning (no PE idle). One other thing that can make a difference is your closed throttle timing table.

More timing advance tends to run the car leaner, in my experience, especially with a cam that has more valve overlap. Try bumping your idle area (400-800) up around 4kpa and see if that helps too. Mine is set to 400-22, 800-24 right now, but I've run 24-26 with good results too.

 

I had a bit of an idea today while thinking about this 'issue.' How about a new approach to the problem of the computer correcting what the o2 sensors are seeing. Hows about a 'black box' if you will that piggybacks onto the weatherpacks for the tps and the o2's. And does mechanicly what you're trying to do with software. In other words it supplies the pcm with a dummy signal from the o2's at idle switching perfectly to say that its got a 14.7 a/f, while stopping the actually o2 readings. Then you can adjust your idle to your hearts content with your maf tables and it'll idle all day like a champ without the stench of gas. The only other hitch is telling it to kick off at a certain tps percent and let the real o2 signals pass back to the pcm, so you still get correction.

I'm not the greatest at circuit design/analysis or the other skills required to construct such a piece. But i just thought i'd fire this idea i had out there as food for thought.