Idle tuning basics

[louvered97gtp]

After reading so many posts on idle tuning, i'm still left cross eyed trying to figure out the logic behind this.
The idle rpm surges and sometimes dips too low and stalls out. Also, if I put the car in gear it will stall immediately. I now there is tons of posts on this matter but my question is a bit different as I've not seen this explained.
Why does the idle surge, and why does the car stall when put in gear. I'm trying to understand the mechanics of this. Is it too much fuel? Not enough fuel? Why does the car stall with too much or not enough fuel? Maybe the steps from cell to cell aren't smooth enough? Will a non-smooth curve cause the rough idle and stall?
I've increased and decreased the MAF up and down by as much as 12% in either direction but the car keeps stalling. It's colder than usual these days so not sure if that has anything to do with it. I just keep scaling numbers blindly hoping to hit a magic set of increases that works...but am frustrated and would prefer to have knowledge of what exactly is happening, so that maybe I can start again with a set of MAF figures at idle that have some intelligent selection to start with.
Long story short... what does an engine look for in an air fuel ratio/quantity in order to start and run properly? I'm assuming it's a balance based on stoich?

Thank you

[streetcar]

Are you referring to a cold start? ( coolant temp = intake air temp )

[louvered97gtp]

In general mostly. My current problem has been getting cold and hot start. So for example.. on cold start... do we need richer or leaner AFR to help start the engine... and therefore do we increase the number to increase the initial vs startup ect values which reduce from the AFR?

On warm startups... how do we know if we're too rich or too lean to keep the engine idling correctly?

[Bandit gs]

Seeing that you have a cam, you'll pull fuel at idle now, you're requiring less now given the cam will cause the engine to pull less vacuum (this depends on your set idle) and therefore use less fuel. A WB will fix this pretty quickly

[streetcar]

Here's a quick rundown on the mechanical side of what's going on mechanically at a cold start, defined as IAT = ECT

With the motor cold, the internal clearances are respectively more then when the motor is hot. Metal expands with heat, clearances come into tolerance. So, you can say an engine on cold start mechanically is going to be very inefficient. Along side metal, fluids are most likely going to be thick, further increasing inefficiency.

On the EFI side, we are stuck with injecting fuel on a cold intake valve, meaning the fuel isn't going to evaporate readily. This is a primary factor on the high IPW during the cranking event seen on cold starts. (Ever wonder why OEMs place engine coolant circuits in throttle bodies?)

So, there a few things at play. Generally, you need more fuel to deal with inefficiency in terms of mechanics, lubrication, and atomization. How much is engine specific.

The other key player in cold start and eliminating surge is identifying what tables the PCM is referencing, and correct IAC setup / control.

This is not a part of the calibration phase that happens over night or in a few days...unless you have an engine test cell at your disposal.

[louvered97gtp]

Thanks Streetcar, so i'll need more fuel on cold startup and thusly richer AFR due to the initial inefficiency if I understood you correctly.

Seeing that you have a cam, you'll pull fuel at idle now, you're requiring less now given the cam will cause the engine to pull less vacuum (this depends on your set idle) and therefore use less fuel. A WB will fix this pretty quickly

I have the WB installed, but the engine keeps stalling out right after starting. runs for about 2 seconds while it surges up, and when the rev comes back down, it dips too low and dies. Not much of a scan here to help me figure out what's happening. Am I supposed to use the WB error histogram at idle to correct the problem? cuz it won't idle I've done this already on previous run and this is what created my problem. I pasted the error % correction to my MAF, and now it won't idle.
I'm searching for older tunes to temporarily load and fix this, but would like to learn how to fix this tune.

[streetcar]

You got it. The trick is to start small and work you way up. You want the motor to fire over on the first crank. This could mean 2x, 5x, or even 15x the fuel content.

[louvered97gtp]

hhmm, LOL. Ok, I guess I'll go back and re-read. Thanks.

uhm.. scratch that..... Problems with browser. Doh!

[streetcar]

My fault - shitty connection here, double posted.

The key is a systematic approach. There are multiple things going on, the trick is to isolate them, understand their affects, and calibrate them to what works.

[louvered97gtp]

You got it. The trick is to start small and work you way up. You want the motor to fire over on the first crank. This could mean 2x, 5x, or even 15x the fuel content.

So for the motor to fire over on the first crank, is it dependent on both the startup AFR correction as well as the idle maf cells?
If the car fires right away but dies immediately, the startup is ok but the startup cells in the maf need tweaking?
If the car turns over but won't fire, then the cluprit would be the startup AFR correction?
Also, the set screw on the Throttle bodies affects the IAC? this set screw and IAC should be adjusted to let more air in if the car surges? Surging meaning it needs more air?

[streetcar]

This is where the "art" comes in. You are correct, the startup AFR correction factor is for initial cranking, then the switch to MAF, if failure, stall. On Restart, switch to MAP (VE) table.

Surge can come from alot of variables, but if IPW is constant, most likely there needs to be some changes either in IAC control (which we dont have access to) or airflow entering the engine.

[TDCRacing]

Any updates on this? Mine is surging also....not really any pattern...does it hot and cold. And also doesent stall right away but rather surges for about 40 seconds and stalls....this only happens at times also...sometimes it fixes itself somehow.

[PBJ]

I thought I was the only one fighting this issue.... I just ground a custom cam for this '04 and it wants to hunt and die on occation. I wish there were air flow tables like the ls based cars. A base air flow table would solve 90 percent of this. Maybe HP can add this in the near future.
Joe

[IndeedSS]

Joe, good luck on getting any of the Idle Air Flow tables anytime soon... We've asked. Anyway good or bad, I drilled a hole in my TB to help out under cold startup, because the RPM's were dropping too low after my cam install. Hope you get that orange beauty going soon. It's not even my car, and I excited.

[TommyGloves]

FWIW - I have the same problem. TrannyMan95 recently did some work on m car (P&P heads, 77mm N* TB, VS cam).

It fires up fine and then surges and wants to die unless I tap the accelerator. (Happens about 50% of the time.) He suggested that I drill a hole in the TB blade since we do not have access to the IAC tables.

[PBJ]

Hey guys,
This is drive by wire and I already have 14% tps, I don't think a hole will help. It starts perfect cold, but it has to have pedal assist as it warms up. The car made good power with the cam...it picked up 40 rwhp and picked up 3 tenths at the track...I just need to come up with a plan for this for sure. Thanks for the feed back !!!
Joe

[IndeedSS]

Joe, I know you got this worked out on Rick's car, now you have to share what you found.