warm/hot startup stumble

[inkscape]

Hello!

I have a 2000 Z28 with a LS1/LS6 T56, with a cam (don't know which one installed by a previous owner), ported tb. When I bought it, it had longtubes and since then, went back to stock exhaust manifolds and cats for state referee in CA. Even before all of this, the car had this stumbling on a warm/hot startup. Cold starts are great, but this stumbling on hot starts bothers me. Stumbles like it wants to stall out but after a few seconds she finds her idle and sits nicely from there. The car was tuned prior to me taking ownership. Car idles at the desired idle 896rpms perfect at about 21-23 degrees advance.

A few things I've dug in on is increasing the startup frictional airflow and decay. What it was set to before was 3.50g/s around 140f+. On a hot startup, I have managed to get it to start fine when ECT was around 176f+ and startup airflow upwards to 5.45g/s or a bit above. and the cold start I leave untouched, but between the cold and that hot start temp, I still get that damn stumble.

A new beginner to the tuning and learning day by day.

Should I be playing with the startup frictional airflow or may my problem lie elsewhere?

I can upload my current tune if anyone wants to dig in on it.

Thanks.

[5FDP]

Look up the russ k idle tuning configs to see if those will help you get everything dialed in.

Also you'll need to make sure that the VE and MAF tables are dialed in as well with a wideband. Without a data log with a wideband to see, it could also be running rich or lean on start ups and throwing it even more off until it corrects itself.

[mowton]

Try messing with the Cranking Fuel...lower it about 10% to start in the temp/off time area's in the table. The Table column label says Cranking time but it is really the length of time the engine has been off prior to this start cycle. Larger cams are less efficient and you are getting greater evaporation so most times a bit less fuel at hot start will improve a slow to start/stumble. Only carry your changes to the temp and time area;s you are haveing the issue. Cold starts will require extra fueling.

Log Desired Idle Airflow, LTIT/STIT (or RAF depending what is available), friction/Startup (some platforms only provide a combined) and Throttle Follower/Cracker to handle stationary throttle blips and coastdowns.

The process I follow is to log the above, determine where (crankk, transition to idle or adaptive idle) the idle starts to go squirrly and make necessary adjustment. Move one element at a time and zero in on the ones that are axised by engine temp. 1) Air (friction/startup) 2) Fuel (Cranking Fuel) 3) Spark (Cranking, Idle Spark and if Cat Lightoff is pulling so much timing)

Hope this helps....this completes one of our module sections :-)

Ed M

[inkscape]

So I decreased my cranking fuel in some areas where I know I have the problem for sure, in the time, not so much. Haven't really focused on the time of engine off yet.
Also increased frictional airflow in that corresponding area by 40% as i've heard in some other cases helps. Added 20% to the decay as well so I dont get a hang in my idle.

2000_camaro__StartupAirflow-cracnkingfuel.hpt

[inkscape]

Just tried that tune I uploaded. Didnt fix. I did one last adjustment before saying im done for now. (car sits outside and its slowly beginning to rain) I decreased the cranking fuel again another 10%, reset my airflow adjustments to stock and added 40% from 140f+ then another 10% from 176f+ and added 10% decay at 176f+. It improved, but was still there. Should I further decrease cranking fuel? I read a few people have fixed it by adding the addition frictional airflow initial but I feel if im increasing air to fix it, the problem is elsewhere...

[cobaltssoverbooster]

if its working then keep going. dont be afraid to test settings out. save every file and if you dont like a change then go back one and delete the file you dont like

lots of people are afraid to test, you just got to go try things sometimes

[inkscape]

messed with the frictional airflow initial and increased it by an additional 10% and lowered my cranking fuel by 25% from stock value. Its much closer to a solid hot start. Drove to the store and went in for about 10min came back out. Started. Went up nice to about 1200rpm then fell off to 500-600rpm after about 2 seconds then back up to 900 and leveled out to desired idle. So in relation to how it was before, it only dipped down once. Other times is dipped like 3-4 times then found its idle. Going to start increasing that airflow by 5% increments now.

Anything below 140f I do not touch to avoid messing up my cold start.

[1364838952]

Try this; HPT > Engine > Idle > Idle Airflow > Start-up Airflow Delay then add "50" to all of the boxes. This will add additional airflow for "50" cam revolutions. It will give the engine a little more time to stabilize at a slightly higher RPM and then allow it to return to your programmed idle speed once the engine has found it's rhythm.

Motown above has the CORRECT way to do it. The way I've recommended was found here;
https://www.performancetrucks.net/fo...tricks-373848/
This way MAY work but it really is not addressing the core issue, it's just covering up the real problem until such a time that the computer can compensate for the issue and take corrective actions. At least this is the way I interpret the issue. Programming correct Air/fueling data always trumps just idling it higher until the computer can catch itself. But I could be way off base here. I guess you can try both ways and figure out which way works best for you.

[inkscape]

I really wish I knew who tuned this damn car when the cam was done. Im sort of determined to redo the whole tune. No wideband though so I can't really make a difference without that. In that writeup, he says something about for a hotstart on big cams, the adaptive idle min should be -0.5g/s. On my tune it is -3.0g/s... I'm starting back to base on my startup tune and going to adjust in the hot ect areas. With the tune I had tweaked prior, did fix my hot starts to an extent but made my cold start mental and mid-warm was still complete shit.

[inkscape]

Cars' original tune when I pulled the first tune from my ECM was tuned to always run in open loop. (longtube, big cam combo) I have since set it to be able to achieve closed loop.

Here's the original tune:2000_Camaro_Original-Tune.hpt

[inkscape]

Well guys, I've logged a few starts if you guys would like to inspect a little bit and let me know what you find basing from my log(s).

startup surge.hpl
startup surge 2.hpl
startup slight surge w friction airflow increased 2gs across.hpl

[inkscape]

It's strange. This issue... Lol. It always starts up instantly. Never any long cranks. As soon as it cranks it starts.

[inkscape]

So I found this on FiTechEFI;

Cranking needs extra air to help the engine spin to a higher RPM and to generate very full cylinders for maximum power to fire up the engine against the cold oil. However, at warm engines, the thin oil and easy combustion doesn’t need as much air to achieve a successful start, and too much will cause a very large flare of the RPMs just after starting.

I can bare with the issue but it does get somewhat annoying. From what that says it could be either of the 2 or both. That it's pulling in too much air on hotstart or too little trying to balance out the cranking fuel tables(s). Could even be my IAC. Sometimes if I want to avoid the starting stumble, just gotta give it little throttle during crank as it hits 2k+ rpms and let go. It never stumbles that bit and goes right to idle.

[inkscape]

Another thing I was reading about and thought. What if it's my iat sensor getting heat soaked and causing it? I'm gonna check my datalogs and see what the IAT is. Then try to get it near that heat with the car then manually cool it down and pull it from my intake lid then see if it makes a difference to the hot start.

[brycedrue]

Any updates?

[cookster489]

well if you have the same exact problem the post will be three pages long and leave open ended. welcome to the tuners life.

[ryan_axberg]

If your having some issue post up a log and a tune. If you log the right things it is usally pretty easy to see what is causing the issue.