Hot starting hard, but only if it sat for ~20 minutes

[jonofmac]

Hey guys,

I'm trying to figure this issue out...

My car cold starts great. It warm starts great as well, as long as I start it within 5 minutes of turning it off...

As soon as it sits for 20 minutes or longer, it turns for a while then eventually stumbles to life.

I'm attaching my tune and 2 datalogs. In the first datalog, car has been sitting for 20 minutes and i start it. It takes several seconds to start, but once it does, I turn it off and try again. In the second datalog, you can see me start it up again about 30 seconds after the last start, and this time it starts right up.


My first thought process was that it was related to my IAT temps. The sensor is right at the throttle body and heat soaks easily. So i know the IAT temps are higher than reality. You'll see this as well because my AFR is pretty lean after it starts, but when my IAT comes down to what it really is, my AFR is on the money.

You can see in my tune, That I tried to adjust the airflow charge bias towards ECT at low airflows, hoping that would push it towards ECT based estimation, didn't seem to work though...

Edit: Problem found. Looks like it was my "new injectors". No idea why, but all I can think is that the injectors I was using were leaking. Bought new ID injectors (ID725s) and issue went completely away...

[compsystems66]

do you have a log showing the hard start and easy start while logging injector PW? Im not sure if the motor is lean or rich when it wont start.

I see you changed some cranking fuling and prime runtime..ect
I dont see the reason to change these tables, have you tried putting them back to stock and see if that solves the issues?

[Earwax]

Why is the BRAF (Table 12084) set to 0 in P/N?
Is the ETC Scalar correct?
Try starting it with the throttle open slightly.

[jonofmac]

I don't have a log of pw. Lemme try to gather that.

I don't think I had changes those. Perhaps they're stock C5 Z values?

And earwax, ETC scalar is right. No P/N because it's a manual C5 Z. The park tables are never used.

[IARLLC]

I have experienced the exact same situation where the injectors drip slowly into the port after shutdown. Up to about 5 minutes it still starts OK but if it sits 15 or 20 minutes it requires a bit of cranking to fire with the wet plugs and some throttle helps. Some say there is no concern but fuel on the ring, pistons, oil after every shutdown can increase wear and of course oil quality. Easy enough to left the injectors an inch or two out of the intake and cycle the ignition to get pressure up then wait and watch. It will be obvious if my theory holds pressure better than the injectors or not.

[dermotw]

Aren't they 5.7L? The tune says 6.8?? But anyway, theres a set of soak parameters in fuel>>general??

[jonofmac]

Sorry. Guess I should've been more specific with my build.

It's a new stroker 416 LS3 build in my C5 Z.

I'll have to try the leak down test with the injectors. That thought had crossed my mind, but I hope it's not that.
If I give it throttle, it starts pretty fast. But without throttle it just cranks and cranks

I don't have my injector clips though, so I'll have to grab some to hold them in.

They're brand new injectors, so I hope that's not the issue ha.

There are soak parameters, but I honestly am not sure what they do and has left them alone

[IARLLC]

New injectors? Probably not leaking down. Maybe it will be a waste of time to pull them up.

I'd agree with dermotw. Time might be better spent looking into that. Also, maybe you already nailed this down but with that many cubes I wonder if LS7 throttle/start-up tables will get a more reliable initial airflow amount.

Good luck!

[Earwax]

The reason I questioned the ETC scalar is that I have a 90mm T/B on my truck.
It was professionally tuned a few tears ago.
The scalar is exactly the same.
Don`t you use a 102mm T/B?
Interesting that the car starts fast with the throttle open?
My truck starts nice and elegantly every time.

[jonofmac]

Welp. Just deleted this long post by accident. Damn phone.

So I was at an autocross event yesterday and was having issues with it starting for runs when I let it sit with the hood closed. I started leaving the hood open and it was fine. It started fine almost every time. ECT wouldn't have cooled off at all, which makes me think it is definitely heat related.

With regards to the etc, I do have a 102 and I started with the calculated value around 0.0144 I think. My idle was really hard to dial in. My maf, dynamic airmass and desired idle did not agree. I played with that value until I was able to get all 3 to agree in SD mode. A lot of my idle issues went away at that point. So that's how I arrived at that particular scalar.

The hot start has been an issue since day one on this tune. But I had never considered looking at a LS7 table. Their tables are generally pretty different from a gen 3 pcm iirc but I'll take a look

[IARLLC]

Oops! Yeah, I hadn't thought about the LS7 stuff not being that relevant. The 8.1s were available in the GEN3/LS years. I never checked which ECMs they ran but I expect they used 0411s or something like that. Maybe their tables would be of some help...at least in order to identify a useful trend. It definitely sounds heat soak related.

I am not sure what all needs to be done to counter that. Here are a couple things I noticed comparing GEN3 5.3 to GEN3 6.0:
Engine>Idle>Airflow>Friction Airflow Decay settings are about 30% higher for a 5.3 than a 6.0 so yours might need to be pretty low.
Engine>Idle>Airflow>Friction Airflow Initial settings about 25% lower for a 5.3 than a 6.0 so you might need a lot more initial airflow.
Engine>Airflow>General>Cranking VE the 5.3 has about 10% lower numbers at 60kpa from 0-200rpm and blended in above and below so yours might start better with 10% more than a 6.0 or 20% more than a 5.3 in that row.

Hope that helps.

[jonofmac]

Thanks for taking the time to make those observations!

I'll give it a go and see if there's any noticable.

As a note, in the version of the tune I had sent, I actually played with the friction airflow decay and initial settings. I lowered the decay down to 0.020 and that helped a lot, when it was stock, it would pull it out way too quickly, causing the engine to stumble and STIT to skyrocket to try and save it. And I had scaled the initial airflow significantly from where i was at earlier. I was having a lot of issues with a very large start up flare. Reducing the initial airflow cut down on it considerably. Perhaps I should look at ignition based methods of helping with startup flare?

Thanks for the help! I appreciate it

[IARLLC]

Hmmmm. Yeah, maybe you will find a balance using more initial to get her fired up then Engine>Spark>Advance>Misc>Start Flare Cont. to mellow her out