engine stumbles bad after hot restart

[RADustin]

Maybe someone has an idea. I'll scan the truck later in the week but for now I'd like to know what to look for. Basically- am I looking for an airflow issue or fueling issue?

Problem- On hot restart(within ~5mins, maybe longer) of shutting off at working temp(200F coolant temp), the truck stumbles/misfires bad. I have to immediately give it air with the pedal to clear it out or it will sound like crap stumbling until it dies. If I try to start it again after it stumbled and dies sometimes it will start sometimes not- until I put my foot into the pedal and it'll fire and roar to life. Either way, if it does start it will indefinitely stumble until I give it some air and bring the RPMs up and it'll clear out.

Sounds like it is getting too much fuel and flooding out to me? I'll log AFR while its happening but I'm not sure how reliable the data will be. I can also zero out B3661 and see what that does. It could be heat soaking the IAT sensor as well. So I will have to check on that.

I can't comment on if it's always been like this or not. Its still a generally new build with low miles on it since built.

Specs-
'99 LS1 stock internals, DBW
big cam
P1 procharger
COS5, MAFless.

Runs great once its past the stumble. First start of the day is also great as is warm up. I did a ton of RAFIG/RAFIP back when I was tuning it in maybe slightly cooler weather. I do feel like it used to have slightly too much air during hot restarts and the RPM would jump before it decayed out upon startup- but I honestly don't remember if I tuned that out or the truck is running worse than before.

Tune attached.

TIA

[wesam]

I had the same problem when i put cold spark plugs of 8 heat range

[joecar]

Check that FPR is not leaking via the reference.

[RADustin]

Thanks for the ideas guys.

Plugs are NGK TR6 gaped to .032"

I have a fuel pressure sender/gauge I will log shortly.

[Highlander]

Try this20251

[RADustin]

sweet. I'll give this a try this weekend.

I do feel like the truck is surging at low rpm in gear. It is an M6- and if I don't clutch it when bringing the RPMs up it just gets worse and seems like it'll never come out of it. So I'm going to have to play with that as well.

[Highlander]

that is relatively easy to fix or alleviate.. there are some conditions where you just have to clutch in and drive around it...

[RADustin]

OK- ran the tune you posted up. It needed a bunch of throttle input to kick off and run- like almost floored.

I reverted back to what was in there for now until I can think on it some more. I thought about making an excel calc to tell me the all in numbers of fuel and air QTY during crank and start- but I'm not sure I need to take it that far or I even understand all the modifiers that much to do it correctly.

There is a major difference between 1 minute after shutting the motor off and a restart- and 10 minutes of being off. After 10 minutes it starts fine and everything is good. Very OEM.

Within that 1-5min window a restart takes pedal manipulation and it misses and stuff until it catches itself.


So to me- I'm trying to think what is different after 10 minutes from 1-5 minutes.

-ECT is the same after a few minutes IIRC. I don't think its dropping a considerable amount.

-Heat soak- My IAT is mounted in an aluminum tube off my front mount air2air IC. I'm wondering if it's getting soaked immediately after shutting off and causing the startup to be very lean due to excessive IAT temps. I'm wondering if I could richen up the mixture in B3661 in the 1-10 minute range.

-I have a true return style fuel system and I always let the pump prime and shut off before starting. Fuel pressure is not a problem.


I'll go and log MAP, IAT and ECT on startup and see if I can make since of what physically is going on. I can also log IAT and ECT after a shut off and see what is the different between 1 minute and 10 minutes after shut off.

[RandyWS6]

I had a few leaky injectors that was causing the same problem , If i let the car set for a few min. it would not hardly start, but if i let it set for a while , like a hour or so it would start on first crank. The injectors was flooding the engine. Not saying thats your problem ,but some thing to look at. I pulled the injector rail and left the injector on the rail and turned the key on to prime the injectors then tuned the key off and you could see three injector dripping . Changed injectors and fixed it.

[RADustin]

Thats a good idea- I did have injector issues on first start up a while back- but I run a fuel pressure gauges. So once I key on and prime I watch it to ensure it isn't moving before I crank.

When I first had a problem the engine hydra locked and blew the starter off. it was scary.

[Highlander]

You have another problem... your main problem was your Cranking VE was way high. It was scaled according to your main ve table. If a car needs that much airflow to start you have WAY too much fuel in it. YOu should start checking injector (like mentioned above) and injector data. Making sure its actual injector data with offsets. When vehicle cranks voltage really drops, if you have aftermarket injectors with stock voltages it will tend to overfuel on the cranking.

[RADustin]

These are my injectors.

http://www.texas-speed.com/p-3980-fic-33-48-lb-modified-bosch-blue-demon-3-fuel-injector-ev1-style.aspx

I run vac/boost referenced FPR in a true return setup. Pump is a 255. Lines are 1/2" and 3/8".

If my injector data is off I assume my entire tune is off. Problem is- my main VE is spot on with commanded AFR. I'd hate to have to retune everything- but I also want it 'correct'.

Its starts very well at all times- except for the 1-5 minutes after hot shut off.


So where do I go from here? Getting really good data on the injectors is going to be difficult if not impossible. I can copy all the stock cranking tables back in and try again. hm!

[joecar]

...

Its starts very well at all times- except for the 1-5 minutes after hot shut off.

...This sounds a lot like leaky injectors (maybe they leak only when hot), or a leaky FPR (ruptured diaphragm).

[RADustin]

Ran out of time today. I will log fuel pressure, IAT and ECT tomorrow after getting it up to temp and letting it sit. Maybe for like 10 minutes.

[statesman]

It looks like you're dumping too much fuel into the engine at startup.

Try the file attached.

[joecar]

statesman proposed an interesting change, I want to see how it goes...

[RADustin]

I'll run out and try it in a few minutes.

See how the truck responds.

[statesman]

statesman proposed an interesting change, I want to see how it goes...

It's been a few days now, so I'm guessing that he's giving it a thorough testing. :laugh:

[RADustin]

sorry Gents, got side tracked last week getting ready for Crusing the Coast down in Mississippi.

I left my cable with my brother to tune a duramax, so once I get it back I'll get back on this.

Statesman, do you mind elaborating on what interesting changes you made?

[joecar]

No worries, take your time.

Open stateman's file (post #15) as the calibration, and open your file (post #1) as the alternate, then use the compare feature to view the differences.

[RADustin]

Kind of half picking this back up. I appreciate everyone's help thus far.

I should have plenty of time after xmas to play with this until it's fixed up.

To tune the injectors I only modified B4001 from 3.3281 grams/second to 6.2188 grams/second.(49.3559 lbs/hour). I've asked Texas Speed for the correct data to verify- I don't remember where I got this information from.

The math doesn't quite work out- sqrt(4bar/3bar)*42lbs= 48.49 lbs. so I will need to verify this information and probably retune the truck. Injectors are rated 42lbs@3bar...

I started comparing my base tune back to what I'm running lately- and I see some big differences that could be causing my issue. I think I made some mistakes early on and didn't go back and fix them.

I remember having a bad restart issue when I first got the truck running. I had to floor the pedal to get it to start, and I didn't know much about RAFIG and air flows in general at the time- so I tried to fix it with fuel.

B3604- Commanded fuel while cranking- I scaled this up in the hotter regions.

B3661- Restart Enrichment(based on time)- I added a big peak in here at engine operation coolant cells up to 8 minutes after engine running. I'm adding up to 1.05 EQ here. I think this could be a major problem and giving me very rich restarts. Also accounts for the delay where if I wait it starts up fine.

I'm going to set these tables back to stock. If that doesn't get it- then I will go to statesmens file. If B3661 is really an additional fuel enrichment, that would be put my start enrichment to something like .8EQ(from B3604)+1.05(from B3661)=1.85EQ. That kind of explains the crazy rich start...

[RADustin]

got it fixed up.

Did a few things-

B3661- zero'd out completely. This was my biggest issue and why the crap restart was time sensitive.

Also, added a little to my startup friction and zero'd out startup correction. The truck starts beautifully now everytime I've tried.

[joecar]

Good job :cheers: