Open Loop Off Idle Stumble

[Patrick G]

I am running MAF and VE up to 3000rpm. I have the ID850s tuned based off the tables that ID provides. I have my tune scaled by dividing the injector flow rate by 2 and multiplying the Stoic AFR by 2.

Unless you're running a 3 bar MAP sensor with the VE tables tuned independently at higher boost levels, then you are probably getting the stumble from there. The VE tune is no good if you haven't tuned into the 200-300 kPa range of your VE tables.

[hartigan85]

I would start by cutting your injector voltage offset in half. When your fuel trim reverses form rich/lean to lean/ rich on your fuel trim table, your close and should help your stumble....

I was always told not to mess with injector offset when scaling injectors? I will definitely give it a try and see what happens though.

Unless you're running a 3 bar MAP sensor with the VE tables tuned independently at higher boost levels, then you are probably getting the stumble from there. The VE tune is no good if you haven't tuned into the 200-300 kPa range of your VE tables.

Yes Pat I have a Zr1 3 bar MAP sensor. My VE table only goes up to the 175 KPa range, but I do have it tuned up to 3000 rpm, which is where I have SD disable set at.

[Patrick G]

Also, check burst knock retard to see if your timing is getting retarded during the stumble period.

[G8-4-Speed]

I was always told not to mess with injector offset when scaling injectors? I will definitely give it a try and see what happens though.

Your pulling so much fuel (STFT) at low injector timing then as soon as you go for throttle it goes lean. Cut the offset in half since it is injector timing that is added to normal injector timing, then richen up the lower airflow gain tables to even out the fueling tables. Any time a LTFT table is lean to rich from idle/decel areas to heavy throttle is a sign of injector offset being off. Same if the table goes rich to lean in the same way. You may have to mess with injector size also since you are scaling. There are several ways to rescale but when I am close, this is what I would adjust. Another thing to work on is the VE table. It should be fairly smooth without sharp spikes or drop-offs. With the VE enabled, it needs a little more refined.

[hartigan85]

Thanks Rick. Where is the airflow gain table?

[G8-4-Speed]

Sorry, maf table..

[Ace270]

I had a very similar stumble at idle and mine ended up being a vacuum leak due to a bad intake gasket on my Maggie 1900. Found it by spraying some brake cleaner around the base and listened for when the rpm's increased.

[hflores3]

I also have a signficant "off-idle stumble" as well. However, I attribute it to the Stoich Bud Lights I consume on Friday nights.

[Patrick G]

Have you tried doing a Speed Density-only tune to eliminate the MAF as the culprit? I've tuned many forced induction setups where the MAF was a big contributor to off idle stumbles. When the car was converted to speed density only, the stumble went away. Try this and if it helps, you may need to reconsider how your MAF is situated in your setup.

[hartigan85]

Its just odd cause it only happens when I have a cold engine. After the car warms up for a couple minutes it doesn't do it, not even a hint of stumble once its warmed up.