Flare on start

Hi all,

I need some help with eliminating the rpm flare/drop on start. Could someone direct me to what tables I need to adjust please? I've attached my latest tune and a cold start log if that helps.

Cheers,

Phill

Anyone?

Do you mean the way some engine's management gives the engine a bit of a rev' when first starting, the sort of thing that's annoying, anti-cocial, embassassing,and increases wear on the engine - especially if sitting for a while?

It would help if you actually mentioned the vehicle, the managment -especially if not OEM, the software you're accessing thECU through, and anything else that may be significant? Without those basics it's very unlikely anyone is going to respond to your wueriy - assuming someone who actually reads your post can.

Unfortunately, I can't offer direct help.

Hi Gord,

Car is a 2016 VF Commodore with L77 6.0Ltr with DoD delete. Also fitted with a VCM CAI, 1-7/8" headers, hi-flow cats. Cam is a 224/232 .608 int and ex with 114deg ls. It was an SV6 Black Edition. The car has a Mafless tune on the E38 ECU.

Have the tune running nicely in the low to cruise area and will be fitting a wideband shortly to tune up in the higher rpm's.

The issue is that on start the rpms flare up quite high, drop low then level out at the required rpm/coolant temp.

I'm sure there are some adjustments in the idle air area I could make. I'd just like some pointers?

Might be a bit of a long shot, but it may be a faulty, or missing, one-way valve for the brake booster/servo - usually the booster will hold vacuum but, if it doesn't, then when starting the engine air will be pulled out of it on top of that coming through the throttle until the vacuum is established.

A simple check is to disconnect the line at the manifold and have someone hold their finger over it to stop air entering the manifold, and seeing if it still happens, or is reduced, when the engine is started. I don't know if there are other vacuum powered devices on that vehicle, but if there are, try them as well.

Sorry, I can't make any other suggestions.

Do you have your VCM Editor set to advanced view (Edit->View->Advanced)? If so, then there is a table Spark->Advance->Startup Flare Control that should probably be adjusted. There is a multiplier to this table that changes the results based on coolant temp. So is your issue at all temps, or only certain temperatures? You might just need to change the multiplier table.

Hi Gord,

Definitely no vacuum issues as the car idles nicely a few seconds after starting.

Hi David,

Thanks for the info, I'll check on your suggestion tonight.

Thanks guys, much appreciated

Phill

I expect David's suggestion will be the solution, but you mis-understand mine.

It isn't a "vacuum leak" in the usual sense of the word. A faulty/missing check valve releases the vacuum within the booster/servo' as air from the manifold enters from the manifold when the engine is stopped. When the engine is started, it draws air not only through the throttle body, but from the booster into the manifold as well, as a vacuum is re-established within. Once the vacuum is established it's stable and "sealed", until the brakes are used and released, when there is a small loss of vacuum which is almost instantly restored.

The condition can be replicated by turning of the engine, pumping the brakes several times until the pedal gets hard, then starting the engine. The rpm will be raised for a second or two before dropping back.