Help with Fuel Cut and Fuel Recover in VLT RB30

bachig24u · Post by **bachig24u** » Sat Aug 29, 2009 10:44 pm

hi,
I've read the manual over an over and evened resorted to Googleland to get answers, but I just can't get enough info to explain what Fuel Cut and Fuel Recover Maps do. If I need to reduce fuel earlier during deceleration which one gets changed and which direction?

Any thoughts?

FHCRSky · Post by **FHCRSky** » Tue Sep 01, 2009 11:10 am

I would think for fuel economy when car is cruising or is in stop and start traffic, as the factory had to take fuel economy into account when designing the stock ECU.

"Fuel cut", "Fuel recover" are two of the settings. The other settings are "fuel cut RPM max" (4000rpm on HCR32 ECU), "Fuel Cut RPM min" (1000rpm on HCR32 ECU), "Fuel cut speed min" (30km/h on HCR32 ECU) that gives parameters (triggers) when the fuelcut should be used during deacceleration. I would think if RPM needle is below 1000rpm or above 4000rpm when speed is below 30km/h, "fuelcut", "fuel recover" tables are ignored. If inbetween 1000-4000rpm while above 30km/h, the "fuelcut", "fuel recover" tables are used.

It probably takes looking at "idle RPM" table to see where idle rpm is and where to set "fuel cut", "fuel recover" tables to match during warmup and when engine is warm. If idle is raised for any reason (addition of cams, front mount intake plenum, etc) on "idle RPM" table (like it is on GTR / RB26 engine at 950rpm), would need to raise the min settings on "Fuel Cut RPM min" 250rpm higher at idle when engine is warm and adjust the rest of the table to suit. Look at the difference between RB26, RB20 "fuelcut", "fuel recover", "idle RPM" tables to see what I mean.

I would think if idle rpm hasn't changed (stock), leave those settings as they are. If idle rpm changes, change the "fuel cut", "fuel recover", "Fuel Cut RPM min" settings.

Matt · Post by **Matt** » Tue Sep 01, 2009 1:58 pm

fuel cut is designed such that no injection occurs for a period following deceleration. There is a difference between auto/manual ecus where manual cars care more about this and require more fuel on deceleration (i've noticed this today after reducing the cut/recover tables lower so i have more fuel on decel)

On my previous auto it would 'pop' as unburnt fuel was spent though the exhaust system

Recover is when it needs to inject again

All is dependent on the current temperature of the engine. The table specifies the RPM to cut at

See 'Fuel Cut and Recovery' in the Nistune Mapping Guide (page 41)

Matt · Post by **Matt** » Tue Sep 01, 2009 2:07 pm

btw I used the Z31 fuel cut/recovery tables as someone else suggested and this seemed to work a lot better with stopping the ECU starving the car on fuel during decel (stock VLT ones with the bigger AFM seem to pull it too quickly and make the car jerk when decel in higher gears)

FHCRSky · Post by **FHCRSky** » Tue Sep 01, 2009 5:39 pm

Thanks for that info Matt.

Injecting fuel on deacceleration does explain why fuel recover is set above fuel cut. So I presume increasing cold start enrichment tables adds fuel to fuel recover and decreasing removes fuel to fuel recover???

On HCR32 .bin files (manual, auto) it looks like the settings on fuel cut and fuel recover are the same. It seems like there's 100rpm gap between fuel recover, fuel cut on BNR32 ECU and 300rpm gap between fuel recover, fuelcut on HCR32 ECU (manual, auto).

bachig24u · Post by **bachig24u** » Wed Sep 02, 2009 11:22 pm

I ask this question because since i've used nistune, the VLRB30plusT wants to stall everytime you blip the throttle at idle or even when cruising and is a really tricky to keep running when you brake suddenly.

I can be cruising at any rev or gear at 12-13-14 AFrs then just start to back off and the AFR's run down to 10s then 9 or 8's.

thanks for explanation it confirms what I've been nutting over for a few days.

any tips to iron this out would be greatly appreciated.

Matt · Post by **Matt** » Thu Sep 03, 2009 1:33 am

I received a JWT Z31 ECU here for install... this one has the cobra MAF modification to it. Going to look over their tables to see what they've done since it should be similar. Will post how I go... its still cutting out on light throttle third gear. Didn't have the wideband connected at the time so try and do a log tomorrow

bachig24u · Post by **bachig24u** » Thu Sep 10, 2009 11:35 am

Is there an issue moving the Z32 maf to the pressure zone just infront of the throttle body?

It would seem that the speed of the mafs response is massively affected by it being just behind the air cleaner assbly.

I'm also thinking a BOV is required to assist the maf in reading correctly when the throttle is snapped shut.

Matt · Post by **Matt** » Thu Sep 10, 2009 7:03 pm

You need to run a BOV to sort reversion issues as well as turbo flutter. I'm running about 2 feet pipe to the Z32 airflow meter currently but I've still got to get cold start running better

bachig24u · Post by **bachig24u** » Thu Sep 10, 2009 11:53 pm

cool, Thanks.