How is Fuel cut determined?

Efi Performance · Post by **Efi Performance** » Thu May 29, 2008 7:45 pm

Hey Guys,
Very new to Nistune so just looking for a little help,
Have a CA18Det with aftermarket AFM and it looks like we are not getting fuel cut. Same thing running the same AFM on an RB30...

So this leads me to the question.... what conditions have to be meet for the ECU to recognaise over run?
We have TPS setup correctly, have tried Min TP, changing fuel cut rpm and recover rpm but still seems to be injecting heaps of fuel under decell.

Thanks guys

Matt · Post by **Matt** » Thu May 29, 2008 11:15 pm

There are a few conditions for fuel cut which is checked

Fuel cut is when below specified RPMs for current temperature that it should cut on decelerate (TPS is on) and will resume when the RPMs in the recover table is reached. These should probably be the tables you should play with. Send me a log through and I can look at what the ECU is doing and can play around with the limits on the bench

So you can drop your recover table lower if desired and perhaps increase your fuel cut if not cutting soon enough at desired RPMs

Other cuts are RPM, speed which are self explanatory, as well as TP cut which is when maximum pulsewidth allowed (often called boost cut) is reached

Min / Max Total theoretical pulsewidth are other cuts which limit min/max injection allowed

datsunboy · Post by **datsunboy** » Fri May 30, 2008 7:28 am

is the speed sensor used for ANYthing other than the speed limiter on a CA18 ECU?
does it affect fuel cut/recover?

reason is, when putting a CA into an older car with mechanical speedo drive, you loose the speed input

Efi Performance · Post by **Efi Performance** » Fri May 30, 2008 8:45 am

Could this be a contributing factor matt? The CA I have only tested stationary, but its definatly enough to stall the engine.

Fusion Ed · Post by **Fusion Ed** » Fri May 30, 2008 8:57 am

Is the new AFM too near the turbo and thus causing problems? Are the AFM voltages stable under with the throttle shut?

Efi Performance · Post by **Efi Performance** » Fri May 30, 2008 9:20 am

Hey Ed,
We are getting a lot of reversion through the air flow meter on decel, (measured with scope) but was hoping that the ECU would dis regard any signals from the AFM when the throttle is shut and RPM above fuel recover rpm.

Matt · Post by **Matt** » Fri May 30, 2008 11:39 pm

Some interesting info from Devins documentation on the 300ZX whilst I was going through it

TTP Min

TTP Min stands for Total Theoretical Pulsewidth Minimum. This sets a certain minimum pulsewidth to keep the engine running in cases where the voltage signal from the MAF may drop too low. A good example is on a hard deceleration or a hard shift where some air may pass back through the MAF and cause the engine to stall. All of the tuned ecus that I have seen keep these values stock

Still I feel that Nissan set it a bit high, probably to keep the some unburnt fuel in the exhaust to keep the cats hot. It is okay to lower these numbers especially on low load, high rpm conditions. When you upgrade injectors you will also want to lower this since the minimum pulse width needed for large injectors is less than that of small injectors.

TTP Max

The TTP Max is a pulsewidth limiter of sorts. The ecu has a number of enrichments like the Fuel Map, Throttle Enrichment, and Water Temp Enrichment, as well as the possibility of problems with over boost or problems with the MAF sensor which can send the pulsewidth figures through the roof. The TTP Max limits the total pulsewidth by rpm and I believe it is based upon the rpm scale for the fuel map

As you add breather mods and increase boost on the Z the engine is going to ingest more air and likewise need more fuel. Increasing the values will allow for this extra fuel and allow the engine to maintain the proper AFR at higher than stock engine loads. From what I have seen the stock values are pretty good but a slight tweaking should be done on any car with higher than stock boost.

Matt · Post by **Matt** » Sun Jun 01, 2008 10:00 pm

As you mentioned, the ECU should use TPS on values when throttle released. This post looks very similar to the one mick has raised, with reverse airflow being 'read' by the AFM when throttle released

GZ@hybridka · Post by **GZ@hybridka** » Mon Jun 02, 2008 7:02 am

http://hybridka.com/devec/4896644.pdf

Fusion Ed · Post by **Fusion Ed** » Mon Jun 02, 2008 11:30 pm

There is of course another option. Move the maf to a blow through design. I have done this with an SR20. It worked very well.

Fusion Ed · Post by **Fusion Ed** » Mon Jun 02, 2008 11:32 pm

Efi Performance wrote:Hey Ed,
We are getting a lot of reversion through the air flow meter on decel, (measured with scope) but was hoping that the ECU would dis regard any signals from the AFM when the throttle is shut and RPM above fuel recover rpm.

This I have had too, In the cases where its been a problem moving the maf away from the turbo has always solved it.

vn_luke · Post by **vn_luke** » Tue Sep 09, 2008 11:50 am

Probably a bit late - but it helped me....

from another forum:

Ok had a look at holden manual last night:
Fuel cut dependant on rpm, speed, temp, idle contact.
Will only get fuel cut if over 8km/h.
Fuel cut table will reduce injector pulsewidth to minimum if engine rpm ABOVE the table line for that temp.
Manuals have a higher cut & recover rpm.
Fuel recover table brings fuel back online once engine rpm (for that temp) is BELOW the line.
So definately gonna have to crawl under and fix ya vehicle speed sensor to get any cut.
Interesting is manuals have a 2 stage cut (3 cyl after .3 sec then rest after another .3sec) whereas autos cut all after .6sec.

[/quote]