What is Fuel Cut?

can boost leaks cause premature or delayed fuel cuts in a MAP based system as the 1jz?
does running with a boost leak in a Map based system cause any effect on AFRs or anything else other than overworking the turbo?
and is there any difference between fuel and boost cuts?

I am running a HKS Fuel Cut Defencer on my stock 7MGTE Turbo which has a stock AFM.
I am currently having a big problem with terrible gas mileage, running rough, bumpy acceleration, and very bad back firing inbetween shifts.
Do you suppose that the Fuel Cut Defencer might be bad or have anything to do with these problems?
Your comments very much appreciated.
Supra Derek
Oakland, California

Originally Posted by terror

can boost leaks cause premature or delayed fuel cuts in a MAP based system as the 1jz?
does running with a boost leak in a Map based system cause any effect on AFRs or anything else other than overworking the turbo?
and is there any difference between fuel and boost cuts?

Nope the 1JZ is an easier to tune simplified system.
The 1JZ meters its air inside the actual plenum.
What that means is the only way to unmeter the air in a 1jz is to disconnect the MAP sensor itself.

You can actually drive a 1JZ around if all the IC plumbling came off and it wouldnt hurt a thing you justy wont get boost.

At what point will boost cut come in? and at what afr does the ecu decide fuel cut is necessary? I am running 15 psi and still rich 10.3:1, (have supporting mods and dyno tune) Hit 18 psi untuned and no bc or fc?

560s, lex, safc2, lc1, aem tru boost.......

Here is a question for you guys. (I have thought about this for many years, but never tried to see if it will work.)

Run 880cc injectors, a Walbro pump and to meter the air, use two stock 7MGTE AFM's, but only pull the signal from one of them. (In theory, it should give you 1/2 the airflow reading for the TCCS.)

Both AFM's have sensors, and the air screws are set to the same depth, so the amount of air each flows is the same.

The air filter would be mounted on a 4" ID pipe, that is necked out to a flat plate with two retangular holes cut, along with the bolts to mount the AFM's. (So the honeycomb screen sides would be mounted to this flat plate, the AFM's back to back, one up, and the other sensor facing down, so you can get them close, but not touching, and allow room for the two 3" hose connectors on the other side.)

On the "exit" side of the AFM's, fabricate a 3" dual to single 4" "Y" pipe. This is then tied into a 4" feed pipe that hooks up to a 4" inlet turbo. (And has the BOV port(s) and PCV port, as well as a ISC port if you still run that.

Just like stock, all metered air would be in that intake pipe, and any bypass air would be put back there as well, so the TCCS would not be confused.

In theory, the TCCS would "see" the same Hertz rate needed to calculate the fuel, since the injectors are now 2x as large as stock.

And the fuel cut should be up near 24psi or perhaps even more as I was able to boost 12 psi easy on the stock AFM and 440's with no fuel cut. (At about 15psi, it would fuel cut all the time, and inbetween 12 and 15, it would cut if there was a spike the stock CT26 waste gate could not deal with. I think the stock waste gate is on the small side, and it's limited in how much it can open, no problem with 12psi or less, but a problem when you start to run higher boost pressure, and the exhaust gas pressure/volume increases as a result.)

Of course, a setup like this would have a larger turbo, and a better waste gate design. (Easy to handle say 21 psi on what is basicly a stock TCCS, and perhaps a Super AFC to fine tune, with an Ebay manifold, medium sized T4 turbo, and a solid motor with metal head gasket etc.)

What do you guys think? Any TCCS issues I have not considered? I'm wondering if it can even calculate at that low of a hertz rate for the rpm?

It would in theory, be about 1/2 of what stock is. So, if stock it's 34hertz at 2000rpm, on cruise control at 70mph, it would be about 17hertz at 2000rpm on cruise control at 70mph.

The trick is, when you are WOT, and 1250hertz boosting at 4500rpm, it's going to be about 625hertz at the same boost pressure, and 4500rpm.

The guy who's going to do this is at the stage where he's either going to buy 550's and run a Lexus meter, or go this route, and we fabricate the mounts/Y pipe needed with 880's. (We have the two stock meters/sensors.)

Signal might be too low for the TCCS to work with, plus the issues with timing at that point. A local had a 500HP+ 7M that a pervious owner used a secondary throttlebody to bypass the AFM to get more air in at higher boost levels and it was tuned with piggybacks. Was a dangerous setup so the guy disabled it.

How else could i avoid FC with my setup? 1.5JZ 550's, Walbro, AFPR, MPT61 Turbo and SAFC to tune. (Version 1).

^With out the use of a BCC or FCD? Or am i forced to use this until i go standalone? I would love to feel more then 13 psi of boost lol

i have an 89 supra with a turbo'd 7m but i am hittin fuel cut at full boost around 10 pounds of boost, and when i ease out of the gas it sllowly picks up, and also what do you mean AFM sorry if i sound stupid but im just tryiing to get into this i love supras

afm=air flow meter, Its the thing at the end of the intake tubing.

is fuel cut and boost cut the same? if not how are they diffrent when i use a fcd to run more boost?

Nicely put info, im just looking for some 1 to tell me, would this fuel cut kick in if I increase my boost from 10psi to 14psi (0.7bar to 1 bar). The engine is a 1g-gte and it has stock turbos. I think its a generation 3, coz the car model is 1991 late. Havent opened the injectors up, so do not know the color thing.

You forgot the part about hitting your face on the steering wheel when fuel cut happens.

Originally Posted by mk3tattoos

is fuel cut and boost cut the same? if not how are they diffrent when i use a fcd to run more boost?

Reread the article. "Boost cut" as you call it is when an overboost condition occurs and fuel is shut off. Fuel is shut off by the ECU for more than one reason, not just over boost.

Originally Posted by Supra Derek

I am running a HKS Fuel Cut Defencer on my stock 7MGTE Turbo which has a stock AFM.
I am currently having a big problem with terrible gas mileage, running rough, bumpy acceleration, and very bad back firing inbetween shifts.
Do you suppose that the Fuel Cut Defencer might be bad or have anything to do with these problems?
Your comments very much appreciated.
Supra Derek
Oakland, California

I would check for a blocked catalytic converter. The one in the downpipe gave me this problem. And at the same time my charge pipe coming directly off the compressor outlet was cracked 75% of the way around. So whenever the engine twisted from torque it would open the crack and the turbo would try to compensate and it would hit fuel cut. But when the throttle was released, the engine would torque back down and seal the crack, allowing a steady idle. It was eye opening to discover what was actually going on.

i seem to be hitting fuel cut at 10psi. from everything i've read, i should be able to see 13psi before it kicks in. i was told that if i disconnect the electronic boost sensor on the driver side fender, that i could go a little higher. i understand the dangers of this, i'm just wondering if it's true.

Boost sensor is not attached to the ECU, it only controls the guage.

that's kinda what i figured. thanks.

i have this problem as well plz help i need a afm asap

Are there other benifits to running the V8 AFM? I currently have one on my 91, and the idle sux. It's very hard to start, and I was thinking of switching back. I was also told to get the matching injectors, and it would cure this problem. I am just curious to know the benifits to this other than being able to turn up the boost before I make the decision to switch back.