Another Fuel Cut Question?!

[TokyoHead]

Okay I have an 89 mkIII with basic mods, 3" downpipe and exhaust, FMIC and hard piping, Intake, 12v fuel pump mod... You get my drift. I have my boost set at 10psi rite now and my car runs rich as hell but I still seem to be hitting fuel cut. I dont have any leaks in the piping or vacume. But the weard thing is that I only hit fuel cut in 3rd gear and sometimes 4th and most of the time its before I get to 8-10psi. It is really only when I have my foot more than 95% throttle that it happends. If I baby it then its fine... Is this still fuel cut or what elce could it be and how can I fix it?!


Thanx
ToKy0 H3aD

 

[speedfreak426]

I dont have a supra but my 77 celica has a 7mgte, I have the EXACT same issue.

FMIC, 2.5"pipes, 9(ish, maybe 9.5)psi, Big mouth 3" downpipe to full 3" exhaust, ARP studs torqued to 75 on a fresh OEM HG with the head well cleaned and prepped, and a generic cone filter.

I hit fuel cut once in a while in 3rd, 4th, and 5th gear at full throttle. Not every time, but mostly if I shift hard into third from a WOT run in 2nd. in 4th and 5th it will hit randomly at full throttle many times, gets pretty annoying.

Also, when I took my car to the drag strip i hit fuel cut in the bottom of second. Spun into second..... fuel cut, kicked back in and then more tire spin, spun through second........ shifted hard into third then more tire spin after after coming off fuel cut, spinning through third then fuel cut in 4th a few times before the traps. Still ran 12s though

That run at the strip was at 10psi and the fuel cut was extremely annoying and frustrating, after that run I turned the boost down to about 9psi(very close) cant be exact, just using an autometer gauge that I dont 100% trust, and dont really have much time to look at it when trying to keep the car going straight) My next run I hit fuel cut twice down the track, once in 3rd and once 4th (still runs 12s but I need more traction and I think I can get really low 12's or high 11s, I am running crappy 195/70/r14 all season tires @ 35psi lol)

I know I don't have any boost leaks becasue I have checked many times, I dont know if I should be hitting fuel cut still at only 9psi?

Over this winter I plan on tearing the motor down for some oiling system upgrades, possibly a MHG too, but I dont know if I will be able to go over 9psi.

 

[grimreaper]

you guys have stock afm's with border line fc boost settings and its getting colder and colder (assuming because its cold in tx), fc levels sound right.

 

[speedfreak426]

It was about 70-80deg when I was having the problems at the track. The car has been put away for the winter(dont want salt on the car and the car is now immobile, I think I shattered a shift fork...oops)

I guess I will have to go lex/550s/57trim. I just need to decide on a piggyback that I can handle as a tuning Noob

 

[forcefedsupra]

running too rich can cause you to hit it also. I learned this using a MAFT.

 

[TokyoHead]

IDK how it worked but I noticed that the guy who had th ecar before me shimmend the wastegate about 3 shims. I had it on boost controller pushing about 8-10 psi and ill I did was take out the shims and up th eboost on th econtroller. So Im thinking maby it could be caused by a bad wastegate or that maby open watsegatehas the same effects as a boost leak

 

[tweak666]

I had a problem similar to this a while back, turned out to be the wastegate flapper was siezed

 

[supradjza80]

IDK how it worked but I noticed that the guy who had th ecar before me shimmend the wastegate about 3 shims. I had it on boost controller pushing about 8-10 psi and ill I did was take out the shims and up th eboost on th econtroller. So Im thinking maby it could be caused by a bad wastegate or that maby open watsegatehas the same effects as a boost leak

An open wastegate does not have the same effect as a boost leak...not even close. If you are still measuring MAP at 9 psi, the wastegate is functioning properly if it is shimmed, or if you EBC is set to 9 psi.

Fuel cut is based on air flow into the engine, which for a given turbo, and RPM (and various other parameters - air temp, head work, IC and piping, etc etc.) is designated by the boost pressure.

I had problems with just an apexi intake, BIC DDP and a shimmed wastegate running 8-9 psi in cool weather (40-60 degrees (although I still need to check the crappy stock inter cooler pipes for leaks) and I would hit fuel cut in 2nd-5th gear. I am guessing my car is hitting fuel cut due to a boost leak due to my modification level (or lack of ) and other cars with similar or more mods not hitting FC until 10-15psi in similar conditions.

 

[supradjza80]

I had a problem similar to this a while back, turned out to be the wastegate flapper was siezed

So were you seeing more boost then desired? If the boost spikes it is obvious that the car will eventually fuel cut. more PSI = more flow. I am assuming everyone is talking about a car that holds the desired boost pressure but still fuel cuts. if it holds a desired level of boost, the wastegate is functioning properly...

 

[tweak666]

So were you seeing more boost then desired? If the boost spikes it is obvious that the car will eventually fuel cut. more PSI = more flow. I am assuming everyone is talking about a car that holds the desired boost pressure but still fuel cuts. if it holds a desired level of boost, the wastegate is functioning properly...

a bit more, like 13-15psi, but the build up was too fast, like right off idle. can't remember the code for abnormal turbo pressure, but thats what would pop up after the fuel cut. I overlooked that the OP is holding the desired boost.

 

[supradjza80]

^^^ that makes sense

 

[TokyoHead]

I know I have few mods but I rarely hear of people hitting fuel cut at 8psi unless they have a leak. IDK what was the problem with my car but it runs now without fuel cut and its colder now than before the fuel cut was eliminated. lol Does anybody know if the MAF on the 7m reads air flow or air density? Im kinda cerious on how in cold weather the air flow dosnt change but the air density does. Density is just unmetered air rite?!

 

[jdub]

It's an AFM on the 7M, not a MAF...the turbo uses a Karman Vortex type that senses volume and converts it into a pulse electrical frequency for the ECU to use. Density is not unmetered air...think of it as more air in a given space.

For this type sensor, denser air will produce a higher frequency telling the ECU more air (which is true) is entering the motor.

 

[supradjza80]

Jdub - Do you have good resource/webpage I can read that explains exactly how the AFM works on these cars? I have read a little bit about Karman Vortex AFM's but I still dont understand exactly how it works.

Also, isn't there an air temp sensor to provide a "gain" setting to the AFM's volume signal to account for density of the air? Since the AFM should measure the same volume or air, but the air in that volume is just more/less dense depending on temperature.

 

[jdub]

Read this:
http://www.autoshop101.com/
Specifically #24 - Engine Controls #1 - Input Sensors.pdf

One thing to keep in mind: Code 34, turbo pressure signal, triggers a fail-safe function of the TCCS...fuel injection is stopped. It's one and only input is the Ks signal from the AFM.

In normal operation, both the Ks and VTA (air temp) are used as corrections to fuel injector volume (duration). VTA "standard temp" is 20 deg C and it's correction is limited to +/- 20% of base injector volume. Ks is also used as an input for basic ignition advance angle above idle (IDL off).

For normal ops, you are basically correct...VTA acts as a gain to account for intake air density...kinda. But it has it's own correction in addition to how the ECU uses Ks to increase/decrease injector volume. For fuel cut, VTA has no input.

 

[Nick M]

The KV meter can not see humidity. Just an FYI.

 

[SideWinderGX]

Read this:
http://www.autoshop101.com/
Specifically #24 - Engine Controls #1 - Input Sensors.pdf

One thing to keep in mind: Code 34, turbo pressure signal, triggers a fail-safe function of the TCCS...fuel injection is stopped. It's one and only input is the Ks signal from the AFM.

In normal operation, both the Ks and VTA (air temp) are used as corrections to fuel injector volume (duration). VTA "standard temp" is 20 deg C and it's correction is limited to +/- 20% of base injector volume. Ks is also used as an input for basic ignition advance angle above idle (IDL off).

For normal ops, you are basically correct...VTA acts as a gain to account for intake air density...kinda. But it has it's own correction in addition to how the ECU uses Ks to increase/decrease injector volume. For fuel cut, VTA has no input.

I thought it cut spark, not injector pulse?

 

[jdub]

I thought it cut spark, not injector pulse?

You thought wrong

 

[SideWinderGX]

You thought wrong

Hmph, that's weird. I remember a big argument in some thread about this...

Research time!

 

[Pantaloon007]

The reason Toyota designed it to cut fuel and not spark is if you have fuel, and not spark, you could concievably get a detonation due to compression. Cut fuel and nothing ignites.

If there was an argument about this, then it was short and one-sided.

Fuel cut will happen at lower psi levels at lower temps because the air is more dense at the lower temp. You might want to rejoice in the fact that your TCCS is working properly to keep you from blowing the engine up. Of course, opening the AFM bypass is a way to fool yourself into thinking youre smarter than Toyota's engineers.

TH- how high are you altitude wise? Density also involves how far away you are from sea level.

It helps to think of density as "the heaviness of volume"