fuel cut question

[gsxr141]

i'm hitting fuel cut randomly and had a couple questions.
my mods are cone filter, bic ddp and full 3" exhaust. i have a cheapo boost controller set to 9 or 10 lbs. sometimes the car runs like a raped ape, and other times it hits fuel cut.
the question is.......... can i hook up a fcd if i plan on running less than 12lbs? i understand the issues with using it OVER that, i just want the car not to hit fc.
i currently have a lex afm, 560's (that actually flowed 620), profec b, walbro 255, and an afcII, not installed yet. i'm just waiting to get an afpr kit to install everything. i'm just looking for a temporary fix until the install.
thanks..........

 

[Dirgle]

No, by itself the FCD is very dangerous. It needs a host of supporting piggy backs to operate properly. It clamps the signal at a set level, past that the engine has no idea how much more air is entering the engine, and therefore can't add the correct fuel to compensate. Using the FCD with out supporting fuel computers is a dead engine waiting to happen, and it usually happens quickly. If you are hitting fuel cut the set maximum amount of air is passing the air flow meter, and fuel cut is stepping in to save the motor. So the question is why is the maximum amount of air passing the meter.

What boost gauge are you using?

 

[gsxr141]

i have an autometer. i was under the impression that you could get 12 or 13psi without hitting fc. i can get to 10 or so, but it's just weird that it's intermittent.

 

[Dirgle]

Fuel cut doesn't happen at a set PSI. It happens at a set volume of air. So denser air(cooler, or at sea level,) will cause fuel cut to happen at a lower PSI value.

The newer autometers have become more reliable, but the older ones were unreliable. The reading would vary almost every time, so you may be closer to 13psi and not realize it. Also manual boost controllers are very finicky to weather/temp changes. You may have it set low during the day, but during the evening it might go right past it's setting. And with the cooler, denser evening air, FC will happen at a lower boost setting.

But if you're confidant in your boost readings, and know you have the boost controller dialed in and operating right, then you may very well have a small boost leak somewhere. But I would start looking into that after you have verified your gauge and controller.

 

[gsxr141]

thanks. i'm going to check into a boost leak. i know the clamp at the throttle body is bad, but i totally forgot to change it. that could be the whole problem.

 

[Dirgle]

Could be, report back and let us know, even if it fixes it. You may help the next guy coming along looking for answers.

 

[Dan_Gyoba]

An FCD isn't really a good idea.

About 10 PSI is what you can reliably get to without hitting fuel cut. More than that can happen when it's hot, or at high altitudes. Calgary, Alberta at an altitude of about 3700 ft, most people can reliably get to 12 PSI.

As stated, it's airflow, not pressure. Also, 15 PSI at 3500 RPM is still less air flow than 10 PSI at 6500 RPM. This could be another reason why fuel cut seems random.

12 PSI is starting to hit the upper limits of the stock CT-26. If you want to play there, then maybe a Lexus AFM and 550cc injectors is called for. Then you can upgrade the turbo later.

 

[Nick M]

You have a problem. Abnormal boost pressure leading to fuel shut off will not happen at 10 psi with a stock meter, let alone the larger Lex meter on a stock turbo.

 

[radiod]

I was gonna say....with the Lexus AFM and everything running properly fuel cut is usually ~18psi IIRC. Did you check your codes to be sure it's a fuel cut code and not something else? Beyond that start looking for boost leaks.

 

[Dan_Gyoba]

Whoops. Missed that he has the Lex.560s already.

It's either a boost leak or a TPS problem.

 

[hvyman]

^ says there not installed yet.

 

[Turbo Habanero]

^ says there not installed yet.

I was gonna say....with the Lexus AFM and everything running properly fuel cut is usually ~18psi IIRC. Did you check your codes to be sure it's a fuel cut code and not something else? Beyond that start looking for boost leaks.

On another note doesn't the size of turbo your running affect how much boost you can get to before fuel cut.

I was really surprised to find out I can run 19psi on my 60-1 CT26 without fuel cut on some nights most nights I can only run 17psi.

 

[Dan_Gyoba]

Yep. A more efficient turbo will make pressure with more mass. A less efficient turbo will make pressure with more heat. The stock turbo will not flow much more at 16 PSI than at 14 PSI, since at that point it's basically just making the intake hotter.

 

[gsxr141]

quick update.....
i changed the two clamps at the throttle body, and i took the boost controller off. i ran the vacuum hose from the turbo directly to the wastegate.

on the highway it hit 8 psi and rose to 10, with no fuel cut. when i launched from a light, it hit fuel cut when i shifted into second. i'll doa little more checking and post what i find.

 

[Turbo Habanero]

If i shifted quickly I noticed I'd hit feel cut but if i just shifted a little slower it would be fine.

Something you might wanna check into.

 

[IndigoMKII]

When you shift it hard, it'll cause a higher load on the engine thus producing more psi. Assuming this is the stock turbo I'd say do a boost leak test..

Are you also assuming this is fuel cut? Is the CEL flashing when it happens?

 

[harley1985]

good thread guys, lots of good info. i dont mean to highjack it but i gotta ask. i seem to this same problem. im pretty much all stock other than a MBC and a boost gauge. my car wont go past 6 psi. anything after that the CEL comes on and it sputters out. (stays running) anyhow it happened after i installed that MBC. i checked the psi against the stock boost gauge and it read the same. currently have 3 hoses on order from toyota. 71 dollars lol once i get those installed ill see it it was just a boost leak. i guess my question is why does my CEL come on and is this fuel cut?

 

[gsxr141]

when you shift it hard, it'll cause a higher load on the engine thus producing more psi. Assuming this is the stock turbo i'd say do a boost leak test..

Are you also assuming this is fuel cut? Is the cel flashing when it happens?

yes, i am assuming it's fuel cut. The engine light does come on when it happens. I hate to admit it, but i suck at figuring out the flash codes.

 

[radiod]

Use this:
http://www.cygnusx1.net/Supra/TechTips/engine_error_codes.aspx

Just to be sure