Trying to Solve Fuel cut - at wits end - this one will stump you

[ArmandoP]

Hey everyone. I will try to provide as much detail as possible. I have searched and searched and tested and tested, and now Im at wits end. I need some help figuring this out.

car: 89 turbo with a jdm 7mgte swap cause the PO was an idiot who thought it easier to swap than fix a BHG.

engine: post 89 7mgte with no emissions stuff

current modifications: turbonetics 62-1 hifi turbo with ported wastegate hole, BIC divorced downpipe with motorcycle muffler on wastegate pipe, minor head porting, lex AFM with K&N filter and Sonic Screw, 580cc injectors with clips, 4 inch MDC downpipe back exhaust, no emissions equipment (egr, canister, cat), no boost controller, Tial BOV with 11pound spring, 2.25" upper intercooler piping, 2" lower piping, Jtube bypass, electric cooling fan, autometer boostgauge in the dash.

The diag process starts back several months ago. While on the stock turbo with BIC downpipe and stock injectors and stock AFM I found fuel cut with a boost controller. No problem, back down the controller and Im good at this point. so good so far. Went to the dyno and saw my top end falling flat. MDC was nice enough to build me a one of a kind 4" exhaust to help resolve the lack of top end power and from this point I cannot get away from fuelcut. As soon as the exhaust was installed I removed the boost controller and was hitting fuel cut and running extremely rich.

Remedy #1: Lex AFM and 580CC injectors

still hits fuel cut.

Remedy #2: back out the Sonic screw to try to trick the AFM out of fuelcut so low
still hits fuelcut. Then I opened up the bov port on the accordion pipe to let in some unmetered air. no dice, still hits fuel cut.

Remedy #3: boost leak test.
found one small leak, fixed it. I drove the car all of about a mile before the stock turbo shat itself. so I never got to see if this actually fixed it.

At this point I already had the turbonetics turbo so I swapped turbos and installed the muffler on the downpipe so as to not get arrested every time I floored it. get the car back on the road and BAM, fuel cut. only now its atleast alot of fun right before it hits it. Boost comes on very strong at 3500rpm or so. Im thinking this car would be alot of fun if I could rev past 4500rpm... now it hits 14-15psi and slams fuel cut at that point without a boost controller.

remedy #4: boost leak test.... again
minor problem found air coming out from behind the compressor housing. I read on "the all knowing internet" that this is ok since it held pressure for a couple of minutes no problem. I also couldnt get the manifold to hold pressure, but this also appears to be normal.

remedy #5: replace 7pound Tial BOV Spring with 11pound tial BOV spring just in case the BOV was leaking
waste of time, still hits fuel cut and now has some surge to boot.

remedy #6: replace injector seals and upper manifold gaskets hoping they were leaking boost
no dice, still hits fuel cut.

remedy #7: remove J tube (Im reaching at this point guys)
still hits fuel cut, still runs rich.

remedy #8: pressure test intercooler
nope, no boost leaks found here. still hits fuel cut.

remedy #9: loosen up wastegate actuator.
It dawns on me that the car should not be hitting 14psi when the guy who sold me the turbo said the actuator was hitting 10psi on his car with the same actuator and no controller with the same dp and a 3" catback... got the wastegate actuator loose then adjusted it open a bit and set it so it was just barely loose. Looked inside the downpipe at that time to see if it might be hitting the wastegate flapper, nope, no interference found. Put the thing back together with a big smile on my face thinking I finally figured it out and went for a test drive and guess what?!?!?!?! The freaking thing still slams into fuel cut at 14psi with no boost controller.

Like I said, I need some help here guys. I have a dyno commitment at a meet in Indy on the 10th and Id really rather not dyno the Scion... Im hoping someone can post something simple Im forgetting.

Summary: fuelcut sucks, been troubleshooting for months please help.

AP

 

[Boost Lee]

So true.
I'm stumped.

As Armando has laid this out pretty clear, he/we've tried just about everything I can think of...yet this monster hiding in the closet still cannot come out.

Surely he's not the only one with Fuel cut issues as harsh as this.

Ideas for this poor man?

Jeff

 

[Zumtizzle]

Hows the hose that goes to the turbo? Is it fresh?

 

[Nick M]

Many of those things have nothing to do with fuel cut. It is a frequency that the airflow meter hits that triggers it. Nothing more. Well, other things effect that frequency, such as that big turbo you are running.

You have a code 34 right?

 

[tlo86]

what fuel pump

 

[Nick M]

The fuel pump has nothing to do with fuel cut.

 

[tlo86]

The fuel pump has nothing to do with fuel cut.

and?

 

[Nick M]

He questions fuel cut, and you said what fuel pump. Get it now?

 

[HommerSimpson]

no FCD just the lexas maf ?

 

[ArmandoP]

Many of those things have nothing to do with fuel cut. It is a frequency that the airflow meter hits that triggers it. Nothing more. Well, other things effect that frequency, such as that big turbo you are running.

You have a code 34 right?

remedies 1-6 and 8 deal directly with the amount of air being measured by the AFM either by way of bypass through boost leaks or intentional bypassing. this would most certainly affect the reading of the measured air by the AFM. Step 9 would directly affect the amount of air being sucked in the by the turbo...

I dont understand the point or the purpose of your post. Seems to me the only one that is a stretch is the J tube, but as I mentioned the car also runs rich...

I have not checked codes since I originally started diagging and again after I swapped the turbo. Back then all I remember was a speed sensor error because my speedo is not functioning properly. I will check the codes again in the AM. abnormal pressure (34) has appeared a few times, yes. That seems to be standard fare for fuel cut.

Please dont hesitate to tell me what I have missed. I am very frustrated with many hours into this one small problem, so my judgement can be clouded.

Hows the hose that goes to the turbo? Is it fresh?

Im assuming you mean the vacuum reference line for the wastegate actuator to the compressor housing right? (hose to the turbo is kind of vague) It was replaced when I swapped turbos. I will double check it in the morning as it is potentially possible to damage during install and I hadnt thought about that. Thank You.

what fuel pump

There is some speculation here. The previous owner said he put in a walbro, but he was an idiot, so I am assuming its a stock 89T fuel pump. Have not verified either way. fuel pump shouldnt affect fuel cut, however, I see your point, runing a BFT with a stock pump is a bad idea... thats why I have no boost controller on it. I simply want to run 8-10 psi until the rest of my fuel system catches up. (weldon, fpr, etc). I want to fix fuel cut before I upgrade the pump, etc.


Thanks for your help so far everyone. I will check these things and get back on here tomorrow. Im hoping someone says something so ridiculously stupid and simple that I have completely spaced and it fixes it. Sometimes we tend to go for the complicated stuff first and miss the easy stuff.
Armando

 

[ArmandoP]

no FCD just the lexas maf ?

I have no FCD. I struggled with that idea. I want to fix the cause of the problem and if I wish to defeat it and blow it up later, then I can do so in a blaze of glory on the dyno or the track on purpose. FCD is good for raising fuel cut, but not necessarily for masking existing problems. My fuel cut seems to be caused by an actual problem that must be fixed, not masked. Thats how you blow em up good. ask me how I know.

 

[HommerSimpson]

I have no FCD. I struggled with that idea. I want to fix the cause of the problem and if I wish to defeat it and blow it up later, then I can do so in a blaze of glory on the dyno or the track on purpose. FCD is good for raising fuel cut, but not necessarily for masking existing problems. My fuel cut seems to be caused by an actual problem that must be fixed, not masked. Thats how you blow em up good. ask me how I know.

What is the max the lexas maf will allow ? I thought i saw 16psi someplace.. not sure so im asking...

 

[ArmandoP]

What is the max the lexas maf will allow ? I thought i saw 16psi someplace.. not sure so im asking...

depends entirely on the turbo youre running. its not based on psi but on airflow measured in Hz. what that measurement actually is I dont know offhand.

 

[jdub]

Nick is correct. Fuel cut is a direct function of the AFM and the volume of air it senses being flowed to the motor. The FPR, J tube, injector size, etc has nothing to do when the motor hits it, and a code 34 will be set when it does...however, fuel can determine a rich AFR condition. Hitting fuel cut at 14 psi with a Lex/580 combo is a bit low on average. Typical is 16-18 psi depending on OAT and altitude...it can be lower on a very cold day, especially at sea level.

You have a minor boost leak some where. A couple of other things to look at:
- Intake manifold and TB gaskets
- 3000 pipe to TB coupler (these crack and are hard to see)
- Check valve under the ISCV
- Confirm the BOV is closed with a good seal

 

[ArmandoP]

Nick is correct. Fuel cut is a direct function of the AFM and the volume of air it senses being flowed to the motor. The FPR, J tube, injector size, etc has noting to do when the motor hits it, and a code 34 will be set when it does...however, fuel can determine a rich AFR condition. Hitting fuel cut at 14 psi with a Lex/580 combo is a bit low on average. Typical is 16-18 psi depending on OAT and altitude...it can be lower on a very cold day, especially at sea level.

You have a minor boost leak some where. A couple of other things to look at:
- Intake manifold and TB gaskets
- 3000 pipe to TB coupler (these crack and are hard to see)
- Check valve under the ISCV

first suggestion already done. seemed fine, replaced them anyways.
second suggestion was done when I did multiple boost leak tests. even removed it from the throttle body and pressure tested it that way once to be sure.
havent checked the ISC valve. What should I be looking for? Be a little more specific and Ill check it in the morning. Thanks for your help.
I have checked the BOV several times using both soap and water, carb cleaner and also taking it apart and looking for surface imperfections. no apparent problem. I know of no way to be 100% sure though... hence why I put a bigger spring in it.

for the record, I understand what causes fuel cut. which is why I said the J tube was a stretch. believe me, fighting something for this long makes one question their own expertise and change things like J tubes in hopes of strange but simple fixes.

 

[jdub]

Under the ISCV there is a disk with "spokes" on it...looks like a little mag wheel. It's a check valve, the "spoke" side should be seen when the ISCV is removed. Make sure the ISCV gaskets are good. These can get blocked open with carbon or just get old. This too is a bit of a stretch...usually what you see with a check valve is idle issues.

Is the EGR port on the intake manifold sealed good with a gasket?

Nick also hit on another point...a bigger turbo is going to flow more air at a lower psi. What were you hitting FC at before you swapped turbos?

 

[Supra60-1]

I went through fuel cut problems at one time, before my car was fast. It felt like the exhaust was stopped up and it was. I cut out the pre-cat on the downpipe and it was stopped up and contaminated with green buildup.

But it still got cut, eventually I looked at the rubber boost hose coming off the compressor housing. Under light acceleration and idle the crack was closed, but once the engine torqued up under boost it would lift on the passenger side and open the crack and I would get fuel cut like crazy. It was so hard to see the crack because it was up under the clamp on the compressor outlet.

I see you have aftermarket boost pipes but, Check all of your unions to and from the intercooler.

Thats the only thing that ever made me hit fuel cut for seemingly no reason.

 

[xcyanideslethalx]

im just gunna throw it out but do you have a wideband?? or something thatll tell your conditions?? well on WOT is it rich?? is so it could be bcause your hitting overboost? thats my problem

 

[Doward]

How rich is 'rich'? (just wondering on the off chance someone sold you a lexus afm that was actually a stock 7M afm)

Back boost down to 12psi and see how it runs there. Don't keep slamming into FC - run a couple runs @ 12psi, and pressure test the entire system (from the turbo intake all the way through) with the engine fully warmed up (lessen the blowby from the rings) - you should have an easier time locating your leak.

 

[GrimJack]

Where is your boost gauge getting it's signal? And are you still using the stock intercooler? How much pressure did you put into the system when you pressure tested it?

I realize these things do not sound like they could cause the problem, but bear with me, they fill out the background and make it easier to work out the entire system in my head.

PS: Have you tried another set of AFM electronics? Your Lexus still has the honeycomb screen in good condition, right?

PPS: We LIKE tough problems like this. It's a challenge that makes it worthwhile to spend effort solving it. Unlike a lot of posts... If I never see another post that starts with "How many b00st is the 7Mgtte RLY gud 4?" again, it will be too damned soon.