Fuel Cut: What is it? How do I fix it? [merged]

[Slanted J]

All fuel cut threads are combined here.

I've tried doing a search but to no avail.

I was just wondering what Fuel Cut is. I've heard it tossed around alot, but do not actually know the details behind it. And, naturally, I can gather that its when the fuel is cut off, but when and why.


Thanx,
Jon

 

[SpeedAddict62]

true, but I figure the more I know about whats going on the better, and loggers are cheap. I can't mess with fuel yet, becasue I don't have anything to tune with yet. Either way, I'm not worried about being so rich (its the factory tune, its safe) but I want to know what is causing me to lose power.

 

[MrBoxx]

Get a logger. Get an AFPR. Do a boost leak test. Recirculate your BOV or buy one that is recirculating. Do another boost leak test. And get a logger.

 

[kidrabbit84]

Hi all,

I am new to the DSM world, new to turbos, but not new to cars. I have an 84 FSP Rabbit GTI that I have competed nationally and locally with SCCA for 9 years now, and locally with NASA for 1. I have done the following to the car: installed SDS efi (tune myself), fabricated my own intake manifold from steel tubing/plate, assembled the 12:1 Compression ratio motor, ported head, Koni double adjustable shocks (grafted front struts to use same shock out back), 25 mm front swaybar, solid motor mounts, installed Quaife LSD, etc..

I have searched posts and the FAQ, but most of them are vague on description of the problem. Ex: I floor it and at 12 psi the car pops… not very descriptive. Hence I want to post a new thread.

MY PROBLEM:

I have a 97 GSX, 85k, which I bought from a Nissan Dealership. Unfortunately the car had been molested. The previous owner put a 2.5 exhaust, front mount intercooler, TurboXS Dual Stage Boost controller, and TurboXS Type H-RFL blow off valve. Since I have gotten this car I have been cleaning up all sorts of hacked stuff, and finally got rid of all the gremlins. BTW I just got done overhauling the trany in my garage…new 1st, 2nd, and 5th/rev syncos/hubs…new bearings. I think the previous owner did not know how to use a clutch. He didn’t know how to adjust it - that’s for sure.

Last weekend I decided to remove the catalytic converter that the Nissan dealer installed. This thing bottled down to a 2” cat. I cut it out and welded in a 2.5” straight section. I also have an O2 sensor simulator wired into the rear sensor to fool the computer in thinking there is a cat:
http://www.o2sensorsimulator.com/
BTW this works nice for the price. This was wired in prior to exhaust mods and no issues.

I went and test drove that car with the free exhaust and what a difference. Boost comes on strong…free exhaust = . I had to reset the boost controller because it would spool to 20 psi and fast. My goal is to hit 18 with high octane in the tank, 15 any other time.

With the old exhaust I used to boost to 18-19 psi at Bandimeere on 100 octane with no issues. Now I hit 17 psi and I get a series of “cuts”. It almost feels like running over rumble strips on the highway, or that I have water in one of the plug wells. Not as hard as a rev limiter though. The first cut is hard, then a series of small ones as I try to continue accelerating. I thought it might be leaning out so I put the wideband on. The car has an air/fuel ratio (AFR) of 10.6/1 under boost until 17 psi. Once I hit the “cuts” the AFR drops to 12.2/1. Hence I don’t think it is running lean, but turning off fuel.

I pulled the plugs and they had a few speckles on the insulator on cylinders 3 and 4, but nothing major. I tried running the NGK BPR7ES plugs (btw…nice Spark Plug FAQ post in FAQ forum), yet no major difference.

Odd things making me scratch my head:
-The car seems to have this cut all the time when the water temp is 180-210 deg. Once above 210 it will hit 17 psi ok.
-The coolant, air charge, and MAF sensors meet factory specs (checked them 2 weeks ago for a cold start issue that is fixed).
-It appears to have a nice AFR under boost per the wideband.
-If I have the boost come on rapidly, 0-17 psi in 3 sec, it will cut. Yet if I do it over a longer time, like 6 sec, it will not cut.
-I do not hear any detonation.

Have I hit the fuel cut? What does this feel like (FAQ articles do not state)? Does boosting quickly produce this fuel cut? Something else?

Thanks,
Adam

 

[DGajre777]

One of your problems is the TurboXS Type H-RFL blow off valve. This blow off valve is made to vent to atmosphere while the stock blow off valve is made to recirculate. You need to get a blow off valve that is recirulated or switch to a GM MAF/MAFT setup that allows you to vent the blow off valve by moving the MAS from pre-turbo to after the turbo on the upper intercooler pipe.

I would also suggest that you do a boost leak test which can trigger fuel cut. A fuel cut feels like you hit a brick wall. If you ever hit it, you will know that it was fuel cut.

Because of your a/f ratios, I would also suggest that you check to see if you are still using the stock fuel pump and if you an a good AFPR. The stock AFPR is on a 1:1 ratio, it will increase fuel pressure by 1psi for every 1psi of boost. If you have a crappy fuel pressure regulator like the B&M one, then it won't raise it on a 1:1 ratio.

 

[tylermartindc]

Oops, nevermind.

 

[kidrabbit84]

I am going to do a boost leak down this afternoon. I will try that first...my guess is there are some pin hole leaks.

Assuming I have a stout fuel and intake system, what 1/4 mile times do you start to have to worry about true fuel cut?

Thanks,
Adam

 

[DGajre777]

I am going to do a boost leak down this afternoon. I will try that first...my guess is there are some pin hole leaks.

Assuming I have a stout fuel and intake system, what 1/4 mile times do you start to have to worry about true fuel cut?

Thanks,
Adam

Fuel cut is not the pump or injectors running out of fuel, it is the ECU cutting fuel because it senses too much air. If you are asking at what 1/4 mile your stock pump and injector runs out of fuel, that depends on many factors. If you plan on doing a lot of 1/4 mile racing, I would suggest at a minimum the Big T28 turbo that www.forcedperformance.net sells with a 190 fuel pump and 550cc injectors (or EVO III fuel pump and injectors). You'll also need some form of fuel control and a logger to tune it.

You really really need to do some reading on venting your BOV using the stock 2G MAS. All your problems might be related to the venting BOV.

 

[1gnow2g]

Adam:

As a quick test, richen up the MAS by exactly
one turn. If the problem goes away, then its
defiantly fuel cut. As to the potential cause
I would think that you either accidently fixed a
boost leak, or got some oxygenated gas in there,
or, less likely, the oxygen sensor is fouled or
drifted.

 

[DGajre777]

As a quick test, richen up the MAS by exactly one turn. If the problem goes away, then its
defiantly fuel cut. As to the potential cause I would think that you either accidently fixed a boost leak, or got some oxygenated gas in there, or, less likely, the oxygen sensor is fouled or drifted.

How exactly do you richen up the stock 2G MAS by one turn? The MAS is a sensor, it is not a fuel control device.

 

[Dan97gst]

I think i have the same problem, I was doing a 0-60 run the other day and I let the car warm up and drove around a little before doing the run. When I went for the first run there was a pop when boost hit in, in second gear and then agian at the end of second gear. I did three more tests after that one and it never did it again, and the car was running stronger than the first run. My car is stock other than a hard pipe intake and 1g bov. Is this some type of fuel cut, or I was thinking it had something to do with it just being really cold out. Thanks

 

[stuckwithgsx]

is there any chance that it could be ingintion with coils not being able to push out spark fast enough as he stated if he slowly gets to ...17? psi it will not cut and if he stomps on it then it cuts......my car did that and i put some cop's on and it ran a whole lot better. although i'm sure i don't have the experience and i'm sure that these other ideas are good just sayin it could be ignition maybe even wires?

 

[DGajre777]

Dan97gst - A pop is not fuel cut. There are many factors that could have cause what happened. If it happened once and didn't happen again, I wouldn't worry about it. It could have been dirty or contaminated fuel too. If you are nervous about it, try a bottle of Super Concentrated STP fuel injector cleaner (black bottle) or STP Fuel System cleaner (grey bottle).

stuckwithgsx - Yes, the ignition system and/or bad wires can cause a pop, but it won't cause fuel cut.

 

[oldman]

Adam:

As a quick test, richen up the MAS by exactly
one turn. If the problem goes away, then its
defiantly fuel cut. As to the potential cause
I would think that you either accidently fixed a
boost leak, or got some oxygenated gas in there,
or, less likely, the oxygen sensor is fouled or
drifted.

Please only post on subjects you know, for sure.

1. Stock 2G MAS is not adjustable.

2. Fixing boost leaks will help delay fuel cut, not induce it.

3. O2 sensor has absolutely no effect on WOT (open loop operations).

kidrabbit84, there are three things for you to consider.

1. As DGajre777 has already said, boost leak is the #1 cause for fuel cut. Fuel cut has little to do with the fact that you're probably also running out of fuel.

2. Assuming you still have the stock pump and injectors, I would not go over 16psi without a logger. You can very well be knocking running 17-19psi due to fuel starvation, back it off until you have the ability to monitor.

3. Where is your mbc tapped? BOV line? further more, your BOV needs to be re-circulated.

 

[kidrabbit84]

Excellent info all!!! Thanks alot Here is the verdict.

So yesterday I did a leak test. I put a plug at the turbo inlet that utilized a PVC pipe plug, 2” ID hose, and a tire valve stem. I then pulled the intake tube at the throttle body and plugged it with a 2.25” Pipe cap. I epoxied in a brass barb and tapped it into my pressure gauge.

I found out that my BOV was cracking open at 13 psi. I decided to mess around with the BOV since it came with the car. I put about 1” worth of shims in the BOV to increase preload on the valve, but got a max holding pressure of 16 psi, 4 short of my goal. Additionally 15 inches of vacuum by my hand pump wouldn’t open the valve anymore due to excessive preload on the spring.

I am planning on getting rid of the TurboX RFL and getting a recirculating style BOV. TruboX should call this thing the “Really Friggin Lame” in stead of “Really Friggin Loud”.

ONE MORE QUESTION:
I did the leak test, plugged the pressure at the throttle body, and noted that I was getting some air out of the breather tube on the valve cover. Is this normal? The engine was cold, probably at 35-40 degrees F. I assume the charge is leaking through the seals in the intake side of the turbo and into the oil feed passage. This turbo makes some noise if I boost when it is cold, but gets quite when up to normal engine operating temp. Are the seals worn out? Or it this normal due to the massive amount of thermal expansion that the turbo sees (0 deg F to 230 deg F)? I have also noticed some oil coming out of the oil fill cap…it never did this before till I freed up the exhaust. Is this normal too?

Thanks,
Adam

 

[kidrabbit84]

I will also start looking at fuel system and ECU options (DSM Link looks cool with a EPROM ECU)...

 

[DGajre777]

You are not supposed to boost when your car is cold, wait until the engine warms up to normal operating temp. As for oil coming out of the oil filler cap, if it is stock, it has a gasket at the bottom, you might need a new one.

 

[97 GS-T]

ive read other posts but none of give alot of info. hers my problem, when im at wot in 2nd gear at 5-5800 rpms fuel cuts. at wot the car boosts to 22 psi with the mbc set for 15, the wastgate opens at 10 psi. i have a 255 fuel pump, and do not have a fuel pressure regulater, its stock.

 

[scottsee]

ive read other posts but none of give alot of info. hers my problem, when im at wot in 2nd gear at 5-5800 rpms fuel cuts. at wot the car boosts to 22 psi with the mbc set for 15, the wastgate opens at 10 psi. i have a 255 fuel pump, and do not have a fuel pressure regulater, its stock.

Fuel cut is when airflow per/rev = 2.55 or higher. You are encounting boost spikes and/or boost creep depending on the durration. I'm really not sure what your asking though..

 

[codyw]

you should have a regulator if your running that pump. also what turbo are you running

 

[oldman]

Sounds like you have boost creep for sure.

 

[boostta]

Sounds like you have boost creep for sure.

I have same problem here,after 5500rpm hit fuel cut.
First at 16psi now at 13psi.
What can we do about boost creep???

 

[oldman]

I have same problem here,after 5500rpm hit fuel cut.
First at 16psi now at 13psi.
What can we do about boost creep???

Fuel cut doesn't always = boost creep, 97 GS-T has boost creep because his boost gauge told him so. You on the other hand don't have boost creep and you need to start with a boost leak test.

 

[boostta]

Fuel cut doesn't always = boost creep, 97 GS-T has boost creep because his boost gauge told him so. You on the other hand don't have boost creep and you need to start with a boost leak test.

Ok, I will do that.
Thank you

 

[oldman]

No problem.

 

[rota92]

Hey guys, I have read through a few topics on fuel cut but nothing has really led me to a conclusive fix.

I have a 90 tsi

Evo III 16g
SBR Stage 2 Head
272's
Front mount
Greddy type RS
GM MAF
MAF Trans
550's
AFC
Forged pistons
3" turbo back
no cat
Aeromotive FPR

I think thats all as far as main engine stuff goes, I bought the car pretty much like this and its always had fuel cut.

Its def fuel cut, and its def obnoxious

I am running about 20 psi right now (MBC) and it will spike to about 22 psi.

I am not looking for the 'cheapest' way to make it run better, I want to know how to stop it correctly.

On my hondas I just throw a good fuel managment system on them to tune from the get-go so there is never any boost cut. I would imagine it would be the same for the DSM's.

I will never go to Victor Research ever again, but Pruven is the only other option I know of to do a tune for me. I dont want AEM due to its price, although I have it on my other cars and love it. My talon just isnt worth it

Thanks a lot guys!