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Boost leak? fuel cut? compressor surge?

2g-TurboJo

Probationary Member
6
0
Sep 12, 2012
Omaha, Nebraska
I just got a 98 gsx auto, really clean body and interior.
Engine has been messed with and modified a little.
138kmiles

Mods include..
Full intake and intercooler kit
16g turbo, full exhaust
Dump pipe
hks bov
Radiator
150 shot of nitrous
Said a 272 cam.. idk if that's true of not
Vis vented carbon fiber hood
Full exhaust

I'm not sure what it is but when I punch it, it takes a little to build boost and when it does it pulls pretty hard and build boost fast but once it hits around 5k/15psi (4800-5200) it cuts hard like some one is hitting the breaks and cutting power. not sure how to resolve this.

thank you
 

Mitsubishigst1999

Proven Member
37
0
Sep 10, 2012
Woodstock, Virginia
check your bov for vaccum leaks all your pipes for any holes. Is your fuel pump stock are your injectors bigger and do you have an AEM wideband gauge to check if you are way too rich and then that could be why your hitting fuel cut, and as you already know that is very harmfull to your engine.
 

bryanwheat

DSM Wiseman
7,110
177
Aug 16, 2004
Columbia, Missouri
check your bov for vaccum leaks all your pipes for any holes. Is your fuel pump stock are your injectors bigger and do you have an AEM wideband gauge to check if you are way too rich and then that could be why your hitting fuel cut, and as you already know that is very harmfull to your engine.

Fuel cut has nothing to do with running rich, fuel pump or fuel injectors. It is caused when the maf sensor is overrun, which is usually caused by a boost leak.

I just got a 98 gsx auto, really clean body and interior.
Engine has been messed with and modified a little.
138kmiles

Mods include..
Full intake and intercooler kit
16g turbo, full exhaust
Dump pipe
hks bov
Radiator
150 shot of nitrous
Said a 272 cam.. idk if that's true of not
Vis vented carbon fiber hood
Full exhaust

I'm not sure what it is but when I punch it, it takes a little to build boost and when it does it pulls pretty hard and build boost fast but once it hits around 5k/15psi (4800-5200) it cuts hard like some one is hitting the breaks and cutting power. not sure how to resolve this.

thank you

Do a boost leak test and back the boost down a little bit.
 

2g-TurboJo

Probationary Member
6
0
Sep 12, 2012
Omaha, Nebraska
***New update***

I found a rheostat switch connected to my barometric pressure switch wire right off my ecu, that determines how much fuel to go in (fuel ratio) right? like lean and rich. I dont have a fuel ratio gauge so how would I know where the proper placement for it would be?
-I also forgot to mention it has a aftermarket fuel pressure regulator

I turned the manual boost gauge all the way down and did some runs and slowly turned it up and here are my results
7lbs, 0-cut out at and rpms.
10lbs, 0-cut out at any rpms.
12lbs, 0-cut out at any rpms.
14lbs, cuts out about 6250 rpms
16+lbs, cuts out at about 5000 rpms.

I have noticed while holding the gas to the floor with the boost set above 10lbs that it should stay where it was set at while in gears (and since its an auto, it should always be the same when at boost peak) but while flooring it boost rises to peak the slowly drops to between 5-7lbs while under full load.
-Boost leak? Intercooler system leak? Bov leak?

It has a stock wastegate to dump pipe and it sounds when boost is rising fast its just dumping it out and not much power is being gained. Its a auto so it takes a while for launches but when the turbo starts to kick in the rpms jump from like 2200 to 6200 in like less than a second??
-Is that normal in these cars?



Does it cut out completely or does stumble? When i say stumble i mean like bog up and down?


The boost was set about 17, I would punch it and it would take off pulling me against my seat then when it cuts out its like someone is slamming on the brakes really hard and boost would drop then rpms would go from 5000 and up and once boost got high again it would cut out again.


check your bov for vaccum leaks all your pipes for any holes. Is your fuel pump stock are your injectors bigger and do you have an AEM wideband gauge to check if you are way too rich and then that could be why your hitting fuel cut, and as you already know that is very harmfull to your engine.


I just texted the last owner and I guess it has a huge 390 LPH Walbro fuel pump. But the rest of the fuel system is stock. No wideband gauge.


Fuel cut has nothing to do with running rich, fuel pump or fuel injectors. It is caused when the maf sensor is overrun, which is usually caused by a boost leak.



Do a boost leak test and back the boost down a little bit.


Whats does it mean when the maf sensor is overrun? I remember on my last gsx I was doing some tuning runs (getting on it really hard) then I blew a intercooler pipe (actually broke the pipe) 30+ psi, my gauge didn't read any higher than 30psi and after that it was like dumping fuel, I was watching the gas gauge go down trying to make it home and it was spitting 4ft liquidy flames out the exhaust. Is that the maf being overrun?
 

ramsack

Proven Member
3,286
16
Dec 27, 2007
West Lawn, Pennsylvania
I found a rheostat switch connected to my barometric pressure switch wire right off my ecu, that determines how much fuel to go in (fuel ratio) right? like lean and rich.

No, the MAF determines that.

-I also forgot to mention it has a aftermarket fuel pressure regulator

Its a auto so it takes a while for launches but when the turbo starts to kick in the rpms jump from like 2200 to 6200 in like less than a second??

In what gear(s)?

The boost was set about 17, I would punch it and it would take off pulling me against my seat then when it cuts out its like someone is slamming on the brakes really hard and boost would drop then rpms would go from 5000 and up and once boost got high again it would cut out again.

Sounds like fuel cut. Since you have mods that increase airflow, it's likely because of the mods, but still test for boost leaks.

Whats does it mean when the maf sensor is overrun? I remember on my last gsx I was doing some tuning runs (getting on it really hard) then I blew a intercooler pipe (actually broke the pipe) 30+ psi, my gauge didn't read any higher than 30psi and after that it was like dumping fuel, I was watching the gas gauge go down trying to make it home and it was spitting 4ft liquidy flames out the exhaust. Is that the maf being overrun?

No, that's not the correct term. The MAF is fine. It's the computer that determines when to hit fuel cut. The MAF can measure a hell of a lot more than stock airflow, so it's not being overrun.
 

Greedy Pimp

Proven Member
253
4
Mar 27, 2009
Newark, California
It's fuel cut, the maf is being over runed. What you need is dsmlink or some way to trick the ecu in to thinking there is less air coming in the intake like a safc. You would need to tune your car with eather one an it would be wise to get a wide band o2 setup. Are you still on stock injectors?
 

AlwaysDTF

Proven Member
72
0
Sep 9, 2012
Prattville, Alabama
believe me when i say that what i'm about to say is not being said in a **** kind of way:

First, it's 93.7% fuel cut that you're experiencing when you hit that brick wall at high boost. this is simply self-explanatory but if you're still like " uhhh, WTF ?! ", you're MAF is telling you to " cut the ####ing boost down before i detonate on your ass ". i'm not sure how this kid was was spraying nitrous at 16+ lbs. and not blowing shit everywhere but touche to him.

i'm no dsm genius at all but if you want an opinion/advice, i think you should rip it apart and sorta start fresh. i'm not saying take the motor completely out & build it; but take all the things out that this dude put in because in reality, you have no idea what the hell this guy's done to this car & if not taken seriously, it could backfire in your face mate.

btw, turn your boost down to fix that problem & keep it down ( like below 12lbs ). it'll fix the " brick wall " feeling but may not fix any other problems you might be having.
 

ramsack

Proven Member
3,286
16
Dec 27, 2007
West Lawn, Pennsylvania
The MAF isn't saying anything about boost. It reads airflow. You could throw on a large turbo, run stock boost, and definitely hit fuel cut. People can even start hitting fuel cut by just putting on a 16g, exhaust, and intake without ever modifying boost pressure. You could have a boost leak and the MAF will still read the air despite it not getting to the engine and hit fuel cut that way. The air could be cold enough outside to go over the fuel cut tipping point but be fine when it's warm outside. It's protection against going lean, not against detonation, which has to do more with timing. Why should he tear the entire engine down and build it? WTF
 

donniekak

DSM Wiseman
5,614
950
Mar 23, 2008
Surprise, Arizona
I didn't say that. Running lean more melts things than creates detonations.



No. The ECU is blind to that.

Fuel cut is triggered by the ecu seeing over a certain hertz from the maf. When you burn EPROM chips, tune with link, tuner pro, or flash cable, you have to raise the airflow cap in the ecu, or you will hit fuel cut.

Maf over run is a different problem. It is causeed by a 1g maf not being able to read over 30lbs/min, or a 2 g maf being pushed over 50lbs/min. It caused erratic afrs, and engine damaging lean spikes. It can be detected by looking at the raw hz signal from the maf, and it becoming erratic. It does not feel like hitting a brick wall, like fuel cut.

No. The ECU is blind to that.

This is the stock 1g ecu fuel cut code.
D6 A0 4F 05 05 05 87 A2 11 04 1D 8D 24 02 DD 8D
 

AlwaysDTF

Proven Member
72
0
Sep 9, 2012
Prattville, Alabama
Fuel cut is triggered by the ecu seeing over a certain hertz from the maf. When you burn EPROM chips, tune with link, tuner pro, or flash cable, you have to raise the airflow cap in the ecu, or you will hit fuel cut.

Maf over run is a different problem. It is causeed by a 1g maf not being able to read over 30lbs/min, or a 2 g maf being pushed over 50lbs/min. It caused erratic afrs, and engine damaging lean spikes. It can be detected by looking at the raw hz signal from the maf, and it becoming erratic. It does not feel like hitting a brick wall, like fuel cut.

amen.

but personally, i think it be better & easier to get some free time and connect the dots on everything the previous owner put on the car. and take the bottle off.
 

rEclipserGST

Supporting Member
1,703
55
Nov 17, 2007
Columbus, Ohio
Sounds like your ECU has a bad/damaged ground wire. It's how the maf reads airflow. I was experiencing very similar issues, I bet its your ecu. Send it out to be checked.
 

ramsack

Proven Member
3,286
16
Dec 27, 2007
West Lawn, Pennsylvania
No shit, Sherlock. All you said was "upgrade the fuel system," which could mean something as simple as rewiring the pump. The ECU is blind to what's going on in the fuel system. It just happily pulses the injectors while receiving no input back from the fuel system. I don't get why you posted hexadecimal. You obviously didn't understand the point I was trying to make about the ECU being blind to fuel system upgrades.
 

donniekak

DSM Wiseman
5,614
950
Mar 23, 2008
Surprise, Arizona
No shit, Sherlock. All you said was "upgrade the fuel system," which could mean something as simple as rewiring the pump. The ECU is blind to what's going on in the fuel system. It just happily pulses the injectors while receiving no input back from the fuel system. I don't get why you posted hexadecimal. You obviously didn't understand the point I was trying to make about the ECU being blind to fuel system upgrades.

When upgrading the fuel system, you either trick the ecu into seeing lower airflow(safc/maft), or reprogram the ecu to control the larger injectors, and removing/raising the airflow cap.

The ecu is not totally blind to the fuel system, the o2 sensor allows the ecu to change the fuel trims so afrs match those in the ecu tables, for closed loop operation.

I would recommend the op have his car checked out by a reputable shop.
 

ramsack

Proven Member
3,286
16
Dec 27, 2007
West Lawn, Pennsylvania
By that logic you should be able to just throw in whatever injectors you want and the ECU should be able to compensate just fine in closed loop. The ECU is blind to anything done to the fuel system. The behavior you describe is meant to make small adjustments because conditions are varaible, not to correct stupidity. He's hitting fuel cut because he has a larger turbo, an exhaust and intake, and maybe cams. He should get something like DSMLink and just go with bigger injectors...but still test for boost leaks first.
 

2g-TurboJo

Probationary Member
6
0
Sep 12, 2012
Omaha, Nebraska
Sorry guys I haven't been on in a while.

I've been getting a lot of feed back that's going all over the place. All I did to fix it was turn down the boost to like 13lbs and its fine. So it probably was just to much air, not enough fuel and a lack of tune. What I am hearing to do, is to run a computer system with better injectors. I do still think it has a decent boost leak, When boost gets up to 13lbs (and I haven't released the pedal from the floor) boost drops to about 7lbs till it switched gears then does the same thing.

But there's a lot I have to do, I just hit a deer last week cracking my hood and breaking my headlight so I'm fixing that and the little stuff that goes with that. I just want it to look good and run good. This is Just my daily, I have a 73 440 Roadrunner that my money is going into. and yes in time I do plan on making the GSX faster but right now I'm concerned on making it nice with looks, drive-ability, and reliability.

I haven't used the bottle yet, I unhooked it. It was a 150 dry shot, ran by a push button switch.


***Also Another thing I just noticed was that right after you start it and take off the radiator cap there's light foamy bubbles that sit there but will slowly go away (10 minutes +). And if you step on it pretty hard for a while it wants to boil over and therefore loosing coolant, but it doesn't overheat unless you don't fill the coolant back up if it boiled over. I'm going to start with the thermostat and a radiator cap and see what that does, any other Ideas?


I really appreciate all the comment on trying to help, thank you guys.
 
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