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

 

[JDNSBR]

Ok before I modded the car, it ran fine. However now that I have added a Aeromotive AFPR, Walbro 255, 650cc injectors, SAFC-II, Evo III exhaust manifold/02 housing, Evo 3 GT 16g, full 3in exhaust with no cat, running at 15 PSI, the car bucks and stutters above 6000 RPMs just before it shifts (it is an automatic).

There is a huge loss in power as well.

I changed the plugs and checked the timing. I am also running slightly rich (FTH 81%) at WOT. Same thing happened when the FTH was 100%. Am I hitting fuel cut and whatever the problem is, how can I fix it?

What are your settings acrossed the board on the SAFC? If you are running too rich it can feel like fuel cut as well.

 

[mac 108]

Well it was -20% (380cc is stock) and with minor adjustments I had the problem and the FTH was 100%

Now it is -15% and 81% FTH and same problem

 

[mac 108]

Here are two logs, -20% Correction across the board

Log 1
TIMA O2-R TPS MAFS RPM KNCK INJP
29° 0.12V 25.5% 188.7Hz 1969 0 3.84ms 8
29° 0.37V 25.5% 201.3Hz 2031 0 4.10ms 8
29° 0.70V 25.9% 207.6Hz 2094 0 4.10ms 9
28° 0.06V 38.4% 251.6Hz 2156 0 5.38ms 12
25° 0.76V 65.9% 339.7Hz 2375 0 6.66ms 16
24° 0.86V 69.0% 402.6Hz 2688 0 7.17ms 20
24° 0.88V 70.2% 522.1Hz 2969 0 8.70ms 27
24° 0.88V 73.7% 629.0Hz 3156 0 10.75ms 36
22° 0.88V 84.3% 767.4Hz 3344 0 13.06ms 46
18° 0.88V 88.2% ####### 3594 0 16.64ms 63
18° 0.86V 100.0% ####### 3906 22 19.71ms 82
11° 0.84V 100.0% ####### 4188 21 19.46ms 87
11° 0.84V 100.0% ####### 4250 21 19.46ms 88
12° 0.84V 99.6% ####### 4469 24 19.20ms 91
8° 0.84V 99.6% ####### 4656 30 17.66ms 87
9° 0.84V 100.0% ####### 4844 36 18.43ms 95
7° 0.82V 99.6% ####### 5063 36 17.41ms 94
8° 0.82V 98.8% ####### 5250 35 17.15ms 96
9° 0.82V 87.8% ####### 5344 42 15.62ms 89
7° 0.80V 9.8% 484.3Hz 4750 40 2.82ms 14
32° 0.78V 9.8% 100.6Hz 4781 7 1.28ms 6
44° 0.76V 9.8% 69.2Hz 4250 0 1.28ms 5
42° 0.04V 9.8% 50.3Hz 3813 0 1.28ms 5

Log 2
TIMA O2-R TPS MAFS RPM KNCK INJP

36° 0.70V 18.4% 119.5Hz 2281 0 2.56ms 6
35° 0.04V 23.5% 169.8Hz 2375 0 3.84ms 9
31° 0.74V 38.8% 295.6Hz 2531 0 5.12ms 13
26° 0.80V 39.6% 339.7Hz 2594 0 5.38ms 14
26° 0.78V 40.0% 371.1Hz 2688 0 5.38ms 15
25° 0.06V 48.2% 427.7Hz 2750 0 7.17ms 21
23° 0.84V 93.7% 540.9Hz 2969 0 9.22ms 29
23° 0.86V 100.0% 798.8Hz 3875 0 13.06ms 54
22° 0.84V 100.0% 1352.4Hz 4313 18 19.46ms 89
12° 0.82V 100.0% 1446.7Hz 4594 25 18.43ms 90
10° 0.80V 100.0% 1459.3Hz 4844 25 18.43ms 95
11° 0.80V 100.0% 1440.4Hz 4969 25 16.90ms 89
11° 0.80V 100.0% 1453.0Hz 5219 38 16.90ms 94
8° 0.78V 100.0% 1484.4Hz 5406 37 15.87ms 91
10° 0.78V 100.0% 1421.5Hz 5594 37 16.13ms 96
10° 0.80V 100.0% 1220.3Hz 5625 36 14.08ms 84
11° 0.53V 100.0% 1251.7Hz 5781 43 12.80ms 79
12° 0.00V 100.0% 1358.6Hz 5906 43 13.82ms 87
12° 0.00V 100.0% 1144.8Hz 6031 43 10.24ms 65
15° 0.00V 100.0% 1503.3Hz 6344 43 13.57ms 91
12° 0.00V 87.1% 1012.7Hz 6188 42 8.96ms 59
14° 0.72V 9.8% 415.1Hz 5188 35 1.79ms 9
35° 0.84V 9.8% 94.4Hz 4844 3 1.28ms 6
44° 0.82V 9.8% 69.2Hz 4313 0 1.28ms 5
42° 0.82V 9.8% 44.0Hz 3625 0 1.28ms 4
40° 0.53V 9.8% 37.7Hz 3500 0 0.00ms 0


FTRL FTRM FTRH
137.3% 97.5% 82.7%

 

[Pure92TSi]

You've got a ton of knock. Probably the most I've ever seen. I think it is leaned out to much up top. Add a little fuel in thu upper RPMs and see what happens.

 

[mac 108]

I need to add more fuel at the top even though I am already running rich?

 

[larsenracing]

What fuel are you running.

 

[girlieracr4g63]

I would like to know as well what fuel you are running. Might not help to pull a plug or two as well since they are new. They are great indicators for whats really going on along with the logs.


Wendy

 

[jackalopeTsi]

Im curious as to what could cause this, because im have a very similar problem. when it stutters in the high RPM's did it backfire. i did a 3rd gear pull today, and it backfired pretty bad along with some black smoke, and my logs look alot like yours. alot of knock and black smoke, it doesnt make sense to me.

 

[mac 108]

I tried -30% today, same problem.

I am running 93 octane

 

[mac 108]

is it possible that the problem could be related to poor compression or that I need to rewire my fuel pump or give it more fuel pressure via the AFPR?

 

[97GsXfreak]

Oh man, I am pretty sure that they said richen it up not lean it out and if you were at -20% and getting that much knock and now you tried -30% that means you are taking out fuel. Better be careful with that much knock.

 

[mac 108]

Well I tried -30% because my FTH is 81%

 

[larsenracing]

What is your fuel pressure at.

 

[mac 108]

43psi

 

[mac 108]

Changed the wires and the plugs. Boost at 10psi, stutters now at 4200 RPMs

New NGK BPR7ES .28 gap

New Bosch wires

base timing is stock (checked with timing light)

I don't understand how it got worse, car has also developed a miss. Fuel pressure 44 currently (I have played with this number quite a bit) with the car running and vacuum hose on.

I did use a timing light to check the timing. Grounded the computer then started the car on and checked it and it was fine.

Before I changed the plugs. It sputtered at 6k+ RPMs with no miss. I took at the plugs, checked the compression (disconnected coil connector and removed EFI fuse) I was 140-140-120-140 from passenger side to driver side. Yes it is getting close to rebuild time. After that I replaced the plugs with the .28 gap. Developed a miss, reinstalled old plugs, miss is still there. Reinstalled new plugs, still had miss and sputter at 4200 RPMs. Changed wires, no change, miss and sputter still there.

 

[mac 108]

Boost leak test revealed my problem: broken o-ring on my injector. It was the o-ring on the cylinder that had poor compression.

 

[tstkl]

Im not sure if my fuel cut is being caused by which, or all of these three things, anyone care to help me out?
1:boost leak. more info: I have good vac at idle and my gm maf is on the uicp. recent boost leak test reveiled no leaks at the throttle body, but still a possiblility.
2:rich afr: recently overhauled my intake system and have a full o2 back exhaust, afr is probably off a little. see http://dsmtuners.com/forums/showthread.php?p=1795941&posted=1#post1795941 for more info on my maft settings (only thing I have to tune right now)
3:flowing too much air: but I'm at 10psi on a 14b? I know I hit fuel cut when the car was completely stock at 18psi, but now I'm hitting fuel cut 10 times earlier (at like 3500 rpms) o2 voltage reads .93, so its a little rich, but enough to cause this much chaos? 2.75 inch dejon intake pipe with an open 9 inch k&n air filter, short route 2.25 inch licp and 2.5 inch uicp with a gm 3 inch maf and a jrc 8 inch fmic, along with a dejon smim causing that much flow? my calculations would be ~80% more since I went from hitting fuel cut at 18psi to hitting it at 10.

what should I do, eliminate fuel cut, I am on the stock fuel system, but I have a vr4 fuel pump ready to go in. If I had a wideband o2 gauge I would just cut it and watch my afr, but thats not the case. any imput is greatly appreciated

I thought its always better to search than start a new thread, and since this one was here and is similar but doesn't answer the question of how do you know when you need to eliminate fuel cut, I'd post here. sorry for bringing back the dead

 

[oldman]

Like already mentioned by both Andy and 92awddsm, fuel cut has nothing to do with your fuel supply so you can eliminate your fuel components, assuming you truly are hitting fuel cut. There aren't that many things which can cause fuel cut, boost leak, boost creep, infinite boost, defective maf, defective wiring harness or problem with the ECU. Are you sure your GM MAF/MAFT is setup correctly?

 

[tstkl]

Like already mentioned by both Andy and 92awddsm, fuel cut has nothing to do with your fuel supply so you can eliminate your fuel components, assuming you truly are hitting fuel cut. There aren't that many things which can cause fuel cut, boost leak, defective maf, defective wiring harness or problem with the ECU. Are you sure your GM MAF/MAFT is setup correctly?

if by set up correctly you mean all the stuff is dialed in and "tuned", then no

if you mean is the maf facing the right way and is everything plugged in, then yes

i just mentioned the fuel pump because I dont want to run lean, and was wondering if changing the injector settings and fuel trims could help at all so I wouldnt hit fuel cut.
with a boost leak, there is such little margine for a boost leak I doubt that is it. the maf is on the uicp and the vac at idle is good. the thing is I wasnt hitting fuel cut wednesday night when the injectors were set to 480cc (my car has 450cc)

 

[oldman]

if by set up correctly you mean all the stuff is dialed in and "tuned", then no

if you mean is the maf facing the right way and is everything plugged in, then yes

i just mentioned the fuel pump because I dont want to run lean, and was wondering if changing the injector settings and fuel trims could help at all so I wouldnt hit fuel cut.
with a boost leak, there is such little margine for a boost leak I doubt that is it. the maf is on the uicp and the vac at idle is good. the thing is I wasnt hitting fuel cut wednesday night when the injectors were set to 480cc (my car has 450cc)

Do you have a logger? If so, tell us what your maf reading is when you hit fuel cut. GM MAF is a piggy back system just like the SAFC, when you set the injectors to 480cc, you delayed fuel cut by reporting less than actual air flow count to the ECU. Is your maft setup for blow through? Bottom line, you shouldn't be hitting fuel cut now at 10psi or before at 18psi, my car didn't hit fuel cut until about 25psi when I was creeping really bad and that was with a small 16g. Don't attampt to use tuning to get rid of your fuel cut because you're just covering up the problem, find your problem/problems and fix them instead. Logger would come in handy right about now.

 

[tstkl]

Do you have a logger? If so, tell us what your maf reading is when you hit fuel cut. GM MAF is a piggy back system just like the SAFC, when you set the injectors to 480cc, you delayed fuel cut by reporting less than actual air flow count to the ECU. Is your maft setup for blow through? Bottom line, you shouldn't be hitting fuel cut now at 10psi or before at 18psi, my car didn't hit fuel cut until about 25psi when I was creeping really bad and that was with a small 16g. Don't attampt to use tuning to get rid of your fuel cut because you're just covering up the problem, find your problem/problems and fix them instead. Logger would come in handy right about now.

yes I have a logger, and the maf is on the uicp. the readings are 1599 hz, but I think thats because this logger doesnt show when it goes to 1600, since it holds it at 1599 for a long time. also it doesnt slowly decrease the rate of change of airflow, the airflow is going up really fast, then all of the sudden is a straight line right at 1599.

how can my problem be causing me to hit fuel cut so early, the vac is good and the gm maf is on the uicp...a leak anywhere before the maf wont affect the value the maf sees.

does the maf take into account the density of the air? I am at 300 feet above sea level or something

 

[oldman]

Did you set the maft to blow through?

Something is definitely wrong if you're reading 1600 Hz at 3500rpm with a 14b @10psi. I just checked one of my old logs for you, GT12 17psi@ 5906 rpm, my MAFS reading is 815 Hz x 2 = 1630 Hz.

BTW What happen to capitalizations?

 

[tstkl]

I don't know what you mean by did I set the maf to blow through. Is there a different setting for if your maf is on the uicp or on the intake pipe? like do I have to change the intake air temp sensor or something? I thought that was part of the maf itself.
I set those four switches to off off off off if I remember correctly. But I thought the first two were for which size gm maf you have, while the latter were for rpm based wot tuning. what exactly do you mean by did I set the maf to blowthrough?

baby steps, baby steps with the caps.

 

[corwininadsm]

i am hitting fuel cut in second 3rd 4th and fifth gear on my 16g supra smic 1g in a 2g running 19 psi (cant run less due to creep) is this normal at all?

 

[corwininadsm]

what does a fuel cut defencer actually do?