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Misfires/Fuel Cut

23K views 97 replies 18 participants last post by  Sleepy7666  
#1 ·
I am kind of getting to my wits end with this car, it has recently started developing random fuel cuts and if it doesn experience fuel cut, it gets some misfires. This occurs at WOT or at least above 75% throttle, typically when I roll into 2nd gear but 3rd and 4th at WOT are fine. When you give part throttle, there is a large surge of power without any additional throttle input and was explained to me by my tuner as a feature of the tune to use less throttle input when driving around town and get more acceleration which I don't like because it feels like boost is overshooting, almost like the BPC is busted. Now, if this feature of the tune may be hiding a faulty BPC which in turn causes overshooting of boost, it could be the BPC going out as well. It seems to come in around 5,000 rpm and am trying to diagnose the culprit but I want to get my bearings straight on what everyone would recommend:


My plan is as follows:

-Replace DIC with a known functioning T7 DIC for future reliability

-Replace BPC if previous step does not work

-Fuel pump: unfortunately, I have come to find that my dad whilst I was at college, had my car checked and in so doing, ordered Saab of Greenwich to remove my Bosch 255 pump. This issue could come from the stock pump not being able to supply enough fuel for my boost requests but replacing this would be a last resort as I will not mess with replacing the fuel pump and would bring it to Walter Wong which = $$$.



Sorry for the long winded response, I am just trying to get all these issues nailed down as I am giving my ng900 to my brother and would really hate to have something die on him like I have had happen to me in the past. Thanks in advance guys!
 
#4 ·
It worked for all of a few drives, basically up until the last month I have ran the same tune and hardware for the last 2 years and never had an issue. There have been no misfires since the new plugs and gapping, however the fuel cut reared its ugly head. I really don't know anymore, this car has been a basket case which I guess comes at the price of having a pristine 36,000 mile ng900. I am going to try the simple things first and see how those go but it is frustrating that the misfires/fuel cut seem to come at random because you can do a dozen WOT pulls and be fine and then one will have those issues come up.
 
#3 ·
k again sounds like the same issue i had and jp is still having.... if you reset the ecu it goes away untill adaption has finished then it comes back...so every time you fix somthing that you are sure is the reason for this missfire or fuelcut and reset the ecu you are sure you nailed it then two days later bam it happends again... sounds like you have the same issue as i do although yours sounds a little less severe then mine
 
#8 · (Edited)
like this?



its unlikely its the fuel pump unless your having a partial failure, since the stock pump can handle stage 4+ tunes ie 250-270ish whp.

the key is that you say it happens when you roll into 2nd

long story short, swap the DI with a known good/or new. this is very similar to a high rpm misfire/flutter i chased on an old 9000 which wound up being a mildly flay DI when pushed hard. basically the spark was blowing out when it came on too hard.
 
#9 ·
Ah, that is great to know. I think I will find a T7 DI for the sake of reliability and give that a try, then I will look towards the other options I have which I hope to avoid. Thanks again, I will let you know how this turns out!
 
#11 ·
Haha, I know T7 BPCs are cheaper and more reliable but I am not really interested in modding the car anymore as I will be giving the car to my younger brother this summer and I don't want to put any more of my own money in it (already have $12,000+ in it).
 
#15 ·
I haven't found any information against moving to a T7 DIC, this includes no one with blown-up engines or any difference in performance. From what I have gathered, a T7 DI has lower voltage to detect knock slightly later than a T5 DI but those who run them on a T5 motor find that knock levels and knock detection are fully functional and the motor adapts to changes in octane and environmental conditions just as efficiently as a T5 DI.

I may just go with another T5 DIC just to keep in line with factory hardware but I was debating the T7 DIC on a T5 engine due to what seems like advantages in spark duration which I heard allows for smoother operation and better spark for tuned cars.
 
#16 · (Edited)
I bought a known working DIC from Mockba and threw that on yesterday to see if it worked. Well, today I rolled into second and fuel cut hit on/off/on/off/on/off until I let off the gas. To recap:

-All new sensors (o2 sensors, coolant temp, blah blah blah)

-Vacuum lines checked and are all connected to their respective locations with no split lines

-New Saab OEM fuel pump installed 6,000 miles ago

-Known working DIC installed


I am not sure what to try next, I will get a new BPC as well because the car feels like it is overshooting slightly so that might be part of the problem. Is there anything else I should look into?

TIA


Edit: forgot to reiterate that the car runs as normal at anything under 5/6 throttle, not even the slightest hickup.
 
#17 ·
BPC sounds like a good idea to swap out too.

Also, ever think of a weakening wastegate? Causing too much boost for a short period of time, fuel cut etc....but that would only really solve the fuel cut, no missfires.
 
#19 ·
I think I am only experiencing fuel cut since you can see the light poofs of black smoke when this happens before I lift off the throttle. Good idea on the waste gate, I had chadwick build my T28 from a turbo he had but I swapped my wastegate from my OEM T25 (30,000 miles on it) before installation. I will double check my base boost as well to rule that out.

have you talked to john about this
I will PM you regarding that.
 
#20 ·
Any chance its the change in elevation and you need a remap?

Idk whats the elevation difference between CA and wherever you were on the east coast but is it sizable enough to cause an issue?

I still think its the stock fuel pump try to keep up with your injectors and tune.
 
#21 ·
I was 100 ft up from sea level in Connecticut and am probably 10ft at most above sea level in CA so that wouldn't be an issue. I have always had that thought in the back of my mind about the fuel pump but I have talked to a number of people who are running similar or higher power levels than I am with the stock fuel pump and they have had no issues for the years they have used that setup.

Leaving the fuel pump and DIC aside, what could cause fuel cut?
 
#22 ·
Leaving the fuel pump and DIC aside, what could cause fuel cut?
Other then overboost? the rev limiter.

Thats all I can think of.

Unless...this is a bit of a stretch.....the roll over fuel shut off valve?
 
#28 ·
Make sure you're smoking a cigarette while doing this :D
No way dude.... lung cancer is serious business.

Christian, come by some time if you wanna try a new DI around the block and see if it's any different.

Best,
Drew
 
#31 ·
It is almost definately fuel cut because the car stops accelerating completely, sorry for the confusion as I did some research on the difference between fuel cut and misfires and had a hard time differentiating.
 
#32 ·
Keep in mind there also a difference in "stops accelerating completely", and fuel cut.

During fuel cut, not only do you stop accelerating, you also get a harsh bucking. Got that too?

Just trying to figure out if you acutally are experiencing fuel cut or something else.
 
#34 ·
Yup, harsh bucking. What happens is that you roll to WOT once it shifts to second and as soon as it hits full boost, it cuts out and starts bucking until I let off the throttle.

My apologies for the confusion. :(
 
#38 ·
It looks like there are some commonalities here; quite a number of JZW tuned guys are all here for the same problem. Hmmm. I just ordered a T5 BPC since I am not in the mood to modify the car's wiring for the T7 BPC, past that I will have no choice but to look into the tune and fuel pump. :(
 
#41 ·
I would stop throwing money at the cars and get John involved as this "sounds" like issues with the tune. Does anyone have access to T5 suite that can flash another tune into one of the ecu's just to see if this bucking goes away, doesn't have to be the perfect tune for a test?
 
#42 ·
I will have this side covered this weekend or next by someone involved in the making of T5suite, then I will go from there. It also worries me that features such as adaptation and overboost fuel cut are being turned off in our tunes (mine at least) and only found out about these features recently. Past that, I am still going to try a BPC from another member to rule that out and go from there.

Thanks everyone for working together on this issue, keep it coming!
 
#46 ·
Nearly face smack, it cuts then power starts coming back then it bucks again in rapid succession. I hooked a temporary boost gauge up a year ago or so and was hitting a spike of 20 lbs.