Who can guess what's wrong with my FC?!
 

My FC chokes/bucks down when I EASE off the throttle or try to keep a certain level of throttle after I give it some gas (it happens in any gear, any rpm). This is very important, the car runs great while accelerating under any amount of gas, or when I let go of the gas compleatly after I accelerate. Idle is great as well.

Example: Lets say I accelerate with partial throttle, reach 5psi, then I ease off the throttle without letting go of it all the way, the car will buck down/choke... if I would have let go off the gas completely after I accelerated it would run fine.

My wide band shows that I am leaning out(15s - 17s) for a split second when this choking/ bucking down happens.

I have checked the TPS numerous times... 1.000 resistance between A&B and 0.800 betweet E&D. I also checked for any breaks in the TPS while giving gas or letting off, and there was nothing that was out of the ordinary... I have also checked the TPS on other working FCs and they were the same as mine. I have swaped computers, checked/swaped the pressure sensor, tested for boost leaks, changed the BOV. Also, I am getting a code 13 when I floor it, which is saying there is something wrong with the pressure sensor... and it does not go away even after swaping it with other pressure sensors... Not sure if this has anything to do with this problem, since this code does not get triggered unless reaching high psi, while my problem occour regardless of high psi conditions.

My mods.

1989 FC turbo (stock s5 turbo motor)
N370 ECU, S5 rtek 1.8 upgrade
720,1000cc injectors
BNR stage II turbo
HKS ssqv
Greddy profec spec B
Apexi SAFC Neo (fuel control)
No emissions, full 3" open exhaust
FMIC

Please let me know if anyone has any ideas... we have tried almost everything and the problem has not went away... the ONLY time the problem got better... the bucking was much less violent... was when I turned my boost controller off... so I am preatty confused lol.

So you only buckle when you cruise not when decelerating or accelerating?

I had the same problem and adjusted my TPS today and the problem went away. Have you tried replacing your TPS? They can go bad.

The bucking only happens after I accelerate, and I slightly let off... Not all the way.

The FSM talks about decelration adjustment. Have you checked the dash pot?

Another thing you may look at is what does the pressure sensor do? Are the wires running to it old and cracked?

Ok, I have checked the pressure sensor voltage at the ECU, and it reads about 0.9v at idle, and about 3.9v at ~11psi.

The TPS is set to ~1.000 resistance at idle (narrow) ~0.77 resistance (wide), after the engine was fully warmed up. TPS was also checked for ubnormal breaks while giving throttle and the only part that overload came up on the multimeter was at about ~4.0 Ohms.... (not sure if thats normal or not)

The dashpot moves freely, and it engages/disengages the throttle at 2.8 resistance at the TPS(narrow).

Problem is still there... nothing has helped :(

My spark plugs are half white/black... and the ONLY time I lead out is when this happens. Basically I have double checked and EVERYTIME I give ANY throttle to accelerate... then I slightly let off (NOT ALL THE WAY), the AFRs lean up to about 16s 17s for a split second, then goes rich again. This is when the bucking occours.

I'm guessing here.....


I've read about s5's being diffent then s4's in the fact that an aftermarket BOV will cause s5's to run rich for a second after boost. Try plugging back in the stock BOV if you can. The ECU componsates for the BOV air that is forced back into the intake. Thus with out that air you will have a rich mixture for a split second.

Did this problem come about after you worked on the car?

Just a thougth :P Could be completely wrong though.

this is true^^^^^^

change the BOV and get back to us.

Well I have had the same BOV for almost 3 years now. This problem only happend a few months ago, when AT THE SAME TIME I upgraded my turbo to a BNR stage II, got my ECU chipped to rtek 1.8 for s5, and changed my injectors from 550/720 to 720/1000 .

I checked my BOV already, I got a brand now HKS ssqv from my friends car... and the problem was still there... I even disconnected the BOV compleatly... and still nothing lol.

I am just lost for ideas at the moment...

And yes you are right, with this BOV, you will backfire as the ECU give too much fuel and that fuel combusts in your hot exhaust... but I am leaning out for a split second... not richining out. Oh, and as I stated before... I dont have to go into boost for this problem to happen... ANY TIME I accelerate, even a little... as I slightly let off the gas I will lean out... the problem is... the more I accelerate to begin with... the more violent the bucking is.

probably the rtek, people have had some funky problems with the s5t version

Lol nice try :p We already have the ECU covered... we swaped the ECU with another S5t, stock ecu, and drove the car. The problem was still there :(

The rtek 1.8 is looking for 720's all around. Since you said this all happened at the time that you changed everything, are the 1000's the issue?

What kind of vacuum are you pulling when this happens? Is it possible that you have a vacuum leak and it is sucking in unmetered air? You wouldn't notice this under deceleration (you are running very lean at this time anyway). However, if you had a slight vacuum and on the gas, you could go lean.

Just a thought.

I have the same problem and i´m running stock ECU and stock BOV, it bogs down when you are cruising or trying to do so, is very annoying and it makes me look retarded at peoples eyes...
and the car even sound different...

My few mods:
3" full exhaust
high flow cat.
apexi filter
short intake
removed air pump

i could be all wrong but i would call this an emissions removal problem? seems like the last thing left. all the plates are sealed, any vacuum leaks? for it to lean out, its getting more air less fuel obviously. what are you getting for vacuum readings at idle, WOT and cruising during the bucking/choking?

I had these troubles before and after the emissions system removal, so i don´t think that this could be the problem, anyways i don´t discard this possibility.

Ok, I have pretty much tried everything. Tested every sensor, even swaped a few of them. Checked for leaks... retarded the timing, swapped computers, turned the SAFC off and removed it from the ECU... and nothing...

But let me tell you what I have found out instead...My friend with a VERY similar setup has the same exact problem on a freshly rebuit engine. With a N374 ECU, SAFC neo, 3" full open exhaust, no emissions, 720/1600cc injectors, and a GT35 turbo... When he drives his arround, to break in the engine and slightly accelerates, then lets off partially... he leans out as well and the car bucks down. I have comfirmed that IT IS the same exact problem.

What we concluded from this is that this problem started happening for both of us since we upgraded the turbo and injectors. And this now leads us to believe this is a ECU tuning problem. Maybe our stock/chipped ECUs are not able to properly deliver fuel when you lift off the throttle because there is much more air in the engine then with a stock turbo...

Anyone could agree with that conclussion? Do you think maybe going to a stand-alone is the only option? Or are there people who have similar setups who DO NOT have this issue? =/

Thanks you for your help :)

i still run the stock injectors and turbo and have the same problem, so, it makes me think in 2 possiblities, the first, we are all wrong :allcho: and the second, if you´re right with your conclusion and this is the real problem, the previous owner of my FC repaired it with... smaller rating injectors?¿

I hope we will get the solution some day!!

BTW, do you have a bov? if so is it external (does it vent into the atmospher)? and if so, what brand model... Also, do you have a front mount IC? Did you even check for boost leaks of vacum leaks?

I have a HKS ssqv...

i have the stock BOV but it is not working because i have not a vacuum port to plug it in, because i am using it for the boost controller and the boost sensor, and i am using the stock IC too. I have to say that this problem we have, i have from stock, i mean that before i touched nothing, it did all these things.

since you have an safc, try to richen the mixture on the low side more.

My car was doing the exact same thing (S5 Turbo). When going full throttle from 2k, I partially lift off at 6k and let it gradually come down, and the car bucks, and my wideband telling me its at 17-18 afr. So at around 4-7k on the low throttle setting, I richen the mixture. I test drove and that stumbling and bucking went away.
I have stock turbo and bov.

Well here is the problem... I'm not so sure we have the same problem. First of all, does your buck down when you even accelerate a little? For example... at ANY rpm, go to 3-5psi (even if you only reach that psi for a split second), then slightly let off to keep that speed... does your car buck down then??? Mine does.... I notice it leaning out AFTER ANY amount of acceleration. But like I said before, the more I initially accelerate, the move violant the bucking is.

Also, I drove my car after turning the SAFC off! Meaning I had 0 correction across the board, and the problem was still there :(

no bucking while accelerating, only when decelerating. But if I keep a constant speed, it bucks sometimes, but I think thats because of my trailing plugs, the gap in them is pretty big compared to new ones, and Im thinking they are misfiring.

if I remember right, there is a one way check valve in the intake pipe before the turbo. I forgot where it goes to, but if its backwards, it could be sending in extra air that the maf isnt seeing and causing lean conditions. its just a thought, but if you dont have the stock intake pipes then you wont have it.

Yeah I have a FMIC setup, and no emissions, so I don't have any of that stuff anymore. I've had this setup (FMIC) for over a year and this problem only started a few months ago after going with a bigger turbo and injectors.

Anyways, I just checked the voltage at pin 2H on the ECU (pressure sensor), with the hose unattached to the pressure sensor... I got 2.2v Now the problem is, under the FSM it saids that it should be between 3.4v - 3.6v ... I don't understand why more then 1v difference. Could the pressure sensor be bad??? I mean I have another one, and it read the same value. Could this be causing me leaning out under partial decel? Tho I though the pressure sensor is mainly used for timing, not fuel delivery....

Also, can timing affect things of this nature? like AFRs?

timing being off can cause something like what you describe.

are your injectors low impedance? if so do you have 4 10watt 10ohm resistors wired in?