What does overboost fuel pump cut off feel like?

[Kliminati456]

The car has a weird intermittent bucking that I haven't been able to find any pattern to yet. No codes have shown up but it feels like I imagine overboost would feel like but the whole car like vibrates when it happens. At first I thought it was maybe bad gas that sat in the tank for the last 6 months but i filled up with premium and it still continues. Then I was thinking a trippy ABS (happens when I'm not hitting the brakes) I do have a worn pad light on the dash that I think is new and all the pads don't seem excessively worn. Or now maybe I'm thinking maybe something with the tranny/diff I just don't know. It wasn't doing this before when I drove the car home after I bought it. It feels more violent than what I think the fuel pump cut off would feel like but maybe I'm wrong as I have never experienced it.

[mcandmar]

The full on fuel cut from overboost is nasty, as in seat belt tensioners kick in and you headbut the steering wheel. Generally caused by a large air leak or MAF issues. The small cough or hickup under full throttle i have only seen with fueling issues, dieing fuel pump, clogged filter, or just crappy fuel. But an air leak is possible too.

[UrS4boy]

What is the wastegate spring situation? If the spring is wound down too tightly or you have 1.9 Bar spring with almost any threads showing on top, you can get into a bucking cycle under WOT (say 3rd gear) where the boost rise rate is too much for the ECU and it backs off boost via the waste gate frequency valve (WGFV) but then it figures out that it isn't so bad and trys boosting again, but again, the rise rate is too much and it dumps boost again. All this happening several times per second = nasty nasty feeling. Cure for that one is turning the spring adjustment UP (anticlockwise) until that nasty cycling goes away.

[Kliminati456]

hmmm maybe I misunderstand overboost, It never happens during wot. From what I noticed every time it has happened it has been at about .7/.8 on the stock gauge. I believe the spring is stock and it was wound down almost as far as possible. I backed off the bolt almost all the way possible and it still occurred. So the symptoms definitely seem something like a fuel pump cut out but maybe not from overboost and something else rather idk

[prj]

If you don't have codes, it's not boost/fuel cut.

[newsh]

Sounds like a bad misfire, plugs leads, boots (ABY/AAN or 3B?) Dizzy cap?

[adam g]

If there is no pattern to it - doesnt happen at a certain boost or rev level it is unlikely to be a fuel cut.

My moneys on air leak.

[5cylinderman]

Overboost cut out feels just like switching of the ignition for a second or two. (quite alarming when it occurs) If you take your foot off the throttle and press it again the car will pull away like normal. When I had this problem it only occurred under WOT in third gear or fourth.

[andyg plym]

had this on my first ever s2,a 3b.it kept doing it as it was fitted with a manual boost valve

it really is like your about to go throughthe screen and you can never floor it.the valve was causing too much boost and the ecu would shut it down,not nice

[newsh]

Boost cut is literally like going flat out then switching the ignition on and off. If it happens at less than full boost then it could be MAF connector corrosion http://www.s2forum.com/forum/showthread.php?t=24544

Or, I had something similar when my Lambda wiring was shorting out on the airbox heatsheild, that was really nasty, like a juddering that went through the whole car and felt like the transmission was banging about underneath.

[Kliminati456]

Thank you for the input guys

Its a mixed feeling of a a quick jitter sometimes and other times the feeling and anchor has just been dropped off the driveline, a quick slack then jerk of power back onto the drive line that vibrates the whole car with a loud bang. The jitter is not to bad but the anchor feeling is violent on the whole car, which is why at first I was question a brake or driveline issue but all the symptoms you guys have described are what it feels like.

But the weird thing is that it has never happened on wide open throttle, only when I seem to be cruising along steadily. It powers right through during wot with no issues. The times I have noticed to look at the stock gauge have been at .7/.8 so I don't know if that would qualify for overboost, and if it was would it not register a code? I have been checking regularly and no codes have appeared except the expected 2111.

UrS4boy-Stock spring, Last night I unwound the bolt through the top of the cap all the way out, (it was practically all the way turned down in) and the issue still continued. I cleaned up the diaphragm and checked for tears which I did not see any. One thing maybe is one of the wg cap studs was sort of stripped so I could not tighten it as much as the others.

prj- No codes have shown up as I have been checking regularly

newsh - 3b, dizzy cap and rotor are brand new, wires were cleaned up and looked in good condition, cleaned and re-gaped spark plugs but they are probably at least 5 years old and were covered in oil (previous owner drove around with a leaky valve cover which got all over spark plugs, like I said I cleaned them but maybe some residue got inside and is tweaking one of them, I should probably replace all them regardless if they are linked to this situation or not. I thought about whether my maf could be causing this I did take it out and clean the wire sensor and the connection but maybe ill have a re look at the connection and double check it.

Adam g - the only pattern I can see is that when it has happened that I have been able to look at the stock boost gauge it has been at about .7/.8. I have just rebuilt the top end of this engine and took off all the hoses and checked all for leaks but i suppose it is possible that something may have been missed.

5cylinderman - yes that is precisely what it feels like when it is bad, but inst always that bad. It never occurs under wot though.

andyg plym - yes it is not pleasant, are you speaking about the n75 boost valve was replaced?

[andyg plym]

if i recall it was disconnected,then a bleed valve put in on a vacuum hose.i didnt know a lot about tuning then and stupidly let the local jap tuner have a look....

[adam g]

Okay, now you mention the "anchor overboard" feeling I know what you mean, much more severe than an air leak misfire. I had this recently but im not on motec so may not be much help. It occured when I went >2.5 boost on a 2.5 map sensor. The ecu didnt know what to do and just cut everything, it felt like id hit a speed bump at 100 mph.
This may help in pointing towards a confusing signal to ecu so maybe a cut out afterall?
Such a situation may not give an error code as all sensors are still working fine and the ecu is happy again when back to normal boost levels.

[newsh]

The fact that it happens at part throttle but not at full throttle is a clue. I think I'm right in saying that at full throttle the ECU uses the WOT map and ignores the closed loop. So I'm suspecting a MAF or lambda problem. Lack of fault codes is confusing though..

[newsh]

I would also check the timing of the dizzy, roll the car forward in gear to rotate and make sure that when the timing 0 (zero) mark on the flywheel is perfectly aligned in the bell housing window that the dizzy marks line up exactly. (happens once every 2 turns of the flywheel )