Lumpy Boost - soft drop in power at 75% max rpm

[AGB-S3]

Lumpy Boost

Hold the throttle at a small-ish opening , in say 4 th , engine pulls cleanly up through the rev-range , till it reaches , round 5k , car in strait line

Then there can be a soft sudden drop in power , as if the boost pressure had dropped , giving the effect of lifting off the accelerator

same can happen , when accelerating 'hard' the engine appears to be down on power ... some times , when approaching the final 25 % of the rev-range , then some times its ok

Once I noticed this , when turning sharply , left and up-hill , the ESP light flashed and the power dropped .. but , no esp light in the above situation's ..

ideas ? odd ?

 

[S3YEP]

I had drop off in power, but around 4k to 5k rpm. I had a new fuel pump and replaced leaky breather pipes (after smoke test) If your car has done more than 100k miles I would def have a smoke test + think about changing the fuel pump (not cheap).

Sent from my SGP712 using Tapatalk

 

[AGB-S3]

All the inlet pipes have been changes , some of the molded unions where split , that made a big improvement to the idle , still rough idle when the air-com is 'on' [ seems to be normal ?]

This power drop is not there all the time , some times , accelerate hard and there is no power , next thing , it packs up , like something had switched on ..im guessing its linked to boost pressure

Would low fuel pressure be recorded by the ecu ?

 

[<tuffty/>]

Would low fuel pressure be recorded by the ecu ?

Not directly no... you can diagnose symptoms of a failing fuel pump to a degree from logging but validating low fuel pressure normally requires a mechanical gauge to be used..

S3 pumps are failing all the time... see loads..

<tuffty/>

 

[AGB-S3]

Caused by - CRANK SHAFT POSITION SENSOR - pulls cleanly round the clock now

 

[badger5]

thanks for sharing.

 

[graham d'arcy]

Caused by - CRANK SHAFT POSITION SENSOR - pulls cleanly round the clock now

how did you come to this conclusion? I'm having similar issues, when i go to accelerate hard it basically cuts power and i have to change gear, everyone telling me the fuel pump however.

hopefully hooking it up to vagcom this week to check fuel and air but failing this would be nice to know how you found your fault, and was it much to fix?

 

[AGB-S3]

I was hunting down the illusive P0012 , intake cam advance flag , the local 'mechanical' engine shop could find nothing wrong with the actuator ..this coupled with the auto electrical shop, came out with the interesting fact, that , in other VW engines , they had seen the same fault , which was the crank position sensor, loosing out put at high rpm , they suggested replacing the sensor ..
Sure enough, the original sensor was showing 50% over spec , new one fitted, loosing various 'odd' faults and a big increase in low range torque .. now pulls cleanly past 4k , where previously , there could be fluctuation ..

I guess , the ecu has to believe something , if the sensor output falls off at mid/high rpm .. causing the ecu to miss the odd pulse , then , there is no way to know the engine has not slowed down , causing the interval to be longer , that missed pulse , causes the cam to 'appear' to be in the wrong place .. when normal service is resumed , the error flag is dismissed , but remains in the history file ..

There must be crank sensor error codes , but none showed in the ecu , which would support the chain of events ..watching the sensor output on a oscilloscope , was to be the next step...

Tnx-G,

 

[AGB-S3]

If the device was in a more advanced state of decay . codes P0321, P0322 might of been flagged , looks like mine was just inside the expected limits of out put .. new genuine item via one of the parts guys , who used to be on this web site , less than the £120 retail .. will know more , next week , when the need to count '1,2' during the up-shift from 2>3 , should be removed , with the s/hand 60k box [ original ~170K]

 

[AGB-S3]

And finally , after binning the aftermarket boost re-circ valve , changing all the leaking pre-formed VAC hose assemblies , including the one to the brake servo , the turbo regulator valve Y , finally it idles smoothly with the air-con on and pulls round the clock with no jumps and jolts
...

 

[AGB-S3]

After all that , issues with the turbo actuator , diagnosed during dyno test , fitted re-furbed S1 AET turbo , luckily the ex manifold was not cracked .. .. and now, runs as good as new , well actually a lot better -)

Old turbo had slight shaft end float , but after 180,000 miles , had lost its edge , new one has up-rated seals and bearings . finally stopped the odd 'run one' if the throttle was held at a fixed opening, while the speed leveled out ..