Sep 30, 2010
My S3 has serious boost issues, its like its missing but i'm sure its boost related!!
Could it be the n75.
No fault codes? What gear did you do the run in?
Looks like actuator pressure so you are either in limp mode or there is a prob with the N75... where it goes to '0' duty cycle it looks like it may be hitting limp mode at that point...
Log blocks 020, 031, 115 too...
Would expect to see faults being logged so do a scan too...
There was some fault codes.
camshaft position sensor
secondary air injection system
I recorded the log in 4th gear.
I recorded some more logs tonight - see below!! Also checked faults - just camshaft position sensor!! The car didn't seem as jerky as it was last run but though I'd post them anyway!!
If it is the N75, is there a way to check it, i.e a multimeter or disconect it.
N75 will account for your boost problem. If non functional, you will run actuator ppressure all the time.
The clutch switch will account for jerky gear changes, as the engine uses the clutch switch for rev matching.
Is There any more info on this clutch switch, as i think it causes over revving? between gear changing
I'm too lazy to write it all again but in this thread
I did alot of work on this last weekend.
The clutch switch was replaced when my clutch pedal snapped. This did not cure the over revving.
I had a new gearbox fitted and 20 miles later speedo stopped working, during this time the car didn't over rev.
I get the speedo working and it starts over revving by quite alot, this suggested to me that the problem was electrical.
No fault codes showed up, so i thought for £11 its worth trying another clutch switch, this didnt work either
As I had two switches i thought what the hell I'll take one apart and see if i can figure out how it works and whether I can sort it out. Turns out its a fairly simple mechanism but my god is it fiddily! On both switches the black 'plunger' that moves as you use the pedal wasn't correctly attached to the simple sliding switch inside, so even thought the plunger was moving there was no actual switching happening. I correctly aligned the plunger with the sliding switch and hey friggin presto! over revving is 90% sorted (it now jumps by no more than 100 revs as opposed to around 800).
I have no real experience of doing anything like this before but once you actually open it up its quite obvious what needs to happen. So if the new clutch switch doesnt work when fitted to the car take it apart get everything in the right place and you might have more luck.
A month or so on and Its still the same jumps by no more than 100 revs, but this is starting to annoy me now, it ruins the experience of changing gear when giving it the beans because all you can hear is over revving. The thing is the clutch switches are fiddly to get back together, install and remove. It is a completely trial and error job you adjust it, put it back together, install it, go for a quick spin, remove it and start all over again.
What would be interesting is to find someone whose s3 doesn't over rev on a gear change and swap out the clutch switch and put it in a car that does over rev and see if that cures it. If it does then its a matter of correctly adjusting the switch and if it doesn't there must be another problem
It resolved my over rev problem in between gears during changes, when I replaced mine.
Welly did it completely stop the problem?
Isnt he just off the throttle at that time.
Your specified vs actual boost is good but it requesting for low boost, wouldnt this be down to the MAP sensor.
MAP sensor has nothing to do with requested boost at all. Requested boost is what the ecu requests, the MAP sensor delivers the data on actual pressure.
Yes mate, drove like a different car immediately after.
Ye, but to get the correct boost doesnt the ECU need to know the manifold pressure to get the right fuel mass to get the best air/fuel ratio, correct me if im wrong.
Fuelling is mainly worked out on airflow through the MAF and using lambda sensors to monitor exhaust gas, amongst other things.
The MAP sensor is there so that the ECU knows what boost is being achieved, and uses that data to control the N75 valve to produce more or less boost based on the difference from requested.
For example, AGU engines have no MAP sensor at all, and fuelling is controllable on them with no problems.
Ye some cars dont have MAF sensors though and use MAP sensor to know the manifold pressure so the ECU knows what the fuel mass should be and tells the ECU how much air is being drawn in to get a stoichiometric of 1. So if the map sensor is giving wrong data the car would run lean as the manifold pressure is less so less fuel will be added and boost could be limited or lowered by the ECU. I maybe worng but thats how I understand MAP sensors.
Its probably not the OP problem but thought I would suggest it as his car is requesting low boost.
We're talking about much less sophisticated engines there though.... The Audi variants all have MAFs as you already know.
The 1.8T engines don't use the MAP sensor for fuelling.
Those logs show boost request down as low as .74 bar (~10 PSI) in places, which is low low. Not quite low enough for actuator pressure, but then again, just looked at logs I have for a standard S3 210 and the boost request is .9 bar, so about 13 PSI.
What else could it be down to??? It is standard, as far as I know, just an air filter and a forge DV.
You need to resolve the problems shown up in the fault scan and then revisit dude really. Can't help any more until the N75 valve at least has been replaced.
Lets be honest - you have boost problems, you have a fault code for the N75. Get it changed out for a known working, or new one.
Not on an audi it does'nt. Me7 does'nt work like this
Does the op have a boost gauge to confirm boost levels vs map sensor?
No I haven't got a boost gauge. Any other way of doing it
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