trident111

Need some help. Trying to diagnose why I cannot boost more than .4 bar. Have a 2005 Turbo S with 17k miles, FVD Exhaust, 100 cell cats, FVD Headers, EVOMS DV and FVD Flash. I am don't drive the car too much maybe 3000 miles a year (hoping to get this number much higher once I resolve this).


Here is what has been eliminated:

No codes apparent (checked with Durametric and PIWIS)
No Boost Leak, smoke and pressure tested no leaks anywhere
DVs hold vacuum properly
Stock Wastegates open at close properly

After eliminating the above we thought it might be that I had some bad fuel in the car and might be detecting some knocking. So I ran the tank to completely empty and put in some fresh 93. Immediately upon driving, the car was back to boosting .8 to 1 bar (have an FVD flash). Great we thought so we call it a night.

Did not drive the next day, but I hopped in the car tonight and its back to boosting .4 bar.

Any ideas??????


Possible Culprits:
-Bad Maf Sensor
-Bad Knock Sensor
-Bad ECU Flash
-Maybe a fuel pump starting to fail?


What do you guys think? Any suggestions on how to diagnose?

Help would be MUCH appreciated!!!

bux996tt

Does it break up at all at higher RPM's? Sputter? Sounds like it is going into limp mode. But the lack of codes is puzzling. Seriously doubt it is a bad flash. My guess would be the MAF at this point.

Macster

Absent any other signs of distress and with no error codes...

I think bux996tt with his suspicions about the MAF may have something though I lean toward an intake leak possibly only under boost, in spite of your assurances about there being no boost leak and the DVs are good I'm going with one of those as the cause.

But the MAF can be (possibly) eliminated with less work.

You can disconnect the MAF sensor at the wiring harness and clear the error codes (to reset the DME's adaptions back to their defaults) and see if the symptom returns.

If a knock sensor were bad there'd be an error code pointing to the sensor, its wiring, or to (spurious) mis-fires.

An aftermarket ECU flash is always suspect but I do know anything about this one to say one way or the other, though if the symptom just appeared *out of the blue*... the ECU flash becomes less likely the cause.

The symptom does not agree with any fuel pump failures I've heard/read about or even experienced.

If the fuel pump was the problem the engine would probably not run above a certain rpm simply because the fuel pump would not be able to deliver enough fuel. You report the engine won't make full boost not that it won't run over a certain RPM.

Let's see... an exhaust leak can bleed off exhaust gases and lower the boost output. Or possibly something blocking one of the exhaust manifolds or exhaust pipes and possibly restricting the exhaust gas flow which could limit boost?

The lack of regular use may be a clue... Maybe. I'm thinking -- long shot here -- but I'm thinking perhaps the car sat long enough rodents got at the car? I'd think you or your mechanic would have spotted the signs though given how close to the car and the engine you have been to troubleshoot this problem already.

Sincerely,

Macster.

kezcapt

I have an '05 "S" and experienced the same problem. After making the obvious checks, (spark plugs/ coils are good, the engine is getting fuel etc) I did a pressure check to assure the system was air-tight. (It was pretty simple to make a boost leak tester.) Now, if the engine is working and boost is available, we are down to engine control. The most likely culprit is what the rest of the world refers to as an "N75" valve. Porsche calls it something else but it is an electromechanical valve that controls boost to the engine. if you google N75 you will find reams of data but it is a simple valve and when mine failed it did not generate any codes. The valve is relatively easy to get to (top of the engine) and is easily available from a reputable source. (I use a famous p-car parts place in Oregon). In summary, it takes hours to diagnose properly but only about 30 minutes to replace. If you want to do things in reverse you can remove the valve and bench-test the solenoid in the valve with a simple 12 volt source. It is amazing that the engine boost comes down to this small part but that's my story.

"02996ttx50

that's invaluable information. i hope i never need it, or that i can find it again if i do. points for you.

Kevinmacd

Here's a link to the N75

http://www.6speedonline.com/forums/9...rging-fix.html