Hello all,
Our 2007 Combi Aero has develloped an intermittent problem and our
dealer is unable to diagnose the problem after 4 visits. We get very
perceptible "jerking acceleration" under specific conditions albeit on
intermittent situations.
- When we drive for about 100km (60 miles) and then let it sit around
for about 7-8 hours. At that time the problem is quite pronounced even
when the car is quite warmed up and all the way back home. Most often
observed.
- When the environmental humidity is high, most of the time

Initially we got a check engine warning light. The dealer reset the
computer stating that it was sensing a non existent problem and that
the computer was merelly sending signals to compensate a non faulty
waste gate valve on the turbo.
Then the problem was back with the same warning light. The test
computer in the shop spotted a malfunction in the same wastegate but
the mechanic was unable to reproduce the jerking acceleration on a
test drive. He reset the system again.
Problem still present, new visit at the dealership and this time they
found a service bulletin that required a change of the spakplugs. Work
accomplished, same problem.
Intermittent snags are indeed quite notorious. Has anyone encountered
a similar problem or has any mechanical suggestion? I am down to water
hosing the engine to create a high humidity environment! Since the car
is relatively new, I discarded the idea (perhaps much like the Sabb
mechanic) of replacing the lead cables (spark plugs cables).

Cheers

Marc Arsenault

On Fri, 12 Sep 2008 05:48:09 -0700 (PDT), Arctic Rat
<marcarsenault (AT) sympatico (DOT) ca> wrote:

I guess this means that the wastegate valve was checked? A faulty
wastegate valve could cause the problems you describe.
How can the dealer be sure that the wastegate valve does not have an
intermittent problem?

If the wastegate and wastegate valve are working correct it seems
possible that a malfunctioning air pressure sensor is causing the
problem.
If the air pressure sensor is erroneously sensing too much pressure
the ECU cuts fuel input to prevent engine damage. If this happens it
does not seem unlogical when the ECU stores a fault code about a
faulty wastegate valve (since it energized the wastegate valve but
boost pressure was too high).
The cutting of fuel results in jerks. If this is happening under hard
acceleration I expect hard jerking, not some subtle jerking.

Is it only one jerk or does it keep jerking under acceleration?
Can you tell if there is suddenly more or less acceleration when the
car jerks?

I am not a car mechanic but an issue with the spark plug cables seems
unlikely if the computer complains about the wastegate valve.

Rob