[s-cars] Low boost when engine is warm

[Kirby Smith]

I have a somewhat similar condition.  And it started, maybe, when Sunoco
dropped their high test from 94 to 93 (R+M)/2 this spring.  Boost no
longer reached the expected level.  Lower boost seemed independent of
temperature or humidity.  As some may recall, I use an A'pexi boost
controller, set at 1.3 bar peak boost to correspond to the capability of
the MTM 1 chip installed in the ECU.  The A'pexi controls boost, but the
ECU can still override it via ignition retard.  Over an apparent period
of weeks, peak boost dropped from 1.3 to 1.1. 

Plugs looked good but were replaced anyway, fuel and air filters were
replaced on general principles, intake tract was checked for pressure
retention.  Coil grounding was checked.  No effect. 

A week ago, I had the ECU checked for codes. There weren't any, but
reseting it anyway fixed the boost problem, at least temporarily.  But
after the initial highway tests, over the course of the past week the
peak boost seemed to drop off.  (Testing it going to work is not a
scientific process, but it seemed down.)  I had the ECU reset again
yesterday (via VAG 1522).  Again, there were no codes.  I got full boost
on the highway immediately afterwards, but I suspect that this week the
peak boost will again degrade over time.

These symptoms point to the ECU sensing a problem, even if I hear no
detonation.  Perhaps as noted below there is some other sensor acting
up, although I would have expected a code to be set.  Or maybe there is
a a problem that 94 Sunoco was covering up, assuming that there actually
is a relationship, and not a coincidence.

So short of putting in an ECU disconnect switch, or getting a VAGCOM to
reset the car on a daily basis, or trying water injection, I am more
than a tad unclear how to find the cause.  I will try to check the
temperatore sensors noted below, if I can get a break from my house
projects this weekend.

kirby, guessing from a different distance


Dave Forgie guessed:
> 
> Taka:  It seems obvious that however the circuitry is arranged, right
> now your ECU thinks the engine is over heated and therefore extra boost
> and fuel (and power and heat) would be a bad idea.  If you had a bad
> MFTS or ECU temp switch and changed it (them), you may still have to
> clear the codes in the ECU by removing power from it (disconnecting it
> or removing the fuse) for say, 5 minutes.  If that doesn't work, then I
> would guess that the wire from one of the new sensors has a break in it
> and the ECU default is "no info = bad info = default conditions = save
> the engine = no boost".
> 
> But  then  again, I just guessing from 3000 mi away.
> 
> Dave F.
>