Intermittent low boost - most likely causes?

[Bernard Littau]

Hi Ben,

> When my 1990 200tqa makes 1.4 bar, it's wonderful.  When it makes 1.1
> or 1.2 bar, it's a slug.  It goes between these two states every so
> often, but not fast --- often I go for a drive and the car either can
> or can't make full boost for the duration of the drive.
>
> My question is: what are the most likely causes for such a problem?
> Intermittent problems are of course difficult to diagnose, so I guess
> I'm stuck replacing one thing at a time until I stop seeing the low
> boost.  What should I replace first?
>

When your car runs at 1.1 to 1.2 bar, that is the wastegate spring doing the
boost control.  These cars come with a 1.3 bar spring, so it generally gets
to 1.2 bar on the meter.

Anything above 1.2 on the meter is achieved by the ECU modifying the spring
pressure through the wastegate frequency valve (WGFV).  Before the ECU can
"add" boost, many conditions must be met.

Some of these conditions are:

1) Full throttle switch closed -- thus, as you surmise, a flaky full
throttle switch will cause the behavior you are experiencing.

2) Engine warm enough.

3) Engine not too warm.

#2 reading too low (car does not appear to be warm to the ECU), or #3
triggering too low (car appears to be too hot to the ECU) will cause the ECU
not to add boost.

Scott's site has many more details on the low boost problem.  The three
items I listed are my guess as to your most likely issues given that the
boost comes and goes -- not likely a torn Michelin man hose, for instance.

Each of these three can be fairly specifically tested for correct operation,
so hopefully you will not need to replace parts that are not broken as a
diagnostic.

See:
http://www.sjmautotechnik.com/boost.html
for all the stuff on boost.

See, specifically:
http://www.sjmautotechnik.com/boost.html#chklist

Good luck!

Bernard Littau
Woodinville, WA
'88 5ktq