5kcstq low boost, sputtering-bad temp sensor?

[barry at moonbeast.com]

Hey gang-
It's been awhile since I've posted, hope everyone's doing well.  I seek your 
wisdom once more.

My '87 5k is almost sorted out now, but I've still got some gremlins that I'm 
trying to exorcise.  Here's the major issues:

1. I'm consistently seeing low boost - in normal driving, not usually more than 
0.9bar, and not more than 1.2bar at WOT.

2. Sputtering at idle.  By messing with the idle adjust screw at the front of 
the intake manifold, I can balance it between wanting to stall and going into 
the infamous "idle bounce" mode where the engine speed rises, cuts out, falls 
then rises again (ISV and its hoses have been replaced).  The exhaust is 
sputtering at idle, even though the car is smooth on acceleration and at any 
road speed.

3. The water temp gauge shows full cold pretty much all the time.  It has risen 
to half scale on occasion, but unpredictably.  Most of the time it stays pegged 
on full cold, even after more than an hour at highway speeds.

4. Fuel economy is crappy- I get about 19mpg consistently, on rare occasions 
topping 20.


My (admittedly inexpert) guess at this point is that the water temperature 
sensor has gone bad, and the system never gets out of "cold start" mode.  It's 
constantly running that intake manifold injector, which makes the system 
consistently run rich, which in turn is plugging my cat.  The plugged cat is 
restricting the turbocharger spool-up and making the engine sputter at 
idle 'cause of the low exhaust airflow and rich mixture.

Does that sound plausible, or am I off my rocker?  Any insight/BTDT would be 
greatly appreciated.

Cheers!
-Barry

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