Erroneous Boost Gauge Readings
 

I have a '86 930, and on occasion, after starting the car, at idle, the boost gauge will be sitting at .4 bar. After an hour of driving, or perhaps as little as 10 minutes, the bar then might read .2, or "zero". The next time, after starting it may be at "zero" and seem to function normally. Might one assume this problem is with the boost transmitter or sender unit, rather than a vacumn line, or the gauge itself ?

QUESTION - when you start the car and it works normally, is the car hot or cold?

I had a similar problem with a 600+ HP turbo race car and the problem turned out to be a leaking plastic vacuum junction! When the engine compartment got hot, the crack would expand enough to give inaccurate readings. When cold, all was well.

BTW: we had to resort to using STARING FLUID to find the problem. We would spray a little along EVERY SINGLE VACUUM LINE until we heard the engine surge a little. Primative, but effective.

Thanks for the hint ! As far as when I start the car; it can go either way. Let's say the car has sat 24 hours, thus a cold start. One time the gauge will stay at "0", next time it will go to .4, or perhaps .2 . It might stay there an hour, then slowly go to "0". Or, the next time, a cold start, or hot start, no difference , it might stay right at zero. With no pattern, that to me indicates a gauge or sender unit issue , but I'm certainly no expert.

Any insight with this info ?

Your car should be on a mechanical boost setup if it is not already. If you have electric, get rid of it! There is no sending unit to a mechanical gauge - the gauge itself reads and displays the information. The installation is very, very simple - mounting a new gauge is the hardest part. However, you will be rewarded with dead on boost data (critical stuff) and you will have eliminated yet another possible problem area.

Reviving a very old thread here, but I have exactly the same issue described by the OP. Is this most likely a problem with the sending unit?