I’ve waited until I received my 123 service CDs so that I wouldn’t ask a question that is answered there, but alas, there is no answer.

On the California version of the 1984 300D, in addition to a slightly more complex vacuum system, there is also an ECU. Someone previously posted information that stated the boost control is RPM dependant, and boost does not build as early as in the non-California models.

There is an interesting vacuum controlled valve on the outlet side of the turbo. I am theorizing that it could be a recirculation valve. Some gasoline powered turbo cars use a turbo compressor bypass to keep the turbine wheels spinning (thereby avoiding losing inertia) when the throttle is lifted for gear shifts. It is NOT the wastegate that I am talking about here.

The service CD refers to the valve in question as the “circulating air safety valve” (on page ‘14.8 – 100/12 USA 1984 California F 3’, Item #71), yet there is no explanation of its function. This leads to two questions, and I’m sure that someone out there knows – maybe our resident genius & Oracle of all things theoretical from Florida could disseminate some wisdom here :).

#1. What is the function of this valve – air recirculation / flow restriction / pressure transducer / or ???.

#2. If it IS a method of limiting boost at low RPMs, what can be done to bypass it’s function – apply vacuum to it at all times, or leave it open to the atmosphere?

I guess that brings up a third question: is it a stupid thing that I’m trying to do here? For example, is the injection pump calibrated differently for California, and thus would the fuel mixture be inadequate if boost is allowed to develop at lower RPMs? This seems counter to my understanding of diesel functioning – I should not need to be concerned with too lean of a mixture should I? I mean, isn’t that a fundamental principle of having an un-throttled engine? I better quit here… I’m starting to go off on a tangent.

Any information would be GREATLY appreciated. This subject has not been addressed previously (I checked over 100 threads) and many other persons beside myself could use the information.

Thanks again!

For future reference in the archives…
 

Just in case anyone wants to know, I did a little mickey-mouse reverse engineering of sorts on the unit. I hooked up a vacuum gauge to see how this valve received vacuum under varying conditions. It gets a relatively low vacuum signal, which does not actually vary too much, but does vary consistently under the same conditions of loading & RPMs.

I then set up a temporary boost gauge and ran another line to the valve from my vacuum tester hand pump. While driving with all three hands & four eyeballs, I discovered that when vacuum is applied to the valve, the boost is inhibited. This leads to my conclusion that I will plug the vacuum line leading to the valve and leave the port open to the atmosphere.

Next on the list is wastegate adjustment, as I only am getting about 600 mbar of boost.

The boost level sounds correct. You may want to try turning up the fuel at the anaroid valve on top of the injection pump. This will give you the greatest amount of performance increase.