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!

They had that in '84 - wierd.

I think it's the same device all the trap oxidiser cars
got in 86/87. Basically a boost limiter - it recirculates
air back to the input side of the turbocharger. The shop
manual for my '87 124 diesel says that it is used to
manage the boost levels to suit the trap oxidiser, without
going into any additional detail. I don't know why your 84
would have it, since there was no trapox that year.

My own theory is that it was used to limit the unthrottled
air flow through the engine to raise exhaust gas temperatures.
The trap oxidiser had to reach a very high temperature before
it began burning soot particles.

The vacuum feed to the boost modulation valve has mysteriously
become disconnected on my '87, and the car runs great. Once
a year I have to reconnect it to pass a visual inspection, and
the car runs like total crud until the gremlins get after it
again. You'll be driving down the road, and all of a sudden
all the boost disappears. You have to go to about half throttle
to maintian 40MPH. Thing also smokes like crazy with the
valve connected - cuts down on tailgaters, I guess.

Try disconnecting the vacuum feed to the unit; either at the
valve itself or further upstream. The computer control on my
87 is an open loop system, so it doesn't have any idea whether
the boost modulation is operating or not, hence does not
complain about the modifications. Give it a shot and let us
know how the car runs.

Thanks for the information. Prior to seeing your post, I also 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, like you, 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.

Thanks again!