In the diesel engines, a vacuum pump is used to provide the vacuum
needed by the power brake booster. In your VW diesel engine, the oil
pump and vacuum pump are both connected together and are turned by an
intermediate shaft that engages a gear on the vacuum pump. There is a
smooth pulley on the other end of the intermediate shaft and this pulley
is turned by the smooth back side of the timing belt. The fact that this
engine relies on mere rubbing of the smooth side of the belt to turn
these critical items creates the possibility of the belt slipping,
resulting in reduced oil pressure and vacuum pressure. That said, the
system works pretty well unless something has gone wrong to increase the
amount of force needed to turn the intermediate shaft. The shaft
bearings could be failing, or the vacuum pump could be failing, or
something could be amiss with the oil pump. I would be more suspicious
of one of the first 2 I mentioned. Is there a lot of oil getting on the
back of the timing belt from some leak?

Mark

Jaap Nauta wrote:

>Although my diesel runs fine for some years, there is one thing that keepsing me, and I wasn't able to find any info on it.
>After starting the engine, it take regularly quit some time to build up oil
>pressure. Sometimes the light goes off immediatly, sometimes more than half
>a minute before it goes off. Outside temp does not matter, nor does oil level.
>I never have problems once it's running.
>The weird thing is that the brakes don't have vacuum as long as the light is
>flashing. Then suddenly there is a slight change in sound from the engine,
>the light goes off and vacuum comes up.
>
>What I do is always wait for the light to go off, sometimes reving a bit higher
>helps.
>
>Any thoughts on what could be the cause? I was thinking the oil-pump, but that
>doesn't explain the vacuum
>
>Thanks
>Jaap
>1987 Caravelle Diesel
>
>
>