The first thing to check is the thin vacuum hose that connects at the rear of the intake manifold, make sure it is in good condition and connected properly. There should a one way valve on that hose, check it's operation.
Try removing one of the vacuum hoses with the clear plastic elbow from the heater switch under the steering wheel and to the left - have engine idling - there should be strong vacuum from one of them.
If not there is a vacuum problem.
Mine does not work too.
I noticed when cleaning the air intake that someone had blocked off the small vacuum hose at the rear of the intake manifold with a small length of hose with a screw in the end.
This may indicate a leak somewhere.
There is a small round plastic thing attached to this hose which would be the one way valve. From there it branches two ways, one going into the cab.
Might go downstars and see if it works after connecting that hose back up the other day.
The clear hose was not connected as per Hako's photo above.
Hooked it up and now all is working.
While I was there, had a look at the speed alert that also was not functioning.
The connector for it was not plugged in.
Now that works too