low vaccum at idle

[ian911uk]

Hi, looking for help in trying to diagnose the cause for my engine
having poor vaccum at idle.
Car is a mk1 vw golf gti (1600), rebuilt engine 6k ago (rebore new pistons valves guides etc)
It runs well fairly smooth idle and pulls strongly to the red line, but idle vaccum is around
15 in hg. When revs are taken just off idle ie 1200rpm it increases to 18-20 in hg,when throttle is opened further
vaccum drops away as expected. Full throttle shows 0 vaccum and overrun shows around 22-25 in.
Running stock cam, compression checked, valve clearances checked and cam timing checked (even advanced and retarded
by one tooth makes no difference).
I can find no vaccum leaks even disconnected the brake servo to rule that out. Currently idling at 14 deg
but have swung this about between 3 and 20 deg but nothing makes any difference.
My ignition map starts at 20 kpa so at the moment i am wasting half my map.
Any ideas what to check next? or should i rewrite my map starting at a higher kpa value and ignore the
low vaccum reading as the car seems to run and perform ok?
All advise welcome. Regards Ian

[NITROPIXIE]

Where is your vacuum port on the carb?? If so then this is wrong as it is on the atmosphere side of the throttle valve. Alot of Golfs sometimes use the carb one

A true vacuum port can be found where a pipe goes to the brake servo. This will be a true vacuum. It should be on the inlet manifold, on the right hand side as you look into the engine bay. There is normally a stiff plastic pipe from the servo which then goes to a anti backfire valve, then a rubber pipe to the inlet manifold. I think there is normally a vacuum takeoff for a distributor/megajolt on or around where the anti backfire valve is. If not then you should be able to get one from a later model.

Hope this helps

[ian911uk]

Hi Nitropixie,
Thanks for your reply. The vacuum is indeed taken from the inlet manifold at the same takeoff as the
brake servo and is receiving the full vacuum that is being created by the engine.
Ian

[NITROPIXIE]

Is there a second pipe, similar to the vacuum pipe, which runs from this point to behind the carb???

[ian911uk]

There is a take off by the throttle butterfly but this is covered
by the butterfly when closed so has no vacuum during idle conditions.
My takeoff is on the engine side of the one way valve in the brake servo line.
I have connected my workshop vacuum gauge directly to the fitting in the
manifold for the servo but it still measures the same.
I am happy that my takeoff is in the correct position, but my engine
is not producing enough vacuum for some reason.
Regards Ian

[NITROPIXIE]

Your setup sounds fine for vacuum takeoff. This is how i would of set it up. Weak points on a golf engine from the exprience i have is a leak in rubber hose as they are of the age to split. The carb to manifold gasket deteriorates. How far open is the idle adjustment?? Do you have to use much/any choke for the car to idle? What sort of carb setup do you have?? Weber or pierburg??

[ian911uk]

Hi
The car is a gti (k-jet) so no carb. All hoses and gaskets replaced.
Idle screw is about 1.5 turns out and runs its cold start cycle well eventualy settling
to an even idle approx 800 rpm. Spent another weekend on it checking things but still no better.

[DannyP]

If you have a Mark I Rabbit with CIS, check the idle air bypass on the backside(I think that's where I remember it!) of the intake manifold. Either one of the hoses is leaking or the valve itself is leaking. Best way to tell is pinch the bypass hose shut with your fingers, idle should go lower when cold. Also, does it have EGR hooked up? If so, make a blockoff plate out of thin aluminium and plug where it goes in the exhaust manifold with a large (1/2" or so) ball bearing under the captive nut. A ball from a wasted CV joint fits well.