Author Topic: Vacuum issues (Read 3694 times)

detour1999 · « **on:** July 30, 2007, 07:16:11 pm »

The car:
1991 Jetta 1.6 NA ~280K miles.

Here's the situation:
-no vacuum at idle
-vacuum is fine (~25in) after revving.
-as soon as brakes are pressed, vacuum is gone, and must be re-revved to have any vacuum again.

Here's what has recently been done (which is to say, in the immediate past... Vacuum hasn't worked since):
-new head (yeah.)
-new 1.9 metal hg.
-new oil pump (36mm)
-new oil pan gasket (actually, the one w/ the baffle)
-new injectors (stock rebuilds)
-new vacuum pump (was, and still is the vane style, replaced after all other things, after the vacuum problem surfaced, was purchased hoping to fix this specific problem)

notes:
-My IP is VERY old and worn (I'm in touch w/ Giles to get a nice reworked one) but for the time being, it's only happy @ 1.1-1.15. It's @ 1.1 right now.
-oil pressure (cold) is 100+psi at the filter flange.
-brake booster has been tested and holds vacuum.

I'm at a loss.
Anyone ever experience something like this?
Anything I should check that I haven't already?

burn_your_money · « **Reply #1 on:** July 30, 2007, 09:04:10 pm »

Where are you measuring vacuum from?

detour1999 · « **Reply #2 on:** July 30, 2007, 09:13:43 pm »

Quote from: "burn_your_money"

Where are you measuring vacuum from?

measured from all vacuum ports... pump to booster.
-d.

Trimster · « **Reply #3 on:** July 31, 2007, 04:41:48 pm »

Even after I got my vac. pump in, timed, etc. I still had low break vac. Found there was nearly a cup of oil in the bottom of the break booster. Sucked that out... all is well in the world.

detour1999 · « **Reply #4 on:** August 02, 2007, 12:00:05 pm »

Quote from: "detour1999"

-new oil pump (36mm)

The culprit.

Oil pump works great (as I mentioned, oil pressure checked on a mechanical gauge @ filter flange is outstanding...), however, it had apparently been sitting on the parts supplier shelf for a while..

There was a small amount of rust buildup on the oil galley that feeds oil to the vacuum pump to push the vanes out. Not enough oil getting up there: no vacuum at idle. As RPM go up (2,500+), centripetal force was pushing the vanes out, creating enough vacuum for one press of the brakes.

Oil pump removed, oil galley cleaned, reinstalled. Now pulls ~30in at idle.

Cliff Notes version
Check/clean the oil galley that feeds the vacuum pump when you get a new oil pump.

Side note, new head, injectors, exhaust (tubular manifold to 2" exhaust), new oil pump, new vacuum pump... I'm in love with my car again.

-d.