Geo1965

Hi 928 Community :-)

My 928 S4 was diagnosed with 0 vacuum from the intake manifold; as a result all vacuum lines to the fuel pressure regulators and auto tranny were not operating at all notably with very harsh upshifts from the tranny.

As a temporary solution and to avoid removing the intake manifold to fix the vacuum connection problem, I disconnected the vacuum T system coming from the manifold and connected the fuel pressure regulators and tranny to the vacuum connection coming from the brake servo.

The tranny is now working perfectly but will hold each gear until 6,500RPM and will keep 3d gear forever before engaging 4th ....so my first question is whether I did something stupid and risk the safety of my car or is it a matter of adjusting the vacuum modulator on the tranny or anything else?

Also I have seen no important power increase or decrease with all this vacuum mods although the car revs very differently with and without vacuum in the fuel regulators. Any thoughts on the effect of full time max vacuum on the fuel pressure regulators too? Can I break something?

Thanks for your thoughts!

Kindest regards,
George
-89 928S4 Auto Baltic Blue

FredR

Your car has two vacum systems- one is active, the other is passive [i.e. constant]. The constant vacuum system is required to ensure the brake servo is ready and able at any moment and connected to that there is a vacuum reservoir that is used to drive the flappy and the HVAC actuators.

The active system uses real time vacuum to modulate the fuel pressure regulator, the dampers and the shift modulator in the gear box. If the fuel pressure regulator and dampers are exposed to a full vacuum full time you will end up with a lean condition that gets worse as engine speed increases and that can end up overheating the pistons with potential for whatever damage goes with it. The gear box modulator needs vacuum to work so it will do its intended job connected like that but whether there are other consequences connected to vacuum full time someone like GB will have to advise you on that.

fiatrn

The follow up to Fred's reply is

Yes, you can damange your car using it as you are. Maybe not immediately, but eventually.

Fix it properly asap.

gazfish

+1 fix it properly

dr bob

George --

You can remove the MAF and the throttle to gain enough access underneath to reattach the manifold vacuum hose. If you have or have access to a borescope/endoscope/camera, it will help you some in finding the hose and the target port. The rest will be by feel, mirrored upside-down and facing the wrong way from the direction you'll get to the belly of the manifold. Use caution especially if the hoses under there are originals. The oil vent and idle stabilizer hoses are, um, delicate in their advanced age. They like to fall off or split if they see much disturbance.

I'm sure there are pictures available of how/where the hose connects. I don't have any, unfortunately, but others will undoubtedly pop in with guidance.

I've seen those vacuum hoses replaced using a picker-upper tool with the endoscope camera. I've never tried it myself though.

FredR

The sub manifold as removed from the motor- The ISV is at the front- ust behind the ISV are the two connections supplying vacuum- the front most one is downstream of the butterly and is the connection you need to take vacuum from to the multi way manifold just behind the MAF.

dr bob

Thanks Fred, that's the Perfect Picture! The clamp just aft of the vacuum connections is accessible with a 7mm (IIRC) 1/4"-drive socket on an extension from the right (passenger on US cars) side of the intake, easier to see/access after removing the fuel rail cover. The MAF boot has oil vent hoses attached, so comes loose and goes down to make room for fingers. I know folks who have thrown a few heavy blankets over the intake to lay on top of the manifold for access. I've managed to get in with one hand while standing to the side, but still with fender covers and such over the bodywork anyway.

Given a choice, I suggest using factory tubing and rubber elbows. The seem to last longer than many of the 'latest high-performance racing pink silicone alloy..." pieces.

FredR

Bob,

I didn't know you were into proctology-

When I do my inlet manifold [as I did a few weeks ago for that pic] those connections are first on my check list of items before buttoning up. Anyone who can do that job "in-situ" gets my vote!

Geo1965

Fred thanks for your replies,

That's exactly the pics I needed to (try to) fix this!

Great help!

George
-89 928 S4 Auto Baltic Blue

Geo1965

Bob,

Thanks for the detailed explanations. I've done the blanket over the manifold in the past to remove the MAF and retune it by JDS in UK, so I guess I am good to start again! :-) Agree that factory tubing and elbows are very resistant so will definitely go with stock parts.

Thanks again !

George
-89 928 S4 Auto Baltic Blue

Fabien92

Hi George,

Two days ago I changed the vacuum line from the MAF to the 5 ways splitter.
To do that, we have removed the MAF, not the intake manifold. 3 hours of work !!

A friend helped me. He had the tools, I had the light ! It's not easy alone to see where to work. You need eyes at the end of your fingers

Now I measure vacuum at the 5 ways splitter (D and E).

Zirconocene

What would the impact be to the engine/performance if the vacuum line for the air pump were not attached or capped properly?

The air pump on my S4 is no longer attached and, until I read to the end of this post, it was not obvious to me that it was connected to the vacuum system. Now I have questions.

Thanks

FredR

It should have little to no impact.

The line that connects to the diverter valve is tapped outboard of the throttle butterfly so when the throttle is closed it makes no difference at all.

When the throttle is opened a very small amount of unmetered air will be drawn into the engine that will lean out the AFR slightly but I doubt you would notice such.

Geo1965

Thanks Fabien, for the follow up and the pics.

My vacuum line from the 5 way splitter went out of the rubber elbow on the intake. Not sure how did you hold steady the rubber elbow to insert your new vacuum line? any tools or pics will be much appreciated.

Another question I had is did you remove only the MAF or the thick round rubber support base of the MAF? Seems to me that way one can have max access but asking if this is possible at all without removing the intake...

Thanks again
George

Fabien92

Hi George,

We removed the thick round rubber support base of the MAF. It’s not possible to access if you don’t remove it. You have three hoses plugged to this base (on a GTS). You have to unplug them.

No special tools. Only a screw driver, our fingers and a small light. It’s only a question of feeling with your fingers.

All my pictures are there. No more pictures. Sorry.