Hi!

Finally got around to working on my '93 928 GTS.
Adjusted the timing belt with the Kempf tool and I'm now working on the vacuum system.
I have tried looking and searching, if there is a thread about this, my apologies.

I have read this, but I can't seem to find what I'm looking for.
http://members.rennlist.com/pirtle/svc_act.html

My car has really rough gearshifts and I suspect that there is a vacuum leak.
I replaced a small vacuum hose today as it was brittle/broken (see pic), and it might have
done things slightly better, but it still shifts rough. Does anyone have pictures or info on
where the the vacuum line (based on the variable engine vacuum described in the link) is
in the engine compartment and where it goes to the garbox? Where are the different
(weak) points that usually brakes and needs to be replaced? Hopefully are they either in
the engine room or by the gearbox I do understand that either the rubber cap or the
modulator can be the problem as well, but I haven't checked them yet.


Christian

 

Christian,
Here's an image of the vacuum diagram.
Regarding the hard shifts this is what I'd recommend. First check your ATF level. Get car up and level. Get running to operating temperature, then check level of the markings on the reservoir by the rear tire (may be dirty so wipe clean). Should be between the Min and Max markings near the top.
Next, I'd make sure all your vacuum is good off the manifold. The line that goes to 'e' is for your transmission. Use your device (a MityVac in the States) to measure off 'e' line. If I recall should be at 20 in H2O. If not, track your lines and fix any defects. This is easily said, but sometimes not so easily done. If you have good vacuum of the 'e' line then I'd check to see what value you have for vacuum at the other end of the 'e' line at the pressure modulator on the transmission. Is on the Left side. You'll see the line attaching to the modulator at 10 o'clock. If vacuum is less then off the manifold, that line is faulted. Next to check would be to see if the pressure modulator holds vacuum. If it doesn't, get another. Start here. The next thing to check would be your control pressure control cable (Bowden cable) off of the cable 'quad' on the side of your manifold, but start here and see what you have.
Best Wishes

 

to OP, what do you mean by "rough gear shifts"?
I assume shifting hard is in the seat of the beholder.
...

 

MainePorsche: Thank you! Will look into this during the weekend.
The ATF level should be fine, as it was changed by a workshop before storage, but I will of course check it. Is the vacuum line(s) off the manifold visible/accessible, or do I have to take things apart?

OTR18WHEELER: Not sure if you're trying to be funny. I'm not a mechanic, but I've had 2 928 S4's, 1 928 GT and this is my third 928 GTS and something is NOT right with the gearshifts. It changes gear with a bang at it feels like the car is going to be ripped apart. One of the symptoms with a vacuum leak is rough gearshifts and I'm trying to get some help to sort it out as I have not been able to find (for me) good enough info to try to find the fault(s) myself.... Telling me that I should buy a Prius pi***s me off, as I'm trying to get help fixing my car and your not contributing to that at all....


Christian

 

Christian,
As seen in the diagram, the vacuum line to the transmission comes of the 'T' under the airbox along with 3 other lines. The 3 other lines go to a fuel pressure regulator and 2 fuel pressure dampeners. You changed a dampener connector - make sure all are good/uncracked.
The transmission vacuum line can be faulty in holding vacuum for it is not one continuous line. It goes from the 'T' as a rubber/plastic line over the bellhousing then attaches to a steel line for a bit, then attaches to a rubber line again as it makes its way to attach to the pressure modulator. There is therefore places it can detach/crack. I replaced with a single sturdy line like this (#3 size) https://www.pegasusautoracing.com/pr...p?Product=3260

You would have to raise the car and get under to replace the line and access the attachment point on the modulator. By what you said about the 'bang' when it shifts it seems that either you've lost vacuum to the modulator, or the modulator is shot and not holding vacuum. If I remember correctly the line can be seen/accessed from under the car by removing or gently bending back the heat shield for the exhaust/catalytic converter on the Right side. Check the status and/or all connections of the line. After you know the status of your vacuum to the modulator and the status of your modulator, report back and we'll take it from there to the next level.

Craig

 

You're right. The force of the shifts is regulated through the pressure modulator which is dependent on vacuum for its function. The control pressure control cable (Bowden cable) tension controls the timing of the shifts. Both can be adjusted for 'perfect' shifts.

Add: Is very important when you look at you pressure modulator (26) that you have the black rubber cap (29) and it is in good shape. It is needed to hold the vacuum of the unit.

 

Seriously? My reading of the OP's question was that he was asking a genuine question and looking for help, which didn't warrant what you posted. If I was the OP I'd be pissed off too.

 

Thanks MainePorsche and Dave928S

I just came in from the garage

Removed the air filter/box in the dark yesterday, further examinations this morning shows that the intake has cracked on both sides by the MAF.
I guess this can be the reason for the vacuum problems (false air)

Might get hold of another tomorrow (approx $500), are there any things I have to take into consideration?
Are they all the same for the 32v cars and anyone will do for the GTS?

As mentioned earlier, I'm not very technical, so thank you for bearing over with me and my questions


Christian

 

If you post a photo of what has cracked we can give you more accurate advice.

You may be looking at the intake leg on each side of the MAF, and if you see what looks like cracking, it's highly likely it's just cracking of the coating on the manifold, which is very common, and not cracking of the manifold itself.

 

Short answer from my phone, hope this works (Dropbox)

https://www.dropbox.com/s/vjvfxnf9ar...6.21.jpeg?dl=0

https://www.dropbox.com/s/gsk1ssftz6...6.04.jpeg?dl=0

https://www.dropbox.com/s/vpwqk14rn6...4.32.jpeg?dl=0

Christian

 

Kind of makes you wonder what would cause that...a big backfire or something? Has it ever been off? Weird.

I would imagine that intake could be welded, if it is aluminium. If it's some other composite...an epoxy may do the trick (if you don't care about aesthetics).

Good luck, whichever route you take.

Brian.

 

On the first image of the R leg it looks to be a true crack of the manifold. Closer shot may be helpful. Would explain loss of vacuum, but I would imagine more symptoms beyond just transmission's loss of vacuum symptom.

If are true manifold fractures, how and why ?? Don't think a simple backfire would do it, and not symmetrically on both sides. Was this manifold ever on an engine that significantly overheated and warped ? Prepare yourself for a replacement. Take some close up shots of the architecture of the cracks for confirmation.

 

?
Would think a weld might work.
An epoxy...lots of vibration, expansion/contraction with heat, thorough application of the agent...would be skeptical of this.

Add: Can you see the footings of the manifold ? How are they ?

 

I believe I've read a few times that the intake manifold is a (cast) magnesium alloy. My background is in - sniff - more sophisticated materials, so my prejudice is that it might well have had casting defects that grew into those big cracks, perhaps encouraged by bad torquing.

You'd want someone experienced in welding magnesium, since we're long past the July 4th celebrations. Epoxy or duct tape would make a good experiment, unless there are more cracks you can't see.

I've got a hiss from the vicinity of the manifold, but don't know if it means a leak. I've never had the air box off. I have harsh shifts, too, which I've been considering fixing or getting fixed. (I already own a Prius, btw.) Thanks for a timely discussion.

 

If it's magnesium...I believe it can be tig welded. If so, a good tig welder can usually lay down some of the best/hidden welds known to man.

As far as epoxy's go...I've used the JB professional grade stuff and I believe it has the ability to stretch/expand some.

Just don't try the epoxy if you plan on getting it welded...this would only make matters worse for the welder.

Brian.