Lost vacuum intermittently for some time. Vacuum measures around 10 when not working 20 when working. Using a vacuum gauge I found the loss all the way back to the manifold. I have the same working and non working 20 on 10 non working at all lines back to the brake booster and line cluster. All lines are now disconnected and gauge is on the manifold measuring 10 when not working. Can anyone help with this?

 

Help yourself with getting the best help possible by giving as many details about your car as you can. There were multiple markets that these cars were built for and many changes over its 18 model year reign.

 

Sorry, the car is a 1987 928S4. with the 32 valve V8. The problem has been going on for a year or so, always made it home in limp mode. Now as I mentioned in the thread, the vacuum guage is hooked to the manifold with all lines disconnected and I am reading around 10 with a little flutter.

 

It would help if you can be specific about where you are taking the vacuum measurement from.

Suggest you take a photo and post it if that is possible with user status- cough up a few bucks [$16?] and be a paid subscriber to get full value of this utility.

 

How long have you owned the car and what, if any, maintenance have you performed or paid to have completed? Also, do you have any knowledge of work performed by the previous owner(s)?

It may well be intake refresh time. That's nothing to be bummed out about. If you're doing the work yourself, I think it is one of the more rewarding jobs to do for your 928.

 

Following up on Fred R's question:

Your car has three separate vacuum systems. The easiest to identify is the power brake booster, which is connected to a small eductor (Venturi) on the *left side of the intake manifold. (*Left is left looking from the rear of the car). As more air flows through the intake, more vacuum is generated by the eductor and applied to the brake booster. The booster has a check valve that holds vacuum at the highest value it sees, so even as load and airflow change, the brakes have the maximum vacuum available for assist.

The second system ties into the same source at the brake booster check valve. This system has its own check valve for the same reason as the booster. This system serves the HVAC (heater and air conditioning) system to move diverter flaps and the heater control valve. It serves the cruise control unit on earlier cars, and the intake manifold diverter flap (the "flappy") via a solenoid in the *left front of the engine above the water pump. This system includes its own accumulator tank in the left front wheelhouse behind the wheel. Easiest place to test this vacuum is at the multiport rubber connector next to the brake booster.

The third system is connected directly to manifold vacuum at the bottom of the manifold, and serves the fuel pressure regulator and the fuel pressure dampers, plus some smog-related valves. This is the vacuum system shown on the hood sticker on US cars. This vacuum follows engine load, with vacuum decreasing as engine load increases. IIRC this number hovers around 17-19" at idle, and decreases to as low as zero under heavier load. It goes higher on trailing throttle when the engine is slowing the car. the easiest place to test vacuum in this system is to tap the hose running to the front fuel damper on the right side front of the fuel rail.

 

Jan, I know you called me on the phone but I'm going to reply on the forum, as many heads are better than one.

The intermittent nature of your low vacuum is a mystery but probably a strong clue. You can search the internet and find ways to track down a fixed vacuum leak, such as spraying the running engine with starting fluid and listening for a change in idle speed or, with the 928, (if you join and can then do Searches) you will see older posts about building a little set of plumbing fittings to allow you to apply positive pressure to the intake while listening for leaks.

Things I can think of that are quasi intermittent with respect to placing a load on the vacuum system is the heating system control network, the brake booster, and the fuel tank evaporation recovery system. Hopefully others will chime in with other culprits. There should be a vacuum diagram in the engine compartment. It is not exactly user friendly but you should be able to trace the lines back and "clamp them off" one by one and see if it resolves the problem.

I second the recommendation about an intake refresh.

 

This statement is somewhat troublesome to me, "...always made it home in limp mode.",
Limp mode is when the MAF fails and the engine will respond to limited throttle input, enough to get you home.

With that said, some miss read a failing ignition system as "Limp mode", as the 32v has two separate coils that feed four cylinders each, if one fails the engine runs on 4 cylinders and dumps fuel in the no-sparking four, if run for a prolong amount of time a CAT fire will surly follow, burning the car to the ground. Porsche introduced a fuel cutoff in later years '89, IIRC and yours is an '87, so be sure all eight cylinders are getting spark
This will also show a low vacuum as the result of running on four cylinders.

Dave K

 

What I described as "limp mode" occured several times. The last time it continued to run poorly at home long enough to measure the vacuum. at the brake boost , the fuel regulator and the manifold vacuum fitting on the right side of the manifold facing the engine 11 when running poorly, 20 when running well. I have replaced all exposed rubber fittings from the brake booster back to the fuel regulator.
I recently had my 928 in a Porsche dealership and replaced the water pump, timing belt and all of the recommended parts including the radiator. The drive shaft was rebuilt with an upgrade bearing. They also did a complete tune up. Spent many dollars and running fantastic when vacuum is right on the places that I mentioned. Thanks to all that have offered advise.

 

Sorry, I picked up the term "limp mode" somewhere and was not aware that it was an actual mode in my S4. That term seamed to fit, It felt like the power loss was more like a fuel mixture issue.

 

Jan --

Consider that the erratic vacuum reading is a symptom of a problem rather than a problem itself. Coincident with the drop in performance, we are looking for a performance-related problem that affects vacuum. Plus we still don't know which of the two available vacuum systems you were testing.

All that said, a standard guidance is to look at the last things you worked on, and in your case the timing belt replacement and tune-up are the tasks you shared with us. Based on that, my troubleshooting list would start with removing, inspecting and re-seating both coil wires. These are the high-tension wires between the distributor caps and the the ignition coils. During both the timing belt change process and a tine up, the distributor caps are removed. It's a popular error to fail to get a coil wire end completely seated at one end. The result is an engine that runs on only four cylinders.

Continuing on the ignition theme, make sure that the distributor caps and rotors are installed correctly. Each rotor screws onto its respective cam gear with three small screws. The rotor needs to fit flat against the cam gear, screws just snug. The rotors should not have damage.

More on ignition, the caps each mount on a cam belt cover with a couple small screws. Again, they need to fit flat on the cam belt cover. While you have the cap removed for the rotor inspection, look inside the cap to verify that the little spring-loaded carbon "brush" is in the center and intact. This carbon pin/"brush" carries spark energy from the coil wire tower on the cap to the center of the rotor.

And even more ignition... The spark plug leads are easy to mix up if the tech wasn't careful. This is lower on the list because mixed up wires are -always- mixed up. Your symptom of runs good some of the time pretty much rules thi s out. Still worth checking that all the plug wires are securely seated in the caps.


More stuff to know: If you are in fact suffering from one side weak or failed ignition, you should not drive the car, and for sure limit the engine running to test. While one ignition bank is failed and you are running on only four cylinders, the other four cylinders are pumping raw fuel and air If the car has catalysts installed, they will fairly quickly overheat and catch the car on fire. The fire starts in the undercoating under and forward of the gear lever, and the heat causes the interior insulation, carpet and padding, console & seats, plus the wiring to catch fire. Fire is not a friend to 928 owners. Many of our extinguished members can share their experiences with fires caused by overheating catalytic converters. Don't be one of them please.

Even if you don't have catalytic converts on the car, Porsche has shared a recommendation related to driving with the engine in four-cylinder mode and driveshaft failure. The uneven pules of torque damage the relatively elastic driveshaft, plus cause premature torque tube bearing failure.

Hope fully these few simple steps will get your car back to top performance again.

 

Thanks Bob! I agree the ignition should be cleared even though the vacuum problem that I am having did not show up until at least a year after service by a Porsche shop. The system that I am checking is the one that connects between the outside of the intake manifold on drivers side of the intake and runs to the brake boost. At the fitting on the manifold there is 11 and that stays the same thru all lines back to the spider fitting and booster.I hope that makes sense.

 

Let's take a step back here- What symptoms prompted you to think that there was a problem of some kind and what motivated you to check vacuum?

 

Sounds like vacuum is stable at around 11"hg. Bob is right. Vac gauges can be misunderstood. It definitely sounds like an ignition and/or mixture issue to me.

 

I suspect that the vacuum readings are being taken at the cross connection next to the brake booster, therefore reading the HVAC and flappy supply vacuum (with leakage...) rather than the actual manifold vacuum. The telltale is that vacuum increases with load, while manifold vacuum would decrease with load. The OP's first post talks about testing there by the brake booster, then moving to "the manifold". Knowing where on "the manifold" he's testing vacuum is critical. Regardless, the vacuum is a symptom of another problem with the engine, and not a problem unto itself.