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I recently did this tedious job and have a different approach that more qualified M96 guys may have comments on -s o heed them not me !
The small diameter vacuum hose is very brittle after so many heat cycles.It easily cracks when manipulating it during re-installation. O.K. if you notice at the time but not so good if you get an SAI code or Flag after you have reassembled it all !
If you try to separate the oem vac line from any of the 'rubber' boots/connectors you may find it is glued on and if you twist it off to repair ,another break/crack occurs.
I decided to replace as much as possible of the entire system with USA-made(Samco) 3mm silicone hose and BRASS connectors/fittings/adapters/splicers.
If you search EBay for "4mm Hose Barb Fit Brass coupler Splicer Connector fitting" or the Imperial equivalent 3/16 " you will find "Y's" , Cross, "T", straight adapters and more.Yes, it is tedious hunting but the brass/silicone combo will last forever and you will never have a vac leak in this part of the system again! . Make your own measurements, Be ready to convert sizes mm/" and remember the good quality black silicone hose(probably 3mm) is very stretchy and flexible. Cut it long.
And do not forget to pressure test the Vac reservoir under water for leaks in the seal at the base. Otherwise you will need to re-do.
If part of the reason/problem for your vacuum line work was an SAI code/Flag ,there is a beta SAI delete hack circuit on 986 Forum that solves it. There is a related beta circuit for inefficient cats to 'help' the post-cat O2 sensor give a get-you-to-pass Smog Test
Ya, I had the same problem, losing mine. It turned out to be hanging just above the alternator, back a few inches where it wasn't visible. I found that both of the long vacuum lines were a little short (especially after one cracked), so I bought some vacuum line and couplers from Pelican and a local shop and extended them. The photo below shows the new vacuum line, which is white. You might be able to infer where the lines originate from the photo.
Thanks! I was able to find the hose, it was pretty much where you said. Of course, I dropped one of the bracket screws in behind the motor..
Thanks for this Duncan. While changing the alternator, I pulled out the line going from the change over valve to intake resonance flap actuator. This diagram was very helpful. How did you reattach the lines?
I got it done, but it felt pretty crude and really lucky. I lowered the engine and was able to pull the rubber part connected to the resonance flap by reaching over the top of the engine. I reattached the line from the top and started threading it back down while flashing a light underneath the distributor hose. I was barely able to fit my forearm over the top of the engine to do this. Once I saw where the line was, I shaped a hook from wire coat hanger and i fished it out towards the change over valve.
I have a 987.2 and have owned it for a year. Just had the clutch, double mass flywheel, RMS, water pump, idler pulley, and serpentine belt replaced by local IDY. Les than a week after the repairs there is a cel with both P0491 and P0492 displaying just 18.2 pressure. Is the vacuum reservoir behind the front access panel or the rear. I am guessing the intake actuator is near the air filter.
I have a 987.2 and have owned it for a year. Just had the clutch, double mass flywheel, RMS, water pump, idler pulley, and serpentine belt replaced by local IDY. Les than a week after the repairs there is a cel with both P0491 and P0492 displaying just 18.2 pressure. Is the vacuum reservoir behind the front access panel or the rear. I am guessing the intake actuator is near the air filter.
Hi Guys! Sorry to resurrect this very old thread (almost 10 year old ) It seems that we all have messed up the hoses when we change our alternators! I think it's really not our fault because as time goes by, these vacuum hoses (rigid types) tend to get brittle.... so it's really Porsche's fault!
I just have a question to the OP if he's still around...and I apologize in advance if I may have missed the answers within the posts. I included this picture to explain more what my queries are...
1. Regarding the Y Connector encircled a the bottom, I seem to have lost mine, I can't see it anywhere. Is the Y Connector really attached DIRECTLY to the Changeover Valve #1?
2. On the Black & White check valve thing, I also seem to have lost this part and plan to purchase one now. Is the line really this short? Because I seem to not see this valve anywhere in the engine bay as well..... and which intake manifold does this connect to?
3. Lastly, there are 2 intake plenums on the engine bay driver's side; one in front, the one nearer the cabin and one towards the rear, the one closer to the back of the car. Which one please?
I also got confused with this other post on P0410 and P1411 error codes tackling almost the same issues because in the diagram...there was no Y Connector directly connected to the Changeover Valve #1...Strange!
Hope you guys can help out. Our cars are now 20 years old ++ and people are starting to get all these issues that you've experienced earlier. Hope to hear from someone. TIA!
Last edited by fullmetaljacket; 04-10-2023 at 12:15 AM.
I have just replaced a couple of the lines. If the lines are longer than specified, would that contribute to a rougher rev from 1-2k rpm?
It should not, On MAP sensors, that are remote mounted on some engines, the lines are up to 5 feet long.
A engine guy I used to work with told me vacuum in those lines move way beyond super sonic when measuring pressure, but
Diameter will affect the speed a vacuum activated valve opens. so I'd assume if your taking about the line to the x-over
valve from the solenoid valve if it was really crazy longer it might affect the operation, I doubt a inch or two would matter.
I would first vacuum test the solenoid valve and the changeover valve to see that they hold and the solenoid valve releases
and retains pressure when activated (Bench test basically) you'll want a mighty vac hand pump or similar.
A vacuum leak or a tired coil would both cause low rpm stutter..
Last edited by pdxmotorhead; 04-10-2023 at 02:02 PM.
There is actually a vacuum diagram under the deck lid of your car showing the specific diagram for your particular model...The black and white diagram shown above is for a Boxster 2.5 engine that doesn't have a secondary plenum with the resonance flap...That could be part of your confusion...
That said there is only a few vacuum lines and valves on a 996, it's a very very simple system compared to the late 70's cars ( at the peak of emission controls without EFI) that had 50 feet of vacuum lines and dozens of vacuum valves with dual,triple, vaccum tree's and at least a half dozen "t's" "y's" and "4-way's".......... they were a literal nightmare !!!
Last edited by Porschetech3; 04-10-2023 at 08:25 PM.
Thanks so much. I did a whole day googling pictures of various intake manifolds and vacuum systems and came across tons of pictures that explains what connects to what. Still very confusing. I hope i'll be able to figure everything out when I take down the intake manifold and guided by the pictures and write-ups on this thread. Will also post pictures here so that people can learn from it. Thanks Doc!
05-28-2017, 02:42 PM
Us Silicone hose and brss barbs
) It seems that we all have messed up the hoses when we change our alternators! I think it's really not our fault because as time goes by, these vacuum hoses (rigid types) tend to get brittle.... so it's really Porsche's fault! 
