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Old 04-11-2018, 10:23 AM   #1
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Location: South Wales, UK
Posts: 852
Vacuum leak problems (continued)

New thread as there's too much old/new info clashing. Need a fresh start.


- Lacking performance after AOS change (lacking in low/mid range torque)
- P0491 and P0492 error codes (Secondary Air Injection pump), but the pump is operational

I'm certain it's a vacuum leak.

Here's the chain of events so far:

1. Car was performing perfectly about 2 months ago.
2. Air/Oil Separator failed, bad performance and smokey start up
3. Replaced AOS, all fixed but performance was around 90% - lacking torque and bogging slightly. All pipework seems to be okay.
4. Replaced MAF, no change.
5. Occasional weird sounding SAI pump on start up, but not all the time
6. P0491 and P0492 stored error codes (SAI ineffective on both banks)
7. Disconnected the pipe to the SAI pump to see if it blows air - it works great and blows incredibly strong, so the pump is fine.
8. Disconnected all pipes and re-connected. Car ran perfectly for a few days, even the codes cleared themselves, then slowly went back to 90% performance and same error codes.

Few things to note:

- P0491 and P0492 codes point to a bad SAI, but both can also be triggered by a vacuum leak.
- Playing with the pipework used to change its performance slightly as mentioned above.
- Since owning the car, whenever the SAI pump used to come on (either cold start or at traffic lights), revving the car used to cause pops and crackles due to the extra air. This no longer happens since I've been having problems.
- Starting the car on a cold start no longer gives its normal higher revs which slowly settle. Instead the revs instantly rest on around 700rpm, even on a cold start, as if the engine was warm.
- I've tried spraying different engine areas with carb cleaner but it makes no difference to the revs.
- I've just tried to remove the oil cap while the engine was running to see if it made a difference. It does, and the engine starts to splutter, but there's a very loud hiss sound that comes out of the engine bay, and not the filler cap when it's removed. See video here:


Should I hear this coming from the engine? Maybe it's coming from a split or crack, or perhaps it's normal? Ignore the blue tape on the valve, that's me wondering if air was escaping it.

Guys you've been amazing so far, but I still need as much help as I can. I'm running out of patience with this car very fast.


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Old 04-11-2018, 12:59 PM   #2
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I went through hours of poking around finding big vacuum leaks in the 2003 986 I bought a few months ago. Bought the car from a friend who could not get rid of the check engine light. I got a lot of advice from Dave and Jeremy at TRE Motorsports......great guys....

This is what I learned. Test the system with a vacuum gauge....temporarily put a T fitting in there. You should have 12 to 20 inches of mercury of vacuum at idle.

1. The plastic fitting into the rubber manifold that is the main source of vacuum fell out. I replaced with a brass barbed fitting from the hardware store and high temp silicon tubing from McMaster. It fell our because of gas and oil on the fitting and well it was just badly designed by Porsche.

2. from the factory the vacuum system has lots of plastic tubing with black rubber fittings. Guess what, heat and age makes them crack. I replaced everything with silicon tubing. Also used some brass barbed fittings where advisable and necessary.

3. the vacuum canister is down there on the passenger side of the engine.Looks like a big plastic can of Fosters beer with a domed top. One vacuum line going in. It is not molded in one piece, but two and glued together. Well the glue fails.....duhh.

Mine would not hold vacuum......duhh...again. Use long needle nose pliers to get the hose off and hook up a hand vacuum tester with a long vacuum line. See if it holds pressure. If not........ that is a massive leak. New won is like $22 from Parts Geeks or other sources. To remove the bad one you have to go in from the front and remove the alternator.....my bad one is still in there, and the new one is up in the corner of the engine compartment with a long silicon hose.....when or if I take out the Alternator.....I will take the old leaky one out.

After this relatively easy vacuum overhaul, I had 18 "hg in the system and all the CEL and emission monitors went away and the car passed smog.

1973 Carreras RS tribute
2000 Saab Aero wagon stick
1990 Chevrolet pu pre runner

Last edited by boxfix; 04-11-2018 at 01:06 PM.
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