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  #1  
Old 06-13-2019, 02:31 PM
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New diagnostic - massive vacuum leak at intake manifold

So my mechanic has located a massive vacuum leak to the rear of the intake manifold. Might have to replace the manifold gaskets. Now here is the question:

I cannot find, in the electronic version of the manual, what the torque specification is for the intake manifold bolts.

My mechanic also talked of rubber hoses by the intake that need to be renewed. I think in the diagram below this is the part that is circled in red:



Anybody have any detailed additional comments? My mechanic won't do the job, so I am going to do it...
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1989 560 SEL (black/black)
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  #2  
Old 06-13-2019, 02:51 PM
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This can quickly turn into a major PITA job if the manifold has leaked coolant into the plenum. The combination of the alloy cylinder heads, aluminum manifold, steel bolts, coolant and a couple of million heat cycles can "weld" the intake manifold bolts (usually the ones on cylinders numbered four and five) into place. If the coolant hasn't been maintained at proper levels the bolts can corrode and twist off using very little torque.

Go slow, use heat and lots of penetrating fluid.

Before I'd tackle the job I'd do a though "smoke" test to ensure those gaskets are actually the culprits.

This is not a "same day" kind of job to do it properly. You really should have the intake hot tanked/cleaned and checked for war-page. The odds are good you won't need to have the manifold re-surfaced but it is better to be sure.
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Old 06-13-2019, 03:59 PM
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Thanks for the heads up...

Coolant has always been kept at the proper level. Overall she is in good shape. I will work those bolts slowly...
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1989 560 SEL (black/black)
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  #4  
Old 06-13-2019, 04:04 PM
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Also, I don't see any torque specifications, and it seems the tightening sequence is simply "from the inside to the outside"...
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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)
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  #5  
Old 06-13-2019, 05:30 PM
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Mechanic has confirmed it is the Plennum seals....

He did not smoke test it, but he sprayed starter fluid into the area, confirming the leak, and that is what I have to deal with. 30 year old rubber!!!

Everything else has been good on this one, I don't suspect a coolant leakage at all. So I am keeping my fingers crossed...
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  #6  
Old 06-13-2019, 05:49 PM
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Quote:
Originally Posted by hbofinger View Post
Coolant has always been kept at the proper level. Overall she is in good shape. I will work those bolts slowly...
Proper level ( volume ) isn't the issue, having the coolant corrosion inhibitors at the proper % is the issue. Even then, coolant can slowly weep through gaskets then corrode bolts because the weeping coolant is trapped , loses corrosion inhibitors and does not get "renewed".

Be aware that coolant passages on the intake manifold and cylinder head may be pitted resulting in more work than anticipated.
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  #7  
Old 06-14-2019, 06:29 PM
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Assuming that this is a 117/116 engine, I've done quite a few of these. At their age, intake leaks are pretty common. You'll want to replace plenum seals, injector seals, injector holders, injector holder hoses, all the breather hoses under the intake, the air intake seal and maybe the injectors. The reason why your mechanic doesn't want to do it is as mentioned in a previous comment, this job can turn south. The intake bolts can be more than likely to break than not. The positive is, when I've done this job when the car has good compression, good working fuel and ignition components, it can run like a brand new car. Especially at idle.
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