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VW MKIII-A3/B4 TDIs This is a discussion about MKIII-A3/MkIII Jetta/Golf (<99.5) and B4 Passats (96,97) TDI's. Non TDI related postings will be moved or removed.

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Old September 11th, 2017, 20:18   #1
Rapidrob
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Default Vacuum pump suction readings.

I cannot find anywhere what the Vacuum pumps suction readings should be?
Some post show as much as 29" of mercury.
What is considered normal with no leaks?
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Old September 11th, 2017, 21:37   #2
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29 inHg would be ideal but a reading over 20 inHg is certainly acceptable

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Old September 11th, 2017, 23:32   #3
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I get about 22-24" on mine.

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Old September 25th, 2017, 18:05   #4
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in order to get a real reading on the vaccum you need a micrometer, the cheep ones are junk and you will pay about $500 at the very least for a GOOD one that will actually work. Rule of thumb is, vacuum it down for 1 hour and turn off all the valves and let it sit overnight or for a full day. if the needle moves AT ALL, you have a leak. To truly get down to a level you should be at for a perfect job, you need to flush all your lines out and change the oil in the compressor and new dryer because of contaminates. In a nut shell, just vac it down for 1-2 hours and let it sit for 1 full day. if nothing changes than your golden. if the system always had a good charge on it than just weigh in new refrigerant, dont do this any other way. If its in need of service, change all your o rings, the valve, dryer, and flush all the lines and evaporator with AC flush followed with nitrogen purge. Pull a vacuum for 1 hour and let it rest, if good, fill with nitrogen and do a bubble test with soap water than vacuum it again. Now weigh it back in and enjoy. there is more to meet the eye about this stuff and if you want you can PM me and i will help you out. My father is an Master HVAC and know too much about this for my own good.
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Old September 25th, 2017, 20:17   #5
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What's all that got to do with the vacuum pump on the engine?
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Old September 26th, 2017, 09:28   #6
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Quote:
Originally Posted by KLXD View Post
What's all that got to do with the vacuum pump on the engine?
Nothing. What type of $500 micrometer do you need for air conditioning service?
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Old September 26th, 2017, 15:46   #7
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Originally Posted by Ol'Rattler View Post
Nothing. What type of $500 micrometer do you need for air conditioning service?
Yeah, I'm trying to figure that one out too. I mean you could buy a nice starret for about 125, but I fail to see how it will measure vacuum, or be affected by it for that matter.

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Old September 26th, 2017, 16:58   #8
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I have a Lufkin 1" and 2" micrometer. Beautiful, had them for decades. Can't pull vacuum with either one.
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Old September 26th, 2017, 19:27   #9
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Micron gauge.

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Old September 27th, 2017, 07:07   #10
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I bought a used one from a member here and am still waiting for it to show up.
To test the pump I was going to use a drill motor or chuck it in my lathe to do a quick test of the vacuum.
Does the pump spin CW or CCW?
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Old September 27th, 2017, 07:48   #11
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CW As viewed from the shaft end.

I wouldn't test it that way. It normally gets a constant oil supply from the engine. A brief spin with a drill probably wouldn't damage it if it has residual oil in it but I doubt you'd get a true reading of its output.

Edit:

Oops. I was thinking of the ALH pump. The AHU is gear driven by the accessory shaft. You could probably tell by looking at the wear on the pump gear which direction it turns.

I think the accessory shaft turns CCW since it's driven by the back of the belt. That and the angle of the gear teeth would tell you.

Again, the vanes rely on a nice layer of oil to effect a seal with their bore...
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Past: 85 Mitsubishi PU, 4D55T; 81 Rabbit, 1.6; 80 Dasher, 1.5; 79 Rabbit, 1.5

Last edited by KLXD; September 27th, 2017 at 14:38.
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Old September 27th, 2017, 09:25   #12
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The best way to test it would be to connect a vacuum gauge directly to it and just run the engine.
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Old September 27th, 2017, 13:29   #13
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If you remember that it spins at half engine speed and make sure there's some clean oil on the pump shaft I think a quick spin at 500-1000 RPM is probably not too risky.

You will know in seconds if it works or not... as long as you get 20+ inches of vacuum you're good to go.

The vacuum supply is used to power the brake booster and also the turbo actuator and EGR valves... fed-back systems for the most part, so the actual value is not super critical.
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Old September 27th, 2017, 13:55   #14
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If you are evaluating the performance of the vac pump, test it by itself, as well as hooked up to the booster - a perceived faulty pump may just be a vacuum leak somewhere.
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Old September 27th, 2017, 16:15   #15
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Good point.

Always tricky to lend constructive help without knowing the context... aka... the actual issue at hand.
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