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Old 05-04-2009, 02:47 PM
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Join Date: Jul 2001
Posts: 897
Vacuum level on non turbo 606?

I'm trying to troubleshoot a vacuum problem with my '95 E300D, 288K. At idle I don't really have enough vacuum to properly operate brakes, engine shutoff, HVAC pods. While running on the road it's ok.

I put the Miteyvac on it yesterday to read the vacuum level. When idling, the vacuum level will slowly (over maybe 90 seconds) build up to about 19.5", with the AC off and no vacuum users. The second I turn on the AC, it drops to about 10" and the needle fluctuates. If I step on the brake pedal with the full vacuum level (~19") it will take a big hit down to 10" or so and then slowly build up once I release the pedal.

When the engine is not running I pulled 15" on the system with the Miteyvac by tapping into the T on the small line from the vacuum pump and it will slowly leak down, over a couple minutes. So some sort of small leak, but nothing major. (I'm guessing it's vacuum pods from the AC having a small leak or two.)

So my questions are, has anyone checked (or can they check) the vacuum level on a naturally aspro 606 at idle, and second, would it seem like a failure mode on the vacuum pump to not pull sufficient vacuum at idle, or to slowly build it up over the course of >1 minute? And would a slow leakdown like I have be of major concern?

Chris W.
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Old 05-04-2009, 03:07 PM
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Join Date: Mar 2009
Location: San Diego
Posts: 2,874
You have a leak somewhere. Tap in to the output line of the vacuum pump with a T, the one goes to the brake booster, fires up the engine and the gauge should show a vacuum. Stop the engine and the vacuum should hold for a few hours.

If not then you have a leak. It works fine when the car is running because the vacuum created is better than the leak.

Your vacuum pump is OK. Fix the leak by divide and conquer, unplug each line one at a time from the pump output and plug it. Then pull vacuum with your Mitvac gauge, you will find your leak.

Good luck.
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

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