I am buying a vacuum tester today from my snap on guy, I assume that with this device I just hook it up to a vacuum line and pump it full of air and listen for a leak. What is the maximum pressure I should pump, are all the lines connected, are there any junctions that are made specifically for me to hook the device up to?
Any words or caution??

Like anything else a tester is no better than the knowledge being used to test. Vacuum testing usually involves either determining leaks in servo controlled systems or evaluating levels/conditions of vacuum for engine diagnosis or vacuum system performance.

The type of tool you seem to be looking at is used to create VACUUM not pressure; actually the lack of pressure. A good tool should get you close to perfect vacuum 29.xx inches of mercury (around thirty feet of water). Any system running on vacuum can be pumped as low as the pump can take it. It usually is not a question of how low but does it stay. In other words if you are checking the fuel shut-off servo on the back of the injection pump one would pull 15-20 inches of vac and let it hold. It should be stable and stay at the vacuum pumped indefinitely.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Thanks. I was thinking that if I had a vacuum leak, I could pressurize the system with the engine off and listen for an audible leak. Is my line of thought correct?

Not quite, Only in a pressurized system will you 'hear' a hiss and that depends on the size of the leak. A vacuum pump will only provide a 'negative' pressure aka 'vacuum' I'm just a rookie but the pros on this forum can walk you thru the process of isolating a system to see where the system has a leak and it does not respond as expected. Your on the right track, best regards LH

I hear they use methods such as you speak in the telephone industry with special listening devices.

We use a number of techniques. The most recent is the smoke machine. You hook it up to the system and it applies 1psi pressure with dense white smoke. The smoke rising from an area identifies the leak.

The way we have done door locks for over twenty years is all together different. We start by identifying the line to the reservoir. We disconnect it (taking the reservoir from the system). We put a gauge where the line to the reservoir was. The point here is to reduce the ammount of vacuum in the system so small leaks become more significant.

We then run the car in both the lock and unlock position and shut-off. We watch the gauge in both cases and no movement is the sign of a tight system. If the reservoir is in the system small leaks would take hours to show. Without the reservoir small leaks are usually apparant as visible needle movement.

Once the leak is verified as lock or unlock (basically the same thing on later single line systems). The side that is leaking is divided at strategic locations usually under the carpeting on passenger floor. The system tees there to the right doors, trunk, and gas flap. The individual lines are capped until the leak goes away. Once the culprit is identified the rest of the lines are hooked up. I never rehook a tested line untill all leaks are gone and then they get rehooked one at a time. The defective unit is identified then by operating the system and viewing which element isn't working. That will be the one that is plugged and was the leak.

On early two line systems both sides "lock" and "unlock" must be checked in this way.



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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Would it work if I hooked up the pump and created vacuum so that a leak would be heard. With the engine running the nois may not be that audible.
This post is getting very educational.

Its possible, I have often heard vacuum leaks; even inside doors, but not usually. One reason for this is that the source lines act as an oriface/restrictor. The air only makes noise where the velocity is high. I would imagine that the source lines/check valves are often a greater restriction than the leak, so your noise will not be at the leak.

If you have a gauge the method I gave above is simple and PRECISE. If its a door lock problem and the car is over ten to twelve years, I would replace all three door vacuum elements and forget the diagnosis. If you fix only a single unit you will soon be back diagnosing the rest one at a time. If you are paying for diagnosis its cheaper to just replace all of them as they are all the same age.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

If you are having vacuum problems, don't forget the fuel filler and trunk locks.

I bought a "Siverstone" hand vacuum pump out of an ad in Autoweek magazine about 10 years ago and it has served me well ever since. It came with an assortment of of plugs and adapters and cost about $30. Oiling the cylinder once a year has kept it as good as new.

Tne nice thing about vacuum is that you cannot spill it, shock yourself, our burn yourself. It will not explode.

Chuck