Vacuum system sleuthing

Greetings Capt. Mike,
Thanks for your various replies on; Aux. air regulator, Thermostats and Downhill compression for my '84/1.9. Now that I've eliminated the switches, sensors, Stabilizers, thermostat.... Your observations on a potential leak in the Vacum lines are well taken. Are there places that are notorious for leaks in the Digijet system ? I do not have access to a VAG 1338. Do I take this to the shop ?

Cheers, Steve

Capt. Mike

Most vacuum leaks can be found at home. The most important tool is thinking about what the vacuum does (per engine speed & load) and what it's trying to accomplish. Then you get a vacuum tester. There are some cheap plastic ones at the auto parts stores -- also sold as "brake bleeders", but I prefer a decent one with gauge from a tool company.

For example, you have a charcoal evaporative emmission system. You route gas tank fumes and hydrocarbons to it, where they're trapped. You don't want to try burning them when the engine is laboring (low vacuum) or possibly allowing back pressure into it from the intake system. So you install a vacuum valve that only opens at high vacuum when the engine can suck the hydrocarbons back in strongly and burn them.

What I'm trying to say is you have to understand WHAT the vacuum is trying to accomplish as well as where it comes from and how. Using a tester allows you to duplicate a vacuum condition to see if it does what it's supposed to independent of the other factors like engine speed. In the example, you would put a high vacuum on the valve and see if it opens. If it doesn't, the valve is bad; if it does, it's not getting vacuum, from a vacuum leak or engine not producing vacuum. Half your diagnosis done.
Thanks for the reply,
Once the latest snow melts I'll dig back into the system. One new thought is I have a locking gas cap and never I have sensed it was under any vacum pressure. Also, if the Charcoal fitler is a service item I suspect after 16 years its ready for a replacement. Any bedside reading on Vacum theory ?
Cheers, Steve

Capt. Mike

Transferred to consolidate similar topics.

Engine stalling when brakes applied.[

McBugs Junior Member # 3002 posted 02-01-2002 12:33 PM

Started last night on my way home from work.
'71 Westy, all stock.

When I apply the brakes, the engine wants to stall. The harder I depress the pedal, the quicker it wants to die.

When sitting at idle, you can hear the vacuum being stangled as the pedal is depressed, like someone putting their hand over the carb while it's running. I realize that the servo is vacuum operated from the intake manifold, but I have never been able to literally "hear" the vacuum being sucked out of the manifold!

Any ideas as to what could cause this?
Please help!!! We were supposed to go camping in Georgia this weekend!!!
Thanks in advance,

Capt. Mike

Vacuum assisted power brakes get their vacuum FROM the engine manifold. An open line means your engine is sucking air and losing its air:fuel ratio. Applying brakes takes more of the vacuum than at idle, so the engine reaches a point where it cannot burn the air:fuel combination now being fed.


New member
A new problem has devolped with my 1972 kombi bus.

The problem its self is that it will not idle, does run when I keep the gas down, but gas is touchy and in second gear she really chugs.

I posted this problem here because I'm just taking a guess that it has to do with the vaccum draw in the engine. I suspect this because of a brake booster that I have installed that seems to be failing, though I have pluged off the engine side of the vaccum line (the hose was previously cut in two) running from the booster, and the problem dosn't change.

I was also wondering if this might sound like a case of putting too much oil in the engine and blowing a gasket? How would I check this?

I was thinking about buying a vaccum tester and doing some tests myself, is their a place in this fourm where it explains how to go about doing a good check?

If anyone had any suggestions I would love to hear.


Capt. Mike

There isn't a specific topic for vacuum testing beyond this one. Your Bentley, section 3-5.1 has a troubleshooting chart that identifies several vacuum/air leak sources of your symptom and the approriate sections to deal with them. It is usually a case of testing the indivdual vaccum controled items.


New member
I recently purchased an '83 1/2 westy and can't seem to get it running right.

The shop that I had it checked at only said that it needed different plugs and a new fuel pressure regulator. It got another fuel pressure regulator (used, but seemed to be working fine), and I had another shop check the plugs while attempting to figure out what was up.

Here's the problem: majorly low power. When I am idling I can rev the engine and get the RPM's high, but when in gear the RPM's won't go up even if I have it floored.

The one shop thinks that it is the fuel pressure regulator (hence, why I am posting in this forum), but when I tried to adjust it, I had the same problem.

When driving on a flat, I can't get above 45mph and hills are a major challenge.

I am not really savy (yet) with cars, and could use some help here. Thanks.

Correction: The shop that it was the air flow meter, not the fuel pressure regulator.

Capt. Mike

This topic is for vacuum sleuthing only. The pressure regulator is tested per Bentley 24.25 with a pressure gauge. It's purpose is to regulate the fuel pressure in relation to the intake manifold pressure. The only vaccum reference is to connect or disconnect the vacuum hose per fig. 24.117 to test pressure variance.

Air flow meters have their own topic in the Fuel Injection forum.

Hank Springer

New member
Vacuum hose sizing

1985 (late version ) full camper with a 1.9 and an automatic.
I am in the middle of testing and troubleshooting the FI system for surging and poor gas mileage. In testing the Auxilary Air Regulator (pinch off test), the vacuum hose fractured. This is probably a good thing, since it led me to examine all of the hoses more closely. I am attempting to replace ALL of the cloth covered hoses. My problem is finding a resource to identify the hose size. I took a sample of the first hose to a reputable foreign aouto parts store, and he used a caliber and sold me 14mm hose. This hose is certainly not tight enough and I will have to add some clamps. I was hoping that there is a better way to identify the hose diameters. Do you know of a resource out there? If it is in the bentley, I sure can't find it.
Thanks much

Capt. Mike

Guideline #8; your dealer can be your best friend. The factory parts fiche usually specs the hose by both diameter & length. If not, the N number (most meterware hose is sold under an N number) can often be cross-referenced to the standard parts list or to DIN standards. Most hose sold in auto parts stores is US, not metric, and will not fit correctly. Buy from one of the well-recommended vendors listed in the Site's PARTS forum; they are usually accurate and have the OEM quality parts.


New member
Diesel vacuum pump

I have an 82 diesel vanagon with a vacuum pump for the booster. I have not found anywhere in the Bentley a description of the pump or how to maintain it. Although I have found rebuild kits for the pump from various venders. My main question other than why didn't VW include this pump in the manual has to do with the two spring loaded valves on the top of the pump. The rebuild kit gives me two button sized valves with o-rings. Does one valve go in facing up and one go in facing down? This is the way I found mine when I took the cap and springs off but I had low vacuum and wasn't sure if they were incorrectly installed as I really have to push the brakes hard to stop. I have replaced the lower diaphragm and now the upper valves, seals and rubber manifold and brake vacuum is higher but I am still not sure if I installed the valves correctly. It would seem to make sense if the vacuum is being drawn:confused: in one direction. Thanks to anyone who can clear this up.

Capt. Mike

The reason there are not detailed rebuild instructions in the Bentley for the diesel vacuum pump is that VW never sold the parts to rebuild -- only rebuilt units. There were two -- 068-145-101EX & -101GX. The -101Ex is the only one still sold. The only parts VW sold were the O-rings for pump-to-block & head plate-to-pump. I presume they felt the diagram & information on 13.19 were sufficient to remove & install the pump itself.

Aftermarket vendors often get repair parts from even the same vendors VW might have used for their in-house rebuilds and offer them as bootleg kits. Unfortunately, the responsibility for "how to" falls on the supplier of these bootleg kits. That's not to say the kits are bad, but they often fail to provide good instructions. A good reason to find out from VW what was offered and what instructions are available to you before you start so you are prepared to document your disassembly for later reassembly.

Bill Beatty

New member
vacuum line diagram

I have a '75 F.I. campmobile with a double vacuum chamber distributer. Runs fine but oil temp. up to 225*F. So I check the timing after setting the points & gapping the plugs. Should be 5* ATDC but reads 15*BTDC. If I set the timing as per spec. it will almost stall, accellerate poorly & start with difficulty, even after adjusting the idle screw on the throttle body. I think it could be the vacuum line connections since when I disconnect the vacuum lines it affects the idle/ timing. Bentley manual is vague. When I look at Ratwell's diagrams, they don't match mine exactly since I have 2 connections on the opposite sides of the throttle body and none to the air cleaner. There is also an orphaned slightly larger vacuum line coming through the driver's side firewall from what looks like the brake booster under the bus. If someone could e-mail a photo of the vacuum line diagram on their '75 bus, that would be helpful.

Capt. Mike

Read the rest of Atwell's excellent site, He discusses this as length. He has the factory decal of the dual vacuum advance as well as the schematic.

Look at the schematic and you'll see that the connection at the air cleaner is on the same circuit as one side of the throttle body. The factory decal shows the other side going direct to the throttle body, but the schematic shows that line connects to the intake plenum. I believe that the other side of the throttle body, when so equipped, is the plenum side so either should accomplish the same thing.

There are tests in the Bentley to determine if your vacuum advance is functioning properly and you can test with a vacuum tester t-ed into the line.


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New member
vac diagram 88 vanagon gl


i may be the first person in the history of earth to almost bring the 1988 vanagon to perfect running condition. ("perfect" being a relevant description used loosely)

i have a vacuume leak somewhere and whenever i roll up to lights, or take off from lights and turn (ie: from an intersection) it chokes out. it doesn't always stall, but it comes close.

i can hear the leak, and i'm losing my mind because i have sealed EVERYTHING i can think of being associated with the intake air distributor and the charcoal canister.
is there a vac diagram for the 88? or is it as simple as it appears and i am simply going to be committed because of my van..



New member
Hi guys. I have a problem so simple, I just cannot figure out the best/safest way to do it. I need to replace the check valve in the vacuum hose that goes to the brake booster. 1975 Westfalia camper but it has a 2.0 Litre Westy engine in it. The valve is located right over the engine and the vacuum hose is the original fairly hard plastic black hosing. One end over the valve stem (engine side) has a clamp, the other side does not. Where the stems are rammed into the hoses, the hoses are stretched plenty wider than the rest of the hosing. Therein lies my problem. How do I get that hose off the valve stems/connectors? I removed the one clamp but I can tell you, pulling/wiggling lots has gained nothing. I sure don't want to damage those hoses. I suppose I could go at it with a hair dryer to try and soften it but surely guys in repair garages don't pull out a hair dryer? Thanks!