Hesitation/Stumble -- Vacuum Leak @ Brake Booster/Master Cylinder

Way too much info to try to digest, but did you try disconnecting the master booster and plugging the hose? Did that help?

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I agree with PeterD's test. Or testing the vacuum reservoir for leaks? Vacuum leaks will cause the 02 sensor readings to be off a little...........so you have to take care of the leaks before setting the CO at idle.

Also you might be able to use some RTV between the master and the booster if you can't find the o-ring. Remember to let it set up well before starting the engine. BTW I have seen this leak before on an '82 Rabbit.

Now about that Hesitation...............sounds like you covered most of the common problems, such as residual pressure and frequency valve settings. Did you test the intake manifold and fuel injector seals for leaks? I think I would advance the ign. timing a little. Also I would check the engine timing belt setting and make sure it's tensioner is rotated CW to adjust the belt.

JMHO later, dave (One out of many daves)

First of all thanks for the replies. The primary reason for all the info was to try and avoid a post where someone asked the questions concerning cold/warm/residual pressure, and the dwell on the lambda, coupled with a desire to pique the interest of a person who had been thru the bad habit rabbit wars. Anyway prior to the post one of the preliminary tests that i performed included clamping the main vacuum line (to the booster) while the engine is running which caused the idle to drop substantially. (I did not block it for fear that the removal of the main brake booster line runs the risk of damaging the delicate (25 yr. old) plastic fittings on the hoses as they cannot be purchased at the local VW dealer. To me this was a further indication of a vacuum leak at the booster. Then when partially removing the master cylinder and finding no vacuum seal I assumed that this was further evidence of the vacuum leak between the brake booster servo and the master cylinder. Also as heretofore stated shooting carb cleaner at the point where the master cylinder and the booster mate caused the engine idle to rise, I felt this was the point of the vacuum leak. Knowing that this leak is present i do not know how to test the reservoir as I assume that creating vacuum to see if it leaks in another place is going to give me a false result. Anyway the injector seal and the injector seal inserts/holders are brand new (viton o-rings). I did not use loctite to seal the threads on the holders, but I did shoot carb cleaner on them and double checked with propane to see if there was any noticeable change in the idle -- there wasn't. I am confident that the injector o-rings are OK. Also as a preliminary check, I pulled the timing belt covers after a professional mechanic replaced the timing belt just to make sure all the static timing marks were correct and they were, including the tension and the operation of the idler. Obviously what I was really hoping for was that someone would know the size of what Bentley calls the "vacuum seal o-ring" (see pages 9 and 18 of Service Manual) or perhaps had an old one in a removed but can't throw away box (I have a lot of these boxes) and could measure it for all the world to know. This would be an immense service to everyone who is in the process of tracking down these annoying vacuum leaks and trying to get rid of the hesitation/stumble/bucking problem.

If I advance the timing what good would this do- anyway i need to get the idle down first--the vacuum leak

I will see if I have one of the brake masters laying around somewhere, but I doubt I have one for the Rabbit and I believe, for years 82-84, are different than the Jettas. Not sure on your '81.

Advancing the timing might reduce/eliminate the Hesitation/Stumble that you are experiencing. BUT definitely make sure you have no vacuum leaks or other issues.

I know that some VWs/Audis that have A/C systems have aux. air valves that might stick and cause a higher idle also.

Can't you disconnect the Brake Booster vacuum line/hose at the intake manifold and plug the vacuum up to see what happens? It might pop out of the Booster too without breakage too.

BTW I appreciate all that information in the original post and agree with you that it helps keep down questions that were going to be asked. ;-)

later, dave (One out of many daves)

Not to make a big deal out of it, but you should try to get to the point quickly. Most will not bother reading a lengthy post if the major premise is not revealed immediately.

Also, breaking up your post into short paragraphs will make it a lot more readable.

Point well taken --- I am trying to learn the etiquette of posting on a forum - i.e. not enough info, versus too much information.

I think your post was very detailed and stopped me from asking many questions that you had already answered in advance. So the thread of your original post will be kept very very short. lol

YES some people don't like reading super long posts, but I found your post very informative and concise. ;-) So I vote you keep them this way, unless you truly want a long thread! One point I would make is that you include your car info in the Subject area.........was it an '81 Rabbit gas 5 spd?

JMHO dave (One out of many daves)

I hate to get off subject, for as you know my quest is to ascertain the size of the "vacuum seal o-ring" for a westmoreland (U.S. built '81 Rabbit) but I will say i do not like to type, & i am normally not verbose. You are right I should have put '81 Rabbit, FI, A/C, 4Speed in the title, but being a long time reader and a first time poster, i am not adept at the many acronyms used in the forum. I had to look up BTW to find out what that stood for. The word Rabbit attracts aficionados (habitu=E9s) to forums and threads, and obviously that is what i was trying to do, attract a knowledgeable person

lol Yeah I remember that you asked about that. ;-) I will send myself some email so hopefully I won't forget while at the garage.

IIRC (If I Recall Correctly) it is not a perfectly round O-ring and it looks like a circular triangular seal.

It looks like you need Sealing Washer 171-611-089A which is the rubber gasket (vacuum seal) between the Brake Master Cylinder and the Brake Booster. double check with the dealer on this part number!

I don't have any new masters that come with seals like that. :-(

i have a daughter who is working on her masters at Indiana Univ in Bloomington, and she is my parts chaser, (she also drives the 1983 Rabbit) I will send my grad student to the VW dealer with the info. Thank you very much for your assistance.