Rough idle and vacuum assist

[audismittens]

I'm currently having a go at fixing the rough idle on my Mum's 1999-2000 Audi A4 B5 1.8T. I've read that rough idle can be caused by a leak in the vacuum assist to the brakes, and it seems likely in this car because the brakes will go hard at idle engine speed.

Is there some way of isolating the rest of the system from the brake booster to check if the problem lies there? Like taking off a hose somewhere and bypassing it to rule it out?

Also someone mentioned in another post that there is a fuse associated with the booster that sometimes blows and causes problems, but I can't find specific mention of where this fuse is.

Did a bit a searching on this forum but couldn't find anything specific so forgive me if this has been asked before.

[BeautifulAudi]

Are you throwing any cel's?

[sonic32]

There's a little pump on the vacuum system which the booster is also connected to. Start with that, that fixed it for me.


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[audismittens]

Are you throwing any cel's?

None. I've got one of those cheapy OBD readers which I'm going to try as soon as I can start the car (it's also got a leaking fuel hose which needs to be fixed before I can really do any testing).

[audismittens]

There's a little pump on the vacuum system which the booster is also connected to. Start with that, that fixed it for me.


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That's what I'm thinking of. You wouldn't happen to have a picture for me? Never worked on an Audi before.

Also, where would you get a service manual for this car?

[sonic32]

I can take one for you tomorrow and check out the Bentley Manual


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[audismittens]

I can take one for you tomorrow and check out the Bentley Manual


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That would be terrific, thankyou.

[OverSpun]

Possible the jet suction pump... Mine recently completely failed (basically blew up) and it also caused brake issues.

OP, it looks like this-

[audismittens]

Possible the jet suction pump... Mine recently completely failed (basically blew up) and it also caused brake issues.

OP, it looks like this-

Thanks, I think that might be the bit I read about. Is there some easy way to test this? Bypass it somehow? Also the fuse, do you know which fuse might affect this and which one it is?

Sorry for all the dumb questions, just hoping there's an easy fix for this. The booster itself has been replaced so it's unlikely to be that.

[OverSpun]

Thanks, I think that might be the bit I read about. Is there some easy way to test this? Bypass it somehow? Also the fuse, do you know which fuse might affect this and which one it is?

Sorry for all the dumb questions, just hoping there's an easy fix for this. The booster itself has been replaced so it's unlikely to be that.

It may be called as a "pump" but its not electronic or fuses for it. However, you may have one or more failed check valves because they can absolutely cause a rough idle. They are brittle and should be replaced if needed. I replaced all of mine a couple years back.

Given that it is your mother's car, may not have a boost gauge.... It would be very helpful if it had one. You should see about 20hg vacuum at idle.

They look like this (some are nearby the jet suction pump):

You have a few of these (1 nearby your airbox, a couple below your intake manifold) and the rest nearby that jet suction)-


and 1 of these-



See if that resolves your rough idle if your check valves look worn out.

[audismittens]

OK, making progress.

I changed the fuel line and messed around with Torque OBD2 Lite a bit. Given that I'm using the program correctly (which it's possible I'm not) the vacuum at idle seems to fluctuating wildly. The display I'm using is "Turbo boost and vacuum gauge" (I hope this is the right one). Is it worth investing in a better scan tool/software?

Where would you get a vacuum gauge and how would you install it?

I had a look for the various check valves but can only spot 3 at first glance. How much of a pain is it to change these? Do you have to take half the engine apart to get at some of them?

Mine are held on with a variety of zip ties and ordinary worm gear clamps. Is this normal?

Also I've used a worm-gear clamp to hold the fuel intake line on to the fixed line from the tank, the spring clip got a bit mangled in taking it off. I'm assuming this is ok.

Other than the rough idle and suddenly deflating brakes the car runs very smooth. There is a long story involving the turbo, coils and injectors prior to this which I won't go into but if we can just get this fixed it's going to be a lovely car.

Edit: I'm thinking of starting with the vacuum pump. Does anyone have a part number?

Edit2: I'm wondering if this was the "vacuum pump" mentioned in that other post?: https://www.ecstuning.com/Audi-B5_A4...8T/ES1904590/#

[OverSpun]

It would be helpful if you can post some pictures of what you are looking at on the car.

Besides, leaking check valves can certainly cause a fluctuating vacuum @ idle. I had that issue before I changed all my check valves. It's not too bad replacing the check valves, you just have to take off the factory clamps and replace them with new clamps when you're done. I carefully used a dremel to remove the factory clamps since they are a pain.

I wouldn't suggest replacing various parts until you have replaced the common and cheap parts (e.g. the check valves) first. Especially since it sounds like you a vacuum leak.

[SN95Audi87]

Awesome man! I'm glad you are chasing it down and not giving up. You will understand your car much better. Keep up with progress. I changed out all my corroded vacuum lines that ran underneath the intake Manifold and on top just as a preventative measure.


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