Normal vacuum pressure at idle?

[Tim Allen]

Hi guys.

I was just wondering what the stock vacuum pressure should be while at idle. I have an 2000 A4 B5 1.8t Quattro Sport (AJL). When sitting at idle, I am seeing 0.4 bar(5.8 psi), which to me seems a little on the low side. Could anyone else tell me if this is a normal pressure? I will add that sometimes it can be at 0.6 bar (8.7 psi), but as soon as the car is moved it drops back down while at idle. When driving normally, and coming to a stop, the car can sometimes almost die (revs drop loads) and then pull itself back up again. Any thoughts?

Cheers

[zollaf]

5.8 psi is pressure, not vacuum. vacuum is measured in mmhg. if you have pressure at idle then something is very wrong, and you would have no servo. you should however see about 500 mmhg at idle, or 20 inches hg.

[Tim Allen]

5.8 psi is pressure, not vacuum. vacuum is measured in mmhg. if you have pressure at idle then something is very wrong, and you would have no servo. you should however see about 500 mmhg at idle, or 20 inches hg.

Thanks for your reply. This is what my boost gauge is showing at idle, does it seem right to you?

30583

[zollaf]

ok, not the best way to measure a vacuum, but yes, thats showing you do have vacuum not pressure. you cant tell how that gauge has been calibrated though so impossible to tell what its measuring. if in doubt buy a proper pressure gauge and plumb that in. they are only a few quid for a cheap one that will measure vacuum in the correct way. what you have there is to tell you roughly how much your turbo is boosting, mainly to make sure the waste gate is opening and tom give you something to talk about down the pub :)

[Tim Allen]

ok, not the best way to measure a vacuum, but yes, thats showing you do have vacuum not pressure. you cant tell how that gauge has been calibrated though so impossible to tell what its measuring. if in doubt buy a proper pressure gauge and plumb that in. they are only a few quid for a cheap one that will measure vacuum in the correct way. what you have there is to tell you roughly how much your turbo is boosting, mainly to make sure the waste gate is opening and tom give you something to talk about down the pub :)

Thats a fair point about having a proper vacuum gauge. Alas, the pennys wont allow that for the time being. I know I have vacuum, but does the rough amount shown in the pic seem enough? Like I said in my original post, it can sometimes be at the (for want of a better measurement) -0.6 mark, but then go straight back to where it is now. What I am trying to figure out is if I have a vacuum leak or if this seems a normal amount of vacuum

[zollaf]

it should always be in the same place on idle. the gauge may be sticking or you may need a new pcv valve. if you use the search function you may find the pcv valve pages.

[zollaf]

Please Help A4 cabriolet 1.8t crushing my will to live. (http://www.vwaudiforum.co.uk/forum/showthread.php/168867-A4-cabriolet-1-8t-crushing-my-will-to-live?highlight=pcv+valve)

found it.

[Tim Allen]

it should always be in the same place on idle. the gauge may be sticking or you may need a new pcv valve. if you use the search function you may find the pcv valve pages.

Ok, I know it's not the gauge, although I do agree that a better one would be a good idea. After converting the -0.4 from the gauge to mmHG, i know know that I am getting aprox 300 mmHG vacuum. So, I am well below the figure you mentioned (500 mmHG). It does appear the that I do have a vacuum leak somewhere.

I know that a pressure test is a good idea to locate leaks, and I will build one as soon as I can. Are there any common locations for vacuum leaks? Just thinking that I could start at any common points while I am waiting to get a pressure test done

[zollaf]

pcv valve.

[Tim Allen]

Thats the "pancake valve" at the front of the engine? Am I right in thinking that I should be able to blow through only one direction?

[zollaf]

no, read the link i posted. its at the back, behind and under the inlet. a right bugger to get to but the cause of most 1.8t problems.

[Tim Allen]

no, read the link i posted. its at the back, behind and under the inlet. a right bugger to get to but the cause of most 1.8t problems.


Oh god. If it's anything like replacing the crankcase breather pipe under the inlet, then I may have to get someone else to do it. Not going through all of that again haha

[zollaf]

the valve is a tenner from vw and a pain to fit, but may make a difference but yes, you should be able to blow but not suck through it.

[Tim Allen]

the valve is a tenner from vw and a pain to fit, but may make a difference but yes, you should be able to blow but not suck through it.


Just to make sure I have the right part...

30584
I take it the part number listed for in in the link you provided will be the same for my engine?

[zollaf]

thats the little bugger. it sits inside the join of 2 pipes so you cannot actually see it.

[Tim Allen]

I know exactly where you mean now. Oh god! haha. Will see if I can pick one up on monday and will let you know if it has worked :) Thanks for your help.

[Tim Allen]

I know exactly where you mean now. Oh god! haha. Will see if I can pick one up on monday and will let you know if it has worked :) Thanks for your help.

Small update. I have fitted a new PCV (£9.50 from Audi), and it has made a very noticable difference. I didn't realise what a difference changing such a small part could make. Car seems to be boosting so much better, and replacing the pcv has gona a long way to helping fix the vacuum issue that I have been facing. Still not seeing enough vacuum to start with, but then something gives/moves and I have full vacuum. I think my next part to change may have to be the "pancake" valve. I did take my existing one off and drained the oil from it that had built up over the years. Perhaps the rubber membrane/diaphram has worn or got clogged up with sludge. Is there any sort of test that I can do on the pancake valve, similar to the pcv (blow through one way, but not the other)? After that, I suppose it will have to be replacing vacuum lines, and maybe even cleaning inlet manifold (would a build up of crap in that reduce vacuum pressure?).

All in all, I am a step forward, and already noticing a difference.