Vaccum.. How much is your car running

[s4kido]

Hey everyone
I wanted to know how much VAC is everyone running on their car???? I got my car running but it looks like im only running 13 on idle.. I think i have a boost leak somewhere.. Can also anyone tell me the most common places where it could leak???

Thanks

[Jamo]

I'd expect from than that about -0.40 bar as a minimum on an 20v engine.

Mine is -0.55 to -0.60 bar on cold

[JP]

Depends on the cam profile you are running as the more overlap one has the less vacuum it will show.

Mine runs 11.5inHg-12inHg cold start and 15.5-15.7inHg when warm with the cams I'm currently running. On standard cams it ran 15.5-16inHg cold start and 18-18.5inHg when warm.

I've seen some running 20inHg when warm but most seem to sit around the 16-17inHg on a proper gauge.

If you have a leak I'd start by looking around the idle air control valve as this controls your vacuum, also any of the small vacuum hoses that come of the manifold, if your ECU has been chipped at one point also check the hose that connects to the ECU.

[s4kido]

ok so i really shouldn't be too worried??? the car has stock cams but it has a 1.9 spring and the ECU has been chipped from what im told....
What is the best way to do a boost leak test.. I'm done it on my B5 S4 thru the intake manifold

[JP]

get a kit from ebay, remove maf from inlet pipe fit kit and pressurise to 14-15psi listen for leaks and spray soapy water near sound to locate leaks. As the vacuum reading is from the manifold itself based on a closed throttle body I would expect to hear leaks aroud the manifold area only. Could even be injector seats they are known to go brittle too.

[s4kido]

I could only get it to about 8 and then it would leak out

[S2central.net]

Same old thang.... Boost leak test should NOT be done at MAF hose.

Approx -0.5 to -0.7 bar vacuum is to be expected at idle

1bar is 14.5 psi by the way

[s4kido]

I wasn't doing it with the MAF on I took it off and then just did it with the pipe connected to turbo. So we me getting about 13 inHg so I should be ok but I should check for leaks either way.

[S2central.net]

D'oh....

Do a boost leak test that starts with pressurised air entering the hose at the turbo output... not the input / MAF hose.

You want pressurised air to pretend like its coming from the turbo as even when the turbo is blowing as much air as it possibly can.,.. the input hose (where the MAF lives) is ALWAYS under vacuum.

A turbo never sees pressurised air at its inputs so its very easy to blow air past the oil seals this way - you hear bubbles into the sump and cannot build up enough pressure to do a leak test.

Also - you really need to bung the throttle body hose as well - otherwise air gets past the throttle valve and into whatever cylinders happen to have inlet valves partially open... so you end up pishing air past piston rings too.

The other thing to block off is where the PCV system vents air from intake system back to the MAF hose.

You need to make the intake system airtight in order to perform a good leak test. There are lots of threads on the subject here.

[JP]

2 schools of thought on the pre and post turbo application as I once had a car which was erratic in it's performance and the system pressurised fine. Then when we did the test pre turbo we found ever so slightly a leak underneath between the MAF and the turbo. Which under full chat was drawing in false air post MAF.
Sorted the pre turbo leak and no more problems.

If you do a test pre turbo I would never crank the pressure past 1 bar otherwise yes you will leak past the turbo seals, also you need to block of the breather pipe. It does no harm to use both methods depending what you are testing for. As we are looking for a vacuum leak in this case I would only assume it is post throttle body as this is where the vacuum reading is taken from, so this part of the system will have to remain connected. It means there will be a slow leak from 1bar pressure to zero, however if no audible leaks are heard then you can safely assume it is leakage into the cylinders past the rings and not a leak around the manifold area. Meaning the problem is elsewhere.

[s4kido]

thanks everyone i really appreciate it... tonight i will do a test...

[s4kido]

ok i did a test last night. I was leaking from the Evap line and the sensor was backwards. so i fixed that leak... I was able to pressurized the system to like 12 psi and then it started falling slowly.. so that completed but i still dont have full vac when the car is started..
I will try to change the plugs and check the coils maybe there is a problem..

[Josh-stateside]

ok i did a test last night. I was leaking from the Evap line and the sensor was backwards. so i fixed that leak... I was able to pressurized the system to like 12 psi and then it started falling slowly.. so that completed but i still dont have full vac when the car is started..
I will try to change the plugs and check the coils maybe there is a problem..

With a cold engine or a warm engine? It's probably pissing out slightly through the piston rings. It's known that these engine ***** oil. You say it was leaking from the carbon canister lines which is directly connected to the crank breather setup. Do a compression test on each cylinder if your really worried about it. Warm the engine up and let the rings set and then use a compression tester to compare each cylinder.

ttyl
josh

[s2_bo]

The other thing to block off is where the PCV system vents air from intake system back to the MAF hose.

You need to make the intake system airtight in order to perform a good leak test. There are lots of threads on the subject here.

Paul are you sure thats necessary if your only pressurising from turbo output to the throttle body?
If the ISV is closed no air should be getting in the the inlet manifold therefore the PCV system is untouched.
Am i right in thinking it only collects from the sump and the back of the cam cover and vents off in to the maf hose as well as having an input to the Inlet manifold via a valve? i think its N90..
There are no other connections for the PCV are there? So it doenst really get effected during a boost leak.

Even if your just pressurising air at the maf hose with out blocking anything else off. In which case "if" the turbos oil seals can hold it it will pressurise the whole system all the way in to the sump via the piston rings > back though the PVC and in to the maf hose. Leaving only the exhaust valves to stop air from escaping providing all the vac lines are ok....

[S2central.net]

@Bo - I am trying to mentally visualise exactly where the PCV pipes connect at/near the throttle body... You might be right that if the TB hose is blanked off to prevent air getting into the IM that there is no need to blank off the PCV outlet.

If you allow air into the IM there is no way to stop it filling the crankcase as one or more inlet valves will be open - same goes for exhaust valves... so even if you could dump enough air into the entire crankcase to try and equalise pressure at PCV outlet to the MAF hose - it still wont be airtight.

The way I like to do a boost leak test is that I can seal up the system as far as possible, fill it up with 1bar pressure and leave it for 15-20 minutes... That gives you cast iron confidence that the path from turbo outlet thru hoses, intercooler and up to the TB is airtight. I appreciate that leaks in the TB hose, MAF hose and PCV system cannot be tested this way but those can be done in isolation with a little creativity.

Must admit that hooking up to MAF hose is most convenient way to quickly find a large boost leak - as you inevitably hear it escaping if a hose has a large split or the intercooler is damaged so its horses for courses I suppose.