what's normal vacumn reading on the boost gauge at idle?

[Josh-stateside]

Hi guys

I am starting the process of checking everything.

my boost gauge reads 10hg at idle. What's normal?

Thanks
Josh

[porkkis]

0,5 bar abs.

[Josh-stateside]

0,5 bar abs.

thanks porkkis.

Cheers
Josh

[SR71]

Mine reads 17mmHg (94 ABY)!

Does that mean I've got a monster leak?

[jubjub]

Freaky I was going to put a post up on this...........My mapped RS2 shows just under 20hg at idle.When driving and you pull of the gas it reads at 21. Does this also relate to boost, as in do you get the equivalent in boost as i peak at about 21psi

[Greenm3]

@Jubjub
I get the same readings on my mapped S2!

[Pisobiker]

Freaky I was going to put a post up on this...........My mapped RS2 shows just under 20hg at idle.When driving and you pull of the gas it reads at 21. Does this also relate to boost, as in do you get the equivalent in boost as i peak at about 21psi

If your boost gauge has PSI units, Hg is in inch, therefore 20 inch of Hg (vacuum) ~0.4 absolute (-0.6 relative) which is about normal at idle.


Marco

[Greg_S]

mine is -0.53 Bar

[Pisobiker]

mine is -0.53 Bar

Mine stay's around -0.6, but you know, those aftermarket gauges aren't 100% precyse, and different brands might well have slightly different readings.


Marco

[SR71]

More important than gauge error would be the spec. For instance, it doesn't make sense comparing the readings from a K&N'ed car with a car equipped with a stock intake filter...

My vacuum reading is on a completely stock ABY (i.e. stock filter, stock plumbing, stock displacement & IC).

My understanding is that with a leak in the intake system, when the intake is under vacuum conditions, the reading would be lower than would otherwise register without the leak i.e., tending towards ambient atmospheric conditions as "high" pressure air from outside the intake "leaks" into the system.

Similarly, when under boost, the reading would be lower than would otherwise register without the leak but for the converse reason.

So in my case, if there are stock ABY's out there with a vac reading of less than 17inHg at idle, then its yet another indication I've got a leak in my system.

I've seen max vac readings on the over-run of ~20-25inHg.

It would appear, in my case, to be a subtle leak though, because the car is boosting to 17psi on overboost and in general to ~12psi which is, as I understand, about right?

But I can notice a very slight hunt at idle if I watch the gauge carefully.

In addition, whenever decelerating and coming to a halt, just as I get on the clutch, the revs will rise and fall as if I've blipped the throttle rather than just smoothly decrease to the idle rpm.

Looks like the plumbing needs to come apart again.

[Greg_S]

Mine stay's around -0.6, but you know, those aftermarket gauges aren't 100% precyse, and different brands might well have slightly different readings.


Marco

This is very true, Im fairly sure its is accurate, sender is placed on the end of the IM using SPA design gauge.

Greg

[Pisobiker]

SR71,

if you had a leak causing at idle significantly less vacuum than normal, your leak could only be located "FROM TB ON". At idle TB is shut, while ISV is providing flow for idling, so a leak could only be:

- hose from ISV to IM;
- thiny hoses that go from IM to ECU and other aux systems;
- IM to head gasket ( );
- A crack in the IM itself ( );


If your reading is about 17 INCH of Hg (not mm), which is about -0.56 bar, I still belive it might be in the range of inaccuracy for some aftermarket gauges. Worth checking of course, in any case.


Marco

[SR71]

SR71,

if you had a leak causing at idle significantly less vacuum than normal, your leak could only be located "FROM TB ON". At idle TB is shut, while ISV is providing flow for idling, so a leak could only be:

- hose from ISV to IM;
- thiny hoses that go from IM to ECU and other aux systems;
- IM to head gasket ( );
- A crack in the IM itself ( );


If your reading is about 17 INCH of Hg (not mm), which is about -0.56 bar, I still belive it might be in the range of inaccuracy for some aftermarket gauges. Worth checking of course, in any case.

Marco

Cheers Marco for the fault diagnosis.
I totally agree with you.
I will check the first two suggestions.
Problem is accessing them.

It sounds like 17inHg of vacuum is reasonably tight though which was my main concern.
The problem I have with the ECU going into "safe" mode is very sporadic.
I have got some knock sensor faults so maybe my problem lies elsewhere.