Installed a boost gauge, hardly any VAC?
 

So I installed a boost gauge on my 2004 ion redline. I ran the line and t'd it off the vac line comming up from behind the blower from the manifold. Now my boost holds perfectly at 12/13psi. My VAC is 6-10.. Any ideas? Wrong VAC line? My buddy said I may have blow-by? I've double checked for leaks, kinks, I have some Teflon tape at every connection to ensure its leakproof.

6 to 10 at idle

idle, decel, N any gear.. sometimes it may go to 14ish but then climbs back to 6-10 range.

Thats weird mine is always at 16 when cold and 18 - 20 when warm. What you boost at wot??

Mine is around 15-20 depending on the weather but 15 is pretty much where mine idles I think. I'll check today

can you do a leak down test

Which unit is the gage in?

The stock LSJ gage is 'psi' on the top half and 'in. Hg' on the bottom half, so make sure you are comparing like units.

good point

This is IN HG same as stock.

I checked my vacuum this morning (on my ScanGauge II) 8.5 psi average at idle measure at different stops on my commute to work.

[that is @ 42 deg F, barometric pressure here: 29.49 in Hg]

While my scangauge read 8.5 psi vacuum, my boost gage was holding at ~17.5 in Hg. vacuum. A quick conversion from Google verifies: 8.5 pounds per square inch = 17.3081621 inches of mercury.

From this, I would venture a guess to say that 6-10 psi vacuum is likely just fine, I don't think you have any issues. Just for reference, the equivalent of 6-10 psi would be 12.2-20.4 in Hg.

the car dosent have psi at idle

these engines hold around 19inches at idle my scan guage shows that along with my mechanical guage a vac leak or low cylinder pressure or cams out of time will cause low manifold vaccum

Pressure can be measured in either PSI or Inches of Mercury (in Hg). The Cobalt SS/SC analog gages are twin-unit gages to view PSI units in boost (positive pressure) or in Hg in vacuum (negative pressure).

The vacuum can be displayed in either units. 17.3 in Hg (pressure) that I saw this morning on my analog gage is equivalent to 8.5 psi (pressure) that i saw on my Scangauge II. Both pressure, both vacuum, two different ways to report the same thing.

If it was a VAC leak or anything besides a broken gauge it would affect my boost too?

yes a leak will affect boost also

If you have a VAC leak, your idle would likely be high or have a bouncing idle, but not necessarily. If it was bad enough, you could hear a 'hiss' at idle under the hood. Your engine would be using un-metered air coming into the engine and would probably have issues in high-rpm, high-boost situations. This would probably cause it to throw the check engine light, but not always.

What RPM are you idling at?


I would park the car safely, set the e-brake and let it idle. Pop the hood and spray carb cleaner around all your vacuum lines. If you notice the engine idle up in a certain area, you may be able to determine the location of your VAC leak.

if you are seeing 8in at idle you have something wrong these engines are supposed to have 18 to 19 in at idle stock cruise you should see around 10 to 12 in the only way that will change is if you put a bigger set of cams in low cylinder pressure or a vacum leak bad head gasket pinched line bad guage

Is it the gauge itself? I haven't seen the question posted yet. Does it go back to "0" with the engine off? Could be a broken gauge. If no to question #1, it definitely could be a vacuum leak. Boost leak test with soapy water to find the leak. SRT-4s are notorious for screwed up stock gauges (just sayin').

good point but you dident read it well he installed the guage the IRL dosent come with a boost guage factory unless it has comp package

The gauge does not go back to zero when turned off. Maybe it is a bad gauge.

IIRC, you have to turn the key to 'ON' but not 'START' to check to see if it zeroes.

This is a mechanical gauge. Should do 0 on shut down.

Sounds like a bad gauge. My glowshift likes to stick at 14ish when im really 19-20.

See if a buddy has one you can trade out or just get a new one.

irl dosent come with a factory installed boost guage you have to install a mechanical guage

The irl did come with a stock boost guage in the sports package option. The guage came on the steering coluum behind the steeting wheel.

it was called a competition package it had a ladder tach and a boost guage on the colum idk if it was available on 04 irl or not but sive his dident have it stock it is a non com car

Yup. Should be same set up as cobalt only that IRL comes with shift lights