Intake Vac

[PCGUY112887]

On the 3100, how many inches of Hg should you be getting at idle for the absolute intake manifold vacuum?

I just got an OBD2 scanner and I'm getting 11 inches of Hg, so I'm just curious.

[black05gt]

Intake vacuum

What RPM was that measured at ? I'm used to dealing in kPa , but I'm assuming it can be calculated ....

[gectek]

it all depends on alot of variables, such as altitude for 1. if you know ur altitude then itll help. at lubbock the alt is around 33-3600ft i think. my vac is usually at 15 or 14 at idle, as long as the reading is steady it should be ok, i prefer to use a vac gauge on the manifold.

[PCGUY112887]

I said at idle... so around 600-700rpm's.

It translates out to 37.25kPa

My elevation - 565ft

[gectek]

umm, urs should be around 16 - 18 i think

[geoffinbc]

Agreed. I pull 18".

I have pulled as much as 22" engine braking in first gear down at steep ass hill at about 4000RPM.

[PCGUY112887]

Humm so anyone know why I may be pulling so much less? I thought the # was higher than what I was getting, hence why I'm asking.

[gectek]

could be a number of things. how long have u had the car and how many miles are on it. has the engine been rebuilt? does it have the stock exhaust? has anything been changed on the engine?

[PCGUY112887]

Everything is stock, about 117k, no rebuilds or anything...

[gectek]

i think that you might have a bad cat. ive seen that drop the intake vac before. also the valves are a good place to start, but a sticking valve would make the reading jumpy. the best way to really tell is to get a vac/press gauge and hook it up to a manifold vacuum source and look at it. do a compression test and a cylinder leakage test to see if everything is sealing ok and that you have good uniform vac. other than bad combustion due to other causes idk more than that

[Guest]

Agreed. I pull 18".

I have pulled as much as 22" engine braking in first gear down at steep ass hill at about 4000RPM.

Mine was 18" to 20", some mornings would pull 22". This was on a hybrid, with 8.9:1 SCR, SP 3100 top end, GMP cam, stock port.

I pulled as much as 29 or 30 ONCE going down a hil, just to see my gauge go that low.

[PCGUY112887]

Well my mom has an 03 Buick with the 3100 on it, so I just went out and scanned it.

Exact same readings. Doesn't read throttle pos, spark advance, or MAF either, just like my car.

[gectek]

what scan tool r u using

[PCGUY112887]

PCM Scan 1.5ish

[gectek]

dont know bout that 1...is it a program or a handheld, and how much did u pay 4 it