Possible vacuum leak not sure need help

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McSweeney

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So i have a 97 305 vortec that I replaced spider injectors on and i never bothered to check the intake manifold pressure prior to install. So after i checked it with one of those fixd scanners and map is reading 4psi at idle which from my experience is way too low. Replaced map and nothing changed. Took upper intake back off and cleaned everything and reinstalled gaskets and now its still at 4psi. I dont hear a leak and i took the brake booster hose off and it seemed to suck like a mf and revved all the way up and almost stalled when i put it back on the booster. So it doesn’t look too me like there is a leak or poor vacuum, does anyone know wtf im dealing with? I can send videos of engine running and exhaust but im not really sure how to do that so ill post pics of the live data
 

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stutaeng

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Check engine vacuum with an inexpensive vacuum guage from wherever.

4 psi on the MAP sensor seems way out wack. It's usually reported in mm of mercury...small pressures.

For reference at sea level the atmospheric pressure is 14.7 psi. It's also called 1 atm.
 

Schurkey

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MAP sensor is a PRESSURE sensor, not a VACUUM sensor. Most folks are more familiar with vacuum gauges, the MAP reads exactly "backwards".

4 PSI is strong manifold vacuum. 0 psi would be 30 inches of vacuum. 14.7 psi would be 0 vacuum--atmospheric pressure (depending on altitude, of course.)
 

stutaeng

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Ok, thanks for the correction. 29 mm Hg was what I was thinking about...so that's not atmospheric pressure, but vacuum. Makes sense.

i just looked at my chest HF vacuum guage and didn't see psi. I guess some scanners must report MAP is psi then.
 

Schurkey

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Ok, thanks for the correction. 29 mm Hg was what I was thinking about...so that's not atmospheric pressure, but vacuum. Makes sense.
Do you mean 29 inches of Hg (mercury)?

30 inches of mercury is commonly accepted as "perfect" vacuum, if you're looking at a vacuum gauge.

30 inches of mercury is commonly accepted as "atmospheric pressure" if you're looking at a pressure gauge.

So 29 inches of mercury is either big vacuum, or big pressure, depending on what measurement device you're using.

i just looked at my chest HF vacuum guage and didn't see psi. I guess some scanners must report MAP is psi then.
Scan tools could be programmed to report in any applicable measurement scale. Mine is selectable between "American" and metric scales.
 

McSweeney

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Ok thanks everyone, it seems perfect to me thats why i was so freaked out. Yea so the map sensor doesnt read actual intake man pressure so ill test it with a vacuum tester and report back. I took off a couple vacuum lines and it sucks harder than a 2$ ****** lol so i think im good and the engine runs as good as a 200k motor so im probably fine
 

Supercharged111

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Ok thanks everyone, it seems perfect to me thats why i was so freaked out. Yea so the map sensor doesnt read actual intake man pressure so ill test it with a vacuum tester and report back. I took off a couple vacuum lines and it sucks harder than a 2$ ****** lol so i think im good and the engine runs as good as a 200k motor so im probably fine

Umm, yes it does measure manifold pressure. It measures absolute pressure just as the name states.
 

Supercharged111

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Meant vacuum, so if i test my truck it should measure out to around 14 for my vacuum

Why not just do the math? You know what it is at idle and you can figure the difference between that and key one engine off to get your vacuum.
 
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