vaccume

[dohc v6]

i had a vaccume leak on my 94 z34 and i replaced the intake manifold gaskets and it went from 11 hg to 17. it idels better. my question is it still has a little fluctuation at a stop light but at part it idels a little smoother. can it be my wire or plugs. i got new of both. do you think it could be any thing else. the idel is like a hunting a little

[DaveFromColorado]

17" of VACUUM still seems a little low, I'd check for any more leaks.

 

also, if you do have a bit of a VACUUM leak somewhere in the lines that go to your map sensor that could cause an IDLE problem.

 

--Dave.

[slick]

Could be your plugs and wires. Also, pull your IAC out and clean that sucker up, should help.

[Robby1870]

17's not real low for a DOHC. But, if you did have a small leak, it would mess your MAP up, and you would notice it all the time.

[DiscoStudd]

Uh-oh, you set Dave off again! That's one thing we should add to the "Newbie" post. Never misspell "VACUUM" on this board otherwise you'll get severely reprimanded by DFC!

 

Hey, just trying to help you keep your sanity Dave, that's all ...

[dohc v6]

it goes to about 19 when in park just sitting. you think it could still be the intake manifold. also i tighten the bolts to 31 because the book(hanyes) said 30. but the gm service manual said 18.the gm manual is for 92 not 94.should i check the map and what else.

[DaveFromColorado]

I forgot to see what I was pullin' for vacuum today on my Z34.

 

I'd check all your VACUUM lines to see which ones may be causing the problem.

 

--Dave. (DFC)

[dohc v6]

ok i got a little more informantion. the gauge fluctuates from 17 to aroun 18. when i have the air on it goes to 16. also when ever i accelerate it like is hesitant. like it take a second then it goes ok.

[WhiteOut]

I swear vacuum should be around 36 psi on the 3.4 DOHC. I could be wrong, but when I was having a bunch of issues with my car (front 3 cylinders weren't firing) my mechanic couldn't figure out why I had half the amount of vacuum I should have (I had around 19 psi).

[DaveFromColorado]

Vacuum is measured in "inches of mercury" not in "psi"

 

the 3.4's do NOT run 36" of vacuum, I still gotta go out and check mine - but personally, I've never worked on an (idiling) car that will pull more then 28" - they can pull a LOT more vacuum when you are decelerating from highway speeds, and downshifting.

 

I'll go out and put a vacuum gauage on my car here in a little while.

 

--Dave.

[Robby1870]

Last time I did my 3.1L (like 3 years ago on my old car), I think it was like 22 inHg, dont quote me, its been a while. I do, however, think that the DOHC is a little lower, like 19-20

 

Just to add a little on what Dave said about the units of vacuum not being psi. Vacuum is like (in simple terms) the opposite of pressure. Measuring vacuum is like measuring how much "sucking" your engine is doing at closed throttle

[WhiteOut]

Jeebus guys...your worse than my chemistry professors. Wrong units so sue me. And for your information vacuum CAN be measured in PSI, or Pascals, Torr, or even Bars for that matter, not to mention inches of water, or Newton Centimeters.

[vegaslumina]

yep its vacuum, not vaccume

[Robby1870]

Jeebus guys...your worse than my chemistry professors. Wrong units so sue me. And for your information vacuum CAN be measured in PSI, or Pascals, Torr, or even Bars for that matter, not to mention inches of water, or Newton Centimeters.

 

When you're an engineer units are EVERYTHING

[DaveFromColorado]

I've never* heard vacuum measured in PSI, as PSI is Pounds per Square Inch, and as soon as you start creating vacuum, you no longer have the pressure to create any Pounds per Square Inch.

 

Please, go to your local tech college and sign up for a fluid power class before you start telling me how to measure vacuum.

 

--Dave

[dohc v6]

so what do you think it is. could it be ignition.

[dohc v6]

so what do you guys think it is. could it be ignition or is it still a leak some where.

[DaveFromColorado]

could be a leak at the upper intake manifold. if that gasket starts leakin' you'll almost never be able to find it.

 

--Dave.

[dohc v6]

i did have a tear in the lower gasket on the plenum. could that be it.

[DaveFromColorado]

that would certinally do it.

 

these engines are really picky about bein' sealed correctly.

 

--Dave.

[dohc v6]

appreciate it. i will try to fix that and will let you know if it works.

[Robby1870]

I've never* heard vacuum measured in PSI, as PSI is Pounds per Square Inch, and as soon as you start creating vacuum, you no longer have the pressure to create any Pounds per Square Inch.

 

Please, go to your local tech college and sign up for a fluid power class before you start telling me how to measure vacuum.

 

--Dave

 

^ What he said

[WhiteOut]

I'm a chemist...pretty sure I don't need any coarses on "how to measure vacuum." And according to your sugestions anything less than 15 psi (you know atmospheric pressure), should suddenly be measured in inches of mercury simply becuase there's a vacuum relative to the outside world. I think not, 1 atm = 14.69 psi = 29.92 in. Hg. What you fail to understand is there is no "negative pressure" or "opposite of PSI" if you prefer. Vacuum is a complete lack of pressure and nothing lays on the opposite side of the spectrum, you either have pressure or you have complete vacuum. Vacuum in this case simply implies that one side of the system has less positive pressure than the other, and thus it is appropriate to measure in any unit you like as long as it relates to pressure.