Newbie Vacuum question...

4byfour · Sep 24, 2008

I'm trying to track down a rough idle issue and suspect a vac leak. I have searched and cant find info.Probably too basic for you guys but I'm just trying to learn and save some money. Its a 2000 4.0 il6 auto w/112k
My question is, what should a healthy vac reading be, and where is the best (easiest) place to attach it. Also should reading decrease as rpm's rise?
In my searching trying to resolve issue i also learned about the need to clean the IAC and TB. (this weekends project).
thx for your advice/help !!!

tbburg · Sep 24, 2008

You can have a pretty bad leak and still read good vacuum anywhere you test it, so a vac reading won't help.

First clean up the TB and IAC. Change the plugs/air filter(basic tuneup stuff) Be careful with the coil rail on the 00 model while doing the plugs. Don't mess that up, they are expensive. Any idea when the last plug change was?

If that doesn't solve the problem , my first guess for a vac leak would be one or more intake manifold bolts worked loose.

n1ywb · Sep 24, 2008

4byfour said:
I'm trying to track down a rough idle issue and suspect a vac leak. I have searched and cant find info.Probably too basic for you guys but I'm just trying to learn and save some money. Its a 2000 4.0 il6 auto w/112k
My question is, what should a healthy vac reading be, and where is the best (easiest) place to attach it. Also should reading decrease as rpm's rise?
In my searching trying to resolve issue i also learned about the need to clean the IAC and TB. (this weekends project).
thx for your advice/help !!!

Vac should go UP when you rev, AFAIK.

You need to hand inspect all the vacuum hoses. I have had issues at that main connector on the front of the intake manifole where all the hoses meet in one place, some of the hoses were lose in the connector and I used glue to seal up all the leaks. Wiggle the hoses while listening for idle changes.

ehall · Sep 24, 2008

Healthy is 17-22 mercury (hg) inches. It's alrifght if it's a little lower at startup as long as it picks up after the engine is warmed up.

Vacuum will drop when the throttle plate opens and will raise when the throttle plate closes, regardless of RPM. Remember that vacuum is created when a piston moves down and pulls air into the combustion chamber through the throttle body and intake manifold. When more air enters the system, the negative air pressure (vacuum) inside the manifold is alleviated.

Newbie Vacuum question...

4byfour

NAXJA Forum User

tbburg

NAXJA Forum User

n1ywb

NAXJA Forum User

ehall

NAXJA Member

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