I'm 99% sure there is some sort of vacuum leak coming from the engine area('99,5.2L). Are there certain things that are prone to get leaks from anyones' past experiences. Just trying to narrow it down before I go and start checking for the leaks.

 

check the bypass nipples. they are used to cover extra ports on the intake manifold, alot of times they rot and crack in half and so on. check your power brake booster vacuum line, pcv valve.

 

Our engines are prone to a leak in the intake manifold plenum gasket. I just fixed mine. Not too hard of a job and it really runs stronger now. The service bulletin can be found on this site. my front two intake manifold bolts broke when trying to remove them. Luckily, there was enough of a stud to heat up and use vice grips on to remove. Good luck.

 

ok, grab some carb cleaner and spray the engine in different places while running, when you hear the idle pick up that means the cleaner founds its way to the motor via the vacuum leak, home in on it this way. just dont be a dumbass and spray it into the intake and think you found it. lol.

 

i take it this has happened to you? haha tool

 

Besides the obvious loss of oil from a leaking plenum gasket. Can it cause a loss in mpg?

 

definately, it can also cause a loss of power too. my 3.9 had started to blow the gasket when i swapped manifolds and it only had like 45,000 on it then. BTW how the hell did someone break off the intake bolts? those things are only tightened in inch pounds.