I am suspecting an intake vacuum leak. I Still have a rough idle. It will "shudder" at times in drive with foot on break adn as i increase RPM is seems to even out with no shuddering, blah, blah blah!!!

I have a rebuilt 350 long block with original 305 tbi intake and original tbi. Literally all new sensors/modules, etc. new 350 injectors, ESC module, Knock sensor, etc. Burned several chips with the help of moates and tbichips.com. Complete tune-up in ALL ways. No stones gone unturned here.

Standard vacuum taken at throttle body is 20-21 psi and pretty steady and strong. I have litterally done all of the "tests" on everything you can think of. IAC reset, cylinder compression test (all around 160-170), etc.

QUESTION:

How do i test for an internal intake manifold vacuum leak? I was told one way and I am not sure I did it right. Let me tell you how I did it.

Car was running and at operating temperature. I took out the pcv valve from the rocker arm cover and plugged the grommet with a rubber plug to give a tight seal. I then took the breather tube out of the passenger side rocker arm cover and hooked up my vacuum gauge to the gromment using suitable adapter to create tight seal. I was told that the gauge should build pressure in just a few seconds. I was also told that if the gauge didnt build positive pressure or if it went "negative" that i have a vacuum leak with in the manifold itself.

Well, guess what? No pressure reading at all. It stayed at 0 or below for at least 30 seconds. Now, when i removed the plug from the passenger side rocker arm cover a "rush" of air came out and it was a little "smokey" and smelled of oil. is this normal?

Did i do the test correctly? yes or no? Any other ideas? I have literally stumped several of the "best" mechanics in the area with my idle problem. No one has performed this test but me.

Thanks so much for all the help!!!!!!!!!

 

Try the test from the brake booster hose. Vacuum gauges, at least the "normal" vacuum gauges, measure vacuum - not positive pressure, which is what is in the crankcase.

 

Thanks. I am getting some blow by as well out of the breather tube. It is not a lot but the engine only has 3,000 miles on it.

How else can i test for manifold vacuum??? Still suspecting internal vacuum leak.

 

Smoke test. If you have a smoke machine you can block off the throttle and pump smoke into the intake and see where/if it comes out. This is one of the easiest tests... if you have a smoke machine.

Other tests include the carb cleaner/propane on the intake seams test. Either one will work. Spray them on anywhere that a leak can happen and listen for increased RPM. If you hear it, then you found your intake leak.

Also, the hose/stethescope to the intake seams works too.