Hope everyone's had some happy holidays. I've been trying to fix a g35 with extremly low vacuum and I've had no luck finding it. I have not got too much luck on g35driver either
I bought the car in August, car had the random multiple misfire code-which I found to be valve covers-which I personally fixed and the misfire code disappeared. I thought I was in the clear, but after trying to drive the car it wanted to die under load-rpms didnt was to climb under WOT. I connected a vacuum gauge and got 10hg of vacuum.

I ran a compression test- all the cylinders were above 150 and within 15% of one another.Factory spec is 140.

I also ran a cylinder leakage test and all of my cylinders were under 4% leakage.

Cleric670 helped me to realize it might be an exhaust restriction.I checked and what do you know- my drivers side cat was cooked- it was in pieces and heavy deformed. I replaced them with megan resonated racing pipes.And I started the car but I still had 10-12hg of vacuum??.

I disconnected the supercharger-pulley has a very loud rattle- engine sounded like it was running smoothly and had no pinging or knock. When I changed the oil there weren't any metal specs.

Yesterday i noticed my pedal was hard(never really noticed) and actually found that i was missing the check valve in the brake booster hose.But I dont believe that will actually fix my problem.

I used a unlit propane torch, starting fluid, carb clean.To try all the obvious hoses above but i could not find anything.

I also replaced all the manifold and plenum gaskets for good measure(torqued to spec with snapon torque wrench).

I feel like I'm at the end of the line and I appreciate any suggestions anyone has.Thank you.

 

could have a faulty brake booster. Have you traced all your vacuum lines to see if the rubber is too hard to seal and also check for cracks? Booster check valve normally is on the passenger side of the intake manifold and runs to the strut tower on same side. May consider even removing your wiper cowl to check the hard vacuum pipe running to the booster.

 

ok wow, that's right- I thought the check valve was on the drivers side by the booster.Yah the hard pipe is good, I ran 1 psi of air through it and there wasn't a hiss to be heard.

 

Now that I have verified that my check valve is in fact good, my only question is can a faulty brake booster leak engine vacuum?

 

yes, but before replacing. Disconnect the vacuum line on the passenger side intake that goes to it, cap the intake manifold and see if that fixes the low vacuum first. Blanking caps are cheap if you don't have any.

 

ok will try that- tried clamping it off with locking clamps to no avail- when i plug it with my finger it felt like it was gonna die.

 

Oh you said the manifold- will try that tmr.

 

Ok so I capped it off and still got the same 10-12hg.Heres an attached video-the rattle is not rod knock just a bad pulley bearing.The popping is exhaust pop.

 

Had an idea to cap off the other port by the TB- vacuum jumped up a bit

 

Looks like you found the major part of your leak, trace that line back and find out why your are losing vac

 

1 more question, I found this port in the back of my engine disconnected.Is this a vacuum line or just a vent hose for the transmission(manual)?

 

Believe that is where the trans vent tube connects to the bracket but to be sure would recommend getting under the car with a flashlight and looking above the trans. Plus this gives you a chance to look closer at the back side of the engine to see if anything else needs to be addressed.

 

Just looked on my z and that vacuum line that you capped off should run to your EVAP purge solenoid that is mounted to the back side of the intake manifold.