Engine Vacuum leaks


Founding Member
Aug 5, 2000
I finished putting together the H/C/I on the 89 mustang (ported gt40’s, and ported cobra intake) and decided to get a smoke machine to test for leaks prior to starting up the engine. I connected the machine to the hose headed to the pcv valve at the upper intake, I blocked off the upper intake port that the pcv hose was connected to and the throttle body.

I found two leaks. First was either at the distributor o-ring or the lower intake on the drivers side near the coolant passage. That leak was hard to tell exactly what location it was coming from, I was getting a fair bit of smoke. i made sure the intake mounting surfaces were clean from oil and debris. I used Fellpro 1250S3 gaskets for the intake without any added silicone on the gaskets themselves with a silicone bead for the front and back sealing surface. I also used studs when setting down the lower intake to make sure it aligned correctly. I torqued the lower down proper torque specs and sequence.

The second leak I found was smoke coming out of the dipstick tube. I checked to make sure the tube was seated correctly but I still had a little smoke emanating from that area as well. I’m a little rusty on diagnosis but would this leak be indicative of an intake vacuum leak sealing into the crankcase?

I will have the car running hopefully sometime next week and will test for leaks again using propane at the upper intake and retest with smoke also.

Has anyone else ever encountered this issue or have any insight?

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I think you've smoke tested the crankcase :doh:
send the smoke into the intake track through the pcv hose to the intake.
You will likely have an oil leak at the gasket along the front of the intake where the smoke was coming from though.
I'd run it and test with propane or my fav. Carb cleaner. With the carb cleaner I spray slowly around the entire intake and if it dies that's the leak.