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View Full Version : Vacuum leak after HGs



S_Mazza
07-29-2010, 01:46 PM
Hello all. I have recently replaced my head gaskets, but the car still doesn't run right. It barely idles and has no power.

After much head-scratching and wire unplugging and replugging, I decided to take the vacuum gauge at its word and check for vacuum leaks. When spraying starting fluid anywhere near the intake manifold, I got the car to rev right up. It seems pretty clear to me that I have a bad vacuum leak from either the return plenum connection or the intake manifold itself.

I used Fel-Pro blue gaskets for the lower intake manifold to head connections. I added a bead of blue RTV around each coolant port. These gaskets have worked fine for me before. As far as I can tell, no coolant is leaking.

I checked the bolt torque twice after installing the manifold. I did not retorque after starting the car, however.

There is one additional piece of important information. The heads I am using are 96-98 Mustang heads. This means that there is one fewer intake bolt on each side ... the one between intake ports is gone. The ports all match up. The bolts are M6, which can only take about 12-15 lb-ft of torque. (Experimentally verified. :eek: :p :D)

I will have only limited time to work on this for the next few days. What do you think I should do? Should I attempt to retorque the bolts without changing the gaskets? Or should I just drain the coolant, crack the engine open again, and replace the gaskets?

If I do replace the gaskets, do you think I should use the SC gaskets again? Or use the Mustang gaskets, which are a different design with molded rubber beads? Or use the SC gaskets again with a bead of RTV around the intake ports as well as the coolant ports?

rzimmerl
07-29-2010, 02:09 PM
What did you use to seal the return plenum to the intake manifold?

I have had good luck using a coat of copper spray on the intake manifold gaskets. I personally would not use RTV as it could create high spots and not let the surfaces mate perfectly.

S_Mazza
07-29-2010, 02:55 PM
The return plenum is sealed with the regular Fel-Pro fiber gasket. They included one in the upper intake kit.


I have had good luck using a coat of copper spray on the intake manifold gaskets. I personally would not use RTV as it could create high spots and not let the surfaces mate perfectly.

See, that is what I am afraid of. I have used RTV before with no problem. But that was with the 8mm intake bolts. I am concerned that the lower torque value may not have squished the intake gaskets enough to cancel out any high spots.