Hey all,

Do the 3.9 Magnum motors have the same plenum gasket issue that V8 magnums do? My 3.9 in the Van isnt running that great, loses power on heavy throttle, and has gone through at least three idle motors. Im thinking vacuum leak.

It also uses a quart and a half of oil or so every 2-3k miles. Its not leaking at all.

Im suspecting the plenum and gasket but I wanted to find out if they had those issues first.

Yes, same problem as the V8s and usually at the passenger side rear corner. On some the threads on the manifold are not deep enough for the bolt length.

You can seal the pcv and valve cover outlets and test for the correct pressure build up with a low psi gauge or a homemade u tube manometer. Finding vacuum instead inside valvecover or block is the bad sign. This method is the official recommendation.

You can also slip a thin less than 45 mm wide inspection mirror down the throttle bore to look,
or even better use a video borescope snake camera.

There will be some oil film inside the manifold interior from the pcv system but large dark puddle pools of oil are a sign of a plenum gasket that has pushed out.