LEAKING MANIFOLD

This article relates to the earlier UPDRAUGHT model.

Working on a customer's car the other day, the engine was running but it sounded like it was firing on 3 cylinders. There was a healthy spark at all four spark plugs and all four compressions were equal, although shorting the plugs on the front two cylinders made little difference to the running (the engine note should dip when shorting any spark plug to earth).
Bit of a mystery until I put my hand under the gasket where the aluminium inlet joins the cast manifold. There are of course two of them but under the front one I could feel the suck of air. Problem found!
Taking a closer look, the centre stud end (of the seven fixings) on the manifold was touching the back of the aluminium induction tube, meaning that when tightening the inlet to the cast manifold it would not push back to form a seal at the gaskets!
Simple fix - hacksaw off the exposed end of the offending stud to give a clearance, new inlet gaskets and quick file of the aluminium joint faces and job done.  Result?  Running like a Swiss watch!
I think a lot of new manifold gaskets are thinner than the originals which can add to the problem. Also check that the stud threads on the seven manifold fixings are long enough to fully tighten the manifold on, otherwise the nuts will ground at the end of the thread and not pull the joint together tight enough. Just cut the threads a little deeper or add a couple of washers under the nuts.
Gary Munn (GM Auto Services)