Cummins ISL 400hp engine

The smaller of the two haves often cracks. Good luck. If it is cracked, there are some after market manifolds that look to be better built.

st687 wrote:
When it is replaced this time have the manifold surface planed.

That's what I would do.
Also, check to make sure the manifold hasn't grown in length. Sounds funny but they do that on Cummins sometimes and when they do they bend or even shear off the mounting bolts and may blow the gasket. If someone is just enlarging the mounting ears or otherwise finessing it in then it's time to put a new manifold in.

ISL MAY have an exhaust brake or it may have an engine compression brake.

Do you use your exhaust brake often?

st687 wrote:
When it is replaced this time have the manifold surface planed.

A good idea, or have it replaced as they are known to develop cracks over time.

When it is replaced this time have the manifold surface planed.