Exhaust Question(s)

I've been having a rubbing, scraping noise from the mid-underside of my truck. When I got under there and wiggled everything around I found the exhaust was rubbing on the cross member, so I bought new exhaust hangers. Brought the pipe up about 3/4". Noise gone.

While I was under there I also noticed that the donut gasket was a bit crumbly and a chunk missing (explaining my exhaust leak). That's my job for today.

Question 1: Knowing the tendency for Renix XJ (MJ in this case) exhaust manifold to crack (either under flexing conditions or just out of the blue), would it be a good idea to put a section of flex line into the pipe after the manifold. Not the cheesy thin repair stuff but a good braided SS section.

Question 2: Could it be immediately after the manifold in the straight part of the downpipe or...?? Would the flex-pipe portion cause any problems there?

1: Good idea.

2: If there was a way to put it right after the manifold and eliminate the factory "crush" in the downpipe, it would be really worthwhile.

The flex pipe I'm looking at is a section about 10" long. The braided "flex" portion is about 6" long. There is +/- 2" of hard pipe on either end. My plan would be to remove the gasket flange from the existing pipe, slide it on to the pipe end of the flex pipe and put in the donut, and bolt it up.

Then just cut out the excess on the old pipe and slip the main pipe into the flex pipe and clamp it up.

I'm pretty sure I could do it to include the dimple area and thereby eliminate that. It might even suck the over all exhaust about another 1" to clear more space at the cross member. Basically one cut, one clamp would net me a.) some flex at the pipe b.) no dimple c.) the exhaust clearing the cross member by almost 2".

Are there any major problems I'm overlooking?? I don't want to start cutting and screw things up!!

I've seen the section put in where the pipe crosses over from the driver side to the pass side, right in front of the trans pan.

But here's another location:

http://www.naxja.org/forum/showpost.php?p=243939392&postcount=4

ParadiseXJ said:

Question 1: Knowing the tendency for Renix XJ (MJ in this case) exhaust manifold to crack (either under flexing conditions or just out of the blue), would it be a good idea to put a section of flex line into the pipe after the manifold. Not the cheesy thin repair stuff but a good braided SS section.

Question 2: Could it be immediately after the manifold in the straight part of the downpipe or...?? Would the flex-pipe portion cause any problems there?

Click to expand...

1) The idea has merit, and I've been suggesting it for a few years myself. However, note that:
- It's not a tendency specific to RENIX vehicles - the XJ and MJ in general have this problem, due to the rigidity of the exhaust system.
- Exhaust manifold weldment failure is typically preceded by engine mount failure - when the mount bushings go soft, they allow more relative movement between the engine and the chassis. This allows the welds to become strained even more, and leads ultimately to mechanical failure. Check your engine mounts - all three of them. If one has failed, replace all three of them.

2) It would help to have it close to the collector, but anywhere in the downpipe between the collector and the catalytic converter flange should serve.

Save yourself some headache - while you're getting stuff, get a couple of 3/8"-16 brass or bronze nuts for the exhaust collector flange studs. Trust me on this one - I've been doing it for years, and it makes it so much easier to tear down next time! (I do the same thing at the catalytic converter flange as well. You see, copper alloys do not seize against ferrous...)