First, a bit of history on my 2000 Disco II...

Previous exhaust leak fixed via new manifold gaskets and gaskets between the manifolds and Y pipe.
A slipped sleeve, or bad head gasket caused significant coolant infiltration into the oil at 146500 miles on the original motor, bearing wash likely occurred.
A decision was made to replace the engine with one from Great Plains Rovers. Engine has Top Hat sleeves, ARB Head Studs, Longer Duration Cam, ceramic lined water jackets and a new oil pump.
Upon reassembly, the old exhaust gaskets were used with the new engine and a new exhaust leak like sound returned.
Gaskets changed, sound remained.
New engine has fewer than 3000 miles on it and was broken in according to Great Plains Rovers instructions.
Smoke test performed on exhaust - No Leak Found.
Manifolds have been inspected and show no signs of cracks and smoke test does not reveal any leaks there either.

We now are wondering if it is possible for one of the catalytic convertors to have been damaged by coolant infiltration from the original motor failure, which is causing some excessive back pressure in the system and causing the leak-like noise under acceleration.

This noise only occurs under acceleration with a load, or rapid acceleration when in park. If there is no load on the motor and I smoothly accelerate into the upper rev range, there is no noise.

The engine runs well and pulls smoothly from low RPMs, but seems to lack any grunt in the mid to upper RPM range, particularly under load. It never stumbles, just seems to lack the grunt I would expect from a new engine.

My next diagnostic step is to conduct a back pressure test by pulling the upstream O2 sensors on each bank and putting in a back pressure gauge. It's my understanding that anything above 2lbs at 3000 RPM indicates an excessive exhaust restriction.

We cannot locate any valve-train noise with a stethoscope, so I am left to believe that this is somehow exhaust related.

Have any of you experienced a similar situation? Any suggestions that I may be overlooking?

Any thoughts would be appreciated.

 

try covering the exhaust with a rag, it the exhaust is leaking you should hear the back pressure leaking under the truck.

 

That method was attempted prior to conducting the smoke test. No leak evident.

The smoke test was conducted with a $1300 Bosch SMT 300 Smoke Tester while I had the Rover up on a rack and down on the floor. I inspected every inch of the exhaust while the tester was connected.

I'm pretty confident that if there was an exhaust leak, I would have discovered it.

 

I got the AB engine myself. It lacks the GRUNT that I would expect from a new engine as well. will keep an eye on your thread.....

 

Back pressure test reveals 0 lbs of back pressure at 3000 RPM, so I clearly don't have an exhaust restriction.

The new thought is that the leak may only reveal itself under load or when rapidly accelerating due to the torquing of the engine in the mounts creating a gap in an exhaust joint only when the engine rocks in the mounts.

We'll be testing for that at the beginning of the week next week.

It's driving me nuts!

 

my manifold (cracked and stripped bolts holding it on) noise was only detectable under load as well. I was sure it was hg failure shwew drove me nuts too. had to take it off before I could find the extent of the problem.
good luck

 

Hallelujah!

We finally found the leak today...

On the passenger side, where the flange that bolts to the manifold is welded to the exhaust pipe (not up where the flat part of the flange is, but rather where that whole flange assembly connects to the actual pipe - the first weld you run into tracing the pipe up towards the flange), the pipe actually cracked just below the weld about half way around the pipe on the side closest to the block. I'll try to post up a picture once I get the Y-Pipe out.

Whenever the engine is idling, or when it is smoothly accelerated, the crack seals and no leak is detectable. However, under load, or rapid acceleration, the engine rocks to the right a bit in the mounts and that torquing of the motor opens the crack up and the leak becomes evident.

We finally found it by putting it up on a drive on lift, with the motor running, and prying on the exhaust with a 2x4.

Next step it to pull the Y-pipe and mig weld that crack.

This has been an elusive leak to find and it has been driving me nuts...

So relieved to know what the cause is and to be able to fix it so cheaply.

 

Thanks for posting that. I wish more people would post their solutions.

 

Here's a quick picture of the leak location. You can see a very slight crack, just below the weld. I am prying on the exhaust to get it to open up just a little.



Should have it all sorted soon.

 

So true. How many threads do we get to read about problems similar to one we're experiencing only to find it come to an abrupt end. This is a great find and probably something many people have on their own exhausts. I'm trying to find a leak right now on mine. I'll give it the 2x4 test!