A blocked exhaust can be easily diagnosed with a vacuum gauge on the inlet manifold, as can a whole load of other engine faults. There are plenty of tutorials on YouTube which show you how to do it and what to look for.

Well, I bought a vacuum guage in Japan, and I looked on't Nyet, and I have to say that a lot of the instructional stuff out there didn't seem to make much sense (to me), and the results from intake manifold measurements were ambiguous and inconclusive, and thus not very useful..

YMMV

Eventually I made a temporary spacer (between two exhaust sections) with a port in it for direct measurement of exhaust back-pressure, which seemed to be the only way to get anything interpretable out of the gadget.

I believe if you have a catalyst-equipped system you can use the oxygen-sensor port(s) for this purpose.

I back-flushed the rear exhaust section by disconnecting it and sticking a hose in the back end. Got some debris out.

Havn't tried any of this with a catalyst-equipped system, since I've managed to avoid them so far.

All that said, the OP's description of "racing" doesn't seem a likely characteristic of a blocked exhaust, which I'd expect to cause power loss, rough running and overheating,

Edited by edlithgow on 29/01/2020 at 14:12

Worried me when I heard about it due to my (bad?) habit of shoving bits of beer can (plus some Portland Cement) down my exhaust downpipe, with the objective of flame spraying the inside of the (non-cat) exhaust,

IIRC (from pictures, never had one) the exhaust path to the cat is very short on the affected Nissan engines. which allows transient reverse flows to cause this problem. Slightly reassuring since distances have been greater on the systems I've done this with, though blockage still a possibility.

As suggested above, IF the OP engine was damaged in this way, a replacemnt cat might not last long.

Maybe a leak-down test (and/or an emission test without the cat) would be worthwhile, though I dunno what emission numbers you'd expect without a cat so maybe diagnosis of excess oil carry-over wouldn't be possible.