My 91 Q45a overheated and now sounds like a rod is knocking. It has 175,000 miles. It runs with the loud knock. Is there anything else this might be?

My best suggestion would be just to toss a new engine/tranny in there and be good for another 150+k...

ONce they overheat, theyre pretty much toast...

Ouch!

Have the engine checked by a mechanic. A rod knock is very uncommon. Overheating is a killer. You can destroy an engine in a few miles, especially an all-aluminum engine.

most probably its a badly scuffed piston

My 1990 Q45 has developed a knock, but never overheated. Bought from a client a month ago who I trust (didn't notice any knock at purchase), it has 166k, he bought it from original owner at 80k, Mobil 1 religiously changed, all engine updates, plenum service, new knock sensor and injectors this last year, knock comes in at 2500 and higher, local infiniti dealer says "rod bearing." This car is incredibly clean, garaged all it's life, body rubber like new, interior still supple, trans cooler installed after rebuild 80k ago. Can I service rod bearings via inserts from below after removing crossmember/suspension, or since the engine might otherwise be so tight, pull engine and do a bottom end rebuild? I suspect the previous driver's high rev habit (frequent redlining) may have caused this, which might have pounded the rod bearings while leaving the rest of the engine tight. Any thoughts, advice?

so it idles quiet but "knocks" above 2500 RPM, rod bearing sounds unlikely because mine did it whenever...

Take it to Jerry Tucker in Miramar and you'll know for sure what it is and what to do.

rod knocks are heard at LOW RPM and lugging. Sometimes valve train will knock and sound like a rod--snap open the throttle and it knock gets real loud then--look for intake valve issue. Often a mid RPM knock is a main bearing or piston/head inteference from carbon deposits. Run a couple of cans of TECHRON through it to clean out combustion chambers. Main bearing can be isolated by cutting out cylinder on each side of the bearing and listening for marked reduction. Rod bearing will fade with the particular cylinder 'killed'. Piston/head inteference will not stop immediately like a rod bearing.