The issue with the 3.9 4.0 and 4.6 are virtually identical in the problems they share.
The achilles heel that everyone seems to emphasise is the slipped liner problem that has plagued some Rover V8's even the 3.5s. it is due to poor casting and sub par machining, to cut a long story short the coolant galleries between the cylinders due to the poor casting were offset in the block when the V8 is overheated the block cracks the thin wall of the water gallery which on one side is nice and thick and on the other is paper thin and as you'd expect the thin weak wall with break first and you get the usual issue of the rhythmic tapping which is the piston moving the liner up and down also usually accompanied by the often misdiagnosed headgasket issues overpressurised cooling system and also coolant in the oil etc etc pot luck really as to whether you get a good engine or a bad one, the biggest contributor to these cracked blocks and slipped liners is actually a fuelling problem a lean running condition which i've had countless times with RV8 will murder them, with the cooling system being a close 2nd faulty thermostats bad Viscous fans coolant leaks bad waterpumps etc etc all contribute to their often early demise.
head gasket issues i've seen engines with 50k needing a set of gaskets and engines with 190k still on their originals so again its down to what life the engine has had really. if they need doing even a novice could do it over a weekend, the hardest part is the exhaust heat shields and that damn lower exhaust manifold bolt for cylinder 8 gah!!
Oh yes and ARP studs are a must due to the varying quality of stretch bolts I could whinge on but I won't. ..
Valvetrain wear "they all do that sir" yep every Rover V8 ever built and most flat tappet valvetrains will have wear exacerbated by poor quality oil and sub par service intervals, how do you tell when its worn??? she'll be a gutless whore all noise and no go and with the valley gasket removed you'll see the carnage..
Oil leaks, standard.
Fuel economy to be fair i've seen many threads on this its a 2ton shed with a V8 what are you expecting? the max I got out of mine was 22 with averages around 18.
Power? well a healthy 4.6 will still surprise you especially with the mid range grunt and good tow ability, the 4.0 is adequate but it is actually just as thirsty as the bigger motor.
And finally is it a good motor??
yes if you look after it which a great many don't it will last, like any machine neglect and abuse it and it'll deteriorate
The same applies to the M51 I see far more threads for Diesel engine issues nowadays on the fora
The Americans dumped it due to the fact there was absolutely no demand in the 60s for a small capacity light "economical" v8 back then large capacity V8's ruled, the 472 Cadillac V8 or the 454 big bock chev 440 mopar etc etc all twice the size and all did sub 10mpg but who cared petrol was cheaper than water.
As for comparing it to the diesel, there is no comparison power wise 218hp and 407nm for the 4.6 vs 134hp and 270nm the former is going to be a nicer drive just for the comparative effortlessness, most will bleat on about MPG but lets be honest you don't buy a 22yr old Range Rover for its fuel efficiency by todays standards even the diesel is woefully uneconomical.
Your choice at the end of the day, if it shíts the bed we'll be here to laugh at you "cough" I mean offer words of encouragement and helpful advice!!
