MP4.2+6.0

Understood. What did the mechanics conclude went wrong? What was found?

altaf

Wrong plugs, negligence of maintenance by the previous owner
And abusive driving

mishar

Nor really conclusive answer. It won't help anybody. What exactly was broken besides valves and spark plug? What was the piston damage ? Melted or just mechanical? Any small intake manifold parts missing?

Abusive driving wise, what was that last tiptronic move? Over revving should be electronically prevented.

altaf

Bent valves, melted piston, the head was pretty bad.

Following was replaced.

40 valves
Guides
8 sleeves
8 pistons
Rings big ends.

Basically new engine.

halik

I would bet the belt is missing a tooth or two.

dvs_dave

That's a lot of damage, although your mechanic has not given you any useful information around cause and effect. What's to say it won't happen again if you don't know the cause? Was it over revved, overheated, low oil pressure, skipped timing belt, injested something, sensor failure, ECU failure etc etc....?

mishar

Exactly!

I think there must be missing part of intake flaps that stuck the valve opened. If that was the case and intake manifold was not replaced, it will happen again.

It is not possible to over rev A8 engine unless there is something wrong with the gearbox, including it's computer. If that was the case and it was not fixed it can happen again. Even if it was over revved it wouldn't be only one valve (cylinder).

If it was timing belt all valves would be bent, but not that much since the engine will stop immediately.

s4master1

definitely timing related

s4master1

how were the cylinder walls?

MP4.2+6.0

if you happen to remove the MAF air flow screens that you find in the tube in pursuit of more power. This board is too tame to have ever seen that discussed/admitted here though AFAIK. The engine over revs in first faster than the computer can move it on to second or the rev limiter finally kicks in to tamp it down, at least w/ the prior five speed. Don't ask me how I know... The old 4.2 D2/C5 V8 S8/S6 used to rev to something like 7,000 IIRC with pretty much the same motor set up but somewhat more high RPM intake breathing (throttle body, air box bypass hose, etc.), so the belt and valve train (and con rods, etc.) should be able to tolerate some in practice.

Yes BTW on your intake theory as a possible. Depending on what the broken parts are, it can take out one or multiple cylinders, or jam the belt drive as all hell breaks loose. Actually had it as a fluke on my 85 5000 I5. Got hit at 90 degrees at side of fender far forward and snapped off a piece of the throttle assembly that was near that area in the old slant motor inline layouts. It was hidden by the air intake boot and clamp. On the first restart/ application of gas, it ate the chunk of pot metal, took out one spark plug, and scratched up that cylinder wall--on an old cast iron block design. Insurance paid for a new short block, and damage was only found after the body work and i picked up the car the first time. They should should have totaled it if they had done a better inspection/tear down.