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What to do about a G35 Infiniti that has had a blown engine twice

What do I do with a G35 that had a bad engine (engine knock) at 50,000 miles and after 6 mo of fighting I got the dealer to replace the engine for free. They first said it was because I did not do enough oil changes which I could prove was done approx. every 5,000 miles. Now the engine is doing the knocking again after 20,000 miles on the “new” engine and again they it is because of not enough oil changes. Is it possible for 2 engines to go bad in the same car especially a Nissan engine or could they have not really changed the engine? How would I know?

You need to provide some more info.
Did you buy the car new or used?
Was this replacement engine new or used?
How often do you check the engine oil level?
Who performs your engine oil changes?

You need to keep in mind that depending on your driving habits changing the oil every 5k miles may not be often enough.
Maybe this knock is a valve lifter due to engine sludging.

The odds of Nissan building one bad engine is unlikely, much less 2 of them.
If you want to know if the engine was changed locate the engine serial number and verify it against the vehicle VIN number.

The car is a 2003 and I bought it new. Average driving with a lead foot but not abusing the car. I agree that the odds of 2 bad engines are highly unlikely. I also do not see how even if the oil was not changed, it would cause an engine to “blow” up. The engine knock is really loud and undrivable.

What do you think not changing the oil would do then? Are you using the right oil and following the right schedule?

I am using the right oil and the same oil each time recommended by the dealership. My son works on my car and is very picky about making sure the oil is correct and changed on time. I am not saying I did not change the oil, I just could not believe that how the oil being dirty or being changed at 6,000 or 7,000 miles instead of 5,000 would make the engine start knocking.

Still some missing answers to the a few of the questions I asked.
Do you ever check the oil level at all?
Was this replacement engine new or used?

As I stated, changing your oil every 5k miles may not be nearly often enough depending on your driving habits (short hop, stop and go, lot of damp/cool conditions, etc.).

It’s entirely possible DEPENDING on driving habits for an engine to go bad based on 5k mile oil change intervals. much less 6-7k intervals.

In some cases even going 3k miles may not be often enough. And you should keep in mind that an oil change interval should also be based on time, not just mileage.
Like my comments or not, these problems are more than likely caused by you and are not the fault of Nissan.

We check the oil level all the time and there is no leaking or much change in the level, if the oil is dirty then it gets changed at 3500 if it needs it. I was able to prove this enought the first time to get Nissan to change the engine at no cost. I was told it was a new engine but I am suspect it might not have been. The car is driven in CA only with pretty steady conditions, dry and hot in the summer with some rain in the winter. 1/2 of the driving is short hop but on the freeway. Other driving is about 30-40 miles all highway driving.

And you are using 91 octane or better right. The VQ engine is fairly sensitive to ocatane levels.

Always 91 octane, sometime race gas 100 octane

Are you absolutely sure it’s an engine knock? It’s tough, but you would have to dissasemble the engine to determine if it’s a spun bearing or something like that. Otherwise, there are other things that can sound a lot like a rod knocking (loose torque convertor bolts in some rwd cars, for example).

I think it’s a stretch to imply that changing oil at 3000 miles or even 5000 miles is risking disaster after living with oil change calculators in cars for a few years, knowing that BMW advises 15,000 mile oil changes and VW diesels have 10,000 mile oil change intervals.

Speaking of stretching, how about this: A con rod big end actually failed so the dealer, tired of fighting, gave it a quick and dirty journal regrind with oiled emery paper, threw in an oversized bearing shell and sent the car out of the door.

Good suggestion for the OP, check the serial numbers.

I find it impossible to believe a Nissan engine would suffer a premature bearing knock all on its own; and especially twice.
There’s a lot of story missing here.

Just curious about this knock. Is this knock present all of the time; even at idle?
Only on acceleration or what?

I can give a good example of my suggestion that 3k miles intervals on oil changes may not be good enough in some cases.
A dealer I worked for bought about 35 Monte Carlos that were used as fleet vehicles and had been maintained by the company’s maintenance facility; with “regular” oil changes.

These cars all had about 35-40k miles on them and every single one of them had shaky engines. Every one of them smoked, especially on startup, and every one of them was sludged up pretty badly. With valve covers removed one could barely even see the valve springs because of the crud. All of them had clogged oil drain holes which was allowing oil to pool in the cylinder head around the guides, etc.
One can only imagine what the lower end and oil pan looked like.

Did fleet people lie about oil changes?