300c, oil light illuminating.

[jonnyjeep]

Hi all, has anyone ever had their oil light illuminate when motorway driving, but not when urban driving.

A couple of times in the last month, the oil light has come on after doing 60mph+ for more than 10 minutes. I cannot hear any extra engine noise when this happens. The light goes off when I drop down to about 50mph.

I've done a full diagnostic scan & although a couple of unrelated fault codes came up, they were nothing to do with the engine oil pressure or the sensor.

Maybe an oil change is probably due because it has been 6000 miles since the last oil change....or should I say, possibly just a filter change ?

I may have answered my own question....filter ?

This OM642 mercedes engine takes 9.5 litres of oil at a change, which is pretty much double what a 4 cylinder 2.0 diesel takes, but only has a single oil filter the same size as a 4 cylinder 2.0. That is double the amount of oil being filtered through a filter that really is only build to handle half that oil. It's no wonder that this engine requires such frequent oil changes !

An oil change on this car requires 9.5 litres of fully synthetic 5W/30 oil & a new filter after about 5000 miles. Considering that fully synthetic oil is premium oil & in most cars is good for between 10,000 - 18,000 miles, then maybe it may make more economical sense to just replace the filter, as the oil should still be good....shouldn't it ?

 

[bignev]

I thought my van took a lot at 8 litres!!! But that gets 10w40 semi synthetic, buy it in 20 litre drums and changed way too often, and flushed. Get that horrible soot contaminated oil out!

I've had it 17 years, from new, and it's had a great many oil changes, so it's not a risky thing to do, I'm not going to dislodge any massive built up sludge! As there isn't any!

The inside of my engine is nice and clean!!

I also buy the 5w30s from my local autofactors in 20 litre drums, it's still dear even that way at £80 recently. It was £66 start of last year.

It's not a big name brand, however it does meet all current specs, and it doesn't stay in either of my cars for more than 5000 miles, that's just my choice.

I've had something similar on the oil warning light, with oil getting too thin when really hot, but at lower revs, a few years ago on a Subaru Impreza WRX. An oil and filter change sorted it, but that was a very high performance turbo engine, I had used an everyday synthetic oil of the correct grade that was not up to the job. I went back to Mobil 1 Motorsport, can't remember the grade but quite thick, maybe 15w50. It went 11 years ago to make way for our first GV when our twins were on the way.  Half the horsepower, and twice the weight.... 

Our petrol Grand Voyager did the opposite when the pressure switch was failing, on at low revs but fine at higher rpm. I deduced it wasn't the oil pump as there was no oil on the dipstick while the engine was running so it was clearly pumping it around at idle!

[jonnyjeep]

Thanks for the reply Bignev, sorry to take so long to reply as I haven't been on here for a while.
I did hit another 6K miles on the 300C a couple of weeks ago, but the oil light hasn't come on yet since the last oil change. I'm tempted to leave it until it does come on at high speed, just to see how many miles it takes.

Engines like this should do at least 10K miles before an oil change, especially as it's fully synthetic. I also don't use expensive brands...I've been sticking with Granville hyperlube VL 5w30 for years & it's always been good & it's British !

An engine like the Mercedes OM642 needs better oil filtration. A single filter that is the same size as a 2.0L engine is not enough for the oil capacity of the engine.

I will change the oil & filter soon though, as I don't want to tempt fate.

I think Mercedes cut costs when they made this engine. Proper efficient German engineering would have put a bigger filter, or even two filters