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Hi all,

Need some advice. I have had my S420 for just over a year and am having indications of my first problem. The gauges seem to show a loss of oil pressure after a certain amount of time (About1 hour town driving) down to 0 at very low revsand reduces as the coolant temperature gradually rises. As yet, I have had no warning lights for either the oil or coolant and the engine is running smoothly with no loss of power. I have no clue what this could be and just pray this is not a famous Cylinder Head Gasket away on my baby.

Any help would be appreciated!!
 

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It is losing pressure as temp rises, and it's lost psi compared to when new, the question is how low is it really? The oem gauge is a rough estimate and they can fail and give false readings, including lower readings as time goes by. Of course you can buy a new sender, but the only way to tell for sure is connect a real mechanical gauge to it. You can get one pretty cheap, either an actual oil gauge or a generic one that reads 0-60psi. Then find the port where the electronic sending unit it connected and plug it in there. Ideally I'd use a brass T fitting so you can have both.
I'd then run a line to inside the car or maybe attach the gauge to a wiper so you can watch it for a day or so. Oil gauges come with line to do that fyi.
Oil pressure varies inside the engine, so the most is between the pump and filter, the least is wherever the furthest point in the oil system is. So of your pump is in front then expect the parts at the rear to see the least psi, usually the furthest rod bearing is what suffers the most. I mention this so you can note where your port is compared to the pump. In my Chevota, which has a Chevy V8, I routed a 1/2" line between just after the filter in back and the front of the engine so the system was fed at both ends to average out the psi. I did this mainly because I had your problem; 0psi at idle when hot, and RPMs make oil hot. Oil is normally cooled by the block, which is cooled by water. So when I ran at higher rpms, like 3500+ for a while, the oil got hot, thinned out, and there was my 0psi at idle. It was flowing, but how much reached the far end was the question, hence the special line. Even when the oil was normal temp it only had ~5psi at idle. Once the rpms came up a bit the psi shot up, like at 1000 it was ~20 and 1500 ~40 or so on up to ~60. I never had an issue btw, my bearings were fine. I later added an oil cooler which helped keep the oil thicker, and I also tried Lucas oil which both thickens oil which is a bonus for higher mileage engines, and it sticks to parts better which is a bonus for cold starts.
Point being you have options if it's actually low, the easiest being Lucas, which imo you should use anyway. I'd also try to keep the water temp down if you're getting hotter than normal. I'd figure out why and fix it since eng/water temp is what cools the oil. If you have an oil to water cooler, like my car, I might check to be sure it's not clogged. They're aluminum and small so I suppose it's possible that after enough years it has corrosion and/or crap plugging passages?
If you live where it's hot, and I'll assuming you do, you may consider using less coolant/antifreeze because coolant doesn't actually cool as well as water. I think most cars are 50/50 mix but since it doesn't freeze where I live the only point to coolant is water pump lubrication and some corrosion protection, neither of which need a 50/50 mix. I use more like 10% in older cars I don't care about, which are usually the ones that need more cooling, and maybe 25% in cars I do care about just to be safe since new car water pumps are expensive and usually a pita to replace.
Or maybe your radiator and/or fan are working right. Cars with elect fans rely on a temp sensor, which will fail sooner or later and often they just read low so the eng runs hot. Some cars I get so frustrated with those sensors I just wire the fan on full blast all the time. Or maybe the fins on your A/C condensor and/or radiator are bent up and/or clogged with bugs and grime. I've had a few cars that I had to manually straighten out fins or clean gunk out. Then there's the possibility of gunk inside the radiator too, but I kinda doubt it. Thermostats are another, I've had several of those fail. Scratch that, many of those fail, and often I just take them out and problem solved. I haven't had a MB stat fail me yet, or maybe they fail open?
One more thing; the tranny is cooled by the water too so if your converter is not locking up it just makes the water hotter which makes the oil hotter, plus heat is bad for the trans. It shouldn't be much added heat but if your cooling system is having trouble keeping up then things like that make a big difference.
Good luck!
 
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