Weird Mercedes C220 Warning Light Behaviors

I have a 1996 Mercedes C220 with 60K miles on it. It runs great, but is giving me fits with inconsistent, intermittent, and (I’m pretty sure) incorrect warning lights, as well as intermittent gas gauge failures.

Soon after I bought the car about six months ago, the oil pressure warning light began to come on intermittently. Each time it came on, I checked the oil level and found it to be full.

At first it would come on, sometimes for just an instant, sometimes for several miles, but then go off and stay off for days or weeks. … Then it got weird … Each day as I was driving to work, the oil pressure light would go on - always within about 100 yards of the exact same place - about 10 miles and 15 minutes from home. It would stay on for 1/4 to 1/2 mile, then go off and stay off until the next day, when it would do the same thing again. I was still checking the oil level every day and finding it to be full; I have never added oil except when I changed it.

About two months ago I changed the oil and the filter. I put in Mobil 1 0W-40. Immediately after I did that, the oil light came on and has stayed on almost permanently. I say “almost” permanently because if I get to highway speed and then accelerate quickly, getting the RPM’s up to about 4500, the light will go off, sometimes for just a few seconds, sometimes for a minute or two, but will always come back on after I slow down.

Weirder still, immediately after I changed the oil, the coolant level warning light came on and has stayed on permanently. I have checked the coolant level every day since then; it has been full every time, has never gone down, and has never needed coolant added.

Here’s another issue that may be related. The gas gauge intermittently drops to empty, even when I know for sure the tank is not empty. Sometimes it shows the correct fuel level, sometimes it shows empty. Sometimes it will bounce back and forth between the two several times within my 1/2 hour drive to and from work. It does this with no pattern I can discern. It has been doing this as long as I have owned the car.

I have hooked up my Actron OBDII code reader to the car several times. It never shows any outstanding codes, any pending codes, or any other problems.

Although these issues worry me a bit, I have been willing to live with them, assuming there’s really nothing wrong with the car. But now I’d like to sell it - for reasons completely unrelated to the warning lights - and I’m pretty sure no sensible person would buy it if they test drove it and saw the warning lights. (And I think most people would notice if I covered them up with electrical tape!)

What do I do?

I hate to be the purveyor of gloom and doom, but by not understanding what the oil pressure warning light means, you may have caused serious damage to the engine.

Your crankcase can be correctly filled with oil, but if the oil pump is failing, or if engine sludge is present, insufficient oil flows to the bearings and other sensitive areas of the engine, and very expensive damage is the result.

If we relate this to human health, when your MD detects low blood pressure, he probably does not give you a transfusion, and if he detects high blood pressure, he probably does not extract blood from a vein. In other words, there is a huge difference between oil level and oil pressure, just as there is a huge difference between the volume of blood in your body and the pressure of that blood in your arteries.

You were correct to first check the oil level, but if that proved to be normal, the very next step should have been to not start the engine, and have the car towed to a competent mechanic in order to determine exactly what the situation is. It is entirely possible that a mechanic will find that you merely have a defective oil pressure sensor.

But, it is equally possible that the oil pump is failing or that infrequent oil changes (or use of the incorrect spec oil) has led to a build-up of engine sludge. Based on the reality that the car has apparently been driven less than 4k miles per year for the past 16 years, I think it is very likely that the problem is the result of a build-up of engine sludge–and that is not a good thing. Think of the oil sludge as being analogous to a build-up of cholesterol in your arteries, and think of how that situation can lead to stroke or a heart attack. What happens in your engine is similar.

Regarding the coolant level, I hope that you have been checking the level in the radiator, and not in the overflow container, as you can have a normal level in the overflow at the same time that the radiator is very low on coolant.

I suggest that you do the following:

When the engine is stone-cold, remove the radiator cap and check the coolant level. Refill if necessary with the correct spec coolant.
Drive the car to an independent mechanic, preferably one who specializes in Mercedes, or at least in foregn cars, and have him test the actual oil pressure and also have him check for the presence of engine sludge. Normally, I would suggest towing, but since you have been driving the car in this condition for 6 months, whatever damage that is possible has already been done and one final drive to the mechanic is unlikely to cause more damage. If you are really lucky, the problem(s) may just turn out to be the type of weird electrical issues that Mercedes and other German cars are known for.
When you sell the car, make sure that you fully disclose the problems that the mechanic finds, or bad karma will follow you for the rest of your days.

Please post back with the mechanic’s findings.

sounds like electrale grimlin in your cars wiring or a ground problem. but i am worried about the oil light or/and gauge if your car has one. is the engine makeng a strange noises?maybe have the oil pressure tested make sure wverthing is ok with the engine.