On Sunday morning I was about to leave to see a nice car show. Despite my ‘91 M5 being too new to access the “display parking lot” we decided to go with it. But when I started the M5 the engine oil pressure lights decided to remain lit. So far the light has always gone away immediately when the car has been started.

There wasn’t any weird sounds coming out from the engine but the light wasn’t going to turn off. I suspected that it was a sensor problem as I had pressure washed the engine just before I drove it for the last time. But I left the poor M5 on my yard and continued with my parents’ Mondeo.

Today I checked that the wire and the connector are intact and the oil pressure switch is constantly grounding. I was planning to pry away the switch but that is harder than I thought. The sensor has a 24 mm head that’s placed in a very annoying place. My only 24 mm socket wasn’t deep enough to clear the connector part but also not any of my turning things would fit between the socket and the alternator bracket either. And wrench doesn’t have enough space to turn. I’m pretty sure that I have to visit a tool store and find something.

Now that I cannot get the sensor out I can only speculate. BMW didn’t leave any usable place for a pressure gauge so the only way to put one is to get either the temperature sensor or the pressure switch out and replace it temporarily (or get a divider). But if I could get the pressure switch out I could test if it works or not.

The switch would be an inexpensive thing to fix. The tools to get it out likely cost more than the switch itself. But if the problem isn’t inside the switch the next possible issues are a lot harder to fix. The certain oil pump model has piston regulating the pressure. The pistons (part #7 in the picture) are known to get stuck at the open position. That causes the oil pressure to go near zero. And this isn’t very healthy for the engine. The stuck piston problem is quite easy to fix by getting it out and rounding the corners.


If the pump issue is the cause my problem is rather annoying. I didn’t run the engine much above idle so likely the rod bearings are the only things that might be harmed. And both the pump and the bearings can be accessed by removing the oil pan. My M5 is a quite high mileage BMW so replacing the bearings shouldn’t be a bad idea in any case.


The other known pump related issues for an S38 engine are the oil pump sprocket retaining nut getting loose or the splines giving up from the sprocket/shaft.

I really wish that I don’t have to open the oil pan at this point. This kind of thing is really likely to run out of control. If I take the pump out I might decided to swap the chain too. That chain is placed behind the timing chain so I might replace that too. While the timing chain is off why not check the cylinder head off as well. And then the pistons could be taken out too...


Aargh! Not yet! I have too much projects already going on. I don’t need any non-operating car at this time of the year. I need to paint my house too.

Btw. The show was awesome. Here’s a nice Bugatti and a weird Crestmobile.