oil pressure drop while running for a long time

[ptmar]

hi im from the tropics and i have a fintail 67 200 gas. when my car is running for a long time like in traffic the oil pressure goes down a bit making my idle drop slightly and temperature goes up. Is this normal? I use 20W50 engine oil.

[ctaylor738]

I think you have it backwards. The engine heats up in traffic (normal), and the oil pressure drops because the oil becomes thinner with the heat (also normal). It is also not unusual for a really warm engine to idle more slowly. I think is has to do with a slight loss of vacuum at higher engine temps.

[joshhol]

There was a thread here recently regarding o-rings in the filter housings of some MB's. You should check your filter housing and look for a rubber ring that sits ABOVE the oil filter. Usually these crack and compress and are forgotten. This may not be relevant to your symptoms, but it's job is to seat the filter tightly so oil circulates through the filter and not around it.

Try searching vintage forum under "oil filter seal ring" and it should show up.

[ptmar]

hello ctaylor738 so you mean this is normal. but what do i do to prevent my oil pressure to go down. should i install an oil cooler? should i use a thicker oil like SAE 40? or just leave it.

[300SDog]

It sounds like your 110 Heckflosse 4 cyl 200 needs a new oil pump, thats all. I owned a '63 190c with about 160k miles that suffered the same symptoms. Brand new oil pump solved the problem and drove oil pressure through the roof.

MB says it's okay for oil pressure to drop below 10 lbs at idle when hot, but when oil pressure started dropping at/below 32 lbs while driving - that's when I finally replaced the oil pump on mine.

Oil pump wearing out is actually a common problem among MB 1960's gasoline 4 cyl engines at/near each 150k miles.

Most mechanics who dont know these engines will freak out and start crying about the crankshaft bearings, eh? But that's a bunch of crap - truth is that all the engine needs is just an oil pump. I would bet my paycheck on this. Hell, I dont think a bad MB crankshaft or crank bearings have ever been reported on this board. I sure as hell have never seen an MB crank go south.

[Tomguy]

How far does it drop? If it's at or above 15 PSI you're fine! Hell, like DD said, even 10 is ok, but I'd personally be wary at 12 or lower, possible gas dilution in the oil.

My car does this too, hell, I think ALL cars do... at least ones of this era. As the temp goes up, less gas is fed to the engine (it doesnt need as much to run smoothly). Therefore, the idle goes down. When the idle goes down, so does the oil pressure. Also, when the oil warms up, it is thinner, and therefore flows easier - there is less backpressure on the pump (and gauge) so the pressure readings drop. You're actually getting BETTER oil flow (as long as your pump isn't bad)!

If your oil smells like gas, REPLACE IT, especially if you get gas fumes out of the engine when you remove the filler cap! Gas diluting oil makes its lubrication qualities decrease exponentially (for every 5 drops of gas, it goes down by a power of 2 or so). It becomes too thin and causes severe wear on startup and idle, PLUS it breaks down at MUCH lower temps (it boils off the crank bearings, cam lobes, etc under stress). It will also show lower pressure at idle. Below 15 is often a sign of this, or a really hot engine, or a bad pump. Although mine is a V8, I can tell you my car's oil pressure is about 20-25 at a warm idle (175F), but when the oil was diluted, it was at around 12-15! It ran out faster than water when I changed it (engine warm, ~120F).

[psfred]

Well, I've seen TWO bad cranks -- one an oil starvation problem that resulted in spinning the thrust bearing (#2) in the block and eventually breaking the crank, and one were a terrible rebuild resulted in horrible cylinder wall wear and totalled crank bearings from too much clearance. Reground that one to the correct undersize and installed it -- now have 45 lbs oil pressure at idle even when warm -- drops to 30 or so after running on the highway.

As far as factory installations go, unless it was run out of oil or until the knocking rods finally broke, the cranks are almost indestructable.

Peter