I hate engine oil :-(

You've ignored the bit where I pointed out that not all the solids settle.

I put to you again, if all the solids settle and sucking oil out is just as good as draining then why:

1. Do all engines have oil strainers and filters
2. Do motor manufacturers provide a method of draining the sump from the bottom?

Robert

No they are there for a reason. Perhaps cars should still come with manual chokes, because people expected them to at one time? I think you will find (speaking as an engineer here) that the sump plug is located in the optimum place to remove all the oil, suspended and settled particles and sludge from the sump. If there wasn't an engineering need for it to be there, it wouldn't be there.

Robert

That's just not true. Sumps are on the whole flat bottomed. There is no optimum position for a hole in the bottom of a flat bottomed container - any deviation from the horizontal results in a varying quantity of oil (proportional to the degree of deviation) being left behind. It's crude, but good enough.

It is "easier" to get the oil out with a suction pump via the dipstick hole (i.e. no sump plug to round off, no washer to replace, no sump plug to mis-tighten, no liability when a sump plug drops out, no undertray to remove). It is in no way better or even as good a way of removing used oil and contaminants as removing the sump plug and draining that way.

This is not because it is a better way to do things, quite the contrary it is because there is not clearance below the engine to place a container to catch the oil. In the case of boats, oil HAS to be removed by an inferior method because the SUPERIOR method is no available.

Boat engines do not have an easy life.

-- James

My sump has a flat bottom, but it doesn't have a level bottom, if you can appreciate the difference.

-- James

With any settled solids it is not simply a black and white case of them either settling or remaining suspended, it is more a question of under what conditions they settle and under what conditions they are held in suspension. When you have solids settled in the sump and you remove the sump plug, the flow rate of the oil erodes the settled solids and carries them out of the sump plug. The flow rate you get coming out of the sump plug is far greater than that you can acheive with your thin hose on the vacuum, and it covers a large area of the sump, so the sump ends up cleaner.

-- James

Car engines produce solids. Condensation, burnt oil, piston ring leakage, valve guide leakage and external foreign matter all end up in your sump.

Have you noticed how quickly the oil in a high milage diesel engine turns black? The black is all particulate solids that the engine has produced.

-- James

No you aren't.

that the sump plug is

What?

Well I suppose the engineering question is why should letting it run out the sump plug get any more of anything out than sucking it out with a vacuum pump.

Gravity pulling the 'bits' downwards.

The oil flow going in the direction it was intended from the lowest point of the oil pan where gravity would dictate the heaviest, loose, particles should er... gravitate.

But that's not how they get out, they're carried as a suspension in the oil

The smaller particles, possibly...

Anything heavier than oil would be affected by gravity though.

But with this I'm (obviously) just a layman.

An engineer.

lowest

Duncan is correct. The main advantage to letting the oil out of the sump plug though is to let it have time to drain. Most 'suckers' will just drive the vehicle in the door and suck until the pipe sucks air, not letting the internal walls and cavities drain sufficiently. Even if the left the top end drain for a few minutes before sucking, the walls and floor of the sump would not dry off to the pipe end very well. In mitigation, low viscosity oils may drain down more thoroughly and sumps drained from the bottom never drain thoroughly anyway. In the grand scheme of things, the difference between one method and the other is not significant enough to worry about, at all or in the least.

Huw

dipstick hole

contrary it

container to

This is not correct. It has been reasonably common to have the sump connected to a tap and pipe with a pump to pump the waste oil to a higher level. Many engines have pipes connected permanently to the sump opening with a tap on the end of the pipe to ease maintenance. I prefer to drain from the bottom for at least half an hour, but it is not important, it just gives me job satisfaction.

Huw

Really?

Yes really.

If you say so