This is in the 240D.

I have never done a compression check, but it will start right up even in cold (say >50* for here ) weather without glowing at all.

It has always used a good bit of oil, like a quart every 500-700 miles, and I used to be able to see a puff at high RPM right before I shifted gears (MT).

I just followed my wife home while she was driving it and there was a pretty steady stream of oil smoke coming from the tailpipe. I know it is oil b/c of the the smell.

What are the likely culprits? How do I start diagnosing?

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

Mileage?

Valve Stem Seals and or Guides worn. Rings, Cylinders and Pistons worn.

There is limited ways for the oil to get into the Combustion Chamber. It has to come up from the Crankcase, get in through the intake Air, past Valve Stem Seals/Guides and there is one other place a few Member have said. (On the older models if the Diaphragm on the Vacuum Pump goes bad Oil will get into the intake manifold.)

They said if the little O-ring in your Fuel Supply/Lift Pump is shot Oil can be sucked into and mixed with your Fuel.
Because more Fuel goes back to the Fuel tank then into the Engine if the above is happening you would expect your Diesel Fuel go be off color; kind of a translucent gray.

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84 300D, 82 Volvo 244Gl Diesel

It has about 275k

MPG is low - 23-25 for a 240D with MT, but power is really good. Never really had any problems getting around gassers in traffic. In fact, my son calls the 240D the 'fast' car and the 300TD the 'slow' car.

There is no oil in the vacuum system, or in the air intake system. Also, the fuel is not getting oil in it, so I ruled out the o-ring on the IP pump that other members have mentioned.

Also, it takes a second or two for the oil to pressurize, and there is some valve clatter until the oil gauge pegs. Then it sounds good. But, once the oil gauge pegs, it never dips, even at idle at operating temperature. Not sure if that is related.

No coolant leak (except for a bit at the radiator fins), and certainly none in the oil.

My hunch is the valve stem seals, esp. since it is worse at high RPM, but I want to rule out other deeper problems before I start down that road.

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

I think you're probably right about the valve seals. The oil pressure not coming down at idle is a little strange.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

You should test your Oil Pressure with a test Gauge to see what is really going on.

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84 300D, 82 Volvo 244Gl Diesel

On of our Members said that the Hole at the top of that tube that goes down the Oil Filter Cap is there to allow Air trapped at the top of the Filter to get out (This is also whare the Oil that goes through the upper Bypass section of the Oil Filter goes.

As the Oil goes down that tube it has to go past a Check Valve #19 and after going past that ends up back in the Crankcase.

If it takes a long time to pressurize your Oil is during your first start of the day it could be that the Ceck Valve is blocked or restricted and it is taking a longer time to get the Air out of the Oil Filter Housing.

If the Oil pressure takes a long time to build up no matter how many times you start the Car the 2 O-rings at the bottom of the Tubing may need to be need to be changed. A leak there allows the Oil in the Oil Filter to drain out of the Oil Filter Housing an back into the Crankcase.

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84 300D, 82 Volvo 244Gl Diesel

Thanks Diesel 911.....sounds like the check valve....it pressurizes faster once warm.
looks like I have to pull the housing to get to it?
I need a new timing chain, so I'll do the valve seals then.

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

No, you do not have to pull the Housing off because the Check Valve is not attached to the Oil Filter Housing.

The Check Valve is in the Bottom of that Tube (above the O-ring area) that is attached to the Oil Filter Cap.
I am not sure how you get at the Valve itself. Compressed Air could be blown through the hole at the top of the Tube to perhaps blow stuff out of the Valve.

It may be that the tip of that Tube unscrews to expose the Valve but there is no place for a Wrench. If you uses a Wrench with teet on it; it is going to bugger up the end.
So I have never tried to loosen mine.

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84 300D, 82 Volvo 244Gl Diesel

Could I swap in the cap from the 300TD to see if it fixes the problem as a test?

Is the 616 and 617 cap the same?

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT

The "valve clatter" you hear is not the valves. Mechanical tappets do not rely on oil pressure for clearance. You are hearing the play in main or rod bearings before they have oil pressure- not a good sign for continued life expectancy.

I have read that bearing noise sounds more like someone hitting the inside of the block with a hammer.

This sounds more like a tick-tick tick like nailing.

How long do you figure that can go on before failure? (I ask b/c it's been like that for >3 years and >50k)

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT