420SEL burning oil
 

I have a 1987 420SEL with 235k miles that I have driven daily since 2005. It has provided safe and reliable transportation for these last 12 years but it has suddenly started burning oil, but only under a very specific set of driving conditions. Most of the time it doesn't burn any oil or make blue/white smoke, but if I get stopped in traffic and am pointed downhill for more than a minute or two it loads up with oil into the cylinders and when I hit the accelerator and drive off it produces an unbelievable white smoke screen that lasts for more than 30 seconds and starts to clear up slowly turning from white to blue then clear. It will also load up the cylinders with oil if I coast down a steep hill and pull the tranny down to 3rd or 2nd for compression braking, when I press the accelerator down at the bottom of the hill it blows a big cloud of smoke.

I think it's most likely the valve guides are worn and it's pulling oil into the cylinder that way. Do you guys have any ideas for what else it could be? I've religiously serviced this car changing oil every 3k miles with Chevron oil. This might be the end of the road for this old boy as it's not worth the cost of having the heads rebuilt.

It's probably the top end that needs a rebuild at that mileage...

Why is it not worth fixing it if the car is reliable?

I figure removing the cylinder heads would be a real pain and expensive, I talked to a couple machine shops and they quoted $3k for rebuilding the heads. Online retailers want $5k. When I got the car it had 119k miles and needed the timing chain and guides replaced. I did that all myself and it took an entire weekend (20+ hours). I would have to do all that again and add removing the CIS fuel injection, intake, and exhaust all of which have not been disturbed in 30 years. When I have worked on that part of the engine in the past the connectors and rubber just crumbled...

420SEL burning oil
 

Copper crush washers for the bolts, a spring compressor and an air compressor to charge the cylinders with air to hold the valves. You can then swap the valve seals. The guides are worn at that mileage but it could buy some decent time for an otherwise parked car.

Guides are toast, yours does what my 560 does... just change the plugs monthly as they will foul fast. $3-5k is what it will cost if you pay a shop to do the work. I would guess materials alone, including machine shop fees will be $1500 of that easily.

One of our members here just did this on his 420, his bill was just under $5k out the door and had piles of rubber replaced that needed to be done. Luckily his threads did not rip from the block.

Only thing that has kept me from doing my now is that I may rip threads from my block and I am not interested in pulling the motor right now.

Dubyagee, that's a worthwhile effort if I can do it without removing the camshafts. The 4.2L V8 has single overhead cams, I'm pretty sure the cams block any access to the valve springs and keepers. I've replaced the umbrella valve stem oil seals on american V8 which were not SOHC, I've got the 14mm adapter to put in the spark plug hole and hook my shop air up to it.

Hit Man X, I'm reluctant to put $5k into the car. The tranny, rear differential, half shafts, etc. could all go at any time.

How would the rear diff fail unless run without oil? CVs only seem to fail if a boot rips, even then it will still run for a long time without lubrication. Periodic inspection will prevent this. I have 317k on a fully stock 722.351 in my 300SEL. :)



There is a specific tool so you can depress the dual springs on the M116. Works well. You remove the rocker first, then the springs. I would not waste my time again with stem seals, but it could buy you some time. They did not do much for me.

From what I was told by a tech, the angle of the rocker being depressed puts a side load on the guide causing them to ovate some...hence the excessive wear. My machinist says pretty much all the SOHC M116/7s need guides past 100k. You and I are both long over due.

Here is the tool - Mercedes Valve Spring Compressor | eBay

Pull the heads yourself and have someone do the work on the head and put head gaskets back on and drive it.

mdawson, I had that exact same problem with my 84 500SEL. Idleing or going slow down hills, and then accelerating it would create a cloud so bad you couldn't see the cars behind you but other wise it used very little oil. This is what I found.
I removed the left (drivers side) valve cover. The cam oil tube mount at the rear of the engine was broken. Just above that spot is where the PCV system gets the fumes from the engine. Oil was spraying out of this broken mount directly into the this PCV port, running DOWN the channel in the top of the valve cover and right into the PCV hose. The fix was to replace the plastic mounts for the cam oiler tubes. I would go ahead and do both sides if you find they are dark brown in color. New ones are white.

PaulM

Valve stem seals and look inside the butterfly
 

Whether this is your issue or not, also remove your fuel distributor and MAF. Open the butterfly on the intake and see if you have oil accumulated in the intake and clean it out. I recently solved my oil burning issue by changing out my valve stem seals myself. Then, because I also had to change out my hardened Idle Air Control Valve hoses, I removed my FD and MAF. I discovered that my butterfly was covered in black soot. When I opened the butterfly, I discovered a pool of oil about 4-5 table spoons in quantity. After cleaning everything and replacing the hoses, my car now idles better than it has ever done since I got it 3 months ago. It's only been about a week since. In about another week of driving, I'll remove my MAF and re-inspect to make sure I don't have a continuing problem.

As long as you've upgraded to a bluetooth. hands-free radio and the central locking and cruise control work. These are my must-have gadgets.

For what its worth, I had my 420SEL's heads re-done a few months ago. It was a little over $4500. That included replacing the rear brake calipers as they'd seized up and the water pump.

After all that the 420 has never better. All kinds of power. Fuel economy is up a little bit too. I get 18-19 MPG with a/c on driving at 80 MPH+.

The engine idles perfectly at spec speeds too.