Camshaft broken

I bought a 300sd 1983 with a broken Camshaft. It is broken just before the 5th cylinder. The owner told me so before I bought it. It has 164K mls. It runs, but without much power.

The other part with 4 cylinders is still attached to the timing chain and gives it enough power to run but idles rough. The Oil pressure is good. The former owner is not sure why the Camshaft broke, but assured me that the engine is good.

(1). Is there a good chance that the 5th piston did not damage the valves and that I may need just a new camshaft ?

(2). Is there a way to use some kind of scope to see if the valves are not bent without removing the head ?

Have you actually looked at the camshaft and verified it is broken? If you want to verify the condition of the engine, remove the valve cover, check the camshaft, check the valve clearance, do a compression test, and do a leak test. It seems unlikely that engine is running with a broken camshaft.


I have verified it by opening the valve cover, it was when I turned the camshaft as I was checking the valve clearance of valves that I observed that the last two cams were not rotating. When I looked closer, I saw that the camshaft was broken just next to the last cam tower.

The car runs, I drove it for about 30mls. from where I bought it from and ofcourse after verifing that the camshaft was broken, I have not and do not intend to drive it before I fix it.

OK, you may get lucky because the valves in that engine pretty much travel straight up and down so they may not be bent, even if the hit the piston. I would still do a compression test (look for test values over about 300 psi, within about 10% of each other) and leak test first. If those tests are OK, you can probably just replace the camshaft. If you don’t have good compression, you are looking at removing the head at a minimum (about $2000 in parts and labor to do it correctly).

I’m not sure what a 300sd is, but I can guarantee that the camshaft is not broken. If the cam was broken the engine would be doing a lot more then just running poorly - especially if this is a MB 300sd. This seems to be more of a deeper internal failure like rings, pistons, etc. And if it does happen to be a broken camshaft, of coarse the valves are going to be bent.

Refer to the OP’s post where he describes observing the broke camshaft.

There are borescopes which can be used to see into cavities. It’s probably not flexible enough to see the valve faces in the cylinders.
It would be difficult, though not impossible, to apply a leakdown test to the cylinders under the broken camshaft. You would have to devise some way to turn the broken camshaft to where each cylinder has both valves closed (TDC, compression), in turn, for the pressure tests. All the pistons would, of course, turn with the crankshaft pulley to their TDC position.

The #5 cylinder is at the far end of the camshaft, he should be able to rotate it so both valves are closed and run a run a compression/leak test. These are very simple engines with a single overhead cam and mechanical valves.

Thanks Craig58 and hellokit,

When I looked at that broken part that has two cams for the #5 cylinder, the two cams were positioned in way that would look like that both valves were closed. Since the broken part of the camshaft that is over the #5 cylinder is not attached to the timing chain and therefore does not rotate, hence leaving the valves to stay closed while crankshaft moves the #5 cylinder up and down, is the fuel constantly being delivered into the #5 cylinder and if so, what is happening in that 5th cylinder chamber ?
Let us assume the piston goes down, then unless the intake valve opens or there are no leaks, a vacuum has to be created, and then restricting the piston to move downwards, however, since it is one cylinder against the other four, it will be over powered and move down. If the 5th piston was down at the beginning, it will be pushed up by the crankshaft and with the exh.valve closed, high pressure will build up unless somehow it leaks, the next cycle as the piston goes down, it evens out the pressure as a vacuum is to be created as the piston goes down.

As I said the car runs with low power, but it reached 60 mph on a highway. The Tach. goes up and and down as gears shift. The pressure gauge needle goes up to the end when I pres on the acc. pedal.

I hope you will excuse me for being ignorant on some of these things, how do you go about testing for compression leak?

Thanks again.

I will suggest that an engine with a broken camshaft or other broken internal component isn’t going to last a whole long time. It seems probable to me that a broken piece of metal created metal shavings / particulate that is circulating in your engine, embedding in bearing surfaces. This is a bad thing.

Also, consider that camshafts rarely just “snap” in half. Seems like something must have caused that cam to break.

Just my two cents.

Do you currently have this car at home or at a shop? If you bring it to a shop they can at least do a leak test, where the put compressed air into the cylinder (by removing an injector) and measure the leakage. That will tell you if the valves are seating correctly without spending 5 hours to remove the head. If the valves are OK, try replacing the camshaft, adjusting the valves, and changing the oil, then see if/how it runs. You may be lucky, if not you are going to have to rebuild/replace the head at a minimum.

Unless my information is incorrect, the 1983 Mercedes Benz 300s are all diesel engines. They wouldn’t have camshafts.
I’m not sure it’s safe to do a compression test, by using pliers (or something) to turn the broken part of the camshaft. Doing so might allow the churning pistons to hit the valves on the other cylinders. A pressure leakdown test would be safer because you’d be turning the crankshaft over by hand to position each cylinder for leakdown testing.

Yes it is a diesel (OM617.951), and it does have a single overhead camshaft (I have two similar engines). Apparently the broken section is in a position with the #5 valves closed. He’s already driven this car with the broken camshaft (which I wouldn’t have recommended), so turning it over for a compression test is unlikely to do more damage. However, I don’t know how useful the compression test results would be without functional valves, so I would start with a leak test to determine if the valves are bent.

Sorry, I’ve been busy, but I’m taking all your suggestions into account and will update you on the progress.

What’s wrong with removing the broken camshaft and replacing it? That should be the first option. It just seems like the right thing to do.

No Camshafts?. Why ever not?.

Anyway, if it is a diesel and I assume a “sD” is Diesel.
With the camcover off, check the loctions of the cam lobes, You say it is a single can so it is easy.
Cam lobes up, then both valves are closed, cam lobes down, both are open, and it if sideways, one or other is open and the other closed.

Now if yo still want to check the compression rings.

remove the injector. Using a piece of wire, turn the engine till the piston is at BDC.
Now rotate the cam till both cam lobes are away from the lifters or valves.
using a leakdown kit, chek the blowby.

Now I have a question. Where did all the injected diesel go?. Into the intake, into the exhaust of into the oil. How long has it been running like this and do you see liquid diesel coming out the exhaust or intake.

I agree with you dodgevan . . . why not try to simply replace the cam from a salvage-yard motor, clean it up a bit (the parts of the head you can reach) replace the valve cover gasket, change the oil and run it for whatever life is left in this 25 year old car. Couldn’t be that expensive and it sounds like it is within the mechanical ability of the OP. Good luck! Rocketman

I would try to fix it correctly, this engine should have at least another 300K miles in it, if it wasn’t damaged (I just retired one of these engines at 438K miles, and am in the process of having it replaced with a 120K mile engine). I wouldn’t bother with a junkyard camshaft (they are likely to be very high mileage), and I wouldn’t spend money (maybe $200) on a replacement crankshaft until I checked out the engine with a leak test. The parts on these engines are too expensive to guess. A good running engine with 168K miles is probably worth $2000, you don’t want to mess it up by just throwing in parts. While it’s open check the chain stretch, it shouldn’t need a timing chain replacement yet (the chain is usually good for at least 250K miles) but it might need an off-set key to correct the cam timing.