Mercedes Diesel Misfire After HG Replacement

If you decide to remove the head, removing the engine after is relatively easy by reusing cylinder head bolts, or any of the other holes you have because the intake and exhaust manifolds probably have to come off before the head removal. Use those bolts for chain anchors. You might have to get creative, but I have kept old motor mount brackets and the such for exactly this task.

If you do remove the head, generally are out the cost of a new head gasket and bolts. You can reuse the bolts if the FSM says so, but the cost of new ones and assurance is cheap.
 
cptbarkey said:
If you decide to remove the head, removing the engine after is relatively easy by reusing cylinder head bolts, or any of the other holes you have because the intake and exhaust manifolds probably have to come off before the head removal. Use those bolts for chain anchors. You might have to get creative, but I have kept old motor mount brackets and the such for exactly this task.

If you do remove the head, generally are out the cost of a new head gasket and bolts. You can reuse the bolts if the FSM says so, but the cost of new ones and assurance is cheap.
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Thanks! That all makes sense. The lifting brackets are bolted via the intake manifold on the rear, and I’ll have to look again about the front.

I’m going to avoid removing the engine. While I get the added cost and all of doing a job again, these are long heavy blocks, and I’m just not set up for it. I’d put it back together and get a shop that is set up for changing engines to help if I had to go that route.

I have a hard time seeing how I’d need an engine, versus removing a piston and changing rings/light hone, given how everything looks. That’s why I keep doing photos - find a smoking gun, which given the fact that I don’t need this car to get from point a to b, and can be slow and methodical, will hopefully be more straightforward and lower cost in the long run. There are too many unknowns with used 3.5L OM603 engines given their tendency to bend rods.

T
 
Oooof. Are we sure it’s not the pre-chamber fit? That sounded like a smoking gun to me.

If it were me, I’d probably pull the head and have it checked/machined, because (a) it sounds like you can manage that and (b) it might be all it needs and (c) it would provide a 90% of the access you need for full diagnosis. If the problem is truly in the block, it’s a block-out job that would warrant the rest of a rebuild. And if it’s a rebuild, the head is already done…. I hate to say it, and this is where this hobby hurts, that’s probably some money and we start thinking hard things about what it’s worth. It might be worth every pretty penny to rebuild it and drive it!! It might not. (I’m in the weird stage where the practicality of things seems to be looming large. What’s wrong with me??)
 
meep said:
Oooof. Are we sure it’s not the pre-chamber fit? That sounded like a smoking gun to me.

If it were me, I’d probably pull the head and have it checked/machined, because (a) it sounds like you can manage that and (b) it might be all it needs and (c) it would provide a 90% of the access you need for full diagnosis. If the problem is truly in the block, it’s a block-out job that would warrant the rest of a rebuild. And if it’s a rebuild, the head is already done…. I hate to say it, and this is where this hobby hurts, that’s probably some money and we start thinking hard things about what it’s worth. It might be worth every pretty penny to rebuild it and drive it!! It might not. (I’m in the weird stage where the practicality of things seems to be looming large. What’s wrong with me??)
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My thought is that the prechamber just came out because there was no carbon to hold it up. It had what, 10 miles of use on it?

And my understanding with leaky prechambers is that you can see and hear it from right there. When I put oil for the leak down, I’d think I would have seen or heard bubbles or hissing.

I will put the prechamber back in, hammered down, torque the ring to spec, and do the tests again. Why not?!? Then decide about the head. I’ll move it to a different garage of mine where the floor is more conducive to a hoist rolling.
 
Huge finding!

My chile and I were messing with the borescope on the weekend, and I felt like I didnt see any valves opening or closing. So I decided to pull the valve cover and have a look.

i pressed the lifters and one on cyl 6 was loose. Loose like I never saw when I checked the lifters on my 1991 300D.

Ok, so turn the engine. The cam at cylinder 6 was not turning.

They broke the cam!!!

The FSM specifically says how to remove the bearings so that the cam doesn’t break.

IMG_0872.webp


I guess it wasn’t done right. Not sure if there is other carnage now…. What damage could be done from 10 miles of light use with the valves not opening?

@Trav @clinebarger @Astro14
Hoping for your thoughts. In a way maybe this is good, in a way, not. It couldn’t have stoichiometric air to combust in this situation.
 
WOW! Just - wow. That explains the miss, and the low compression.

I think that the only "other damage" would be a tiny bit of debris in the engine - which should be caught by the filter. I saw no evidence of a valve/piston hit in your earlier photos, and the cam should want to rotate to a position where the lifters are resting on the lobes, minimizing the chances of a piston/valve hit.

This is just such a shock - such bad work on their part - but, assuming you can still get a cam - a very straightforward fix.

Man, not following the FSM - how many guys on this forum brag about not needing the service information for a car - look what happens when you "assume" that you know it all...
 
Since everyone else is, I'm just going to throw this out there.

From experience, you are going down money pit hell. You'll put a new cam in and it will be something else. Then something else. I've done this a lot in my younger days, and the lesson learned was to do it right the first time or make it go away. Not saying this is the definitive way to go, just my experience x3 or 4.

If I loved the car I'd do a whole engine. If I just liked it I'd take my lumps and sell it as is without putting another dime into it. Either way, good luck!
 
@Trav can you please remind me - for this kind of engine, cam, and lifter design, if I buy a new cam, OE (if it exists), or Febi Bilstein type aftermarket, then I MUST use new lifters.

If I buy a used cam, can I keep the lifters in there, or must I replace??

Also, is there any chance something else caused the cam snap besides improper loosening or tightening? Is there a good way to check? I’m pretty happy if I don’t need to pull the oil pan or pistons or the head, but I’d hate to put a new cam in only for it to be another issue and snap the next one….
 
JHZR2 said:
@Trav can you please remind me - for this kind of engine, cam, and lifter design, if I buy a new cam, OE (if it exists), or Febi Bilstein type aftermarket, then I MUST use new lifters.

If I buy a used cam, can I keep the lifters in there, or must I replace??

Also, is there any chance something else caused the cam snap besides improper loosening or tightening? Is there a good way to check? I’m pretty happy if I don’t need to pull the oil pan or pistons or the head, but I’d hate to put a new cam in only for it to be another issue and snap the next one….
Click to expand...
Not familiar with MB but are the lifters roller? or are they flat tappets?

I've always heard with flat tappets , that you can use new lifters with a worn cam, but not worn lifters on a new cam.
 
spasm3 said:
Not familiar with MB but are the lifters roller? or are they flat tappets?

I've always heard with flat tappets , that you can use new lifters with a worn cam, but not worn lifters on a new cam.
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Flat hydraulic tappets. What you say sounds right.

I’ll be interested to verify if that cam you found is actually correct. If so, there was only one cam/profile for a range of engines and displacements. That also means that there are a lot more good used engines that I could get a cam from. Mg car is only of the lowest mile examples still out there, but cams aren’t a wear item on these engines, so Illl weigh the big picture.
 
JHZR2 said:
Flat hydraulic tappets. What you say sounds right.

I’ll be interested to verify if that cam you found is actually correct. If so, there was only one cam/profile for a range of engines and displacements. That also means that there are a lot more good used engines that I could get a cam from. Mg car is only of the lowest mile examples still out there, but cams aren’t a wear item on these engines, so Illl weigh the big picture.
Click to expand...
If thats true, a good used camshaft with new lifters and bearings would probably be a lot less money than a new oe camshaft at $600+
 
Astro14 said:
WOW! Just - wow. That explains the miss, and the low compression.

I think that the only "other damage" would be a tiny bit of debris in the engine - which should be caught by the filter. I saw no evidence of a valve/piston hit in your earlier photos, and the cam should want to rotate to a position where the lifters are resting on the lobes, minimizing the chances of a piston/valve hit.

This is just such a shock - such bad work on their part - but, assuming you can still get a cam - a very straightforward fix.

Man, not following the FSM - how many guys on this forum brag about not needing the service information for a car - look what happens when you "assume" that you know it all...
Click to expand...
Yes. I guess the only compression was whatever air lived in the cylinder from slipping past the rings.

Hopefully the wash down of the cylinder with diesel fuel for the time I ran it didn’t do too much damage. That’s probably my greatest concern right now. That and if something else could be broken that caused the cam to snap, besides not following the FSM.

c502cid said:
Since everyone else is, I'm just going to throw this out there.

From experience, you are going down money pit hell. You'll put a new cam in and it will be something else. Then something else. I've done this a lot in my younger days, and the lesson learned was to do it right the first time or make it go away. Not saying this is the definitive way to go, just my experience x3 or 4.

If I loved the car I'd do a whole engine. If I just liked it I'd take my lumps and sell it as is without putting another dime into it. Either way, good luck!
Click to expand...

Thanks for your insight, but in this case I disagree.

I had a nice car that ran beautifully. It just put a tablespoon of oil into the bottle after a few hundred miles of use

This is a known issue and weakness with this specific engine.

I don’t have reason to believe that after this, something else would go wrong. This is what my shop should have have diagnosed and repaired. There was no reason why it should have sat. That was on me and them, but it’s water under the bridge.

Nobody knew the cause, so doing it right was unknown. What is doing it right? Replacing a short block unnecessarily because of a broken cam?

Doing it right requires diagnosis. I diagnosed it, used the tools, found the issue. Now I can do it right. Which for now means replace the cam and verify that compression comes back with actual cylinder operation and use.
 
spasm3 said:
If thats true, a good used camshaft with new lifters and bearings would probably be a lot less money than a new oe camshaft at $600+
Click to expand...
Im also curious if that Febi Bilstein cam is Chinese junk or reasonably ok. Some of their stuff used to be German made.

I’ve already found the OE cam for in the $400s. $150 for all new lifters… maybe new bearing or the set if needed. I found one good set on eBay…
 
Last edited:
Astro14 said:
I saw no evidence of a valve/piston hit in your earlier photos, and the cam should want to rotate to a position where the lifters are resting on the lobes, minimizing the chances of a piston/valve hit.
Click to expand...
I just reviewed my photos. This is what happened:

IMG_0878.jpeg

I suspect the lobes sat right at the point where the lifters touch but the valves aren’t engaged.
 
I'm just now seeing this thread. My condolences on the hacks who worked on your car, and broke the cam...

Personally, I would want new lifters with a new cam. It's also good practice.

I don't know what parts might be OE quality, or counterfeit junk. Hopefully someone still in the auto repair business, and familiar with classic MB's, can give some insight on that.
 
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