Mercedes Diesel Misfire After HG Replacement

[JHZR2]

Ouch, that sucks.

I take it that the cam rests with at least one lobe pressing against the valve spring? If that’s the case, it was likely the cause of the cam not sitting all the way in the bearings before you applied the final torque.

The FSM is very specific about the order of removal and install of the bearing caps. The lobes contact hydraulic lifters, which I replaced brand new with INA (OE) units. The bearing caps that the FSM indicated are the ones that I think press down the lobes in such a way to support the cam properly.

I’ve removed and installed cams a few times now on these engines without incident. Now the stupid tax.

I think when I hand threaded the two bolts per cap, I put those in unequally. That manifested itself at the final torquing.

Years ago I lost the order of bearing caps while installing a crank. Granted i didn’t break it, but upon assembly, prior to dropping it back in the car, I put power to the starter and noted that the engine wouldn’t turn. After brainstorming a bit (I was 15 and this was my first rebuild that wasnt on a tecumseh) it occurred to me that I was uncertain at that step. So I separated it from the trans, put it back on the stand, flipped it over, and tried to resequence the bearings. Lesson learned - start testing to make sure it rotates and doesn’t bind while torquing down. Idk if this is a good idea from the pros or not, but it stuck with me.

Maybe it didn’t seat in the bearings, maybe the caps are out of proper sequence, perhaps a lifter was supposed to compress and didn’t, is this an interference engine?

By the way - I’m so sorry - you are doing wonderful work on an amazing classic, with less available information than say a gasser. Labor of love (or car disease, depending on perspective!)

I’m lucky in that MB puts numbers on each of the bearings.

My main curiosity is if I could have done something wrong with a lifter. After all, I’m replacing lifters and cam. I dunked each lifter in oil before inserting, but nothing else.

Removal:

Dunking:

Installed:

So if the oil in there could cause the lifter to not operate, that’s a concern. I have my doubts of that being an issue, as they weren’t filed underneath and I didn’t dump all kinds of oil in the bores that they sit.

I guess one lifter could somehow be stuck? Not in quite straight?

I think it was because of my hand tightening sequence. Occam’s Razor is likely true here. The simplest thing is that I lost count of the tightening in the bearing cap bolts, and so they didn’t all come down evenly and together.

But I don’t know how to validate that as the case.

 

[KrisZ]

I’m not sure an unevenly threaded bolts by hand would cause this. Everything is loose at that point.

I see dowels on the head where the caps go, maybe one of the caps was seated unevenly because of the dowel and pressure from the lifter.

 

[JHZR2]

I’m not sure an unevenly threaded bolts by hand would cause this. Everything is loose at that point.

I see dowels on the head where the caps go, maybe one of the caps was seated unevenly because of the dowel and pressure from the lifter.

Dowels? There are thread inserts, I assume that are there because its an Aluminum head... Wouldnt a dowel be solid?

 

[KrisZ]

Dowels? There are thread inserts, I assume that are there because its an Aluminum head... Wouldnt a dowel be solid?

I don’t know this particular engine and I’m just going by the pictures. I see these inserts sticking out and thought they were dowels. But regardless of their primary function, they will also act as dowels, since they protrude from the surface.

 

[Exhaustgases]

Sorry that happened. So everything on the new cam dimension wise is the same as what was in it?
Also that new cam could have been stressed (crack started) either by the person that removed it before sending it to you, or from shipping.
So maybe it was not caused by you.

 

[JHZR2]

I don’t know this particular engine and I’m just going by the pictures. I see these inserts sticking out and thought they were dowels. But regardless of their primary function, they will also act as dowels, since they protrude from the surface.

I get it now. No, they arent dowels, they are steel thread inserts.

They could result in alignment or misalignment, for sure. Ill need to look closely, in case a misalignment resulted in some metal chipping. Ugh...

Thanks!

 

[JHZR2]

Sorry that happened. So everything on the new cam dimension wise is the same as what was in it?
Also that new cam could have been stressed (crack started) either by the person that removed it before sending it to you, or from shipping.
So maybe it was not caused by you.

Yes, same cam, same P/N, same dimensions. Sat in there perfect.

 

[Trav]

The saga continues.

Had an excessively busy fall, so I didn’t mess with the car much. Today I went to install a decent low mileage cam I got from Latvia, with all new lifters.

I removed the junkyard cam. No issues. Put in the new lifters, lubed everything up, put on the replacement cam.

I started to tighten the cam tower bolts by hand, then started the half turn regimen by ratchet. When that felt tight, I got the torque wrench set at 18 ft-lb. When I got to bearing 4 (iirc) I tightened the next half turn, could feel the torque, and then bump! A weird jolt. I continued doing the towers per the FSM. Later I was torquing #7 to completion and, pop!



Ugh. I snapped my cam.

Twice in fact.

View attachment 256616


Unbelievable. Stupid tax for sure.

So now I’m shopping for another late model one with low mileage. I think I have a lead…

Beyond frustrating though!!

View attachment 256617


I’m trying more than anything to reconcile what I did wrong. This is the third cam install I’ve done. I haven’t snapped any others on install or removal.

I see it could be one of two things:

1) the new lifters all got dunked for a second in fresh oil just to coat all surfaces. I didn’t work them or try to purge them. Could I have gotten excess oil someplace that made them not want to give?

2) on all the other jobs, I started the loose bolts and screwed them all in at the same number of turns. This time I was a bit more casual with it, thinking that if I just started torquing together when the bolts were down at the caps, it would be ok. Maybe I got the caps sort of crooked relative to the cam (one of the two bolts was turned in more) causing a misalignment and stress even though the cam was pretty well tightened in.

I’m leaning towards #2. @Trav eo you have any thoughts on this? Im disgusted at the mistake and waste of money and time. I don’t want to let it happen again.

Those cams are almost as brittle as Carbide, drop one and it will break, this is a MB trait that goes back 8 decades. Follow the FSM as far as releasing the tensioner and winding it back (if applicable to yours), make sure the axial washer is seated. Tighten the ones with the black arrows primarily 1/2 turn each bolt, start from #4, #3, #7, #2. Repeat until the cam is almost seated then tighten #6, #1, #5 1/2 turn each once pressure is felt until they are almost seated and repeat moving back to 3,4,7,2 until they are seated, finish seating all the caps and torque to 25nm in the same order.
Do this work like it was made of glass, it darn near is.
I would push each adjuster once to collapse them, do not pump them up they will do that within a few seconds of startup. Pumped up you risk breaking the cam. I hope this makes sense.

 

[LDB]

Sounds like another of those things I hire professionals for. Good luck to those doing it.

 

[JHZR2]

Those cams are almost as brittle as Carbide, drop one and it will break, this is a MB trait that goes back 8 decades. Follow the FSM as far as releasing the tensioner and winding it back (if applicable to yours), make sure the axial washer is seated. Tighten the ones with the black arrows primarily 1/2 turn each bolt, start from #4, #3, #7, #2. Repeat until the cam is almost seated then tighten #6, #1, #5 1/2 turn each once pressure is felt until they are almost seated and repeat moving back to 3,4,7,2 until they are seated, finish seating all the caps and torque to 25nm in the same order.
Do this work like it was made of glass, it darn near is.
I would push each adjuster once to collapse them, do not pump them up they will do that within a few seconds of startup. Pumped up you risk breaking the cam. I hope this makes sense.

After all this time I had a free afternoon, and went to work this issue with my kids.

Very carefully cleaned and installed another low mileage very nice cam I had imported from the UK, and torqued it in that 4,3,7,2 order you recommended. Got it all torqued down to spec easily.

I had a real hard time getting the sprocket back onto the cam. I had to turn the engine forward and backwards a bit, which is scary since it could cause timing to jump. But in the end it was ok. It didn’t turn more than 5-10 degrees and I had to do that with other engines too. The TC tensioner was always engaged, for better or worse. Finally got it on and the main 12pt bolt torqued to 25nm+90 deg.

My 12yo glowed and started the engine, and it shook like a misfire! Oh no! Did I crack a cam again? I pulled the VC real fast, re-verified timing mark alignment and all lobes turning.

Starting and giving just a blip of throttle made all shaking and misfiring go away. I guess some fuel leaked down or air was stuck from sitting almost 9 months. But it’s perfectly smooth idling now.

I will pop test and balance the injectors just as maintenance best practice at some point, but I need to clean out the fuel tank and refresh the cooling system first.

Hopefully before the winter I’ll be driving well, then new tires and go! Of course I don’t drive these cars in the winter if there is chance of a remnant speck of salt on the road!

Still enough R-12 to engage the compressor after all this time!

 

[RooflessVW]

Still enough R-12 to engage the compressor after all this time!

Got some air duster (R-152a) for when it needs serviced?

 

[JHZR2]

Got some air duster (R-152a) for when it needs serviced?

I have some old r-12 that I’ve never needed.

 

[burbguy82]

you better address the rust in tank issue before you do much else......that is the nastiest cap i gave ever seen.

 

[JHZR2]

you better address the rust in tank issue before you do much else......that is the nastiest cap i gave ever seen.

That’s the plan. I’m completely not concerned. Well understood why it forms, I’ve seen it in other diesels that have gone well over 300k, and my prefilter is totally clear.

I’m going to backflush my tank and primary filter screen with a supplimental pump and filter and see what I get.

I’ve added supplemental filters to these engines in the past too, when I’ve seen rust in tanks. Never caught a speck because the bits are too large.

 

[Astro14]

Sounds like another of those things I hire professionals for. Good luck to those doing it.

The professionals are the ones that broke the cam through improper installation.

Sounds like another one of those things I do myself, so the “professionals” don’t break my car.

 

[JHZR2]

The professionals are the ones that broke the cam through improper installation.

Sounds like another one of those things I do myself, so the “professionals” don’t break my car.

Exactly. And in this case, the cam is very fragile, as @Trav indicated, and I have my doubts that most shops would deal with the FSM errata, or have the desire to troubleshoot what happened here for any reasonable amount of cost.

I paid the stupid tax for not tightening the last (Latvian) cam down exactly per the FSM. I did tighten the right bearings, but I thought I could take the extra threads out fast before the bearing touched the cam, then do the one turn at a time approach. I had done this job before, never an issue, actually doing half a turn a time. But I’m not the only one who has busted one on this engine…

 

[burbguy82]

That’s the plan. I’m completely not concerned. Well understood why it forms, I’ve seen it in other diesels that have gone well over 300k, and my prefilter is totally clear.

I’m going to backflush my tank and primary filter screen with a supplimental pump and filter and see what I get.

I’ve added supplemental filters to these engines in the past too, when I’ve seen rust in tanks. Never caught a speck because the bits are too large.

View attachment 297022

nice filter setup...........were your injectors nasty?

 

[JHZR2]

nice filter setup...........were your injectors nasty?

Nope not at all. That car had over 300k in it and started instantly and ran beautifully. I sold it and the new owner ran it for nearly 100k miles more before he hit something and cracked the oil pan. I think it was still functional, but the replacement of that pan requires an engine lift up off the mounts so I’m not sure what came of it