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1990 300SE
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275 Posts
Discussion Starter #1
Well, took me a while to do a head gasket job only to find out that I messed up the timing chain tensioner real bad. Timing chain snapped while backing out of the garage. Is it time for a new engine? :surrender:
 

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1990 560 SEL
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731 Posts
Ouch. You could have gotten lucky since you were at near-idle, but the only way to be sure is to open it up and look. Don't start the engine again.
 

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1990 300SE
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275 Posts
Discussion Starter #3
Pretty sure I've royally screwed it up. Prior to the whole timing chain snapping, I had problems starting/ idling. The engine would not stay on unless you flexed your foot a little on the gas pedal. Turned out to be the Idle Control Valve. But, because the engine was running while the tensioner and guides were all messed up, I'm pretty sure the damage is irreparable now.

The timing chain just happened to snap when the car was in gear and reversing out of the garage. I should of suspected something, because once the ICV was fixed, the engine sounded terrible, really weak.

I'd love to be proven wrong though. But, at this point I think I should start looking for a replacement 300SE engine. Anyone know a place that will have these in running condition?
 

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'91 560SEC, '98 SL500
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1,974 Posts
I feel for you...why not take a few hours to determine that the health of your engine before calling around for a used engine?
 

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1989 560SEC
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2,056 Posts
Yeah, commiserations. Fingers crossed though
+1. I have never heard of a chain snapping. Jumped yes, snapping no. Was is a old chain?

Mike
 

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1990 300SE
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275 Posts
Discussion Starter #7 (Edited)
+1. I have never heard of a chain snapping. Jumped yes, snapping no. Was is a old chain?

Mike
Not really. It seemed like it was the original chain, but the chain was very tight and checked out okay. Only 109k miles on that chain before I botched it. :(

Thanks for all the kind words, everyone. When I have some time, I'll probably dig a little deeper.
 

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Premium Member
2012 CL550 4MATIC Coupe
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9,992 Posts
Chains can snap, and it sounds like you had valve collisions. Sad to hear, but you are in LA at least and can get a motor easily compared to the rest of the country.
 

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Mercedes
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9,043 Posts
The M103 motors do require chain/guides to be replaced but they're not as problematic as the V8's. If you verified that it is a damaged motor and not something that can be fixed within normal means, I'd do as mclare suggested and find a good used motor to put in. You have a number of cars to choose from for a M103 3.0, 300SE, 300SEL, 300E, 300CE, 300TE. What I would adivce though is to use a 1989 or newer M103 3.0 as they were upgraded. Might as well put the latest/upgraded motor in if you're going to swap.
 

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1990 300SE
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275 Posts
Discussion Starter #10
Chains can snap, and it sounds like you had valve collisions. Sad to hear, but you are in LA at least and can get a motor easily compared to the rest of the country.

Could you point me in the right direction as to where to get a decent motor? I'll pm you if you don't mind?


The M103 motors do require chain/guides to be replaced but they're not as problematic as the V8's. If you verified that it is a damaged motor and not something that can be fixed within normal means, I'd do as mclare suggested and find a good used motor to put in. You have a number of cars to choose from for a M103 3.0, 300SE, 300SEL, 300E, 300CE, 300TE. What I would adivce though is to use a 1989 or newer M103 3.0 as they were upgraded. Might as well put the latest/upgraded motor in if you're going to swap.
I will try and verify some time this weekend. I guess the bright side is that M103s are so common. Earlier, I was wondering if all the other M103s were compatible with the W126's. Thanks for the info!
 

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'95 993 / 2011 F150 Raptor SC / Land Cruiser D4D V8 / 500 SEC
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439 Posts
Commiserations. You will be in for :

1. A cheap fix but a lot of work, or

2. A not so cheap fix and even more work.

I think you're in for no. 2, unfortunately...
 

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1990 300SE
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275 Posts
Discussion Starter #13
Thanks for the commiserations, everyone.

Yeah, Peter R., unfortunately, I think I'm in for number 2 as well. Right now, I'm just going to take things slow and let things simmer for a while so I don't make any hasty decisions. At any rate, I'm keeping the benz. I like this car too much. It's just going to be a nice little (hah! little..) paperweight in my garage for a while until I have the time and funds to deal with it.
 

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1985 500sel and 500sec 2012 E63 1989 Porsche 911
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5,336 Posts
owwww, i felt like i got kicked in the nuts just reading your title.....
 

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1997 SL500 40th Aniversary 1998 C230 2002 E430 sport 1987 420SEL sold 1989 300E sold
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573 Posts
Sorrry to hear that, have seen it happen before to someone I knew. Most likely what happened is the 103 motors use a ratchet type chain tensioner, meaning they keep racheting forward to take up the slack in the timming chain. When you remove the head, and chain sprocket and tensioner, it goes into the fully extended position. They have to be disassembled and then reloaded from the back of the tensioner before being reinstalled.If you fail to do this part, you can still put the chain tensioner back in, but it does require a lot more force. In turn you are know putting a excessive amount of tensioner on the chain, and will snap fairly quickly. They will bent the valves even at Idle speed unfortunetly.
The 616,617 Diesel motors have the same type of tensioner.
 

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2004 SL500, 1993 S420, 1990 420 SEL, 1985 500SE, 1994 Toyota Supra Aerotop
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753 Posts
What's the price to have a 6 cyl head re-done? Lucky it's not a V8! It might sound like an easy swap for new motor but in the end they do drag on a bit and there's always something that doesn't match up. How much power do the later M103 engines make? Would the 24 valve 104 engine be a possibility?
 

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1990 300SE
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275 Posts
Discussion Starter #17 (Edited)
Sorrry to hear that, have seen it happen before to someone I knew. Most likely what happened is the 103 motors use a ratchet type chain tensioner, meaning they keep racheting forward to take up the slack in the timming chain. When you remove the head, and chain sprocket and tensioner, it goes into the fully extended position. They have to be disassembled and then reloaded from the back of the tensioner before being reinstalled.If you fail to do this part, you can still put the chain tensioner back in, but it does require a lot more force. In turn you are know putting a excessive amount of tensioner on the chain, and will snap fairly quickly. They will bent the valves even at Idle speed unfortunetly.
The 616,617 Diesel motors have the same type of tensioner.
Yeah, unfortunately, that's something that I missed about the tensioner. It ended up being totally too tight.


What's the price to have a 6 cyl head re-done? Lucky it's not a V8! It might sound like an easy swap for new motor but in the end they do drag on a bit and there's always something that doesn't match up. How much power do the later M103 engines make? Would the 24 valve 104 engine be a possibility?
Well, I took my head to a machine shop that a mechanic friend of mine recommended. It was $45 to get the head machined and cleaned and I was also quoted $150 if I included the valve job into the list of stuff to be done. And, of course, a motor swap is not to be trifled with, and tbh, I think it's a little over my head. But, either way, I'm kind of stuck with this car. I like it a lot, and I've got at least 3000$ in repairs into it, it's a bit of a waste to just scrap it, I think. I don't think there was a power difference between the early 300SE and the later 300SE. They were all around the 177hp mark. I believe the 104 will have a lot of wiring hassle to go with it as it is a more modern engine.
 

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1997 SL500 40th Aniversary 1998 C230 2002 E430 sport 1987 420SEL sold 1989 300E sold
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573 Posts
Yeah, no difference between the early 300SE and later ones except for the 1990 and 1991 models had the air injection pump (aka smog pump ) I think the most cost effective, and least time consuming is just pull the head again, replace the bent valves, most likely only 2 or 3, pull the front cover and replace the chain and the tensioner and do the guides while you have it open. That way you know the lower end is solid, since you don't have a lot of miles on it, and you could easily get another 200k on it.
 

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1990 300SE
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275 Posts
Discussion Starter #19
Yeah, no difference between the early 300SE and later ones except for the 1990 and 1991 models had the air injection pump (aka smog pump ) I think the most cost effective, and least time consuming is just pull the head again, replace the bent valves, most likely only 2 or 3, pull the front cover and replace the chain and the tensioner and do the guides while you have it open. That way you know the lower end is solid, since you don't have a lot of miles on it, and you could easily get another 200k on it.

That sounds like a good point to start from. I posted this thread because I had no idea what to do after that spectacular failure. And, that seems as good as any route. I'm nervous about touching that timing chain again though. If anyone has access to any service manuals or knows where to buy them, I'd greatly appreciate the heads up. I've heard of those CD manuals before, but they seem to pop up only every now and then on ebay.
 
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