420 SEL timing chain dilemma

[Luntburger]

Gentlemen,

I’m seeking advice on a timing chain dilemma on a 1991 420 SEL, my daily driver.

I bought the car in August, and I’ve read all the warnings about changing the timing chain and related components. If I were to keep this car indefinitely, I would certainly drop some money into the car and change everything.

However, I won’t be living here for more than four years. In that time, I don’t expect I’ll drive it more than 5,000 miles a year, if that.

I’ve read on this forum that the timing problems arise from those plastic pieces chipping off, not from chain failure. Based on my limited mileage and limited time of ownership, can I just replace the guides?

BTW, I’m a longtime lurker, first-time poster. And hello from “where America’s day begins.”

[porkface]

depends on the mileage on the engine and whether the odometer REALLY is right. the chain does stretch and will break. if you replace the guides about every 70k or so, you can get by on the oe chain 'till about 150k. if the guides are let go for a while after 70k, then the chain gets REALLY loose and will need replacing about 100-120k miles. if all is unknown then i would replace the chain, tensioner and guides and not worry about it. better safe than sorry. the chain breaks, then the heads come off to replace the bent valves on the left side, the right side to make sure compression is the same as the fresh left 1. good luck, chuck.

[gmatthewb]

I agree with Pork and wouldn't push your luck if it's past 150k miles. The V8's seem to fare better than the L6's b/c the larger engine isn't stressed as much during take-off or hard acceleration.

Like Pork I don't believe it's worth the gamble. The cost of the chain replacement (& related components) is so much more manageable than the cost of the 2-bank head job (and maybe the cost of a punctured piston too).

[wbain5280]

Here is a link to my chain guide replacement thread.

Chain Guides Replaced

Contrary to popular opinion, the chain does not break. It wears causing it to elongate. This causes the chain to flop around at the inner guide on the drivers side. The guide breaks off and gets drawn by the chain into the space between the sprocket and the chain. The sprocket slips a tooth or two and the valves on that side contact the piston and bend. The engine stops and needs a top end rebuild. The root cause is the long chain. My Ford 4.6 has two short chains so this is not a problem although I do need to do chains and guides on that engine too.

http://www.import-car.com/ic/ic40024.htm

Contrary to some other popular opinions, the M103 I6 is a very stout engine and smooth as silk at highway speeds. It does rev high though esp. in the 300SE due to a large car with a smallish engine. The engine is really quite good above 60 mph and the acceleration to 90 is very good. I cannot opine on the M104. Nercedes has lots of experience making I6 engines. The car does need a 1st gear start though, something I'm working on for both my cars.

The chain on the M103 is short and doesn't have the wear problems of the V8's. Take it from me, I have both engines in the W126 cars.

[86560SEL]

I have had V8, 4-cyl diesel and I6 Mercedes and like the I6 over the V8 because the I6 does not have the issues like the V8 have with the timing chain rails/guides.

Originally Posted by gmatthewb

I agree with Pork and wouldn't push your luck if it's past 150k miles. The V8's seem to fare better than the L6's b/c the larger engine isn't stressed as much during take-off or hard acceleration.

Like Pork I don't believe it's worth the gamble. The cost of the chain replacement (& related components) is so much more manageable than the cost of the 2-bank head job (and maybe the cost of a punctured piston too).

[Luntburger]

I was all set to replace all the timing chain components, until I read this thread:

Replace 420 lower timing chain guides

This remark from stevebfl stopped me in my tracks:

"Out of the hundreds of chain/timing failures my shop has done over the last 30 years only 2-3 were caused by the lower rail. When it wears through the chain rides on the steel retaining pins and the chain is too long to be tightened by the tentioner as a result. It probably made noise, but by the time I saw them they were broke. The results of the failure looked just like most of the others.

"The upper rails are relatively simple to replace and are dirt cheap. They are good maintenance. I see no point in doing a job as maintenance that involves more than 70% of the cost of the repair it is intended to prevent."

[deanyel]

If you put these people on a doctor's table, hit them in the knee with a hammer and their leg goes up, then say "timing chain" and they shout "replace it". But it's expensive job and used engines are awfully cheap, at least around here.

[handsworth]

Have you pulled the valve covers and looked at the chain and the rails. I bought a 420SEL (170K), this summer and that's the first thing I did. My chain had slop and part of the left upper rail was laying on the chain. I stopped driving it immediately. With the help of these guys, I replaced the chain, the tensioner and all the rail guides. If you drive like I intend to (5k/yr), you won't have to worry about this ever again. Cost me my time and about $150

[Luntburger]

Ideally, I'd do it myself. I don't mind turning the wrenches. However, I've got no garage and a shared dirt parking lot. Combine that with periodic torrential downpours a couple times a day, and I'm not eager to open up the engine in those conditions.

[handsworth]

For peace of mind , just have a mechanic look at it. For all we know the PO could have had this work done already and you're good to go. I could not fathom driving my car daily fearing that it could go at any time.

[Hit Man X]

Why press your luck?

Do all four upper guides (replace the liner on the tensioner unit as it's super cheap), chain, tensioner, cam oilers, plugs, wires, cap, rotor... then you're done with it for a long, long time.

PO of my 560 did it, I had no records... but when I popped the valve covers and saw white guides + an IWIS chain I was pleased. They just skipped the cam oilers so I swapped them too.

Maybe you can find a garage local or know someone that will let you use their space for a weekend to replace it all?

[Luntburger]

Skimming through the manual, it sounds easy enough to change the chain. How hard can following 9 sentences be?? (I especially like step 9: "Complete the engine.")

But looking at the part list, I don't have a chain grinder or the pin puller or the torque wrench or 27 mm socket or 5 inches of extensions or three vise-grips ...

I'll buy all that stuff once I return to North America and start trolling craigslist for W126s and SLs. For now, I'm living lean.

[porkface]

wbain5280-"Contrary to popular opinion, the chain does not break."

i beg to differ on the breaking issue. i've had to replace 3 broken chains over the years. 2 had issues with the rails, 1 had new rails but the chain wasn't replaced nor were the old rail bits cleaned out of the cover and this resulted in a broken chain. after that car, i always suggest-and charge additional-to pull the covers on a used car check. better to know now than when it's too late. good kuck, chuck.

[wbain5280]

I stand corrected.

[hey_allen]

Originally Posted by Luntburger

But looking at the part list, I don't have a chain grinder or the pin puller or the torque wrench or 27 mm socket or 5 inches of extensions or three vise-grips ...

I'll buy all that stuff once I return to North America and start trolling craigslist for W126s and SLs. For now, I'm living lean.

Any grinder works to take off the top of one link, and you might even be able to get by with a file if you were particularly determined to not find a rotary grinder.

As to the extensions and the 27mm socket, I believe the socket is also known as 1 1/8", and I used a 6" extension bar that was commonly used to change lug nuts. The pliers I can't make any comment on, as I figured that they'd come in useful and bought an extra couple to make this easier.