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I'm looking for information and opinions on the MB W126 V8 engines (420, 500, 560) in terms of the longevity of the timing chains, general reliability and any items that need special attention in a pre-purchase inspection. I hope Aprilia is around and can throw some of his usual expert light on the matter.
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Engine
The engines are known for their durability. If an engine is properly maintained, it can easily run for 300.000 - 400.000 kilometers without a rebuild. The V8 engines are alloy engines and the walls of the cylinder housings are coated with a silicium-based layer. Expect to change a distribution chain every 250.000 kilometers. The early Bosch K-jetronic continuous fuel injection systems can develop a rough idle over time. Typically, a V8 should run about 650 - 700 rpm at idle when hot. If the car will develop a jumpy idle (jumping from about 1500 rpm to 500 and up again), the electronic control module is likely to be at fault. Post-1985 models were equipped with Bosch KE-jetronic pulsed fuel injection systems with a completely electronic engine management system.
home.concepts.nl/~vlimmere/tips.htm
also read : too much to copy!
www.mbcoupes.com/buyersguide/buyersguide.htm
madf
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Post-1985 models were equipped with Bosch KE-jetronic pulsed fuel injection systems with a completely electronic engine management system.
No, KE-Jet. is a continuous (not pulsed) mechanical system with electronic enhancement (it has an electrohydraulic actuator - "EHA" - on the side of the fuel metering head). The EHA only controls cold-start enrichment, acceleration enrichment and fuel cut-off on overrun.
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The 420 V8 engine is a pretty rugged brute - not as smooth as the later 500 and 560 designs, but smooth enough and easier and cheaper to fix if you do have a problem.... Timing chains last a long time (anything from 150k-300k or more, depending on how its been serviced and driven) and are relatively easy to change. If the chain is rattling then take care! It can break or jump and the D rails can break causing serious engine damage. Just get it changed as soon as it gets noisy. Other than that the engine is pretty bomb proof if its had regular oil and coolant changes. The big alloy engines are more modern and pretty robust - but not as rugged as the 420 IMHO and they consume serious amounts of fuel!
Watch out for the ignition amp failing - they are very dear new (about £700 from ECP, IIRC) and rare as hens' teeth secondhand. Remove the amp from the inner wing and clean its back face - then refit with fresh thermally-conductive compound (white paste in a tube from Maplin).
Other than that its the usual W126 stuff. Check for slow autobox engagement (anything longer than 2 seconds into D or R from N means a possible rebuild).
These cars are likely to be 15 years old or more now, so check for rust - (sills, door bottoms, the bit of the front wing under the headlamps). They can leak water (esp around rear screen) so look under the carpets. Get in the boot and look up under the parcel shelf - look at the panel with the circular holes in - this can all rot and is very very difficult to repair (double skin).
After that its all 'regular old car stuff' - wear in steering box, shocks, bushes, etc etc.
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Many thanks to you both.
Aprilia, any idea how much time a change of chain should take?
By "rugged", do you mean less prone to failure, more tolerant of harsh treatment and neglect, or something else? Is there much to choose between the 500 and the 560?
I think I saw HJ say elsewhere in the forum that in actual use the difference in fuel consumption between the bigger and smaller engines in the 126 was not a lot, presumably the size/weight of the car being the main determinant. I've seen people report a range from 15 mpg in town to 25+ on steady trips, and one self-declared heavy footer reckons he was getting 25 from a 500 or 560 on longer trips.
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UPDATE
Someone on the MB Club forum has reported a labour charge of a mere £135 for a recent chain job, which is less than half of what I would have feared.
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An ex neighbour worked for an MB dealer around 1990, had a 300SE as a company car for 6 months or so, no a rocket ship though very nice, wonder how the 12 valve 3.0 straight six rates in comparison to the V8's.
Must say though I drove a new shape 24 valve six cyl S-Class (W140?) when they were launched and it was light years ahead, great 5 speed auto box, one of these in good condition could be a good buy, perhaps 93 K plate.
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On the subject of the chain - its an easy job if its just the chain that's stretched and you change it directly. The way I have done it is to split the old chain and fasten the new one to it and then rotate the engine. This is where your £135 job comes from. If there is other wear in the timing chest then it will be a strip-down and cost £500 or so.
The 420 is an older and rather cruder engine than the 560, but everthing I hear, plus my experience, tells me its the on to go for on an older car like a W126. Plus the parts are more readily available from independent suppliers.
The fuel economy is not much worse than the M103 engine (3.0 litre) fitted in the 300SE when you are cruising. They do get thirsty around town though - there is no denying that. I would take 25mpg from a 560 with a pinch of salt - must have been a very long downward slope with the wind behind!
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Thanks again, Aprilia.
Seeing a forecast of oil at $80 a barrel yesterday, I went a bit cool on the prospect of a V8 engine. Then the vendor of the car I was looking at decided he didn't want my mechanic to look at it; the vendor of the car before that seemed unable to answer some very straight questions -- I suppose I shouldn't be surprised. Caveat emptor.
Anyway, I shall continue to sniff around, and I'm having a beer with the W126 Captain of the MB Club next week to get some more low-down. I continue to be attracted by the W126 as one of the pinnacles of MB "over-engineering" before the bean counters got hold of the operation.
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Aprilia
According to the German W126 Club at
www.mb-w126-club.de/gb/index.html
the W126 engine introduction history is:
* Late 1979: 2.8(6), 3.8(6), 5.0(V8), 3.0 diesel
* Late 1985: 3.0(6, same as W124), 4.2(V8), modified 5.0, 5.6(V8, Coupé only)
* Late 1987: all V8s upgraded.
* Late 1988: 560 SE made the 5.6 available in a saloon for the first time.
Mainline production ceased in late 1991, although some specials went through to April 1992.
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P.S.
And the "specials" were made in South Africa, one of them being donated to Nelson Mandela, apparently. Most or all of them were numbered as candidates for collectors of classics.
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During the past 24 hours, I have been able to use the 1989 500 SEL owned by a friend. The MPG facts are:
* Fill up at 116461.
* Motorway and A roads for 190 miles (cruise control on for much of it).
* Cold start this morning, then 7 mile trip.
* Cold start later and 7 mile trip.
Overall fuel consumption: 22.17 mpg.
Interesting contrast with my 124: definite big-car feel in terms of handling, but not significantly quieter or smoother, the V8 notwithstanding.
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The W124 is a newer design and handles better - more sophiticated suspension. It is also resists the rust better.
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