W140 engine reliability
 

Hey everyone,

I am fairly new here and have a question about W140 diesels. I own an '84 300D with the OM617 and was wondering how the engines they put in the W140s compare. I know they came with the OM603 and OM606 I6 engines that also came in the W124s. I know people consider the 124s super reliable but I'm not sure if that just goes for the 2.5L I5 engines they came with or if those 6cyl engines were bulletproof as well. My dad really likes my car and is looking to get something similar but he's considering the idea of something more modern feeling like the W126(not much different than my 123), W124, or W140.

Are those decent engines? Or should he be looking for a 2.5L 124 instead? I have looked around a good bit and can't find a ton of information on them except a couple 10+ year old posts on here where people mention head gasket issues in the 6cyl. Something about the fact that Mercedes switched to an aluminum block?

I'm a noob so thanks for the help,
-Bret

The W140 diesels are usually really expensive and the engines are known for having issues. The w126 is better as long as its not the 3.5 liter one, but they are getting old and parts are disappearing. The 1998-99 W210 E300 Turbodiesel is not bad if you can find a completely non-rusty one. They are easier to find parts for, and are generally up to being used for anything a brand new car could do.

What issues do the 3.5L engines have?

my understanding is they used weak connecting rods,causing cylinders to wear uneven.some engines were replaced by dealers

Just google "om603 rod bender" and you'll have enough bedtime reading for weeks...

Depending on reasonable availability where you live. A Mercedes mechanic highly recommended the straight six cdi diesel engined cars of the 2005-2006 years only. He worked in a Mercedes dealership. He said their five speed automatics where also good.


Those cars were moderate with needed service issues. Compared with a lot of others he thought. I have never drove one but imagine they are reasonably refined,give good fuel milage, powerful,with lower engine noise. Plus you have a better chance of finding lower milage examples.


People that have or owned them can correct his opinion or verify it. Most the members that have been on site for years are kind of unaware just how old some of the earlier cars are now. Yes we know in the back of our minds. In general a car more than twenty five years old is considered an antique vehicle. As just a general rule or yardstick.


We also live in a world that is what it is. If my parent was just getting by for example. I would not encourage their acquisition of one of these cars. If they had repaired their own cars or had some financial room is another story. To buy one and only have paid for service engaged. Could be shocking to them.


If you are not prepared to meet the majority of these cars service needs yourself. Or have a really good connection to get good work done cheap. At todays costs they might not be the best ideal.


Really though with the older iron it can also be you are limited to what you can find. For example if I wanted a really decent 123 model at this time it might be quite a chore to find one.


In my geographical area the gap of wanting a certain model and actually locating one I find acceptable is probably continuing to grow. That Mercedes mechanic might have been on the money. Decent more current ones are not falling out of trees but they are out there.

Early 2.5s had wear issues with the timing chain and cam sprocket. Nothing replacement won’t fix.

IMO, all 60x engines need a head gasket replacement at some point. Reasonable DIY job that doesn’t require special tools. You can get by with tools from any auto part store. Chains typically last forever (except for early 2.5s) so I wouldn’t change it just because.

From ‘90 turbo wastegates are controlled by vacuum and electronics for which there isn’t much diagnostic information. If emissions inspections and your conscience allow, swap the vacuum actuator a pressure actuator at the first sign of low power.

Most if not all US 603.96s (3 liter) came with a trap oxidizer that a recall replaced with an oxidation catalyst. The recall might still be active. There is no further coverage if the cat clogs. Some hollow the cat or replace with a straight pipe, again where inspection and conscience allow.

603.96s came with a #14 casting aluminum head that can develop cracks if overheated. Many survive without issue. Someone recently installed one from a wrecking yard and is racking up uneventful miles. I wouldn’t reinstall a #14 head if I had the head off. I’d just factor a possible $1500 repair featuring a later casting head (used parts) into the purchase of a car with a #14 head.

Some 603.97s (3.5 liter) suffer bent rods (see my avatar). IMO EGR bakes oil vapors into carbon on the exhaust valves which force the front edges of the pistons lower. The symptoms are oil consumption and smoke, same as a head gasket failure. You have to measure piston protrusion with the head off to confirm. Rods bent this way won’t strand you. Consumption and smoke will eventually become unbearable. One solution is to retrofit a 3 liter block. Less torque, more willing to rev, not a bad deal. Again, I’d factor $1500 (used parts) into the purchase. .97s came with #17 casting heads which survive overheating better.

Turbos are nice but I wouldn’t dismiss a 95-97 E300. They’re not likely to impress off the line but rev like a Honda... relatively. There are other reasons to prefer a ‘95 124 over earlier 124s. The 606’s only vices are leaky fuel fittings and failed intake manifold actuators. Repairs are simple but time consuming. Oh, and glow plugs breaking in the head. If you’re getting a premium car rather than a fixer upper, make the purchase contingent on replacement of the plugs by a shop of your choice.

Diesels from ‘97 have a 5-speed automatic which to me is a huge improvement on the highway. It’s a more robust box and easier to rebuild at home. Not easy but there’s less to do by experience and feel than with the 4-speed boxes. And being electronically controlled, you need access to SDS or generic equipment with ‘personality’ to reset codes. Disconnecting the battery won’t do it. Bus as you move beyond 124s, more and more of the rest of the car needs SDS for diagnosis, even repair. Dealers typically charge $150 or an hour of shop time to rear and clear codes.

All that said, I wouldn’t mind spending the rest of my highway time in a sorted 78-85 300SD. I hate doing valve jobs but that’s the 617’s only bad habit. And seats of that vintage don’t hold up well to time.

Sixto
98 E320s sedan and wagon
02 C320 wagon