Diesel Reliability - Other - PeachParts Mercedes-Benz Forum

rmmagow · #1 06-03-2002, 04:27 PM

I'm looking at a nice 93 300SD with 135K on it. In diesel terms, yes, that's not bad mileage. How does the rest of the Mercedes hold up? Will brakes be needing heavy overhaul, rear-end, shocks, transmission etc. I think what I'm asking is is the Mercedes reliable in all these other areas or am I still loooking at a car with 135K on it. Diesel lile is legend but most late 80's to present Jap gassers can easily go 200K too without going terminal. I've found that what eventually did in my VERY high mileage Jap cars was the cost of maintaining everything else but the motor. Am I to expect a similar scenario with the Mercedes Diesel. Thanks for your most trusted opinions.

engatwork · #2 06-03-2002, 05:28 PM

A pre-purchase inspection may be in order. At that milage is when some of the "other" maintenance stuff needs to be done. Brake rotors/pads. Have the alternator brushes been renewed? What condition is all of the rotating equipment attached to the front of the engine in? How bout the air conditioning? You get the point, the list could go on and on.

mikemover · #3 06-03-2002, 07:31 PM

Those early 90s 300SDs actually had a 350l engine in them, even though they were badged as a 300 until around 94, I think.

They came with an aluminum head and have a reputation as a notoriously troublesome engine. There are a handful of people who have had no trouble at all, but many more have had very EXPENSIVE problems. You might want to stay away from one that wasn't recently rebuilt by a VERY good tech. They aren't quite the "indestructible tanks" that the older 300SDs are.

Mike

psfred · #4 06-03-2002, 07:54 PM

My information on the 350 is that it either runs for ever or bends the rods -- a rebuild will have the new rods, and they appear to have solved the problem. Otherwise, head cracks from overheating are a problem but the rest of the engine, bent rods aside, will last as long as any other MB diesel.

Presuming this is a W126, as far as the rest of the car goes, it can be restored to like new condition at any time before the frame rusts away or it gets wrecked -- some of the repairs are expensive (ball joints -- not the joints, but getting them installed!), but unless it has severe rust, it can be repaired.

The usual things are AC compressor, alternator (you may get by with new brushes), brakes (not a big deal, expect about 30,000 miles on a set of front pads and don't "cheat" on rotor thickness). You MAY have front end problems, but only if the previous owner abused the car.

The W140 has electronic do-dad disease -- expensive, non-repairable things like climate control and engine controls that die on a regular basis. Not as much a problem on the diesel, as the engine controls are simpler, but a huge pain in the butt.

Peter

jobah · #5 06-03-2002, 09:58 PM

For clarification,

1993 is a 140 not a 126.

There are numerous threads regarding the dreaded 3.5 l 603's. It is not the aluminum head (this it shares with the 601, 602 adn 603), but the bent rods (esp. cylinder 1) that cause the major problems. It appears that they have been sorted out in the 1996 model year.

Look at the threads -- there is a lot of information out there.

mikemover · #6 06-04-2002, 12:13 AM

Yes, I've heard that the rods were the problem with those...They got the extra displacement by using longer rods instead of a larger piston bore, which compromised durability. I think after a few years, they redesigned the rods, and used the improved ones in any problem engines that were rebuilt, and those are what you get now if you order new rods from MB for that engine. Appartently that resolves the issue in most cases.

I've also heard of some of the aluminum heads being a problem...Easy to warp them if you ever run it a little too hot. Maybe that was on the 6-cyl. diesels?...I'm not sure. Aluminum and iron expand and contract at different rates, so I can imagine the potential for head-gasket issues too. ?

Mike

jobah · #7 06-04-2002, 12:29 AM

All of the Al heads are easier to warp when overheated. However, if kept below the "red-zone" they are reliable. The 603's had some problems with cracking heads, but that problem has largely been solved in a redesigned head.

With respect to the 3.5 l 603, I believe that there is no consensus regarding the exact failure, as there have been many updates. It appears for sure that there is a problem with the rods. Remember, the US 2.2 601 had an increased stroke which is more radically increased than the 3.5, and similar problems did not result.

psfred · #8 06-05-2002, 07:38 PM

The rods bend in the 350, causing the pistons to be tilted in the bores, resulting in very rapid ovaliztion of the bore and resultant oil consumption and failure. Happens between oil changes, so it must be catastrophic (in that they bend radically on a single stoke). There have been some suggestions that coked oil from the intake might be a culprit, but I think the main cause was incorrectly stressed rods and the tendency to use all that power -- these are relatively hot for a diesel!

That said, my local mechanic does the routine service on a 350SDL with 265,000 miles on it that has never given a bit of trouble.

You take your chances -- cost for a rebuild is on the order of $5000 for parts alone if you need new head!

Peter

mikemover · #9 06-05-2002, 07:57 PM

Five grand for just the parts? Ouch! That had better be one VERY fast diesel engine for that kind of loot!

I've never thought mixing aluminum heads with iron blocks was a good idea. The different expansion/contraction rates just seem like a recipe for trouble to me. Either all-iron or all-aluminum would make more sense, wouldn't it?

Mike

psfred · #10 06-05-2002, 08:48 PM

0-60 in less than 10 sec in the W140, about 175 hp and more than 200 ft/lbs torque, yes, it is a hot diesel. Pure pleasure to drive, at least the 300 version in my 87.

Aluminum for the head has properties that make the differential expansion and cracking problems bearable -- much better heat transfer, lighter, and eaiser to cast the rather intricate design. They work fine except for the need to replace the head gasket fairly often (as in every 150,000 miles or so) when it finally dies from the differential expantion. Not a high price to pay for the increased performance.

My Volvo TD also has a similar design (the VW Audi diesel found in Rabbits and 5000s, six cylinder version). Makes 105 hp and 145 ft/lbs torque on 2.4 L -- about the same output per displacement as the 601/602/603 turbo versions.

Peter