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Old 10-17-2017, 08:40 AM
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Location: SE Mich
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So how good is the M103 engine?

I have a chance to buy a clean 91 300SL for a pretty reasonable price. Mileage 137,000.

The car had a fuel delivery problem which the current owner solved by jumping the system with a wire directly to the fuel pump, which he has to disconnect separately each time he stops.

I am not familiar with the fuel controls, but this sounds like a fuel relay issue.

But the real question is: How reliable is the M103 engine?

I have heard negative stories, just wondering if any current or prior owners can shed some light.

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Old 10-17-2017, 09:54 AM
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Location: Lafayette Indiana
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The 3.0 M103 in my 87 w124 was a very reliable engine even at almost 200k. After I got the cooling issues worked out I wasn't able to mess it up as a 16 year old kid. However its slow and doesn't sound all that sporty.

I'd jump on a r129 with one though. Mostly because a cheap r129 sounds fun.
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Old 10-17-2017, 12:10 PM
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The M103 bottom-end is as good as it gets. A friend of mine had an '87 260E that she bought new. I did a valve job w/guides at 250K but the bottom went to 486,000 miles, when it lost a cylinder due to worn rings........
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Old 10-17-2017, 06:11 PM
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If this is a USA spec car, it will have a M103 lower end with a early version ( CIS ) of the 24 valve M104 head. If this is a Euro car, it will have a 12 valve M103.

The fuel system on both is electronic controlled CIS that more or less like a M103.

Some make a _BIG_ fuss about the M103 24 valve / M104 timing cover / head oil leak when in reality is is no worse than any other engine regardless of brand . This is a non issue as far as I'm concerned, change the head gasket or just keep adding oil.

These cars have a 722.5 hydraulic shift 5 speed that is same as a 722.3 but has OD added. It is also the same in design as the smaller 722.4. No reverse is common in all of these due to clutch plate wear.

With the fuel system issue, this is a $ 2,500 car. If you really want a R129 , go for 93 ish and up as these had full electronic FI .
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Old 10-18-2017, 03:00 PM
Join Date: Jan 2007
Location: Bethesda MD
Posts: 17
Great engines; one caveat

My wife had an M103; I've two M104 cars in the driveway. Most folks I know swear by both, with two caveats on the M103: the original valve guides were a known problem area, and my wife had to replace hers around 50K despite diligent maintenance. Second, the single-row cam chain is not forever; my senior tech urged me to replace even the double-row chain on the M104 when it hit 100K. Common practice is to run them longer, but he's seen some well maintained cars lose the chain and crunch the engine even at 120K. Much depends on how long you hope to keep the car, and use patterns, but at 137K I'd budget a chain replacement and drive gently until then. Best of luck.
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Old 10-21-2017, 08:40 PM
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The m103 CANT hold a candle to a Borgward!!!
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Old 10-22-2017, 10:02 AM
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Join Date: Jul 2009
Location: cleveland
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The m103 engine always leaks oil. It’s only a matter of time. It’s relatively difficult to get it running perfect, but even more difficult to make it not run at all. I really like them for their reliability, but they do require frequent maintenance

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