Friday, August 14, 2015

What to look for when buying an old (pre-2000) Mercedes Diesel

You're browsing craigslist and you find yourself trying to commit financial suicide by bringing home a project. You find a couple old Mercedes diesels and you have no experience with them, so you figure what better way to screw yourself than go and buy one! Great plan.

I'm not going through each chassis, this is going to be a quick run down of the most common diesel engines found in Mercedes from the 1970s to the 1990s. Some are chassis specific, some carry over, but this will hopefully give you a quick idea of what to look for (and what you can expect) when trying to buy yourself a Mercedes diesel.

We'll start with some things to look for that apply to basically all of these engines.


It's no secret that cold diesel engines don't like starting. They don't have spark plugs, they run simply on the immense compression of air/fuel to create an explosion. They need some sort of heat to help in the starting process, otherwise the starter will be in agony cranking and cranking and cranking. The engines are equipped with glow plugs to be that heat source. If there's a bad glow plug, you'll have a relatively quick start, but it will misfire and billow white smoke for a while until that cold cylinder fires over. BUT, there's a huge BUT..... If that white smoke and misfire don't go away within a few seconds (10-15 at most) then there are other problems with the engine aka possible injector or weak compression.

There's a quick and dirty test to check the overall health of the engine. With the engine warm and running, loosen the oil filler cap completely and let it set on the valve cover. If it's dancing around a bit, there's a bit of blow by, which is fine. If it's completely blowing off, don't even bother with that engine. It's more loose than a prostitute's........ you get it. No blow by, or no oil cap dancing, is fantastic, but scary. The owner might have thrown in some of that engine honey to sell the car. Unless the engine has been recently rebuilt with proof, or has immensely low mileage, it's very rare that you'll come across one of these with no blow by. It's not a definitive test in engine health, but it's a quick test to give you an idea.


Shut the engine off, or at least try to. If it doesn't, the shut off system, which is usually vacuum, isn't working. There's a stop lever attached to the injection pumps on some way on every Mercedes diesel (except OM606) so you'll be able to shut the engine off that way. If the engine isn't shutting off, there's either a vacuum leak, the shut off valve may be bad, or there's a leak at the key which gets vacuum. In most cases, the shut off valve internal diaphragm has a tear and will leak vacuum. Replacement is relatively simple and the part is $35 at most. 


You have to be absolutely anal about changing the fuel filters on your diesel. They get dirty and they get dirty quick. There are usually two filters on a Mercedes diesel, a small prefilter and a main. The prefilter will usually be small and clear, if they're black, it's overdue, the tank is filty, or the idiot selling the car runs used motor oil in the fuel tank. I change fuel filters every second oil change. I take my oil 3500-4000 miles, so 8-10k miles on fuel filters is long enough. They can cause fuel starvation problems, no starts, rough running, etc. Some will say that's overkill, but ask me if I've ever had a problem with my diesel cars (I've had about 25 or so). The answer is no, it's a small price to pay now for a big tow bill later.

The injectors should be serviced every 75-100k miles. If not, the chances of you needing new injector nozzles is increased 10 fold. Unlike some gasoline fuel injectors, these diesel one's can be rebuilt, however, you need a pop tester to accurate set the opening pressure of the injectors. NA cars usually open at 115 bar, turbo cars at 135 bar. If you need someone to do the injector service, I can do it for you (shameless plug):


Do yourself a favor, buy a block off kit and get rid of it. It doesn't do anything but soot up your intake and make your diesel less efficient. If you don't want to buy a block off kit, get a ball bearing and stick it in the vacuum line running to the EGR. That way, if you have emissions equipment checks, they can't tell the thing is just there to fool them.


I realize this one is going to be a bit difficult for those who aren't familiar with the engines, but you can usually tell how an engine is running by how much it's vibrating in the engine bay, or how it's vibrating the whole car. If you find yourself sitting in the drivers seat and the entire car is rocking back and forth like a rhythmic rocking chair, there may be an imbalance in compression (dead cylinder) or an imbalance in fuel delivery (fuel injector, injection pump). If it's just a jarring vibration (steering wheel shaking), it's probably just a motor mount. Diesels go through motor mounts rapidly. For every one mount you replace on a gas engine in an equivalent chassis, you replace diesel mounts twice.


OM615 / OM616 / OM617

You'll find all three of these engines being used in the W115, eventually the latter engines were used on the W123 and W126. I call these the cockroach engines, it's very hard to kill them. If you manage to do so, you've either bought an engine that wasn't maintained AT ALL, or you did something unbelievably stupid like drive with sand in the crankcase.  I've had one or three of these that were run without oil for a period of time and they were in tip top shape at the end of the day, so seriously, if you kill one, please don't get another.

The OM615 was brought in on the W115 in 1968 and eventually phased out by 1976. The OM616 you'll find from 1976 to be phased out by 1984. You'll find the OM617 from 1976 all the way out to 1985 - in multiple forms which include the ever illustrious TURBO form.

Anyway, these engine all need their valves physically adjusted. While actually getting to the valve's is easy, there are some specially bent valve wrenches that are for the adjustment. Without the wrenches, the valve adjustment get's a bit more difficult. It can be done with a crowfoot and maybe a straight wrench, but you'll have busted knuckles and the job will take about two more hours than it really should. Valves should be adjusted every 15-20k miles, I do them every second fuel filter change, or four oil changes. It's a nice system that keeps my diesel's happy: Oil changes every 3500-4000 miles, fuel filters every second oil change, and valve adjustments every second fuel filter change. You do this and the chances of you having problems with your OM615/6/7 will be nearly nil. 

These are the necessary wrenches: 2 adjustment wrenches, 1 retainer wrench

Turbo OM617

Everything still applies to the turbo cars, only with turbo cars you have to worry about oil coking. Owners don't realize that after long highway runs, or spirited drives, the turbos have to cool down at idle for a few minutes before you shut the engine down. This prevents the oil from coking, or turning solid. You can remove the air cleaner really quickly and check for play in the turbo, forward and backward play should be non existent. A little up and down/side to side play is fine as this will go away once the engines started. The turbine shaft will float on a layer of oil. Turbo cars are much faster and much more responsive than NA cars. 0-60 times should be about 10-14 seconds, depending on how quick the trans is to respond. Any slower than that and she's struggling.

Now we come to the more modern versions of these earlier engines. These engines can be found in the mid to late 80s. They don't require valve adjustments as they have hydraulic lifters. They more powerful and more efficient than earlier engines. They aren't without their own type of faults. The 601 and 603 series, over time would develop a bit of a valve tick with age. The lifter galleys get clogged up over time and won't let oil enter or drain very well. This is where that valve tick comes from. The tick should go away when the engine is revved up to build oil pressure. If it doesn't, that lifter is kaput. What I'll usually do is 50 miles before an oil change, I'll put in about half a quart to a quart (depending on how low I'll let the oil burn down) of transmission fluid into the crankcase. Trans fluid has stronger detergents that will eventually unclog those lifters. If it doesn't work within about two or three treatments, you'll either live with it (and keep trying) or be forced to replace the lifter. Again, if the tick goes away when the engine is revved up (higher oil pressure) then don't worry about replacing the lifter. Just keep up the trans fluid treatments and it should sort itself out. Others have had success by switching over to synthetic oils. I don't care enough to use synthetics as I like changing my oil often.

OM601's are slow. That's their problem. If you can live with being the slowest Em Effer on the planet, enjoy your nearly 45-50mpg highway. If you're still alive after trying to get on the highway. This is a gross exaggeration of OM601 performance, they're not THAT slow, but they are. If you catch my drift.
OM603 (straight 6) engines had a multitude of problems depending on the year they were made. Early #14 casting heads were notorious for cracking if the engine got anywhere near overheating. If the engine didn't overheat, you're fine. Keep it from overheating and you'll be fine. To figure out which head you have, it'll be a series of numbers like 603 XXX "XX" XX. The numbers in "parenthesis" are the casting numbers of the head. Late heads "17" and "22" are expensive and rare, but won't crack, or atleast are far more resistant to cracking. 

You'll find the casting number here, it's somewhat odd to see with the intake manifold and stuff in the way:

Late OM603's are the rodbenders. Mercedes bored the block and stroked the engine to 3.5 liter over 3.0 liter. There was nothing wrong with that, what was wrong were the flimsy rods they used in the whole thing. Over time the rods would bend and oval out the cylinder causing catastrophic oil burning. Some cars had new factory crate motors dropped in, while others are running around being rebuilt or with bent rods. Proceed with caution.

I wouldn't hesitate with a rodbender (I have three of them) because they have the best parts. Take an early 3.0 block and throw all the 3.5 parts (head, injectors, pump, turbo) and you'll have an absolutely bulletproof engine.

I love these engines. They're smooth, they're powerful, they're more efficient than all previous engines and they're the most resistant to problems. It's like Mercedes took 40-50 years of their best diesel technology and threw it in one package, BUT, leave it to Bosch to F*#K it up....

The fuel supply on these cars runs through a series of plastic lines with double the orings sealing each plastic line to various parts of the engine. Over time, these lines will get brittle and break but that's a given. The apocalyptic pain in the ass is with these are the orings. They let minute amounts of air into the system and you'll get small bubbles running through the fuel. That's a problem because the engine can't run on just air. The engine will struggle running, it just may not start, or it'll misfire because instead of fuel it's not able to inject any fuel. I have a post a while back from when I did the fuel lines on the old Daily Diesel project here:

There are multiple posts on the internet of o ring replacements, etc etc etc. Check them out and get to know what you'd get yourself into.

Glow plugs are also a huge ordeal on these, only because they're so damn long. When someone without much experience tries to remove one like a normal fastener, half the body (usually the unthreaded part) breaks off inside the head. Out comes the drill, tap, and your choice of religion. Usually getting the engine nice and warm will help with removal. I left my engine on the block heater for 4 hours before I even considered doing the job. Coat the body of the new glow plug with a little bit of copper antisieze and it should be good to go for a while.

That's all folks. If you have any questions or anything to add, leave a comment below! Thanks, Allen

1 comment:

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