The Challenge of Overhauling a 615, 616, or 617 Diesel Engine

Applies to 220D 240D 300D 300CD 300TD 300SD 1972 to 1985

Let's face it... As these old diesels age, the engines are wearing out. Yes, I know you have probably heard they will go a million miles - but that is very rare and takes an exceptional level of meticulous maintenance to pull that one off! I have seen some 61x diesel engines wear out before 200,000 miles (primarily due to lack of frequent oil changes or because of an engine overheat problem). With regular service most do pretty well up to 250,000 miles. From 250,000 to 400,000 miles it becomes a little bit of the "luck of the draw." Cylinder wall and piston ring wear become the big issue here. As this happens compression drops off as well as the "seal" between the rings and cylinder wall sleeve. Crank case pressure increases (blow-by) which results in numerous engine oil leaks. Oil consumption rises. And starting in cold weather becomes increasingly more difficult.

Problem & Solution

Common among these chassis:


Many have written asking me if there is something they can do about this. Some miracle cure or quick fix is desired. No, unfortunately there is none. A complete engine overhaul is the only cure. Don't just think you can throw in a set of piston rings like you did in that old 350 Chevy.  With diesels it just does not work that way. I tell owners to just keep driving the car.  If it starts and runs and doesn't burn more than a quart every 300 miles - add some engine oil saver, maybe a high speed starter, change the air filter often ---- and just keep truckin. You will know when it is time. 

Major repair on these engines breaks down into 2 key jobs.

  • 1. Cylinder Head Remove and Overhaul
  • 2. Engine Removal and Complete Overhaul

The following information and links should help you decide if you want to take on the job and what parts and resources you will need to complete it successfully:

Let's start with No. 1. Cylinder head removal is required if you determine there is a crack in the cylinder head, a damaged valve or valves or you have a leaking head gasket. Symptoms for these problems can be poor compression, high engine operating temperatures, mysterious loss of coolant, engine oil in your coolant or visa versa, and white smoke during acceleration. Proper diagnosis is by compression test, radiator pressure test and possible coolant analysis. If you have weak engine compression, don't plan on doing a head overhaul to fix the problem. I have found that doing a "valve job" will not help much with a diesel engine. Combustion temps are lower and valves don't normally "burn" like they do on a gas engine. In fact - a cylinder head overhaul on a high mileage diesel can make matters worse (ie, a tighter seal at the valves can often stress the lower end of the engine and lead to increased blow-by and piston / rod bearing wear. 

If you must do a head job, here are 10 very important things to consider:

  • 1. Get the proper manuals and repair information before you begin. I recommend you remove the head and have it checked at the machine shop BEFORE you order replacement parts.
  • 2. Find a very good machine shop that can provide expert work as well as expert advice.
  • 3. Have the head checked for cracks even if you don't think you "see" any.
  • 4. Always replace the timing chain and tensioner (unless recently done)
  • 5. Be very careful when removing the head that you know about the hazards of the timing chain jamming or skipping some teeth. (Many have learned this the hard way with much grief and added expense!) ***
  • 6. You must exercise extreme care when setting the head back on the engine in order to avoid damaging the new head gasket. If you have a hydraulic engine lift and can slowly lower the head in place - so much the better
  • 7. Follow are head bolt torque settings and sequences to the letter. If this is not done you may be doing the job again soon.
  • 8. Replace thermostat and water pump when head is off the engine
  • 9. Carefully adjust each valve clearance using my special wrenches and a proper feeler gauge.
  • 10. When finished, rotate the engine through its full cycle two or three times by HAND and then recheck cam to crank timing marks again. DO THIS BEFORE you ever put a starter to the engine. Failing to heed this warning could bring tears to your eyes.

*** Let's chat about the timing chain a little more. The chain can be left in the engine when you remove the head but certain precautions must be followed. If you plan to replace the chain with a new one, go ahead and re-install the rebuilt head and then proceed to install the new chain using the detailed instructions included in my timing chain replacement kit. DON'T EVER JUST PULL THE OLD CHAIN OUT OF THE ENGINE. if you do, you will end up taking much of your engine apart to get a new chain installed!

Now for one of the most important things you MUST DO before you even begin to think of removing the head! Rotate the engine over so No.1 Cylinder comes all the way up on compression stroke to top dead center. The timing marks must line up on the crank pulley and the front of the camshaft and front cam tower. (refer to the factory engine manual or Haynes diesel manual). These reference marks must be aligned at Top Dead Center so when you put the head back on and attach the chain you will be able to easily line everything up the same way it was before you started. Once you remove the head, you can not rotate the engine without the danger of jamming the timing chain. If you don't have the marks set right before you remove the head, you'll have NO reference to help you get everything back together properly and you will have one of the BIGGEST wrench dances you have ever experienced! If I sound like I am trying to scare you - I am!

Now lets take it another step and scare you a little more. Before lifting the head off the engine you must remove the timing chain from the sprocket on the front of the camshaft. This is done by first removing the upper plastic guide rail. Then remove the sprocket from the cam, slowly lower it down a slight amount so you can carefully lift the chain off the sprocket. DO NOT DROP THE CHAIN. YOU MUST keep the chain under tension the entire time you are removing the head, working on the head, and reinstalling the head. This will mean you will need a second set of hands (helper) and a couple bungie cords (or rope) to hold the chain under tension at all times. If you drop the chain down into the engine, it can jam itself in the sprockets and you will have another REALLY BIG wrench dance on your hands. It will take some thinking and ingenuity on your part - but you WILL figure out how to keep that chain pulled tight during the whole operation.

To wrap this up just one more little scare. Keep the cavity at the front of the engine covered with shop rags as much as you can. If you drop a nut, bolt or other small object down into that cavity, you will be dancing into the wee hours of the morning! What ever you drop MUST come out. Locating and getting to it could mean hours of additional work.

Other than that, the job is a piece of cake...

Now let's discuss No. 2.  Engine Overhaul

Introduction: I am going to start by sharing with you a typical email we received concerning trying to fix a low compression or excessive blow-by problem:

"When rebuilding a gas engine the bore is mic'd in each cylinder and a total out-of-round is gathered from all cylinders. All cylinders then get bored to that size with new pistons for each cylinder along with rings, new rod bearings and crank bearings not to mention the gasket set also. In a 240D if blow-by is found in one cylinder, is that one bad cylinder treated only, or do you do the same as on a gas. That is new sleeves for each cylinder, new piston and rings along with a new oil pump, gaskets, rod and crank bearings. I guess what I'm trying to ask is are only the worn parts replaced? If the entire engine is to be done, it can get a little pricey to say the least. I would think that if the pistons are ok other than the one with blow-by, then install a new sleeve and replace the piston in the bad cylinder bore. As for the other three (assuming their within spec), couldn't the remaining sleeves be honed to break the glaze, and install a new set of rings on each piston? Just trying to figure out a game plan".

Here was my reply:

"Hi Bill, I am an old hot rodder from days gone by. When I first started working on diesels, I quickly learned a Mercedes diesel is a very different beast from a gas engine. You just can not do things "half way" like you can with a small block V8... or it WILL come back to bite you. I remember on some gas engines you could just pull the pistons, use a ridge reamer, hone the cylinders, install new rings and throw it back together! If you try to do that will a diesel you will be very sorry... high oil consumption, poor combustion, short rebuild life, lost time, and more money in the long run usually are the result. If you plan to open up your diesel engine do it all or go find a good used engine. Just my humble opinion of course. And if you do plan to rebuild it, be sure to enlist the help of a VERY good machinist who is familiar with Mercedes diesels. Sleeve installation, boring, and honing the cylinders so the NEW pistons fit PERFECTLY are the most critical part of the rebuild process. A good machine shop can be your best friend during the rebuild process. He can advise on your cylinder head, crank, and other critical parts as well".

There is no "standard overhaul kit" like you might see with a gas engine. Your engine will need to be taken apart, cleaned, inspected and carefully measured before any parts are ordered. And be prepared --- the parts will not be cheap!

Kent Bergsma