MERCEDES BENZ STARTING PROBLEM

MERCEDES BENZ STARTING PROBLEM

The Mercedes-Benz C-Class 250 TD (W202) sedan  and the T-model station wagon with the OM605 5-cylinder engine manufactured from 1992-2000 is probably one of the rearest cars in the Mercedes stable. The W202 model C-Class cars were the first to display the modern Mercedes-Benz  naming scheme, where the badged numbers are followed by letters. For example C250 instead of 230E or 280SE or190E, etc.

The W202 shape with the "facelift" replaced the original W201 190 series sedan; "Baby Benz" (1982-1993)  though less than 2 million W202 models were produced since its inception. The W202 shape came in a number of engine capacities, with slight difference in their bodies. Below is a list of some of the W202's manufactured between 1992 and 2000 with engines ranging from the 4 cylinders in-line (straight)  engines of the C180 to the V8 of the C55T AMG .

Mercedes-Benz OM605.910 engine

The Mercedes Benz W202 range of Petrol  & Diesel Cars

C180 and the C180T,  
C200 Kompressor, C200 Diesel, C200T Diesel, C200 CDI  
C220, C220T, C220 Diesel, C220 Diesel T,
C230, C230 Kompressor, C230T, C230T Kompressor, 
C240 and the C240T, 
C250D and the C250 Turbodiesel 
C280, C280 (V6) and the C280T (V6) , 
C36 AMG. 
C43 AMG and the C43T AMG 
C55 AMG and the C55T AMG 

However, the only  engine of interest at this point in time is :- Mercedes-Benz C250D 83kw normally aspirated in-line (straight) diesel engine (OM 605.910). Only 44,801  cars with the aforementioned engine were manufactured between 1992-1996. The Mercedes Benz C250D 110kw Turbodiesel  engine (OM 605.960) is essentially the same engine but sports an added turbo charger. Again only 59,772 were manufactured between 1995-2000. These two engines are in fact the little brothers of the Steyr-Daimler-Puch tractor (OM 606.964) 3.0 litre engine which is essentially the four valve per cylinder successor of the OM617 two valve per cylinder engine. An engine proven  to  exceed  500 000 to 1 million miles before needing an overhaul and is therefore considered to be one of the most reliable engines ever produced.

Mercedes-Benz OM605.910 dual overhead cam engine with intake manifold,
valve cover and high pressure injector pipes removed 

This extremely successful Mercedes-Benz OM606  3.0 litre engine, is an inline-six cylinder double overhead camshaft (DOHC) diesel engine sporting indirect injection. Likewise the Mercedes-Benz OM605.910  / OM605.960 is the 2.5 litre inline-five cylinder double overhead camshaft (DOHC) diesel engine also sporting indirect injection. These engines are relatively noisy because of indirect diesel injection yet they are strong and powerful,  with low fuel consumption and excellent reliability. 

Mercedes-Benz C250D diesel fuel seals

Diesel engines have two peculiarities, they are sometimes difficult to start and sometimes difficult to switch off. Both of which are relatively easy to repair. Often times starting difficulty is associated with the glow plugs. Glow plugs are essentially 12V heater elements fitted to the combustion chamber that heats up, to ignite the first sprays of diesel from the injectors. A continuity test between the the engine block (earth) and the glow plug connector node will determine whether any of them are open circuit. Continuity or a low resistance reading (typically between .5 and 4 Ohms) doesn't necessarily mean the glow plug is operational. 

The best test is to remove the glow plug, dip the heater tip into a bit of water and connect 12V to the glow plug using jumper leads. Earth/negative terminal to the body of glow plug and Live/Positive to the connector node. The water will turn into steam even before the glow plug tip glows red. That's sufficient to know that the glow plug is functional. Be careful not to touch the tip, it will burn your fingerprints off . I've encountered glow plugs with good continuity (1.5 ohms) but when I connected the 12V  across them, using  jumper leads, all I got was a spark on connection and no steam nor heat, it was  dead glow plug even though  it had continuity. Also bear in mind, there are three things needed to start a diesel engine.

1) Compression - The air in the combustion chambers is placed under such enormous pressure that it's hot enough to ignite the diesel spay spontaneously.

2) Diesel - Supplied under pressure (115 bar) must be devoid of air bubbles. Bleed off air before attemtping to start the engine.

3) Electricity (12V) - Needed to power the glow plugs , the electronic shutoff valve and the diesel pump. When engine is hot, glow plugs aren't needed any longer since the engine is fitted with a diesel pre-heater.

So if any one of the above three is missing, the car won't start. The assumption is that if the car starts everyday and displays sufficient torque, the likelihood when it doesn't start wouldn't be compression  associated. Diesel engine compression range between between 275 psi and 495 psi,  with typical compression ratios of between 15:1 and 23:1. When diesel is sprayed into the combustion chambers through the diesel injector nozzles - the moment the air in the chambers are under great pressure - it's normally hot enough to ignite the diesel spontaneously. However, when compression is suspect, an average pressure of 300 psi is good but no cylinder should vary by more than 10 percent.  Anything greater than 50 psi difference between cylinders will also cause starting problems,  poor acceleration, diesel knock, excessive smoke and can lead to rough idling. 

The part numbers for the 6 hoses are 605 070 12 32, 605 070 10 32,
605 070 15 32, 605 070 07 32, 605 070 08 32, 605 070 14 32,

Another problem with this engine are the clear diesel pipes or rather its seals that leak,  allowing air into the fuel line causing the injector pump pressure to drop substantially. When this happens, the battery can often be drained before the engine is cranked long enough to buildup 115 bars of pressure. However, a squirt of Spanjaard Quick Start spray into the intake manifold should get her idling. But, this screams very loudly that there is some sort of problem that needs to be solved sooner than later.

The part numbers for the 6 hoses are 605 070 12 32, 605 070 10 32,
605 070 15 32, 605 070 07 32, 605 070 08 32, 605 070 14 32,

So remove intake manifold, check the  glow plugs, test  them as mentioned above. Next remove the six clear fuel lines and check the "O" rings for leaks and if they are wet with diesel  replace them with those  supplied by Mercedes-Benz. I've tried various "O" rings from several sources and believe me, they don't work and always leak. The ones from Mercedes-Benz agents are silicon rubber based, manufactured with a tight tolerance.  Next check the shut off valve for leaks. The "O" ring behind the shut off valve tends to perish because it endures the heat transferred from the engine block to the injector pump. Do yourself a favour and get the right one. It will save you time and money.

Clear diesel pipe attached to clogged filter

Fuel filter cleaned out 

The grime that clogged the fuel line.

Also check the hoses clamps on the fuel lines between the non return  fuel cut off valve and the  fuel heater/exchanger. I had a very minute,  hardly visable, crack in my car's heat exchanger.  Early Mercedes-Benz W202, were fitted with biodegradable wiring harnesses.  Unfortunately, this biodegradable insulation deteriorated much quicker than it was anticipated due to moisture and heat in the engine compartment. I had to replace mine because there was more visible copper than insulation on the wires. So check the harness between the shut off solenoid valve and the ECU for cracks and brittleness; replace if necessary.  Also clean the small fuel filter, its bound to have dirt in it. 

Perished  wiring harness and fuel hoses that needs replacement.

And lastly, check the shut off valve at its ends for leaks, if it does leak, replace the seals. The seals are held in by four pins that needs to be pushed out in order to get to the seals. Unfortunately Mercedes Benz agents do not supply these seals separately. One has to purchase the complete shutoff valve at some ridiculous price which I just couldn't justify. The ones I used were roughly 31.88mm in diameter, slightly larger than the ones that came out. Putting them back was an itch with a B. As can be seen I used a G clamp that worked wonderfully indeed.

Shutoff valve with the seals removed

O rings slightly bigger than the ones that were removed.

 The only way I was going to get the end caps back in place was with a clamp.

Shutoff valve closed with "O" ring on nozzle that fits into diesel pump.