Diagnosing Cranking No Start Conditions with OM642 Diesel Engine

Andrew Technician New York Posted   Latest  
Tech Tip
2008 Mercedes-Benz ML320 CDI 3.0L (642.940) 7-spd (722.9)
Crank / No Start

The OM642 family engine is used in many Mercedes-Benz / Daimler Chrysler vehicles including:

Dodge, Freightliner and Sprinter Trucks …

164 Chassis: … ML320 CDI and ML350 BlueTEC

166 Chassis: … ML 350 BlueTEC

164 Chassis: … GL320 CDI and GL350 BlueTEC

166 Chassis: … GL350 BlueTEC

251 Chassis: … R320 CDI and R350 BlueTEC

212 Chassis: … E350 BlueTEC

221 Chasssis: … S350 BlueTEC

… Chrysler 300

… Jeep Grand Cherokee

… Jeep Commander

Cranking No Start conditions are a common concern in our experience on Mercedes vehicles with the OM642 engine, and we’ve assembled a diagnostic process to help identify the root cause. Unfortunately, these problems rarely come with relevant DTCs. As with any Diagnosis, we’re going to start with the basics, use our senses, and gather information.

Crank the engine and lListen:to the engine crank,

● Does it sound like the compression is consistent? 

● Is there any noise or sign of combustion? 

● Does cranking sound slow? 

● Are there warnings in the cluster? 

● Do the interior lights dim? 

Does it have a good fuel level? 

Have someone else crank the engine, and look at the tail pipe.

● Do you see blue, black, or white/grey smoke? 

● Do you smell fuel? 

● Is the tailpipe clean or sooty?

Open the hood, inspect the battery cables and terminals for tightness and corrosion.

● Does the battery look old? Is it fully charged?

● Look the engine over, check the oil level, look for fuel leaks, check that the intake ducting is sealed properly and the air filters are in good condition. While you’re in there, feel the turbo for play, binding, or damage. Inspect the mass airflow sensors.

Note: grey/white smoke may indicate an injector is stuck open and spraying fuel into the exhaust but not atomizing it which is necessary for combustion. A sooty tailpipe indicates the DPF has failed.

Think about the problem, what types of things could cause a no start on a diesel vehicle? What do diesels need to run and start? Fuel, Heat, Air, Timing. What are we missing? Use your Autologic early in the process, there is a lot of “free” information you can get with very little time and work.

Hook the Autologic up and do a complete vehicle scan, record the scan and do not clear any faults yet. Verify the drive authorization (immobilizer) system is allowing the engine to start.

Note: some faults on this engine are active even when shown in a “stored” state, and some faults can’t be cleared at all unless the vehicle is driven and a drive cycle is successful. Avoid clearing the faults especially with SCR problems as this can be perceived as “tampering” and complicate repairs.

What is it that ignites Diesel? Is it compression? It’s the heat generated by good compression. If the engine is cranking too slowly the heat from compression will dissipate before the fuel can ignite. The 642 engine uses a complex Glow system to aid starting as well.

Run the Diagnostic Test via actuations in the Glow Module and check the fault codes, failure of the glow system can cause hard starting and no start conditions particularly in cold weather.

Note: The Glow Module is a common failure item and the typical cause is a shorted glow plug, it is best to replace all the glow plugs as a set with the module. There is an SST Fixture / Kit for extraction and drilling if necessary. Also, the Glow Module sometimes shorts out the LIN bus which is internally bridged to the alternator LIN inside the CDI module, when this happens, some bizarre electronic problems can occur including a no start condition, unplug the module to quickly check if it is causing a problem.

Is the engine cranking too slowly? Are we getting good crankshaft and camshaft position signals and synchronizing the injectors?

Check the cranking speed, if it is less than 200 rpm diagnose the starting system: Battery, Starter, Voltage drops. A good cranking speed also indicates the crankshaft position sensor is working. Use the relative compression test to check for a cylinder balance problem, 7 rpm difference is the spec. You can also use a scope to check the relative compression and battery health via the amperage waveform, or an amp clamp to check starter current draw. Is the engine seizing / binding?

The CDI controller will set a code if an attempt is made to start the engine but it did not turn, the “Blocking” code is the one you will see. There could be a blown fuse or defective starter relay.

11E900 (P11E900) [Stored] Start attempt without starter actuation 

155700 (P155700) [Stored] Engine cannot be started. (Blocking) 

Note: On the Sprinters The B+ connection at the starter is a splice that feeds power to the alternator and sometimes it melts and causes charging system (and possibly) cranking voltage drops. Generally, it overheats the starter solenoid and chances are high that the solenoid will break when the cable is changed so sell them both.

Compression could be low but even across the engine, the only way to know would be a manual compression test. We have to make choices in our diagnostic approach based on how much time and effort testing takes and what the probability of it being useful is. In this case, pulling a glow plug that might break in the cylinder head to check compression doesn't seem necessary or worthwhile. If the engine timing was bad enough to interfere with starting, we would probably have a code. Some tests, however, are an absolute necessity regardless of difficulty, don’t skip them, do not pass go, do not collect 200 dollars.

For reference: the compression for 642.9 engines should be 27-32 bar, maximum 3 bar cylinder to cylinder variation. For 642.8 engines the compression should be 23-30 bar, maximum 3 bar cylinder to cylinder variation.

What about Air? What about breathing?

The 642 engine uses two mass airflow sensors but their reading is combined into one PID. The MAF sensors are rarely a problem, if you unplug one at a time you should see the same reading as when the other is unplugged. Check the pressure sensors KOEO; intake air, charge air, and exhaust back pressure.

The maximum difference between the three is 10 hpa. The backpressure sensor is a common failure on the 642 engines and frequently reads out of range (much higher). Usually it doesn’t cause a noticeable problem but it can interfere with proper EGR operation and most importantly it can prevent the engine from starting when the reading is extremely high. There will be no fault code 99% of the time when this occurs because the monitor allows a deviation of 3 PSI. The CDI module uses the backpressure reading to avoid thermal overload of the turbocharger. On one recent vehicle with a restricted exhaust the turbo temperature reading was 1100 degrees Fahrenheit without even a road test. If the exhaust cannot flow out of the turbocharger it can’t transfer the heat. Disabling engine start is a secret self-preservation strategy.

We’ve got Heat, we’ve got Air, we have Engine Speed and Timing, what do we have for Fuel? Continuing with the process of elimination, the Fuel system is the most complicated and difficult to diagnose. The fuel system starts with the fuel itself in the tank, the low pressure electric pump, the filter, the low pressure regulation, the high pressure pump, the high pressure regulation, and finishes with the injectors. So where do we begin testing? Literally any of these components could be causing our cranking no start problem.

Choosing a fork in the road and picking a direction to go in is often an important part of the diagnostic process. For fuel system issues on this engine we start by checking the rail pressure, which is a good indication of whether the entire system is working. Then we divide and conquer, pursuing either the low side or high side.

Check the rail pressure while cranking, if it makes 270 bar, most engines will start and you can generally move past the fuel system with regard to no start concerns. If you’re not making 270 bar, however, you have work to do.

There is one easy test of the system you can do up front. Engine off, unplug all of the injectors. There should be zero fuel delivery now. Check at the tailpipe for traces of fuel. If you have a five gas analyzer you can use it. If you find fuel, it indicates there is a leaking injector. You can also remove an accessible exhaust sensor and check to see if there is fuel coming out, just keep in mind that if the engine does actually start for some reason it may shoot flames out of the sensor bore

If you are confident an injector is leaking stop the diagnosis here and go directly to identifying it. To find out which injector is leaking you can disconnect them (engine off!), remove them and mount them on the rails pointed upward and crank the engine. Keep clear of any spray, see the later note about hydraulic safety. This is also the definitive test of whether there are any that are leaking into the engine.

Note: the piezoelectric injectors on the 642 engine are fired with 200-250 volts and could give you a dangerous electric shock. Do not probe them, do not disconnect them with the engine running.

Try to be as logical as possible, the closer we keep to logical thinking the more successful we tend to be. Premise: the rail pressure is low because the system is either not pumping enough fuel or the fuel is leaking out. Also we generally start with the basics and build a solid foundation from there; in this case that means we need good quality diesel fuel delivered at the correct pressure and in the correct volume. 

Proceed to testing low side system pressure. Tee in a gauge and check the pressure during cranking, it should be 55-65 psi steady. Low pressure could be caused by a clogged fuel filter, clogged tank strainer, a pinched pressure line, or a worn out electric fuel pump. Normal current draw is 6-9 amps. 

At very low temperatures diesel fuel can gel and restrict the filter or gum up the tank strainer. There is a heater installed in the fuel filter to prevent gelling. Typically, in cold weather fuels are blended with additives to prevent gelling.

Depending on chassis and model year a PWM system may be used instead of an ON/OFF type, but in either system there is a pressure regulating valve on the high pressure pump that mechanically limits the fuel pressure.

Volume is equally as important as pressure, check fuel volume by taking the low pressure line from the filter off of the high pressure pump. Actuate the pump with the Autologic for 10 seconds and check your fuel volume. Mercedes’ spec is at least ½ liter (500ml) in nine seconds, it should do a little better than that. A typical bottled water container is 500ml. Look for spurting or aeration as well.

Note: the leading cause of high pressure pump failure is running without adequate lubrication, particularly with contaminated fuel. Never run the engine without a good pressurized fuel supply. If there is severe fuel contamination and the engine has been run and the vehicle driven, debris from the pump often spreads and scatters across the fuel system damaging many components. Mercedes has a zero tolerance policy once metallic debris is found they require replacement of nearly every fuel system component and flushing of the lines.

The best way to check for metallic debris is to follow the high pressure pump outlet line to the rail, remove the rail, shake it out over a clean white cloth, and flush it with brake clean. You can also check the fuel filter element, but debris would have to make it all the way through the return line, to the tank, slosh around the bottom of the tank, make it past the strainer, through the electric pump, and to the filter – much less noticeable.

Check the fuel sample for good clarity, any water separating at the bottom, or gasoline contamination. You can put some in a styrofoam cup, if there is gasoline it will dissolve the cup. Safely try to ignite the sample, diesel is very difficult to ignite but if there is gas contamination it may not be hard. Leave some of the fuel sample out in a warm area and look for a hazy vapor – diesel is not volatile and should not evaporate much, unlike gasoline. There are other tests including chemical and laboratory tests but what it comes down to is that if you are troubleshooting a hard problem you may need to drain and replace the fuel with known good.

Once you have verified the low pressure fuel supply to the engine and only after that, you can try starting using starter fluid or ether, if the engine starts and runs you should continue troubleshooting the fuel system.

It’s a good practice to avoid the most complicated and the most difficult testing until it is necessary, now we will get into the high pressure diagnostics. This is the second path of the fork in the road.

Check the rail pressure sensor for an offset / drift by reading the value engine off key on, it should be from 4 to 10 bar, Mercedes’ spec is less than 15 bar. If you have any doubt crack a line loose to bleed residual pressure off and recheck the value. The sensor’s operating range is from 250-1600 bar so it isn’t accurate at low pressures less than 100 psi. If the sensor is replaced, perform the teach in and reset Quantity Mean Value Adaptation.

If you have low rail pressure and the low side fuel system is good (volume, pressure, fuel quality), you need to check for leaks in the high pressure portion. Visually inspect for and correct any external leaks. 

Note: Do not risk bodily exposure to high pressure leaks, do not search for them with your hands, and do not expose yourself to spraying injectors. Spray penetration into your flesh can cause blood poisoning, tissue death, gangrene, and require amputation. Google it.

After inspecting all of the lines, injectors, and the high pressure pump, you need to check the other high pressure components. The system uses a “Quantity Control Valve” also known as a “Volume Control Valve” on the high pressure pump to throttle its fuel intake to regulate pressure. It can also adjust rail pressure using a “Pressure Regulating Valve” also known as a “Pressure Control Valve” which is a controlled leak or variable orifice.

The pressure regulating valve is generally open by default, although according to Mercedes some of them have a spring that can maintain 40-80 bar after engine shut down. We’ve been looking lately and we haven’t seen any real residual pressure yet. There is anywhere from 3,000 to 22,000 psi of fuel pressure trying to force its way through the PRV, but the CDI adjusts the opening with electromagnetic force to achieve the target rail pressure.

Mercedes states that only the PRV or the QCV are in control of rail pressure at any given time, not both, but you will often see both actuated in the scan data. The reason is that if you wanted to use the quantity valve to control the rail pressure and the PRV was not actuated, all the fuel would dump through it and return to the tank. In order to use the quantity valve for pressure regulation the pressure regulating valve must be actuated enough to resist the current rail pressure. It isn’t controlling pressure in this case, just sealing the rail off.

The pressure regulator valve is sealed when the engine is cranking and so it should have no leakage. To test it, remove the rubber hose from the rail next to the pressure regulator and pinch it off. Install a clear hose over that metal elbow there and hose clamp it on. Run the hose to a bucket and have someone crank the engine, watch for leakage. During a cranking no start there should be NO leakage. (Mercedes’ spec is < 15ml). Once the engine starts the pressure regulator valve takes control and starts bleeding fuel off, so be ready to cut the engine and make sure you have some space in your bucket. The quantity control valve is generally active whenever off-idle and not decelerating, however this varies with fuel temperature.

In some cases you really need to “know your enemy”, the 642’s injectors use an uncommon / unique method of control that greatly influences diagnostic practices. Some jobs will require taking time to research and learn theory and operation.

Earlier we checked the injectors for external leakage, but they can also leak internally. The injectors on the 642 engine use a “leak oil line”, which is like a drain line that returns fuel to the tank. When the injectors operate, a very small amount of fuel is bled off as part of the control of the internal valve. That fuel is barely noticeable on a good injector, but when the injectors fail internally they can leak into that return line and drop the rail pressure. It can also cause the injector to malfunction. Often there are no fault codes because the CDI module has no way of monitoring for these leaks, no sensors measure the return side. Internal injector leakage can cause extended crank, cranking no starts, misfires, rough running, loss of power, and of course, rail pressure regulation problems.

To check injector internal leakage you remove the leak line and pinch it off so fuel will not leak out. There is a Tee where they join to the low pressure system. Also, never run the engine with that line pinched. If pressure builds up in it the injectors won’t be able to vent and this can break the piezo crystals inside of them.

First simply crank the engine for a while and look for an injector that overflows before the others have even filled up. When the injector leakage is checked it’s generally very bad or minimal (normal), so we check for an obvious excessive failure first. If you don’t get a conclusive result you need to run a line of clear plastic tubing from each injector to a jar and crank the engine. Compare the fill of each line of plastic tubing, they should be close to equal. If you see one or two that are really excessive, those injectors need to be replaced. It's common to see more than one failing at a time, perhaps due to fuel contamination. The replacement interval for the fuel filter is 20,000 miles. The injectors have extremely fine tolerances measured in microns, so clean everything before opening it up and try to protect parts from dirt. You can also do this test if the engine is running but has a performance issue. Start it and run it at a fast idle of 1000 rpm or so, compare the amount of fuel. Likewise, look for an outlier. Repeat the test a few times to be sure of your results.

The previous test is a good example of a case where you don’t need the precision of an elaborate and scientific test right off the bat. You can eyeball this one before you go to all the trouble of a proper test. There’s nothing wrong with advanced test procedures but you don’t always need them to be your first choice.

One last note regarding the fuel system diagnosis – the high pressure pump is a timed component. It seems to be timed so that the cam in the pump strokes at the same time that the injectors deliver, resulting in more stable rail pressure. Rail pressure stability is very important in a common rail system because the extremely precise fuel delivery is based on instantaneous rail pressure and injector opening time. Technically, the “rails” are accumulators that buffer the supply against the pressure drops and fluctuations from injections.

In some cases technicians have removed and reinstalled the pump without knowing about timing and it is out of sync. Being out of time can cause noise, but it can also cause unstable and low pressure during starting. The correct procedure for timing and installing the pump can be found in Mercedes’ service information.

Note: make sure to prime the pump after installation by inserting the key and holding it in the run position for 15 seconds. Perform this procedure three times.

Most all of the cranking no start issues we have seen have been covered by these diagnostics, if you have inadequate cranking rail pressure and everything we have discussed has been proven out it is sometimes due to an internally failed high pressure pump. There is no practical way of proving it however. 

If you have a fleet of vehicles with this engine, such as a delivery company with sprinter vans, you can stock the pump (~1800$) and the high pressure rail (~1000$) with pressure regulating valve and use them as substitute test parts, then sell them when they’re needed. The quantity control valve is usually a serviceable part, separately from the high pressure pump and not very expensive. It is open by default though, so it could only cause low pressure if it mechanically jammed in an actuated position which is very unlikely.

The purpose of this guide above all is to help other technicians by providing a comprehensive overview and encouraging the development of critical thinking skills.

Thank you,

Gary Mobile Technician

Excellent write up. Thank you

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Timothy Diagnostician

This is a great write up thanks for taking the time!

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Bob Diagnostician

Andrew, that's a great comprehensive guide to assessing a diesel no start. Many of those steps could be applied to other diesels as well. In fact, some of the steps would also be appropriate for assessing a gas no start. It's always a good idea to utilize all your built in tools such as sight, sound, smell, touch and common sense, in addition to your scope and scanner.

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Martin Instructor
British Columbia

Hi Andrew. The diagnosis above, save for some dedicated steps essentially serves as a good "one-size fits all" guide to most modern diesels and while exhaustive in details, many of the listed steps can actually be performed quickly, non-intrusively and routinely in the service bays. Obviously, depending whether the test point for a gauge is positioned on the pressure or suction side of a system

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Andrew Technician
New York

Indeed Martin, thank you for your helpful and insightful feedback. This guide is of course intended as a general overview of the fuel system diagnostic process as it pertains particularly to no start conditions. I would agree that igniting the fuel sample for testing could be hazardous. I think I did well documenting the wide variety of other hazards and safe working procedures. I always use

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Martin Instructor
British Columbia

Hi Andrew. Regarding the "test point for the gauge", the displayed value will depend on where the test port is located in the supply system from the tank. Given that systems such as Duramax with CP3 injection pumps mostly do not use an in-tank pump to lift fuel to the HP pump, the low pressure pump on the back of the CP3 pump draws fuel from the tank through a filter placed on the "suction" side

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Gil Manager

Thank you for taking the time to give such detailed instructions, will be out to good use Gil

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