Vehicle details: Mercedes Benz Sprinter 906
Engine code: OM642
Symptom:
Author: Daniel Black, Marcedes Benz of Reno

Mercedes Benz Sprinter 906 | Lack of power

INTRODUCTION

This vehicle came in with a customer complaint of lack of power; customer’s own words, “feels like it has no boost” and the CEL was illuminated. Using the Xentry diagnostic tool we found two fault codes stored. P14B100 and P190100 were stored in the CDI (common rail diesel injection). We were not able to obtain any specific information on the criteria for setting these fault codes.

We reviewed the freeze frame data and it was unhelpful at identifying the conditions that the fault was set under.

The guided tests were run for both stored codes, with the guided test recommending replacement of the mass airflow sensor or inspection of the harness, as seen in the screenshots below.

We decided to check the intake system for air leaks. During this test two leaks after the mass airflow sensor were revealed. These were rectified with no change in operation. Next, we decided a dynamic EGR test could reveal some helpful information. During this test, the vehicle would occasionally stall and fail the test in all the specified ranges.

At this point, a known good EGR valve was installed for testing and the test was run again with no change in results. With no improvement, we searched for any technical topics related to these fault codes, finding a few cases relating to pins at the CDI for the EGR recirculation positioner (Y27/17) and the EGR recirculation cooling solenoid (Y27/13). The CDI was disconnected and pin tension was checked; all pins were secure and the CDI was reconnected. After this, the vehicle ran better and passed the dynamic EGR test in the upper ranges and the engine would no longer stall.

A test drive revealed that most of the power had returned but was still not at full power and the fault codes would eventually return. At this point, the electronic compression test was run and the results were very telling.

With all but one cylinder failing, we proceeded to a manual compression test. Bank 1 (Cylinders 1-3) were around 18 bar (261psi); bank 2 (Cylinders 4-5) were below 12 bar (174 psi). The wear limit for this engine is 17 bar (246 psi). A leak down test was performed on bank 2 with 8% leakage on cylinder 4, and 4% on cylinders 5 and 6. The extra leakage on cylinder 4 was found to be coming past the injector seal but was not enough to cause this loss of compression on bank 2. With good leak down readings, we were initially at a loss for what would cause a loss of compression.

HYPOTHESIS

After thinking it over, I decided a restriction on the intake side of bank 2 could possibly explain the loss of compression and would still give us good leak down numbers. I decided to take in-cylinder compression readings with the PICO scope to get a better idea of what was going on.

MATERIALS

  1. PicoScope 4425
  2. WPS500X Pressure Transducer Kit
  3. Glow plug adapter

PROCEDURE

  1. Attached WPS500X to cylinder 1 glow plug range 1
  2. Battery maintainer attached to the vehicle
  3. Fuel system disabled
  4. PICO set to 200 ms/div, sample rate 1MS, scale 500 psi
  5. Cranked engine until the speed was stable

DATA

Cylinder 1

The same test was performed on Cylinder 4.

Cylinder 4

RESULTS

Right away we can see a clear difference in peak compression and a distinctive pocket near the intake stroke on cylinder 4; cylinders 5 and 6 were nearly identical. Using the measuring tools and comparing to the known good from cylinder 1, I found some very interesting information revealed.  Looking closely at the good cylinder we can see a peak cylinder pressure of 269.8 psi (18.6 bar). We can also see the intake valve is opening approximately 362.9 degrees after compression TDC.

Comparing this to the cylinders in question, we see a peak compression of 183.2 psi (12.6 bar) and the intake valve opening 422.8 degrees after TDC.

Looking closely at the vacuum pocket we can see that there is a significant vacuum on the bank 2 cylinders, approximately -11.2 psi.

Analyzing what the PICO showed us, in particular, the point at which the intake valve opens, I determined the intake valves on bank 2 were opening 59.9 degrees of crankshaft rotation (29.95 degrees at the camshaft) later than the intake valves on bank 1. This created the large vacuum and the reduced time to fill the cylinders resulting in reduced compression.

At this point, no major disassembly had been performed and we decided it was time to confirm the cam timing. Upon checking base timing, we confirmed bank 1 was in time and the intake cam on bank 2 was out of time (not the exhaust cam). As you can see in the picture below, the intake cam lobe for cylinder 4 (viewed from the rear) should be at a near parallel with the cylinder head surface. A rough estimate shows it is approximately 33.9 degrees off; we estimated 29.95 degrees with the PICO scope.

Now for the root cause of this failure- the intake camshaft on bank 2 drives the high-pressure pump via a gear.  The intake camshaft on this engine is gear driven by the exhaust camshaft; as a result, it spins counterclockwise. The drive gear on the exhaust camshaft is friction welded in place. We suspected the high-pressure pump had locked up at one time causing the friction weld to slip on the exhaust camshaft thus resulting in a loss of intake camshaft timing. Once we removed the high-pressure pump, we could feel a distinct difference in the effort to turn the pump compared to a known good pump. Comparing the exhaust camshaft to the new camshaft, we could clearly see the change in relation to the cam lobe and the timing marks on the gear. I should note no gasoline could be detected in the tank and a sample of the diesel was sent out for analysis to determine the cause of the high-pressure pump failure.

CONCLUSION

The loss of airflow into the engine due to the cam timing was causing a loss of power, causing the EGR dynamic test to fail and setting the air mass codes. This test not only revealed the low compression but clearly showed us the root cause was in the mechanical timing. With one test we were able to pinpoint the cause and save countless hours blindly tearing this engine down. It would have been very possible for a technician to miss that the cam timing was off upon disassembly as well, further resulting in lost time.

Comments

3 comments | Add comment

Jesse Weston (Mercedes-Benz of Arrowhead)
November 25 2019

Great write up! I will be using this as training in out shop. Thank you

ED Aulestia
November 25 2019

This is a very good article and I can see your knowledge,  just a quick question, how can you tell or see the intake valve opening on cylinder #1 this is a diesel engine with no throttle plate and no engine vacuum, so it will be very difficult to pinpoint other than going back to valve timing specs and calculating when the valve starts to open.

BOGDAN BODNARESCU
November 25 2019

Amazing conclusion, but this looks like a design flaw? How it is possible that the Cam is only friction welded?

Your email address will not be published. Required fields are marked *

Case study: Lack of power