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  • Writer's pictureTeam Vebo


Updated: Aug 5, 2021


Part 1 : Introduction and Electrical Component Troubleshooting

The repair journey starts when Mr Martin(not his real name) came to us with his sick BMW 5 Series (F10). Martin already made multiple trips to his mechanic friend to sort out his F10 issue but unfortunately, despite changing all his spark plugs and coils, his engine is still vibrating badly. Martin’s good mechanic friend unfortunately are unable to continue to work on his car causing him to leave the car at home for many months.

Video 1: BMW F10 Sick Engine Startup Sound

Turn on audio and watch the video above to hear the sound of the sick N20 engine inside Martin's F10.

When Martin reaches our workshop with his car, we immediately started some basic troubleshooting by performing a vehicle scan to check for error codes and started data streaming from his F10. True enough, we noted that the computer pointed out misfiring issue on piston No. 1. We then swap the coil No 1 with No 2 to rule out coil failure. The engine is still vibrating badly and the computer is still showing error on piston No. 1.

We then decided to swap out the injector with a test unit and the car is still vibrating badly. This basically ruled out a bad injector as well.

Then we performed a manual check to see if the spark plug is actually sparking and it does, so we know that the spark plugs are ok. A gap test is also performed at the spark plug to ensure it meets the exact manufacturer’s specification. We then proceeded to check the wiring connection from the coil to the ECU to ensure that it is not compromised. For the benefit of our less technical readers, ECU stands for Electronic Control Unit. The ECU is basically the brain of the car that controls how the car works. Everything looks OK so far. However, while looking at the ECU, we noticed there is a smudge on it.

Figure 1: ECU had been tempered

Figure 2: ECU in its original condition

So, this raise the first question,

Why would the ECU be compromised?

As we continue to probe into this matter, we reached out to Martin to understand the history of his car. According to Martin, he purchased the car used. During the course of his ownership, car had never been tuned before and he is not aware that the ECU had been tempered with. He was really surprised when we show him a photo of his tempered ECU.

Based on our investigation, the ECU in this F10 was designed so that it is temper proof. Our partner ECU specialist takes a lot of time and patience to pull the ECU apart in order to reach the pin for data access.

Due to this, many workshops that perform ECU modification would normally just drill a hole on the cover to access the pin. Why would they want to access the pin? It is to allow the ECU to be flashed. ECUs are generally flashed for repair purpose or to remap the original factory data to squeeze additional performance from the engine. So, this would explain why the smudge is on Martin’s F10. The smudge is basically a sealant that closes the hole after the ECU modification.

Is the ECU still OK despite the modification made? Based on our current test results of the relevant electronic components, the ECU still functions as it should.

Based on the test results above, we’re quite convinced that the misfiring is not due to the electronic components. This basically brings us to the next level of investigation where this issue could be mechanically related and can be more complex. Subsequently, we would perform an engine compression test which will tell us how healthy the current engine is mechanically.

Part 2 : It’s Time To Test The Mechanical Components

At the end of part 1, we concluded that the electrical components are fine. So, the next test we have in mind is the engine compression test. But, what is an engine compression test?

A compression test reveals the condition of your engine's valves, its valve seats, and piston rings and whether these parts are wearing evenly. Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings.(1)

Video 2: Compression Test

The video above shows the compression test on the first piston.

The full test result is as the following:

  • Piston 1 : 50 psi

  • Piston 2 : 125 psi

  • Piston 3 : 186 psi

  • Piston 4 : 186 psi

With this, we can conclude that Piston 1 fails the compression test as it is way below 100 psi. Our sharp readers will also notice that piston 2 has a lower reading as well. More on this later.

Three things are required in proper combination before ignition and combustion can take place---Heat, Oxygen and Fuel. (2)

  1. There must be Fuel to burn.

  2. There must be Air to supply oxygen.

  3. There must be Heat (ignition temperature) to start and continue the combustion process.

So why does the piston misfire when the compression is lost?

Easy way to understand this is that, it couldn’t complete its combustion when the 3 keys component is incomplete. This is detailed below:

  1. No combustion when there is no spark (This explain when spark plug failure or coil failure),

  2. No combustion when there is no fuel (this explains when injector failure)

  3. No combustion when there is insufficient air to supply oxygen (this explains when compression failure)

To explain why a car misfires when one of these components is affected, we would like to go back to basics on how a 4 stroke engine works.

Figure 3: 4 engine strokes (photo credit

Intake Stroke :

Air/Fuel mixture is drawn into the cylinder via the intake valve when the piston moves down.

Compression Stroke:

Air/Fuel mixture is compressed when the piston moves up.

Power Stroke:

Spark plug lets off a tiny spark and it ignites the compressed air-fuel mixture. Since both valves are closed. The flame has no place to escape and pushes the piston downwards.

Exhaust Stroke:

Piston moves back up and pushes the exhaust out via the exhaust valve.

Going back to our F10, the ideal compression cannot be achieved during the compression stroke due to loss of pressure. This is quickly detected by the ECU which instantly cuts off power to the spark plug and fuel from the injector in order not to flood the combustion chamber which can be dangerous.

Hence, we concluded that the engine is running with 3 pistons instead of 4, causing the vibration.

Why is compression lost at Piston 1 and 2?

This raises a very good question because we only saw misfiring on piston 1, but why would piston 2 have lower compression too? Are both linked in this issue?

So we started probing this deeper by getting more history from Martin, as he claimed that he was driving this car for nearly 4 years, so his knowledge on the car history would be useful to guide us on this investigation.

We raised the following questions and here’s his feedback.

  1. Has the car engine overheated before? Answer - No

  2. Have there been a mixture of coolant and oil in the cooling system or engine before? Answer - Yes in the coolant system and it was fixed by replacing the oil cooler.

  3. Have the car been driven in the flooded area before? Answer - Yes in a brief moment however nothing happened to the car as the water level wasn’t high enough to stop the engine (meaning no hydrolock on the engine before)

  4. Does the car's coolant level drops irregularly over time? Answer - Yes, Martin regularly top up the coolant after driving the car over a period of time. (this can be a good clue)

After reviewing his answers, we found some hints which leads us to suspect that the head gasket must be blown(not a good thing). This causes the compression from the piston to go into the coolant system which then causes the coolant to overflow while driving.

With this lead, we have decided to remove the head of the engine and check the condition of the head gasket.

Part 3 : Looking At The Head Gasket Condition

Removing the head of an engine is rather complicated, especially for modern engines where it is supported by a valvetronic and turbo system such as the N20 model. Below are most of the components that are required to be removed before we can access the head screws.

Figure 4: Engine Components

During the removal of all the components on the head, we will try to observe any abnormalities along the way. When removing the cylinder head, one of the main items we check is the head screw’s torque. This is to isolate the potential issue of pressure loss due to loose bolts. All good with Martin's N20, so no issues here.

When the cylinder head is removed, we can clearly see the condition of the head gasket and surprise! It is indeed that the head gasket had blown.

Figure 5: BMW N20 Blown Head Gasket

In Figure 5, we can see that there's a bridge between cylinders 1 & 2. Compare that with cylinders 2 & 3 where it is still in a good condition.

This explains why both pistons 1 & 2 lost compression during the pressure test.

However, does changing just the head gasket resolves this issue? It may not be as simple as that.

Our argument is that, if there is a bridge between cylinders 1 and 2, why would cylinder 1 have a much lower compression when compared to cylinder 2. It is lower by more than 50% which doesn’t make sense.

So, our investigation continues to probe deeper where the next thing to check is the piston ring itself.

The Piston Ring

There are two symptoms when the piston ring starts to fail where lots of metal residue will reside in the oil sump or even scratches on the cylinder liner on the engine block.

Video 3 : Martin's BMW F10 Oil Sump

We checked Martin's F10 and his oil sump is clean and there are no visible scratches on the cylinder lining.

This is a good sign, but the results also gotten our brain thinking again…what about the pistons itself? We need to get behind the issue of uneven compression reading. There has the be something wrong with the pistons to cause this. So, we need to remove the pistons to check the condition.

Part 4 : The Piston

Before removing the piston, we need to carefully remove the oil pump drive chain and the entire oil pump counterbalance shaft.

One by one, the pistons are removed and we anxiously check each pistons for defects with a focus on pistons 1 & 2. The piston rings remain intact but viola! Piston 1 is cracked!

Have a look at the images below to see the crack.

Figure 6: Piston 1 Crack (Exterior)

Figure 7: Piston 1 Crack (Interior)

This finding concludes the reason for the lower cylinder 1 compression when compared to cylinder 2.

Lets study the piston symptom

Referring to Video 1, it is clear that the engine was experiencing detonation or in other words, engine knocking.

"Knocking (also knock, detonation, spark knock, pinging or pinking) in spark ignition internal combustion engines occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive." - Wikipedia

From the definition above, it is clear that the pinging sound is a result of abnormal combustion which results in detonation.

There is a lot we can study from the failed piston itself.

Figure 8: Piston 1 vs New Piston Sample

Referring to Figure 8, piston 1 is pitted while a brand new piston has a nice smooth surface. This can be caused by detonation.

So what is detonation?

"When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an almost instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket, and the cylinder pressure will rise sharply – and possibly beyond its design limits – causing damage. (Detonation is actually more efficient than deflagration, but is usually avoided due to its damaging effects on engine components.)

If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. The simplest deleterious effects are typically particle wear caused by moderate knocking, which may further ensue through the engine's oil system and cause wear on other parts before being trapped by the oil filter. Such wear gives the appearance of erosion, abrasion, or a "sandblasted" look, similar to the damage caused by hydraulic cavitation. Severe knocking can lead to catastrophic failure in the form of physical holes melted and pushed through the piston or cylinder head (i.e., rupture of the combustion chamber), either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system. Hypereutectic pistons are known to break easily from such shock waves." Wikipedia

It is clear that incorrect air fuel ratio is causing this abnormal combustion. Remember the compromised ECU? Could this be a result of a botched remap work that slowly kills the engine over time? Remember that when remapping the ECU, the tuner plays around with ignition timing, air/fuel ratio(AFR) and boost pressure. With the increased performance, this may kill the engine.

Let's look for more clues from the piston.

Clue 1: Piston Crown Damage

Figure 9: Piston Crown Damage

As you can see from Figure 9 above, that piston cylinder wall is totally black or burned when compare to the brand new piston and piston 2. Piston crown damage can be caused by the following (4):

  • Overheating due to combustion defaults

  • Bent/blocked oil injector jet

  • Installation of incorrect pistons

  • Malfunctions in the cooling system

  • Clearance restriction in the upper sliding surface area

Clue 2: Dry Running/Fuel Damage

Figure 10: Dry Running/Fuel Damage

Figure 10 above shows that piston 1 is more worn when compared to piston 2. Dry running/fuel damage can be caused by the following (4):

  • Over-rich engine running

  • Combustion defaults(misfiring)

  • Insufficient compression

  • Defective cold-start device

  • Oil dilution with fuel

Clue 3: Cracks In The Crown and Crown Bowl

As we've shown previously, piston 1 is cracked.

Figure 11: Piston Crown Cracked

The piston crown can be cracked by the following:

  • Faulty or incorrect injection nozzle

  • Incorrect injection point

  • Incorrect quantity of injected fuel

  • Insufficient compression

  • Lack of piston cooling

  • Installation of pistons with incorrect bowl shape

  • Improvement in performance (e.g. chip tuning)

Part 5 : The Conclusion

Based on the test results and observations above, there are enough clues that allows us to deduce some assumptions that causes this catastrophic failure but can we actually pin down the actual root cause of such failure?

It is clear that detonation is driving such failure. But why it didn’t fail earlier but after 4 years of ownership? This answer can be answered with another question which is why only piston 1 having such failure when compared to the rest?

This is our assumption:

  1. The car's ECU was mapped to a performance setting which is likely to reduce the fuel and increase the air (increasing the boost) and the engine able to give more power out by running lean mode.

  2. This setting increases the wear and tear on the engine and also runs at the high operating temperature.

  3. There is a possibility of injector 1 nozzle failing.

  4. Finally piston 1 started by cracking at the crown and causing the leak on combustion which ECU had no choice but to disable the piston 1 from running. Thus misfiring on piston 1 becomes permanent.

Team Vebo proceeded to replace the faulty piston 1 and rebuilt the engine. All necessary gaskets(including head gasket) are also replaced in the rebuilding process. The different engine components are thoroughly cleaned to remove contaminants.

When rebuilding the engine, it is important to ensure that the correct torque is applied on every screw point. The video below shows how this is done using a torque wrench.

After the rebuilt is done, we proceeded to perform the compression test again.

This time all pistons are showing a consistent 200 psi reading. We then proceeded to run the first engine startup. Check out the video below.

The first start-up was successful and we continued with short and long distance drive test. After a few adjustments, we then proceeded to handover the car to customer. As a conclusion for this case study, we noted that a modified ECU may contribute to the engine running in excessive high temperature. The high temperature will then causes certain OEM components (like piston 1) to eventually fail.

Thank you for reading!



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