Mini Cooper Engine N14 Weak Piston Issue (Myth)
Mini Cooper Prince Engine N14 Weak
Piston Issue (Myth)
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Here are some items that we have found cause piston failure. please feel free to contact us if you have any questions or need more information.
If you own a Mini Cooper N14 you may have read or heard of the myth that N14 engine has a weak engine or a piston cracking issue. We personally don't agree with everyone's opinion that the internals on the N14 are week. it is a miss understanding due to a lack of knowledge of the MCS engine, turbocharger, and emissions system. I am not saying that they do not understand the engine they just don't have a full understanding of the entire system and how it works together. If you understand the actual issue you will be able to keep your car running longer and at higher boost pressures without these type of problems.
First there are two main problems (and we are not talking about the fuel pump or timing chain guide issues) with the N14 that has lead to the myth that the N14 engine has bad pistons or is week. The first issue is the rear crank case ventilation. As you may know due to the design of the rear crank case ventilation system on the Mini Cooper N14 07 to 10 (or to 12 for the JCW) the engine is susceptible to pre-ignition (knock). This engine has a rear crank case ventilation system that vents right into the combustion chamber directly through the intake. This is absolutely the worst place ever to vent hot oil vapors on direct injection engines. The reason is because the oil vapors build up on the hot intake valves and block the air flow.
In as little as 15k miles the valves can coke up with oil and block the airflow into the engine. This blockage leads to an uneven distribution of air flow into each combustion chamber and this is one of the worst things that can happen to a turbocharged engine. Not only do you lose power because you now are getting more air in one combustion chamber than the other but even worse the air to fuel mixture is not correct and it leads to pre-ignition.
This is the first reason that has lead to the myth that the pistons are weaker in turbocharged N14 engines than the N18 engine. The problem is not that the pistons are week it is actually the improperly mixed fuel due to the air being blocked by dirty valves. This improper mixture of fuel in the combustion chamber can cause pre-ignition which can, will, and does cause pistons to crack.
The second reason for the myth that the N14 is a weak engine or the pistons are weak iscause by per-ignition and worse hydro-lock in the combustion chamber as the piston tries to compress liquid oil.All of this is an effect of back pressure caused by the location and design of the exhaust systemdown pipe.
Unfortunately the location of the turbocharger is at the highest point in the exhaust systemand that turnsthe turbo into a heat sink for the heat in the exhaust. All that heatin excess of 1500deg f is funneleddirectly in to the turbocharger and cooks the oil (oil burns at 200f) in the turbocharger after the engine is turned off.Your Mini cooper does turn on an axillary pump to prevent the oil from burning but it only runs for 5min after turning off the engine. 5mins is notlong enough to cool the turbine assembly to temps that wont burn oil since the air in your exhaust is in excess of 1500deg F and that takes about 10 to 20 minutes to dissipate the heat.
Here is what actually happens to the turbocharger when heat soaks into it after the engine has been turned off. In the exhaust side of the turbine assembly there is a pressure seal (what people call an oil seal which is a miss-understanding there are no oil seals in a turbocharger just a cavity that allows the oil to flow out faster than it flows in, See figure 1).
Figure 1, Exhaust Turbine and Exhaust Pressure C-clip Seal
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This seal fills a gap on the turbine shaft that holds a c-clip that when inserted into the turbo bearing housing is compressed into a groove in the turbine shaft. It doesn't touch the turbine shaft once in position it just fills the area in the shaft to prevent exhaust gases from getting into the engine through the turbocharger.
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If the turbocharger is exposed to heat (after the engine is off) oil will burn on the turbine shaft. Over time the burnt oil builds up in the turbine pressure seal C-clip groove and eventually locks the C-clip in place on the shaft (see Figure 1). Once this happens the C-clip will start spinning with the shaft and as it spins the shaft will wobble. This wobbling motion will compress the C-clip into the turbine shaft opening up the back of the turbine assembly to the exhaust. This allows exhaust to get into the engine via the turbo when the car is under full boost and pressure builds in the engine as the exhaust enters through the turbo (see figure 2).
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Figure 2, Exhaust Coated Turbine .
When you let off the throttle the pressure in the exhaust decreases and the pressure in the engine escapes through the turbocharger back in to the exhaust carrying large quantity’s of dirty oil with it.
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When oil enters the catalytic converter it blocks the air flow due to all the particles in the oil and creates back pressure. This “back pressure” pushes the turbine shaft into the thrust bearing causing excessive wear, pushes more oil into the exhaust, and/or into the intake piping (See Figure 3).
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Figure 3, Thrust Bearing With Back Pressure DamageEventually due to the thrust Bering wear the compressor touches the compressor housing which stops the turbine from spinning or worse completely breaks the turbine shaft.
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However before the turbo is damage by the back pressure the back pressure pushes oil into the intake system through the compressor side of the turbo causing other issues. In vapor form it can cause pre-ignition or detonation, and which can and does break the pistons on a turbocharged car. But when liquid oil makes its way into your engine it will enter the chamber increasing the compression ratio or prevent the piston from reaching TDC and it does cause pistons to crack.
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However before engine damage the extra oil raises the EGTs making the catalytic convert more efficient and the extra heat cracks the turbine housing and manifold, and burns valves (see Figure 4).
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Figure 4, Exhaust Turbine Damaged by Back Pressure Damage EGT Heat.
The oil can also cause hydro-lock which causes head gaskets to prematurely fail and pistons to crack. Not to mention that it can damage the map sensors, throttle plate (N14 only), and fill and gum up the inter-cooler with oil. And sometimes the only symptom is high oil consumption (~1qt per 1000 miles), engine miss fire, low power, and boost. However usually there is no noticeable symptoms and absolutely no catalytic efficiency check engine light.
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As you can see just one of these issues can cause serious problems and damage to your engine but with the combination of them both the pistons don't stand a chance. These issues are the primary reason that lead to the misunderstand and myth that there is an issue with the pistons. But these problems are so easy to avoid just by following some simple steps. To prevent back pressure do to turbocharger failures just, idle your car until the EGTs get below 900deg F (about 5 to 10 min, EGTs can be read off of Bank 1 sensor 2 on the MC N14 and N18 engines), and to prevent the valves from coking up with oil walnut blast them and install a cap to block off the rear crank case ventilation. Once the rear crank case ventilation is blocked off they will not likely coke up with oil again as quickly.
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