N62 with LPG. Unsolvable problems

The owner of BMW X5, E70 approached me. For his car with an N62 engine, the propane gas equipment was installed. After this installation, the range of problems started. After 20’000km the engine was not usable anymore.
The engine was shivering, the misfires could be felt clearly. A strong vibration can be felt. The engine doesn’t give full power. In the error message memory, the error messages regarding the fuel mixture, Lambda probes and misfires started to appear.

The specialist, who installed the gas equipment claimed that the work has been done in high quality, so the owner tried to solve the problems with the engine. Both spark plugs and ignition coils were checked and replaced. Even the compression on cylinders has been measured because the repair gave no results.

Datasheet of compression gives no suspicious indications.

For the customer situation looked hopeless. Could it be the non-repairable engine again? What happened to the trusty work-horse N62?

When receiving the car, at first I checked the error messages. Of course, they have been deleted numerous times (also directly before giving the car to me) – as if it would help in any way. Actually, it’s directly opposite – deleting of the error messages makes the solving of problems much more complicated. But is as it is.

For now the only error message – fuel mixture problems in idle in one (2nd) of the banks.

Here, how the idle of the car looks like, with a warm engine, around one minute after starting the engine.

What does the indication of, for example, cylinder No.1, means? Its mechanical efficiency is around 40% below the norm! Why? Either there is an incorrect amount of fuel or air. DME has not to succeed to solve this problem, using adaptations and corrections. Or – DME does not even try to solve this problem for some reason.
Significantly different mechanical efficiency of cylinders creates vibrations and reduces the power of the engine. Cause of the efficiency difference – incorrect fuel mixture in cylinders: cause of misfires, an overload of CO catalytic converters, the error messages regarding Lambda probes, fuel mixtures, etc.


Let’s look further.

Basic data of the fuel mixture of the banks:

And here is the confirmation of the recorded error message – the offset type LTFT in the second bank is very incorrect! Quite close to the critical level!


What has happened to the engine?
No matter how shocking it sounds – nothing! The engine is in perfect order! Can it not be?
Here, the next image.

Here, the data of the performance of cylinders several hours later, when processing the “repair” works. As we see, the data of cylinders of the 1-st bank are perfect! With the 2-nd bank the engine still “works” (but also the efficiency of cylinders of the 2-nd bank differs from perfect not more than 10 .. 15%), below in the entry you will find a video with a result of “work” of DME after finishing the service procedures.

Here, idle after 10 minutes.

Vibration is gone completely, the engine runs perfectly even, no sign of misfires! How the engine was “cured” during one hour without replacing any of its components, and why it “got sick”?


At this moment I will try to explain the essence of the problem very shortly.
To ensure perfectly smooth and even performance of the engine, in addition to a “regular” correction of the fuel mixture, the BMW engine management system performs:

  • measurement of mechanical efficiency of each cylinder in idle; if any smallest deviance from ideal is detected, the adaptation maps for idle of the exact injector are modified;
  • when driving, after a certain time (or – immediately, if the engine management detects any problem, for example – uneven performance, loss of power, misfires, etc), the tests of chemical efficiency of each cylinder are performed, and as a result the adaptation maps in the current circumstances of each injector are corrected (temperature, RPM, torque, etc);
  • of course, the adaptation maps of all basic components (throttle, air mass meter, Lambda probes, Valvetronic, fuel tank ventilation, EGR, fuel pump, etc) are regularly (or immediately if DME spots any mismatch) created and modified.

If the LPG unit is connected, the engine management system actually has a “ghost” component set – other injectors, via which the other fuel flows, with other pressure, with other physical properties! Stock DME does not know it – without reasons, understandable to it, the whole energetical model of the engine and parameters of injectors break down. Suddenly the fuel amount, efficiency of each cylinder, torque of the engine, detonation data are different! DME starts the emergency procedures to fix the situation – measures the fuel pressure, measures injectors, tests Lambda probes, tries to calibrate them, in an emergency mode starts to test air mass meter, calibrate throttle, test EGR and fuel tank ventilation mechanism.
The precision of the pressure maintenance system of the LPG units is not high: +/- 10 .. 15% or even vorse. With a guarantee – DME will detect an incorrect fuel mixture, even if the LPG system will be calibrated perfectly. Here we have to mention one more problem: during calibration of LPG unit (in the installation process), stock DME “resists” to this procedure – actually the correct creation of LPG adaptation maps is NOT POSSIBLE even theoretically! As soon the operator (or the software) increases the amount of LPG, stock DME reduces it and vice versa, immediately modifying the reference values in the adaptation maps. Everything is fine with gas adaptations, but the petrol adaptation maps are destroyed!
With the adaptations of the injectors, the situation is even worse – when switching to the gas, DME detects, that the injector data are completely inappropriate; it records the error messages regarding injectors or the fuel mixture and switches to the emergency mode. Also, the DME doesn’t create adaptations of injectors or any other important hubs, the performance of the engine regress during the time. The problems are only increased during the time.


These problems can NOT be solved by :

  • deleting the error messages;
  • long driving only with petrol or only with gas;
  • deleting of DME adaptations.

To restore the normal performance of the engine, the re-adapting has to be performed. Re-adapting is a collection of specific actions, which takes approximately one hour. The opinion of the “specialists” – it is enough to simply delete the adaptation. No, after deleting the adaptation, a CORRECT creation of new adaptations (flywheel, fuel ventilation system, fuel mixture, injectors) need to be done. If it is not done, the result will be missed.

I hope that I managed to explain the essence – implementation of an additional “ghost” (LPG) system completely paralyzes the performance of DME and creates a lot of other problems.


What other problems are caused by the LPG unit?
As the fuel mixture in cylinders is not optimal, a large amount of fuel gets in the exhaust and is burned in the CO catalytic converters. The catalytic converters are overloaded and damaged. Misfires only worsen the situation, also, they damage the Lambda probes and other exhaust sensors.


How the LPG unit creates additional expenses?
When spotting the error messages regarding Lambda probes in the DME memory, many repair ”specialists” will replace the probes and air mass meters. Of course, without any justified reason! When spotting the error messages regarding misfires, they will replace spark plugs and ignition coils, when it will not help – they will replace injectors. Of course, without any justified reason! Finally – due to incorrect adaptation the performance of the gas unit also collapses. The installers of the gas unit will replace the gas injectors, pressure sensors, valves. And again – without any justified reason!
When reading, that the uses of gas units replace the injectors after several tens of thousands of km, how they continuously discuss, in which fuel station the “best” gas can be purchased (and what is more interesting, the opinions are radically different, and these “good” gas stations regularly change their location), I have no slightest doubt – the engines of their cars work in complete emergency mode! Nor gas, neither injectors of LPG unit in 99% cases have no problems!
And more – due to incorrect fuel mixture, part of the spark plugs (the ones, in which cylinders the fuel mixture is enriched), are “clogged”, it causes misfires and significantly reduces the work-life of the spark plugs.

Here, how the idle of the injectors look after finishing the service procedures.
As we see, the real performance of all cylinders is for only several % different from ideal and the bar fluctuates around the ideal value. So – DME has managed to even the performance of the engine completely! A perfect result!

Let’s see the fuel adaptations:

As we see, offset type LTFT:

  • is practically identical for both banks (so – there are no problems with a separate injector);
  • they are much closer to 0 (ideal) values. During several tenths of motor hours, they will close to 0 even more.

The control probes indicate a correct voltage, which means:

  • wideband (main) probes are calibrated correctly;
  • the fuel mixture is correct not only in the range of bank but also in cylinders;
  • misfires – not observed.

Here, a miraculous “recovery” of the engine! Of course, the whole effect will be ruined, once the LPG unit will be turned on. Suggestion to the customer – get rid of the LGP unit.
During the next 3 .. 5 driving sessions (both for the warm and cold engine) after starting the engine – leave the car in idle for 5 minutes for DME to measure the parameters of the injectors. And yes – for some moment the performance of the engine can be uneven in some modes, while the adaptation maps of injectors and other components will be restored. After 200 .. 300 km: visit the service repeatedly, check the newly made adaptations and the error message memory of DME.


In the end.
Gas burns worse than petrol. Every technical person knows that. To solve this problem, the gas “specialists” install a “gas” spark plugs, which ensure as if more powerful spark.
This “solution” has the following side-effects:

  • increased load for the ignition coils and regular damage due to overvoltage;
  • increased load to the DME coil management switches, it can damage also DME;
  • the spark plugs have other parameters (also – the temperature class and work conditions) – the spark plugs don’t clean correctly. Also, an increased spark gap can cause (and will cause) misfires, for example, for a cold engine, while working with petrol.
    Unfortunately, such “upgrades” can lead to the replacement of all spark plugs and ignition coils (to the ones, recommended by the manufacturer). The ignition coils, which are under increased stress, can “quit” at any moment – accordingly: to reach a quality result, the ignition coils have to be replaced, even if they are as if working in the moment of the test.
    The situation with an indication of misfires in the current case is harder because the engine has a Bosch management system. Why so?

At one time, Siemens got a patent that provides that the misfires of the engine are detected by the speed variations of the flywheel. This method works in almost all modes of the engine performance (only in conditions of swift torque/RPM changes the detection of misfires can be difficult);
Bosch got a patent that provides that the misfires are detected, using the accelerometer. As we understand, this method will not work in idle.
Accordingly, if the engine has misfires directly after starting, the Bosch management system will not identify the damaged cylinder! This aspect can lead to additional expenses to the owner of the car – the search for the damaged ignition coil is longer or several or even all ignition coils of the current bak will be replaced.