 Exhaust gas measurement
As a rule, emissions measurements are no longer carried out these days, only when vehicles are too old or there is an error relevant to emissions. In addition, the readiness codes reveal whether such errors may have been
deleted at short notice. If this is not the case, the emission tester skips this step.
If, for example, an exhaust gas measurement is to be carried out to determine errors, the target values for the vehicle to be tested are of course important. As usual, the visual inspection should be given sufficient time. An
important part of this is the tightness of the system.
The paths and ramifications of a modern exhaust system are long and complex. They not only extend from the exhaust manifold to the tailpipe, they also pass through one or more turbochargers and exhaust gas recirculation
with air or liquid-cooled part of the air supply.
The visual inspection is followed by the conditioning of the device. Residual gases are emitted and the temperatures required for the tests are reached. Measuring the engine temperature, for example, can become critical if
the oil dipstick is replaced by a dipstick with a temperature probe that is too long and gets caught in the crank mechanism.
You have to keep the provided engine speeds for a certain time, then the device goes to the next test. In the case of petrol engines, idling is followed by a check of the control circuit. Some kind of disturbance must be added
on, such as manipulations caused by the supply of unmeasured air. The device monitors how the lambda control attempts to compensate for this deviation within a certain period of time.
In the past, the diesel engine required three precisely prescribed gas bursts from the device, whereby not only the acceleration and the speed to be reached and how long it had to be maintained were important, but also the
speed at which you started. Before free acceleration, the device had to be purged with fresh air.
There have obviously been diesel engines where this type of free acceleration has caused damage. It is no longer carried out when measuring with a probe in the exhaust gas pipe. Nevertheless, with its turbidity
measurement, it was a guarantee for good functioning and, as far as possible, perfect exhaust gas values in practice too.
It is very helpful, especially with petrol engines, to freeze the values in order to track down an error and to be able to compare it with data after the repair. In addition to the engine speed, the lambda value and the CO2
percentage as well as the CO2 are then displayed. This is followed by the rather unimportant CO percentage and then the particularly important HC emission. In the event of irregularities in the lambda
control, this value reacts much more sensitively than that measured by the lambda probe.
The EOBD standard is usually implemented in petrol engines from 2001 and diesel engines from 2004 onwards. This saves the probe in the exhaust gas flow and targeted acceleration. The values are read out and can also
be displayed graphically. In the case of some malfunctions, the onboard control unit also notes the occasion on which it occurred.
Another advantage of this system is that temporary errors are also recorded, which an analogue system would have long since forgotten. Sometimes even the additional conditions are mentioned. The criterion for the
classification as temporary or permanent is the number of engine starts without this error.
With EOBD came the mandatory program of standardized error codes, which was supplemented by manufacturer codes. And then the readiness code has to be changed if a test has confirmed, for example, that an error had
been previously deleted.
| Under no circumstances should the workshop forget the annual maintenance with standardized test
gas. |
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