As to theA/C noise, the sound of refrigerant passing through the expansion valve is audible to most people. This can't be helped. The system in the new cars use a variable displacement compressor to control evaporator temperature rather than cycling the compressor as previously. During constant spped driving with constant heat the hissing of the expansion valve should be constant with this new system. The flow is varied by the variable displacement to achieve a constant pressure in the evap just above the freezing point of water.

Hard to say about the performance problem. If what you are feeling is any form of engine volumetric efficiency problem it should be easy for a tech to investigate. Part of the modern diagnostics is a funtion that allows the tech to view the relatively misfire calculation for each cylinder. Due to federal law the identification of misfires became a legal issue. The mechanism for misfire detection is to watch the speed of the flywheel.

After each firing event the crankshaft is accelerated to a point where it starts to slowing down until the next firing event. At a constant engine speed the crankshaft speed actually can be plotted to be a sine wave of sorts (acceleration and deceleration rates being different keep it from being a perfect sign wave.

Deviation from the average speed is used to proclaim a misfire once an arbitrary threshold is crossed. The diagnostics gives a bar graph representation of the speed differencial per cylinder. Whereas the threshold may be a number like 250 the standard deflection of a smooth engine often shows one cylinder with a deflection of 1-30 say. Way less than the misfire criteria but often enough to suspect a cylinder if the deviation are not random.

With this as a basis, the diagnostics allow the technician to reduce or add fuel in 7% stages up to over 25%.. If say the engine was running lean on one cylinder but the misfire ccriteria was far from crossed, a normal bar graph reading may show that cylinder with a higher deviation than the others. By reducing the fuel to all the point for lean misfire will be reached on that cylinder before the others. When this happens the misfire threshold will also be passed and if normally occuring a P030x code would be set (x being the number of the cylinder in question). This form of test finds those mixture irregularity misfires that are due to vacuum leaks or leaking or restricted injectors.

The diagnostics also allow timing to be altered similarly while misfire (crank speed) deviations are monitored. Sure wish there was such capabilities on K-jet cars.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician