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Old 12-21-2003, 07:58 PM
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Gilly Gilly is offline
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Join Date: Mar 2001
Location: Evansville WI
Posts: 9,616
I didn't include the early 3.0 liter M104's on the 300CE's, and the 1992 300SE because they are not HFM-SFI. The early M104 used the old mechanical CIS-E fuel injection, and the 3.2 liter in the 1992 300SE was an LH system, used only for this one year.

I believe it makes a difference in the long term reliability of the engine.

The HFM-SFI fuel injection system (hot-film mass air flow sensor) is fully electronic, with integrated electronic ignition and sequential fuel injection. This system combines fuel injection and ignition control in one module.

HFM fuel injection systems are designed so that idle speed can't be adjusted. Idle speed is completely controlled electronically. This HFM injection system also has adaptive technology that compensates for conditions such as engine wear and unmeasured intake air and is designed to maintain driveability as the engine ages.

HFM-SFI can retard engine knocking to just the knocking cylinders, unlike EZL technology, which retards spark timing across the entire engine. This keeps the ignition timing point as advanced as possible for maximum power output.
The only difference between LH and HFM systems is the type of air flow sensor (LH= Luft Hotwire, or just "hotwire" to most folks vs HFM= Hot film, unsure where the "M" fits in. Management? Dunno for sure.) Neither of these can in any way compensate for unmetered air leaks, ie leaks downstream of the mass airflow sensor. Not even to this day can this be detected and compensated for.

The statement regarding the ignition system being integrated into the fuel injection control module is incorrect, this did not take place until after the HFM system was discontinued. The next system, the Bosch ME1 system, finally integrated the ignition system to the fuel injection system.

I would also state that as far as idle speed compensation, although the LH and HFM systems began to integrate electronic accelarators (EA) that it is no simple matter to adjust the idle speed on the CIS cars either (electronic idle control valves are not adjustable).

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