3.0L SL engine . 3.2L SL engine

[sldan]

What are the technical differences of the two USA spec engines?

I am interested in bore, stoke, injection system, HP, torque, performance, reliabilty, and known problem.

I currently drive a 1991 300SL and am wondering if an upgrade to the newer 320SL is worth the effort.

Dan

[suginami]

The 3.0 liter-24 valve M104 engine is an early version of the 24-valve DOHC 3.2 liter M104 engine used from 1990-1993 in the 300SL, and from 1990-1992 in the 300CE. This 3.0 liter DOHC version of the M104 engine still used the CIS-E fuel injection system, using a distributor with one coil wire and 6 spark plug wires. This is basically a mechanical injection system with electronic control.

The "modern" 3.2 liter M104 engine was used in all 1994-1997 SL 320's, 1993 W124 chassis 300E/300CE/300TE (3.2), '94-'95 E320's (W124 chassis), 1996-1997 E320's (W210 chassis), and 1993-1999 S320's (W140 chassis). Note that all 1993 300E / CE / TE's are misbadged as the engine is a 3.2 liter engine. The badging error was corrected starting in all 1994 models, but only the logisticians at Mercedes have the answer for this one.

The 3.2 liter M104 engine uses a different ignition system - HFM (hot-film mass air flow sensor), fully electronic with integrated electronic ignition and sequential fuel injection. This system combines fuel injection and ignition control in one module. HFM-SFI systems use coils that are mounted directly on the spark plugs, replacing the distributor at the front of the engine. Each coil pack provides spark to two spark plugs at the same time, one connected directly to one plug, and the other with a short high tension lead to the next spark plug. So there are 3 coil wires and 3 high tension lead wires.

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 3.2 liter M104 engines also have variable valve timing on the intake cam, making the torque curve broad and flat, developing HP at a much lower rpm. This makes the power much more useable and noticeable.

The early 3.0 liter 24 valve M104 engines are nice, but the advancement of technology is just so great in the 3.2 liter M104 engine, that if all possible, go with the newer engine.

[suginami]

1990-1993: 228 hp @ 6,300 rpm, 201 lb-ft @ 4,600 rpm.

1994-1997: 229 hp @ 5,600 rpm, 232 lb-ft @ 3,750 rpm.

You can see that the hp only increased by one, but it develops it at a lower rpm.

There is a big 15.4% increase in torque, and it is available at a much lower rpm.

If you were to look at the torque curve, it would also be much be much more flat than the early 3.0 liter engine.

This obviously a function of the variable valve timing.

[sldan]

Hello Suginami,

Great information.

I can clearly the see the benifits of the increased low end torque in the SL320 engine. The 300SL has respectable low end performance but the SL320 must be much more enjoyable to drive.

Thank you for the great information.

I have experienced some drivability issues with my 300SL. If I start the car cold and drive a short distance so the engine temp only gets to 1/2 normal and park the car for a 15 to 20 minutes, the distance from my house to the local hardware store, when is start the car with the engine temp in this mid range the engine stumbles and does not want to move out. I must baby it until the temp gets a little higher then all is fine. This does not happen often or regularly.

Any ideas?

Dan

[suginami]

My best guess (which is worth only $.02) is that it might be a fuel supply or fuel pressure problem. Maybe your fuel accumulator is not holding pressure. The accumulator maintains residual pressure in the fuel system after the engine has been switched off.

Maybe your fuel pressure regulator is not maintaining enough fuel pressure when the car is running. The pressure regulator contains a check valve that works in conjunction with the accumulator to maintain pressure when the engine is turned off.

Maybe your EHA (electrohydraulic actuator) is acting up. It is mounted to the side of the fuel distributor, and constantly fine tunes the air-fuel mixture to compensate for warm-up enrichment, as well as other conditions.

I think your car also has a cold start valve, which provides supplemental fuel to the engine during cold starting. The valve is operated via the fuel pump relay.

What happens when your car is fully warmed up, and then re-start it after a brief stop?

[sldan]

Once it is at normal temp there is no problem restarting and running.

I plan to have the fuel filter changed and when the weather gets better check out a few things.

The engine has new injectors and run great except when this stumbling occurs.

Thank you for your comments and information.

Dan