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The Mercedes-Benz M103 was a straight-6 15 degrees inclination to the right; automobile engine produced from 1986 through 1993. It was a single overhead cam design with 2 valves per cylinder. The M103 was replaced by the DOHC M104 starting in 1990.

The 3.0 L M103 (2960 cc)(181 cu in) .981 version produced 188 hp (138 kW), 179 hp (132 kW)(with catalyst version) at 5700 rpm and 260 Nm, 188 ft·lbf (254 Nm) (with catalyst) at 4400 rpm. It used the CIS-E (Continuous Injection System - Electronic) and had a bore of 88.5 mm (3.48 in) and a stroke of 80.2 mm (3.16 in). The compression ratio was 9.2 : 1 with a redline of 6550 rpm in 1987 and 6200 rpm from 1988 on. The firing order was 1-6-3-4-2-5. Lubrication system was pressure circulation lubrication system. Amount of oil in engine was 6.0 litres and amount of coolant is 8.5 litres. Number of valves is 1 intake, 1 exhaust with V-shaped overhead configuration. Valve operation was 1 top camshaft. Camshaft drive was simple roller-type chain. Starter motor was electrical with 1.5 kW; since 01.88 was 1.7kW. Ignition system was electronic ignition sysem. Recommended fuel octance for the M103 is 91 RON/MON (96 RON)(86 MON). This engine was found in the W124 E-Class, W126 S-Class.

A smaller 2.6 L M103 (2597 cc)(158 cu in) .941 version of the M103 produced 166 hp (122 kW) and with catalyst is 160 (118 kW) at 5800 rpm and 228 Nm at 4600 rpm, with catalyst is 220 Nm at 4600 rpm. It also used the CIS-E engine management system and had a bore of 82.9 mm (3.26 in) and a stroke of 80.2 mm (3.16 in). The only technical data changes from the M103 3.0 L to the M103 2.6 L was the bore and the stroke. This engine was found in the W201 Mercedes-Benz C-Class/190 and W124 E-Class.