PeachParts Mercedes-Benz Forum - View Single Post

G-Benz · #4 08-23-2009, 07:53 AM

Running "rich" or "lean" are terms implying fuel-to-air ratio when the mixture is in the combustion chamber (where the pistons do their thing). The optimum ratio is 14.7:1 air-to-fuel, or 14.7 parts air to 1 part gasoline. The engine management system strives to maintain this ratio and makes all sorts of adjustments to do so. The emissions system (O2 sensor) monitors the exhaust output and tells the management system how well it's doing based on what is sniffs out.

A "rich" condition is when too much fuel is being used in the mixture. The symptoms are poor gas mileage, engine miss, and the smell of raw gas out of the exhaust pipe. This will foul spark plugs and the O2 sensor as well as ruining the catalytic converter over time. The check engine light comes on because the system is unable to adjust within parameters in order to get the mixture back to optimal.

A "lean" condition occurs when not enough fuel is being sent into the mixture. This manifests itself in poor performance, higher engine operating temperatures and if left unchecked, over time can burn pistons!!

Since you seem to notice a degredation or fuel economy, you are probably experiencing a "rich" condition. The check engine light confirms this, and since the engine works harder under load, switching on the A/C magnifies the problem.

Throrough diagnostics by an experienced tech can pinpoint the source of the problem. I don't recommend "throwing parts at the issue", as there are several things that can cause this...it can get expensive buying parts for these cars and installing to no avail.

Offhand though, I believe a new set of plug wires would be needed, as well as a new O2 sensor. If the bad plug wires caused the "rich" condition, the sensor is probably fouled as well...