"Dieseling" is usually a preigntion problem. Emission controlled engines run less timing at idle than pre-emission engines, which increases EGT and combustion chamber boundary surface temperatures. This helps oxidize HC and CO in the exhaust system with or without a converter, but can lead to preigntion. Dieseling is often accompanied by obvious detonation. This is because the engine is turning very slowly, so combustion is slow and the end gas is exposed to high temperature and pressure for a relatively long time. Thus preignition is followed by detonation, even though the throttle is closed! Dieseling was primarily a problem on carbureted cars from the seventies and some OEMs installed "idle stop solenoids" which closed the throttle an additional amount when the igntion was cut off to starve the engine of air.
Modern cars, including the KE system on eighties vintage Mercedes cut fuel flow when the ignition is shut off, so dieseling should not be an issue.
Most US version Mercedes engines (I think an exception are the early 190E 2.3s) are tuned for premium fuel, which would be a minimum of 91 PON. The 300SEL engine is a variation of the 103 engine in my '88 190E, and it might be able to run on regular as I reported in my prior post.
Suggest the following experiment. Allow the fuel to approach empty, then add about three gallons of regular. If you hear detonation, attempt to "drive around" it as I did. If there is no detonation add three more gallons of regular, and continue to experiment.
An automatic transmission equipped car will have less tendency to detonate because the torque converter limits low rev loading. On the other hand, the W126 body is much heavier, so it will take more throttle to accelerate at the same rate than a 201.
If the engine detonates on regular and you can't find a way to drive around it, try mid-grade and see if that will work.
The 103 engines have modest compression ratios (9.2:1 on my 2.6), which is rather low to require premium. Four-valve engines without knock sensors and up to 9.5:1 CR usually only require regular. The "problem" lies in the basic combustion chamber design of the 102-103 engines. It can be classified as hemispherical, but does have a small quench area. "Hemis" allow generous valve size, but they aren't very resistant to detonation compared to a wedge or pentroof chamber. The four-valve pentroof chamber with small quench zones adjacent to the both the inlet and exhaust valves is very compact with a centrally located spark plug that results in shorter flame propagation distance. This reduces the time for combustion completion and shorter combusion time reduces the tendency to detonate. The relatively quick combustion time of a pent-roof chamber also requires less ignition advance and has better thermal efficiency because less time is available to transfer heat to the cooling jacket.
Current four valve engines run CRs up to about 11:1.
Last edited by Duke2.6; 04-25-2003 at 01:12 PM.