1992 500 sl help desired!
 

(First time on forum. Apologies if I make a posting mistake.)
I am posting because my intuition tells me that the problems I am having with my car may be solved by proper tweaking or replacing the right part rather than continued replacement of parts that haven't solved the problem to date. I am hoping a very patient person willing to read this long post will steer me in the right direction for an efficient and economical solution to the problem I have.

Background:
The car has been maintained meticulously throughout its life. Currently, it has 140,000 miles and runs great at higher speeds. However, since the great manager retired from the local MBZ franchise about 10 years ago, the car has never run as well when serviced by either the franchise or independents.

Problem History:
In 2010, the car began to vibrate when running. After what seemed to be trial and error efforts, an independent replaced an OEM part with a Brinson unit but the car vibrated worse until the mechanic noticed the connectors to two of the tubes attached to the unit were broken and replaced them. (Suspect the connections for the old tubes were the original problem . . . . )
Although the vibration was resolved, not long afterward, I began getting the "check engine" lite when the car was going very slowly, usually backing up downhill or coming to a stop sign or light. (It felt like it would lug/chug a little as though it was not getting the proper mixture of fuel and air, and then the light would come on.)
In 2011, the independent couldn't figure out the problem so I went to a local dealership for more sophisticated diagnositic. Eventually, the dealership said the computer module that generated fault codes was broken, so I replaced the computer module and had the fault code addressed.
However, after this 2011 "repair" to "correct" that fault code, the mechanic said the car was perfect and not to worry about the check engine lite coming on, but it began happening more often. (Initially, this happened semi-annually, then quarterly, and now frequently.)
Since 2011, the mechanic has replaced a few parts to fix "the" problem triggering lite but the lite still comes on after the exact same lug/chug symptom that started 5 years ago. The last replacement was EZL (ignition control module).

Current Symptoms:
Now car starts perfectly when cold but not well when hot and the lug/chug still occurs at slow speeds intermittently. The current fault codes are 7 and 17, although fault codes after the mechanic replaced the EZL and last tweaked the engine to adjust the rich fuel were 11, 17, and 18. (Not sure why 11 and 18 haven't resurfaced).

Your recommendations will be greatly appreciated.

Thanks.

lets start with a cheap test of the four wire temperature sensor at your water thermostat housing. 1 to 3 and 2 to 4 for some continuity with an amp meter. there is a lot of resistance so it may move slightly. no resistance, replace. I assume all the vacuum hoses, wires and connections are in good shape. figuring codes are being pulled from the cis-e control module 11 is a vague reference to the secondary air injection (emission) system, oxygen sensor is a common issue.. 17 would be the oxygen sensor and 18 is "current to the idle control valve". 7 "implausible engine rpm" maybe the crankshaft sensor. make sure to check that the egr valve diaphragm is ok, just suck on the line going to it on the exhaust manifold and you can't pull air through it... the vacuum pump and hoses have been checked? it will probably turn out to be something obvious..

Many thanks for your prompt and explicit reply. I am looking into your suggestions.

brookings,

I followed up on your suggestion with a factory authorized shop and it turned out the vacuum hoses and connections had a leak that caused all the different fault codes. Fixing the air flow eliminated all the fault codes and my car is running perfectly.

Appreciatively,
1992500sl