'91 500 SL - Idle Misfire

[jgl1]

'91 500 SL, 87K miles, with KE-Jetronic...

Have personally maintained this vehicle since new. Lent vehicle to family member for several months, during which time the alternator failed while the vehicle was out-of-state. Vehicle was towed to an M-B dealer, who replaced the alternator. Shortly there after, the belt tensioner failed, stranding the vehicle again. The vehicle was towed to a local repair shop, where the tensioner and belt were replaced, along with both ignition coils (to address difficulty in starting).

Once back in my possession, the vehicle presented with difficulty starting, a chronic idle misfire with idle speed of 650 RPM in Drive, the new style OVP relay that I had presonally installed a year earlier had been replaced with a unit dated "1989", the ignition control unit's retention nuts were missing and a different ICU was installed in the vehicle... correct part number, but not the unit that the vehicle was originally equipped with. There was no charge for ICU or OVP relay replacement on the service statement.

The following work was performed following receipt of the vehicle from the local repair shop:

1. Fault code check revealed a faulty oxygen sensor and sensor was renewed.

2. Fuel pressure was checked and found to be out of spec; fuel pressure regulator was renewed.

3. Spark plugs, distributor caps and rotors were replaced, on principle. Spark plug wire resistance was checked and found to be WNL.

4. Knock sensor wiring harness insulation had disintegrated and harness was renewed. Motor mounts were fatigued and replaced in conjunction with the harness renewal.

5. A new style OVP relay was installed.

6. Vehicle was subsequently driven for approx. 800 miles with fuel treated with LubroMoly Jectronic cleaner. Plugs were pulled and examined: 3 plugs presented with blackened, soot covered insulators. As such, all fuel injectors were replaced, along with accompanying idle air distribution hoses, seals, etc. Inspection of valves with injectors removed revealed essentially no deposits on the valves. Duty cycle was adjusted to 45-50% and the fuel mixture adjustment tower renewed.

7. Intake air and engine coolant temperature sensors specs were WNL but sensors were also replaced, on principle.

8. Cold start valve has not been inspected, as ambient temperature at start is consistently above 78 deg F.

9. EGR valve was inspected and found to be functional.

10. Idle speed control valve was removed, cleaned and found to be functional.

11. Air intake system was inspected for leaks using throttle body cleaner; no leaks were found.

12. Air pump has remained de-energized during the course of this repair, due to clutch failure.

13. EHA has not been inspected other than checking condition of pins and presence of current.


With the above listed repairs, the engine starts readily, runs well, and idles smoothly (but with the infrequent hiccup) except when starting the engine at ambient temperature... with the engine cold, the idle has a constant miss. The miss remains, regardless of engine temperature or duration of run - the run can last an hour and the idle miss persists. The idle miss also persist if the engine is shut down and immediately restarted.

BUT... if the restart occurs approx. 20 minutes after the 1st run from a cold start, then the engine idles glass smooth for the balance of the day, regardless of the number of restarts.

I don't have an oscilloscope to pursue this further should it be an ignition fault. Any suggestions as to what to inspect next? EHA? Ignition control unit (which apparently was switched out by either the dealer or the independent repair shop)? KE-Jetronic control unit?

Thanks for the input.