I've had a high idle problem on my '90 420SE (M116.965) since I bought it three years ago. The car has 210'000km (130'000 miles) on the odometer. I've gathered some information in an attempt to diagnose whether I should buy a new ICV, Idle Computer or OVP.
My theory:
The ICV doesn't move freely enough to adjust idle properly, and needs replacement.
Symptoms:
- High idle (900-1100rpm) when engine is at operating temperatures, or have been driven for more than a few minutes.
- Often revs high (up to 1800rpm) during startup, before immediately falling down to idle speed.
- Sometimes have normal idle (500-650rpm) if engine is cold, and sometimes if engine has reached operating temperature without being driven (idling for 10-15 minutes).
Testing done:
Sprayed start gas everywhere. Can't find any leaks.
The valve does make a click with the 9V-battery test, but the valve also looks kind of dirty inside.
ICV Voltages recorded, idle speed with P engaged on the transmission:
4,35v: Normal idle, 500rpm. Engine not at operating temperature (70C/160F). Air temp: 19C.
4,75v: Normal idle, 650rpm. Engine just below full operating temperature (80C/176F) after a few minutes of driving. Air temp: 11C
4,76v: Normal idle, 690rpm. Engine just below full operating temperature (83C/182F). Air temp: 14C.
4,83v: High idle, 1050rpm. Engine at full operating temperature (87C/190F) after a long drive. Air temp: 14,5C.
4,87v: High idle, 1000rpm. Engine at full operating temperature (87C/190F) after a long drive. Air temp: 17C.
Once, I measure the voltage while the idle speed was rising. This was interesting:
It seemed like the idle speed started rising as the viscous fan disconnected and slowed down, hence the engine got a slightly lighter load. The idle speed rose from 690rpm to 920rpm over the span of around 20 seconds. The voltage then gradually increased from 4,76 to 4,85. The interesting thing is that the idle speed rose first, then the voltage rose slowly and gradually a few seconds later. It seemed like the computer tried to adjust the idle speed, but that it didn't help because the valve didn't respond properly to the voltage changes.
Any thoughts?
My theory:
The ICV doesn't move freely enough to adjust idle properly, and needs replacement.
Symptoms:
- High idle (900-1100rpm) when engine is at operating temperatures, or have been driven for more than a few minutes.
- Often revs high (up to 1800rpm) during startup, before immediately falling down to idle speed.
- Sometimes have normal idle (500-650rpm) if engine is cold, and sometimes if engine has reached operating temperature without being driven (idling for 10-15 minutes).
Testing done:
Sprayed start gas everywhere. Can't find any leaks.
The valve does make a click with the 9V-battery test, but the valve also looks kind of dirty inside.
ICV Voltages recorded, idle speed with P engaged on the transmission:
4,35v: Normal idle, 500rpm. Engine not at operating temperature (70C/160F). Air temp: 19C.
4,75v: Normal idle, 650rpm. Engine just below full operating temperature (80C/176F) after a few minutes of driving. Air temp: 11C
4,76v: Normal idle, 690rpm. Engine just below full operating temperature (83C/182F). Air temp: 14C.
4,83v: High idle, 1050rpm. Engine at full operating temperature (87C/190F) after a long drive. Air temp: 14,5C.
4,87v: High idle, 1000rpm. Engine at full operating temperature (87C/190F) after a long drive. Air temp: 17C.
Once, I measure the voltage while the idle speed was rising. This was interesting:
It seemed like the idle speed started rising as the viscous fan disconnected and slowed down, hence the engine got a slightly lighter load. The idle speed rose from 690rpm to 920rpm over the span of around 20 seconds. The voltage then gradually increased from 4,76 to 4,85. The interesting thing is that the idle speed rose first, then the voltage rose slowly and gradually a few seconds later. It seemed like the computer tried to adjust the idle speed, but that it didn't help because the valve didn't respond properly to the voltage changes.
Any thoughts?