Engine starts at 2,000 RPM

[kropsch]

My '94 C 280 starts the engine at 2,000 RPM, and immediately
drops to about 600 RPM. This happens only when the engine is completely cold, at the first crank in the morning. During the rest of the day it starts at about 1,000 RPM, dropping to 600 RPM.
The CIS injection cars I had before this C 280 didn't have this high 2,000 RPM start. Is this normal for electronic injection ?
Is this the reason why only synthetic oil is recommended for this engine ?

[GeorgeMurphy]

You will have to go to your MB dealer and have the fault codes read to find the problem - most likely the EA/CC/ISC module - very expensive . . .

[kropsch]

George, thanks for the answer. I couldn't find this module on the service manual. Is it the Engine control module ( N3/4 ), or Air relay module (K17 ), or Heather relay module (K35 ).
Best regards.

[Suginami]

I wonder if George Murphy is talking about the throttle actuator, sometimes referred to as the slide actuator.

It sits underneath your intake manifold, and costs about $1,200. I had to replace mine (ouch!).

[Suginami]

Oh, and I believe the initials stand for: Electronic accelerator, cruise control, idle speed control.

All three functions are integrated into one unit, and if one of the functions fail, the whole unit is replaced.

A failing cruise control is another symptom of a bad actuator.

In my case, my idle hunted up and down, as if the engine couldn't figure out the right air / fuel mixture. The car was virtually undriveable.

[kropsch]

Hi Paul:
Thanks for the answer. I'll remove the Engine control module to clean the contact pins. Would you know if a problem that happens in this module only once in the morning would be recorded to be detected later by the dealer fault codes reader ?
I have doubts that the dealer will carefully look for the problem. I believe they will just try to sell me a new module.

[Suginami]

Originally posted by kropsch
Hi Paul:
Thanks for the answer. I'll remove the Engine control module to clean the contact pins. Would you know if a problem that happens in this module only once in the morning would be recorded to be detected later by the dealer fault codes reader ?
I have doubts that the dealer will carefully look for the problem. I believe they will just try to sell me a new module.

Don't quote me on this, but I've been told that a certain fault needs to occur several times over a given period of time before a fault code will be stored.

About the dealer - I have had generally good experience with their ability to diagnose difficult problems.

Are you concerned that they will try to sell you a new engine control module, or a new throttle actuator?

If you need a new throttle actuator, then you ought to buy a new one. They have potentiometers that go bad over time. They also had trouble with the wiring harnesses on these units on M104 enignes from 1993-1995.

If you need a new engine control module, which I find very unlikely, I would find a used one at a Mercedes Recycling center as these units don't go bad.

[Jim Grillot]

Your problem has nothing to do with what type of engine oil you use.

Fuel injected cars have a device that lets air bypass the throttle plate to produce a fast idle. On American cars the device is known by names such as idle air control motor, idle stabilization valve, fast idle control device, etc. It is common for these units to become gunked up with gasoline/motor oil residue and produce an unacceptably high idle, especially on the first cold start of the day. I have had good success on Ford Taurus V-6 and Jeep 4.0L engines by removing the device and cleaning with a volatile solvent such as carburetor cleaner or brake cleaner to remove the gunk from device. You might try locating the FICD on your engine, removing it and cleaning it. This device will be located in the vicinity of the throttle plate. It will have an electrical line or lines with a plug at the device.

[kropsch]

Hi Jim:
Thanks for your detailed answer. This C 280 is the first Mercedes I have with pulse injection. My 3 other cars have continuous injection. I'm used to clean the idle speed control valve, as you suggested. All them (450 SLC, 500 SEC and 190E ) have a cold start at 1,000RPM, decreasing to about 700 RPM.
I would like to know if it is normal a C 280 have a cold start at
1,500 RPM. My doubt was if the synthetic oil was recommended
to improve the engine lubrication during this high RPM start.
I will clean the idle control motor to see if the RPM comes down.

[kropsch]

Hi Paul:
I'm not really afraid to go to the dealer. I believe that probably they will not care if the engine has a cold start at 1,500RPM.
I have been cleaning the modules and relays contact pins, and now the cold start RPM is 1,500RPM. Perhaps it never has been 2,000 RPM, and I was mistaken. Anyway, thanks for the suggestion about new or recycled parts.I agree with you regarding this modules not going bad. In most cases the culprit is just a bad contact.

[vince]

The throttle actuator controls the idle speed, even on the cars without the traction control option (TC was implemented through the electronic accelerator section of the control module). There is no separate idle speed control valve (like the CIS cars used to have). There are several things that could be going on which have been mentioned by others, but the first question is: How long is the engine at 1,500 rpm. Is it for a fraction of a second? I have observed that from a cold start the module advances the idle for a brief (!) period until the engine fires and then immediately backs the idle down to a more normal speed determined by the engine temp at startup.

[kropsch]

Hi Vince:

The answer to your question is yes. The 1,500 / 2,000 RPM remains only for a fraction of a second, decreasing immediately to 700 RPM.
I think this high RPM , even for a fraction of second, will generate a high wearing of the engine. I believe that this could be the reason why Mercedes specifies synthetic oil. The mineral oil probably would not provide a good lubrication during this cold start.
Does this high RPM start occur with your C 280 and E 430 ?

[vince]

Yes, both my cars do the same, even though they have different versions of the engine management system. To confirm what I suspect is normal for your car have a knowledgable technician check it out. Basically, the ISC/EA/CC (idle speed control/electronic accelerator/cruise control) module tells the throttle actuator to open to a fixed position when it sees a "cold" engine. That fixed position varies based on engine temperature. If your car has cruise control the same thing happens, but it is the cruise control section of the module that gives the "orders". If the car has the ASR (automatic slip regulation - i.e. traction control) option, then the electronic accelerator section gives all the "orders" as to what your foot is doing at the pedal; if the car doesn't have ASR, there is a throttle cable. By the way, the requirement for synthetic oil is only for the 1998 and later M112 & M113 V-6 and V-8 motors on cars with the FSS system. Oil change intervals can be VERY long with that system and some engines apparently had some problems. Your engine can use conventional oil. My C280 has 137,000 miles on conventional oil and is fine. I change at one-half the recommended interval (3,750 miles) because the car gets used in Washington, DC traffic a lot, which I consider severe service.

[kropsch]

Hi Vince:
Thanks for the answer. I'm feeling more confortable now.
Will probably look for something else to be worried.