C280 rough idle/check engine light - PeachParts Mercedes-Benz Forum

#1 05-01-2002, 04:08 PM

Hi again.

One other thing I forgot to ask. My '95 C280 (with 75K miles) started idling really rough (felt like I was in a old diesel car) and the check engine light came on. This happened yesterday and although the light stayed on until I disconnected the battery for a while it doesn't happen every time I drive the car. So its an intermitten problem with rough idling but when it does occur that light comes on.
I have not taken it in yet to get the codes b/c I was hoping to diagnose the problem, then disconnect the battery to see if I could avoid going to the dealership.
I haven't owned the car too long and the dealership won't give me the history on the car w/o the previous owner's permission- what a crock! I just want to see what maintenance the car has had in the past.

J.HIDALGO · #2 05-01-2002, 10:02 PM

I don't know if your car has an OVP but, it is textbook symptom of a bad OVP. Do a search (right upper corner of this screen) under over protection relay or OVP.
Good luck!

Gilly · #3 05-01-2002, 11:44 PM

Chris:
There are many reasons the check engine light can come on, an O2 sensor is only one of them. As a matter of fact, a rough running engine in and of itself can cause a check engine light. Since he is experiencing a rough running engine, I would tend to suspect he has a check engine light due to a misfire code. A bad O2 sensor typically won't cause an engine misfire. Neither will the magnet on the cam for the engine control module, although this CAN lead to a check engine light. I also don't believe the magnet on the cam will cause an idle variation. The magnet and sensor for the cam are only used at very specific times, and not at idle.
Since the history of the car isn't known, i would suspect the basics first, possibly it is just worn out or incorrect spark plugs.
Gilly

David C Klasse · #4 05-01-2002, 11:50 PM

An engine misfire (cylinder misfire) will store a misfire cod, but will NOT set the CHECK ENGINE light on.

I have been over this with my dealer. We've retrieved codes (misfire codes), but the check engine light did not trigger.
I need new coil ends, plug wires, and plugs.

Gilly · #5 05-01-2002, 11:58 PM

Yes, it will. It'll set a PO300 code for misfires, and it may also set a code for a individual cylinder ie PO301 for cyl #1, PO302 for cyl. 2, etc. These are OBD codes and will set a check engine light per Federal Law, it HAS to turn on the light. A misfire means fuel is not being burned completely, and therefor becomes an emissions issue, which really is what OBD is all about, ensuring emissions compliance.
Gilly

David C Klasse · #6 05-02-2002, 12:15 AM

Well on my car it doesn't... I know this because I was there when they got the codes from the car's computer.

1995 M104 motors (C280 included) are fully OBDII compliant, but I'm thinking that maybe for some reason it doesn't trigger the CE light, since it's not "per Federal law" in 1995...
??

Gilly · #7 05-02-2002, 06:59 AM

Assuming that the posters check engine light is on because of an O2 sensor is bad, based on a previous post, is a little like saying that his car is black because the previous posters car is black. Get it?

David C Klasse · #8 05-02-2002, 07:04 AM

Gilly,
THat is correct. I know what you mean. I just wanted to clarify that the check engine light *must* must have been set by something other than misfire, perhaps what is causing the misfire.
I guess it comes back all the same anyway.

BTW Gilly, you are great.

blackmercedes · #9 05-02-2002, 09:39 AM

Quote:

Originally posted by sugardoo
I haven't owned the car too long and the dealership won't give me the history on the car w/o the previous owner's permission- what a crock! I just want to see what maintenance the car has had in the past.

As an MB owner, you should appreciate the position that the dealer is in. You do not want people accessing information about your car without your permission. Previous warranty work information should be released to you without any problem, but service information will require the PO's consent.

The PO and the dealer entered into a contract for automobile repair, and the information contained in those records belongs to them, not the car itself. Were I a dealer, I would follow the same practice.

As to the PO not allowing you access to the information, I would have hesitated to buy the car. A car without a service history is worth considerably less than one with a complete and up-to-date history.

If you can talk the PO into allowing you access, that would be great. If those records show a good history, then you've just increased the value of your car a thousand dollars!

Good luck with the misfire. I'd probably change the plugs, just for good measure. If the misfire continues, then I'd either replace the O2 sensor or have the code read, probably the latter, as I'm not much for "shot-in-the-dark" diagnosis. The CE light can come on for many reasons, and isolating them at home can be VERY difficult. This is one downside of all 1996+ automobiles, not just MB products. On the other hand, automakers have been forced to make their products more reliable thanks to mandated emission warranty periods. You take the good with the bad.

Gilly is right about the archives in this case. It gives you a starting point for investigation, but does not indicate what the true problem is.

Arthur Dalton · #10 05-02-2002, 09:54 AM

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The ignition fault will trigger the ignition module BEFORE tripping
the CE emmissions module, but after so many repeats, it will most certainly pop the CE.
Thats why you can go get codes that are monitoring a fault that has not tripped Emmissions yet, but will....

#11 05-02-2002, 10:29 AM

Thanks guys for all your imput! I think I will start with the basics and check the plugs and wires. Its funny how it not always idles rough. Last night I drove it around and it acted fine. I was thinking it may have been b/c I washed the engine a few days ago and maybe some water hit a bad spot. Anyway after dealing with the plugs/wires if I still get the check engine light- I'll get the codes read to give me a better clue whats going on.

Gilly · #12 05-02-2002, 08:34 PM

David:
In my previous post I was referring to Chris' observation on what was wrong with this car.
After researching the facts, what I have determined is that on a HFM-SFI engine (early 104's), it doesn't store misfire codes at all.
On an ME-SFI engine, it will store random misfire and cylinder-specific misfire codes, and if it does, it also will turn on the check engine light. So I guess we are both right, depending on the injection system. I'm just too used to the newer stuff I guess.
Gilly
ps Sugardoo: In general, washing the 104 engine with an actual water hose is a BAD idea, I am relatively sure this is the source of your problem, as the spark plug holes fill with water and cause shorting of the secondary ignition system.

Arthur Dalton · #13 05-02-2002, 11:44 PM

104 eng. with HFM-SFI do have code storing and diagnostics for each cylinders ignition.
The earlier 104 without this feature are the pre HFM Hot wire -
LH-SFI systems.
And even those will trigger a CE when detecting ignition faults.
As Gilly said, misfire is an emmision fault and CE is Emmission monitoring....the different modules talk to one another and a
fault of any system that will affect emmissions will eventually wind up at the CE lamp.

Gilly · #14 05-03-2002, 01:47 AM

The only difference between an LH and a HFM SFI injection system is the mass air flow sensor itself.
The only real difference between a HFM-SFI and and ME-SFI is the ME system has the ignition system integral to the engine control module.
When ME-SFI was introduced, that's when the engine became OBD2 compliant, and that's when the engine control module had to store misfire codes.
Gilly

Arthur Dalton · #15 05-03-2002, 08:53 AM

Me is when OBD2 compliance , but the earlier HFM -104s do have individual cylinder misfire monitoring [both ign. and injectors] and
retrieval codes for them through the HFM-SFI diagnostic module.
These are just before OBD2 and have good storage diagnostics.
An example would be late 124 chassis 104s w/HFM-SFI.

These earlier DTCs seem to be approaching the OBD2 format, but nothing mandated yet..