1993 300E 3.2 mistifiyed! - PeachParts Mercedes-Benz Forum

deportes · #1 05-09-2004, 05:27 PM

Replaced OVP and now the car starts fine until a little later you stop at a light and the idle is rough and it takes off as if it is only running on 4 cylinders or less. If you stop and turn the ignition off and re-start it runs fine again until the next stop.

checked for codes and only 7 flashes is all I get.

suginami · #2 05-09-2004, 05:57 PM

Have you replaced your engine wiring harness?

deportes · #3 05-09-2004, 06:14 PM

Paul, I havent replaced it. Do you think that is the problem? How hard is to replace the wiring harness?

suginami · #4 05-09-2004, 06:18 PM

I don't know if the wiring harness is the problem or not, but it is surely bad. They are all bad from 1993-1995.

It is not difficult. It is a matter of disconnecting the old one and reconnecting the new one as you move along the harness.

A tech can do it in 2.5 to 3.0 hours.

deportes · #5 05-09-2004, 06:21 PM

I'll keep that in mind.

deanyel · #6 05-09-2004, 06:28 PM

I agree with Paul - first question is whether you have the original wiring harness. If it's the original you might as well replace it - if it's been replaced the chances are that something else is the problem.

deportes · #7 05-09-2004, 06:37 PM

Is their a way to tell the replacement from the original, I purchased the car a year ago, it has 107K miles.
Also, I will replace it myself, pretty handy with tools and repairing cars. Where does the wiring start and end ? Does anyone have a schematic?

stevebfl · #8 05-09-2004, 09:39 PM

the typical harness that needs replacing includes the wiring for the injectors and the coils.

Look to any part of the harness where you can see the insulation on the actual wires. The cam timing solenoid and the temp sensors at the front of the motor are about the only wires with a small part of insulation exposed. There also is a single ground wire on the intake that can be seen. You might have to cut the outside jacket to really see it. I am attaching a photo I used in my article on 104 headgaskets. It is from 104 motor in a SL320.

Although I recommend replacing any harness seen to be in any state of this condition. I have never seen one sorta bad. The bad ones are trash and the good one will show no affect at all. I tend to believe your problem is probably a lot simpler. I find great incidence of single cylinder misfires from arcing plug connectors under the three coils. It only takes a couple misfires til the controller sees it and turns the injector off on the failing cylinder. Once that happens the misfire is permanent till the next key cycle as you have seen. You may have only had a couple misfires, but once recognised they are continuous because of the injector being turned off to protect the catalyst.

deportes · #9 05-09-2004, 10:52 PM

Thanks Steve for the info, I'll check it tomorrow. Now I understand the reason why some cylinders or 1 stay dead until the next start. great info. great place to share info.

suginami · #10 05-09-2004, 11:33 PM

What Steve says makes sense about the coil wire boots.

M104 engines use coils that are mounted directly on the spark plugs, replacing the distributor at the front of the engine. Each coil pack provides spark to two spark plugs at the same time, one connected directly to one plug, and the other with a short, high tension lead that piggy-backs to the next spark plug. So there are 3 coil wires and 3 high tension lead wires.

If the boot under the coil wire is bad, I guess you'd lose spark to two cylinders.

deportes · #11 05-10-2004, 12:04 AM

THANKS Paul, I am wondering if I should be able to check for specific fault code that will indicate a wire problem.

suginami · #12 05-10-2004, 01:23 AM

Quote:

Originally posted by deportes
THANKS Paul, I am wondering if I should be able to check for specific fault code that will indicate a wire problem.

Good question......

deportes · #13 05-10-2004, 12:02 PM

stevebfl
I have another question for you, could a defective OVP cause a misfire that triggers the rest of the symptoms? I have just replaced the OVP and the misfiring started, I installed the OLD one and will check for misfiring, the reason I changed the OVP was that I resoldered the board and didn't want to be driving around with a suspicious OVP. Thanks.

stevebfl · #14 05-10-2004, 01:01 PM

As far as I know the HFM cars with OVPs will only not run as a result of a defective OVP. You must understand that the OVP is similar to a fuse. In the case of electronic injection if the fuse is blown the controller doesn't work and the engine doesn't run.

The controller will not pick up the arcing connectors unless it does so as a misfire. In that case it will tell you which cylinder.

Paul: actually it probably only will miss on one cylinder (for one arc problem). I am rather weak in analysis of waste spark systems, but if I understand the system properly a spark starts at one plug and fires from the head to the center electrode through the coil out the other plug center back to the block. Two sparks for each coil firing like two gaps in the same wire. When there is an arc the firing voltage is really not very different through the side of the connector to ground than through the plug to ground. The act of arcing finishes the circuit although one of the gaps is now outside the cylinder.

suginami · #15 05-10-2004, 01:32 PM

Quote:

Originally posted by stevebfl

Paul: actually it probably only will miss on one cylinder (for one arc problem). I am rather weak in analysis of waste spark systems, but if I understand the system properly a spark starts at one plug and fires from the head to the center electrode through the coil out the other plug center back to the block. Two sparks for each coil firing like two gaps in the same wire. When there is an arc the firing voltage is really not very different through the side of the connector to ground than through the plug to ground. The act of arcing finishes the circuit although one of the gaps is now outside the cylinder.

Thanks for the explanation. It's a bit over my head, but I'll take your word for it.