Missing O2 sensor causes of rough idle and running rich?

[dgwestfall]

On my 1993 300 SE Mercedes, it appears that someone has removed the O2 sensor and replaced that section of exhaust with a straight pipe. Would this cause my car to run rich and idle rough? I replaced the fuel pressure regulator and this helped a good bit, and a car drives really well now after replacing that and fixing the cam advanced magnet system I know on some cars it doesn’t really affect them too bad if the O2 sensor is cut out of the system, but being a Mercedes, I can’t imagine this isn’t causing my problem. I’ve already replaced the plugs of wires, cap rotor, and coil and I don’t have any vacuum leaks. I have a new O2 sensor on the way, and I plan to have my exhaust shop install a new bung for the O2 sensor.

[Sugar Bear]

Absolutely.

[ykobayashi]

That’s nuts. It probably runs like trash without a sensor to close loop on.

If you pull the downpipe any welder can MIG or TIG one on. There are also clamp on bungs but they are kind of dodgy looking.

This is the last one I did. I’m a poor welder but I can make an O2 bung stay on a pipe. It doesn’t require quality aerospace welding. I buy my sensor bung and plugs on Amazon. They’re pretty cheap like $10 or so. If you have a pal with a welder it costs a six pack.



ETA - yeah running rich sounds right. Your ECU is reading 0v on the sensor because there is nothing there. It thinks it’s lean. So it injects extra fuel till it sees 1v(rich) which it never does and thus doesn’t back off and you get your misfire condition.

[dgwestfall]

I figured this was the problem. I have a good exhaust shop that should be able to take care of it in short order. I’ll report back.

[dgwestfall]

Well, the O2 sensor did not help my rough idle and running rich problem. I will say I’ve only driven the car about three times since I had the O2 sensor installed, so maybe I need more drive cycles for the computer to recognize the O2 sensor because I’m still getting a code 22 on pin four which says O2 sensor heater open or short circuit. I don’t know what else to look at on this car. I’ve already replaced the plugs, wires, coil, new cam, magnet, cap, and rotor. The only thing I haven’t done is put a smoke machine on it to see if there’s a vacuum leak.

Does this car have an IAC, and if so, where is it located?

Anyone have any other ideas?

[Sugar Bear]

I don't know re: an IAC.

I'd suggest triple checking the firing order, the fuse in the OVP relay if it has one, do either a leak down test or a compression test and a smoke test. Lastly check the wire harness.

After these I'd go back to chasing the codes.

[ykobayashi]

Check to see if your o2 sensor heater is open circuit or shorted with a meter. Perhaps you wired it incorrectly?

I recall the heater should have a resistance in the tens of ohms. If it’s open it’s fried. The other wires on your sensor should read open when the sensor is cold. You may have the wrong sensor or the plug is wired incorrectly.

[dgwestfall]

As a follow up, my codes are gone and the car is running well while driving, but I still have rough idle and running rich. I discovered that the vacuum supply line to the vacuum solenoid (switchover valve) is broken so I do not have any EGR function. I ran a vacuum line to the EGR valve directly from the intake port that would normally operate the valve, and the idle actually decreases but still runs rich and poor.

I plan to replace the vacuum to the valve correctly, test it to make sure it's functioning, and test the EGR valve with a vacuum pump to make sure it's functioning (I really hope it is, because these are no longer made, and the ones I do find are extremely expensive). Then I'll smoke test the engine to find out where any other vacuum leaks are. At this point I'm convinced it's either a vacuum leak, the purge valve or something with the ECM.

As for your suggestions, the firing order is correct and the car runs buttery smooth while driving. I replaced the cap, rotor, plugs, wires, coil, cam advance solenoid, and air temperature sensor. It has plenty of power and everything else seems to be working correctly. It's just the idle and richness that is the problem now. Once this is fixed I'm good to go for a while I hope! It's a great car and perfect as a backup for the family vehicle. Love it so far but this idle thing is driving me nuts.

[Sugar Bear]

How is the wire harness condition? '93's had bad insulation, had to replace mine.

[dgwestfall]

Surprisingly it looks good.