Ideas as to huge MPG drop since overheating? - Page 2 - PeachParts Mercedes-Benz Forum

pawoSD · #16 10-14-2010, 03:20 PM

Nice, what did that beast cost?

ps2cho · #17 10-14-2010, 11:36 PM

Dropped $220 on it...

So after driving for the day and some adjustments back to 50% duty cycle (initial start was out of compensation adjustment -- Had to richen up for it to even begin reading), the car STARTS much faster and returns to the idle much quicker than before, but I am still plagued by the O2 sensors reading of rich (.81v - .88v)
Could we be looking at leaking fuel injectors? The mixture obviously believes that everything is fine.

Power is about the same at the bottom end, feels a little less at the top end.

Ideas from here?

ps2cho · #18 10-16-2010, 03:53 AM

I am reading that normal O2 sensor readings should bounce from 0.1 through 1v -- Lean to rich. Since I am getting a consistent RICH reading with no LARGE fluctuations, can I assume the O2 sensor is bad or carbon-fouled and giving inaccurate readings?

George560sel · #19 10-16-2010, 07:21 AM

Hi..
did you test o2 heater coil?
here is from Mr. Arthur Dalton
The 3 test for heater are Resistance, short to ground, and current draw.
All these test are with connector unplugged.

test 1:
Ohms across white wires.. this is heater coil R test.
test 2:
Ohms across either white and gray. This is a heater short to ground test. There should be NO reading.
Test 3:
Bring 12v from battery to the white wires with an amp meter in series ..this is a heater current draw test. Looking for specs I mentioned in early post.

If all these test are OK, the heater circuit in the O2 sens. is OK
If any fail , replace the sensor.
These test only verify heater part of sensor and passing of these does not test the 02 sensors capacity to generate a sig to respond to rich/lean conditions

He talked about 95 E320 but I think it work the same way on your car too. Your o2 probably has 3 wires.
1.signal wire
2. heater core
3. ground
and the case of O2 is the ground also.
The heate coil R should be low (about 2.6 ohms on my car)
The current draw should be 1.2-2.2 A
The 12 volt supply to the heater core has to be measure when engine is running. You won't get it if the key is on ii positoin.

I too had a problem with idle and gas consumption (from 22 avg down to 13.5mpg and my o2 read up to .9 volt). After many months and many test, finally I got it fixed. In my case (M104 engine though) I don't have the 12 Volt to heat the o2 and another signs that point me to believe my ECM is bad (car is running though). After got the used ECM, everything is seem to be fine now. I will write what I found when I run the car more.
Don't know if this will help you, just my suggestion.

ps2cho · #20 10-16-2010, 11:06 AM

When you refer to heater coil, are you talking about heated O2 sensors? If so, these are not heated.

It is 3 wire, 2 white and 1 black. I assume 2 grounds and 1 sensor/12v wire. Does the test still apply?

I actually just found I have a brand new sensor from the 1990 Mustang GT that I can splice in. Forgot I bought one. Maybe I'll just splice it in and rule it off. The one I have in there may be original....Or at the very least has 50k on it.

mgburg · #21 10-16-2010, 11:19 AM

I'm from Wisconsin...what the hell are "Torx cheese-head screws" anyways? They better not smell like Limburger or that whole job is going to stink to high heaven before you're done!!!

George560sel · #22 10-16-2010, 03:46 PM

Yes, I mean heated o2 sensor, you should have heated o2 sensor too. 2 white (heater coil) and one black signal wire. One white should be +12 V and th eother white is grounded(remember this is when engine is running) The black wire is signal wire which use the case(of o2 sensor) as a ground. You can do bench test by take the o2 sensor out and put 12 v ( I used battery terminal ) and in just a few minutes you can feel the sensor is hot. I would do the current draw test too.
If you look at this diagram, you'll see what i mean.

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1988/300TE/WIRING%20DIAGRAMS/3160.pdf

Left corner bottom, under ovp relay
Like I said, I too suffer from drop MPG, got a new o2 sensor but no fix until I did the o2 sensor test and found no 12 V for heater coil which led to another more test.

ps2cho · #23 10-17-2010, 08:43 PM

Got the sensor out...

Anything jump out color-wise? New one I am splicing in as comparison.
Looks really rich am I wrong?

Test 1: Across both white wires = 4.6ohms.
Test 2: White Wire + Black = O.L.

Is test 1 a little high?

Either way I have a new sensor I found in my box I forgot I purchased. I will splice them in.
Which of the two white wires is the +12v? Or does it not matter and I am good soldering them together in no particular order?

ps2cho · #24 10-17-2010, 09:12 PM

Here are those tests....

Test 1: Heater coil resistance test -- I am not sure if that resistance is too high?

Test 2: The heater circuit is working. Hooked it up to my 12V battery /w amp meter inline....After about 30seconds the O2 sensor was too hot to touch.

sptt · #25 10-17-2010, 11:29 PM

That O2 sensor looks very bad. I took out mine on my '91 300E and it was original and just looked heat treated from being in there so long. Spliced in the same O2 you have there and had an instant improvement in idle but had to adjust air mixture to fine tune. I think I may have an EHA issue too but going to replace injectors, seals, fuel filter, and fuel pumps first. Always have hard cold and warm starts for some reason. Idles fine once started.

ps2cho · #26 10-18-2010, 01:59 PM

Drove last night got it all warmed up with the new sensor and I am still plagued by the rich reading.

So it is definitely running rich, it is not just a bad reading.

Ideas where to go from here?

ps2cho · #27 10-20-2010, 12:58 AM

Tested the coolant temp sensor tonight. I replaced this sensor about 6months ago because the old one broke. I know it has gotta be functioning fine.

Cold'ish: 1900ohms 5 minute drive 2hours earlier. Not stone cold.
Warm: 535ohms (79C @ cluster) 10 minute drive

Looks right....So cross that off the list I guess

hookedon210s · #28 10-21-2010, 09:40 AM

Quote:

I too had a problem with idle and gas consumption (from 22 avg down to 13.5mpg and my o2 read up to .9 volt). After many months and many test, finally I got it fixed. In my case (M104 engine though) I don't have the 12 Volt to heat the o2 and another signs that point me to believe my ECM is bad (car is running though).

Quote:

until I did the o2 sensor test and found no 12 V for heater coil which led to another more test.

Quote:

Test 2: The heater circuit is working. Hooked it up to my 12V battery /w amp meter inline....After about 30seconds the O2 sensor was too hot to touch.

As George560sel wrote, check that you have 12 volts being supplied to the O2 sensor with the car running. Your tests have only proven that the heater in the O2 sensor works when supplied with 12 volt source, not that the O2 sensor is being supplied by the ECM with 12v source necessary to operate the O2 sensor's heating element. Mark

macdrone · #29 10-21-2010, 12:59 PM

Hey I see you found alot of things that probably has alot to do with fuel consumption but you just fixed the A/C didnt you? 10% fuel consumption if its not on E/C setting for sure. That compressor is pulling huge power off you. Just watch mythbuster had a full segment on A/C and windows down. In town A/C is just sucking gas from you. Just so you know if you didnt. No offense or sarcasam I swear. I just didnt see you say anything about your A/C.

ps2cho · #30 10-21-2010, 04:14 PM

Quote:

Originally Posted by macdrone

Hey I see you found alot of things that probably has alot to do with fuel consumption but you just fixed the A/C didnt you? 10% fuel consumption if its not on E/C setting for sure. That compressor is pulling huge power off you. Just watch mythbuster had a full segment on A/C and windows down. In town A/C is just sucking gas from you. Just so you know if you didnt. No offense or sarcasam I swear. I just didnt see you say anything about your A/C.

I did, but I ran several tanks full of gas with the A/C functional and lost only about 1-2mpg...so I was still getting 19-21mpg. 12-13 now is something different.

I will check voltage at the sensor while running like you said. Will report back.