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  #1  
Old 12-11-2016, 02:08 PM
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M104 O2 sensor symptoms

1994 E320 with 55k miles. The only thing I am certain has been replaced is the engine harness (part thanks to a kind member on this forum).

I noticed that the car drives different between idle-3500 and 4000+ RPM. At the lower range, it's a bit sluggish (but still quite driveable), but then once I downshift into 4000+ or rev it past there, it's like the car comes uncorked and takes off. Since the car is new to me, I'm not sure if it's just the uneven power band of a straight 6 or if there's an underlying problem. To my knowledge, the MAF hasn't been cleaned, it's on the original O2 sensor, plugs, wires, coils, etc. Oh, and sometimes at idle it hunts a bit, especially when cold; it's not rock solid like a Mercedes should be.

Recently, the CEL came on and I got two blinks=heated O2 sensor inop. I reset the light and it hasn't come back on since.

MPG in New Jersey in this weather is about 19 with mixed driving--a little low?

Worth changing out the O2 sensor just to be sure? Does this sound like O2 sensor symptoms?

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  #2  
Old 12-11-2016, 04:33 PM
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That sounds like the airmass meter, replace with good used one as test or test voltages and grounds, it may help to clean, DONT BUY aftermarket , get genuine MBZ one
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Old 12-11-2016, 04:36 PM
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It does take off when hitting between 3500 4000 rpm by design, but should never feel sluggish down low , check and adjust Bowden cable of transmission, it may be shifting lazy
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Old 12-11-2016, 05:11 PM
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I've serviced and driven dozens of 'em. They all are "low torque breakaway"....because of low stall rpm of torque convertor. 2200 rpm stall.
217 hp and 230 tq. Not a lot for a heavy car but lively over 4000 rpm.....noticeably so.

My old 124 3.0 with 170 hp would have beat it to 20mph, barely.

Engine breathing is optimized for 2k to 4k rpm cruise speeds. What you can do is watch the O2 on a scope. At WOT the O2 shpuld stay up near 0.850 to 0.975 vdc or so.

At idle or cruise it should cross over 0.050 vdc often.

Here's a vid' of a WOT to 80 mph in real time.....yours like this ?

https://youtu.be/2CoB-TnlXvA




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Old 12-12-2016, 01:38 PM
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Yes, it seems to be something like that. It could just be my driving style.

Perhaps it might be the Bowden cable? The car has a nice stiff shift, but sometimes I have to floor it (but not hit the kickdown switch) to get the thing to downshift out of 4th in order to accelerate.

Can you please explain low torque breakaway? Is that why when under moderate acceleration, the engine will spin up to ~2500RPM and hold there while the car speeds up? Kind of reminds me of how a sprag clutch works in a helicopter.

Also, how do I check the voltage of the O2 sensor while the car is running?
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Old 12-12-2016, 02:55 PM
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Bowden cable should be felt to go over center as throttle approaches WOT.

If it down shifts without activating the kick down switch, seems everything working okay.

Low torque breakaway is an engine with a cruising to WOT tune and relatively low stall torque convertor.

Your stall rpm should be around 2300 rpm...all brakes locked and WOT for 3 seconds.

W123 cars take off from standstill in 2nd gear and feel sluggish. Only WOT from a standing start will W123 use 1st gear..... yours should use 1st gear most of the time from a stand still.

If yours is performing as the video example, your running okay.

As for the O2 scan, you need to tap into the O2 signal and use a DVOM set to Auto or tenths of a volt. ie, screen should read : 0.000 vdc.

Using a scanner off the OBDII, you can either watch real time of have it record a drive period. O2 needs to be in Closed Loop and engine operated through it's full cycle of Idle, Cruise, WOT and back to Idle.


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