O2 sensor - engine idle problem
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Junior Member
Joined: Aug 2008
Posts: 19
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From: Lewistown, PA
1990 560SEL
O2 sensor - engine idle problem
I have a 1992 300E with 82K miles. I've done a bunch of work related to fuel as it had bad gas and a bad fuel pump. At this point and new pump filter, get excellent flow at the fuel distributor. All injectors test with good pattern and open within a couple lbs of each other. I did find one vacuum leak on a hose that I fixed. About a year ago I did a fuel distributor rebuild as I had poor flow to two injectors. Plugs and wires are all good.
Car starts and will idle fine. Suddenly opening the throttle will cause the engine to sputter quite a but will rev out. Driving under load seems to have poor power at below 2000 rpm but above that it runs like a champ. After it warms up if the throttle is released quickly it will almost immediately stall out.
On pin 3 (9 pin round connector) I see the 50% at startup until warm. Once warm it will start fluctuating around 45% to 60% while opening and closing the throttle. If the throttle is left closed after a few seconds the reading will go to 0% (12V DC). Opening the throttle again and revving up will start to get a reading bouncing around 50% again. Once it stalls out I get 30% (which I think corresponds to the 70% engine speed sensor fault). After the stall it takes a bit (5 seconds or so) of turn over until it will fire up again.
I've replaced the O2 sensor and have the same behavior.
I looking for an opinion of what to try next. I suspect there is a fuel problem again that is causing the O2 sensor to read out of range which is causing the 0% reading. Maybe my fuel distributor rebuilt didn't hold out. I figured I'd ask for opinions before I pull the fuel system apart again.
Constructive ideas are appreciated.
Car starts and will idle fine. Suddenly opening the throttle will cause the engine to sputter quite a but will rev out. Driving under load seems to have poor power at below 2000 rpm but above that it runs like a champ. After it warms up if the throttle is released quickly it will almost immediately stall out.
On pin 3 (9 pin round connector) I see the 50% at startup until warm. Once warm it will start fluctuating around 45% to 60% while opening and closing the throttle. If the throttle is left closed after a few seconds the reading will go to 0% (12V DC). Opening the throttle again and revving up will start to get a reading bouncing around 50% again. Once it stalls out I get 30% (which I think corresponds to the 70% engine speed sensor fault). After the stall it takes a bit (5 seconds or so) of turn over until it will fire up again.
I've replaced the O2 sensor and have the same behavior.
I looking for an opinion of what to try next. I suspect there is a fuel problem again that is causing the O2 sensor to read out of range which is causing the 0% reading. Maybe my fuel distributor rebuilt didn't hold out. I figured I'd ask for opinions before I pull the fuel system apart again.
Constructive ideas are appreciated.
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Joined: Apr 2017
Posts: 3
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1992 300e-24
I have a 1992 300E with 82K miles. I've done a bunch of work related to fuel as it had bad gas and a bad fuel pump. At this point and new pump filter, get excellent flow at the fuel distributor. All injectors test with good pattern and open within a couple lbs of each other. I did find one vacuum leak on a hose that I fixed. About a year ago I did a fuel distributor rebuild as I had poor flow to two injectors. Plugs and wires are all good.
Car starts and will idle fine. Suddenly opening the throttle will cause the engine to sputter quite a but will rev out. Driving under load seems to have poor power at below 2000 rpm but above that it runs like a champ. After it warms up if the throttle is released quickly it will almost immediately stall out.
On pin 3 (9 pin round connector) I see the 50% at startup until warm. Once warm it will start fluctuating around 45% to 60% while opening and closing the throttle. If the throttle is left closed after a few seconds the reading will go to 0% (12V DC). Opening the throttle again and revving up will start to get a reading bouncing around 50% again. Once it stalls out I get 30% (which I think corresponds to the 70% engine speed sensor fault). After the stall it takes a bit (5 seconds or so) of turn over until it will fire up again.
I've replaced the O2 sensor and have the same behavior.
I looking for an opinion of what to try next. I suspect there is a fuel problem again that is causing the O2 sensor to read out of range which is causing the 0% reading. Maybe my fuel distributor rebuilt didn't hold out. I figured I'd ask for opinions before I pull the fuel system apart again.
Constructive ideas are appreciated.
Car starts and will idle fine. Suddenly opening the throttle will cause the engine to sputter quite a but will rev out. Driving under load seems to have poor power at below 2000 rpm but above that it runs like a champ. After it warms up if the throttle is released quickly it will almost immediately stall out.
On pin 3 (9 pin round connector) I see the 50% at startup until warm. Once warm it will start fluctuating around 45% to 60% while opening and closing the throttle. If the throttle is left closed after a few seconds the reading will go to 0% (12V DC). Opening the throttle again and revving up will start to get a reading bouncing around 50% again. Once it stalls out I get 30% (which I think corresponds to the 70% engine speed sensor fault). After the stall it takes a bit (5 seconds or so) of turn over until it will fire up again.
I've replaced the O2 sensor and have the same behavior.
I looking for an opinion of what to try next. I suspect there is a fuel problem again that is causing the O2 sensor to read out of range which is causing the 0% reading. Maybe my fuel distributor rebuilt didn't hold out. I figured I'd ask for opinions before I pull the fuel system apart again.
Constructive ideas are appreciated.
A static (not fluctuating) duty cycle value with the engine running and the o2-sensor at operating temperature, indicates a problem according to the following list:
0%: problem with the meter
or diagnostic coupling (X11)
or too rich setting (beyond the EHA’s ‘leaning limit’)
10%: TPS (throttle position sensor), throttle fully closed signal
or (if at 2.000 rpm) no/false supply voltage to POT (air flow potentiometer)
20%: TPS, ‘throttle fully open’ signal
30%: CTS (coolant temperature sensor)
40%: no/false output voltage from POT
50%: o2 sensor (aside from not having reached its operating temperature yet)
60%: car speed signal (displayed during driving or engine still running after driving)
70%: CPS (crankshaft position sensor)
or EZL (electronic ignition module)
80%: IATS (intake air temperature sensor)
95%: micro switch of throttle linkage (4- and 6-cylinder engines)
100%: problem with the meter
or diagnostic coupling (X11)
or ECU ‘N3’ (missing connection to voltage supply or to ground)
or OVP (overvoltage protection relay)
or o2 sensor signal (short to ground)
or too lean setting (beyond the EHA’s ‘enriching limit’)
(information by H.D. from benzworld.org forum)
Last edited by Gerson; May 7, 2017 at 06:57 PM.
Junior Member
Joined: May 2017
Posts: 25
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1987 MERCEDES 300E
Sounds electrical as opposed to fuel related. Invest in a CIS fuel pressure tester to rule out fuel related. The fact that the reading goes to zero and the vehicle is still running seems to be as listed above, a meter issue or short in the diagnostic port. This car being a 1992, you can access the trouble codes and hopefully by doing so will point you in the right direction to solve your issue.
