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  #1  
Old 11-11-2004, 07:44 PM
Neil ('92 300CE-24 cab)
 
Join Date: Sep 2004
Location: London, UK
Posts: 54
Can O2 sensor be causing my poor idle problem??

Bit of an essay to follow but want to get all the relevant info down:

Car: ’92 LHD 300 CE-24 cabrio model 124.061, engine M104.980 (CIS-E injection system + EZL ignition system) with 70k miles on clock

Problem: For 2+ years have suffered from the warm engine erratic idle / rough at low revs problem, so often reported/discussed in the various Merc forums – i.e. car runs perfectly from cold ‘til engine reaches normal working temp (approx 85 deg reached after 7mins or so of driving). Once you then slow down to idle the hesitations start. In the last few months the problem has worsened such that on 3 occasions the car has actually stalled, and then refused to restart until its been left for 30mins or so (presume something needs to cool down). If relevant, each of these occasions has been on the car’s first outing after having been not driven for 4+ days.

Tried remedies: Replaced plugs, leads, lead ends, dizzy cap, OVP, fuel filter, and crankshaft position sensor, plus cleaned the butterfly flap in throttle body …. all without success.

Latest thoughts: Main problem is that nothing ever shows up in the diagnostic codes for either the MAS, EZL or CIS-E controller checks. Based on that I’d started to think it was time to go the expensive route of replacing the controllers one-by-one, but then recently happen to read 2 forum threads that have got me questioning the operation of my oxygen sensor: 1) “how do you know your oxygen sensor is BAD???”, where similar problems are happening to a similar car; 2) “E220 Hesitates and shudders while accelerating” on http://forums.mercedesclub.org.uk where someone states “the lambda (O2) sensor only generates a fault code if it completely fails, not if it continues to work all be it impaired.”. If this 2nd post is correct, then it might be time to look more closely at the O2 sensor….

Today’s investigations: Having read up various posts regarding the operation and testing of the O2 sensor it seems the thing to do is check out the mV values sent from the sensor to the CIS-E control unit using a multi-meter; this being possible either directly off the sensor, or at one of the pins of the CIS-E control unit, or about half way between the two where the cables have a connection in the black plastic box under the floor of the front passenger’s foot-well. As this last place seems to be the most recommended and the easiest to access, that’s where I went today (see attachment).

Test method: Drove the car for 10mins or so from cold so engine temp got up to 85 deg plus erratic idle started. Stopped but left engine idling; disconnected the circular connector of the single wire – black wire with female connector on sensor side / thicker green wire with male connector on chassis harness side; left alone the companion two wire connector that I think is for the sensor’s heater; put one probe of multi-meter into the female connector for the sensor’s signal and other probe onto a nearby screw-head to earth it.

Results: - Engine idling @ 600rpm: steady value of 0.70V +/- 0.02V read
- Up revs to 1500: steady value of 0.9V +/- 0.05V
- Let revs drop back to 600rpm: steady 0.70V +/- 0.02V returns

Turn off engine, leave for 10 mins, restart – engine temp still 85 deg:
- Engine idling @ 600rpm: reading initially rises from 0.0 to 0.20V
but then drops back to a steady 0.08V +/- 0.01A (i.e. 1/10th
of previous idle value!)
- Up revs to 1500: steady value of 0.9V +/- 0.05V
- Let revs drop back to 600rpm: drops right back to a steady
0.08V +/- 0.01V

Reconnect cable, take car for 5min spin then disconnect and repeat
both tests:
- Same sets of values result – i.e. firstly 0.70V at idle then after
a 10min rest 0.08V at idle

Questions:
1) Is this method of testing valid with the sensor’s signal cable completely disconnected – i.e. no valid signal (?) is being sent to the CIS-E control unit so it’s not trying to do any adjustments, or should I somehow be tapping into the signal without completely opening the connector?
2) Do the values I’m noting actually tell me anything about the integrity of the sensor?
3) What should I do next to either confirm/exclude the O2 sensor as the root-cause of my problems?


Hope those in the know have persevered through all this text and don’t mind sharing their wisdom…..

Kind regards,
Neil (London)

Attached Thumbnails
Can O2 sensor be causing my poor idle problem??-dsc00005-2.jpg  
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  #2  
Old 11-12-2004, 10:05 AM
Super Moderator
 
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,318
The only data points I have for O2 sensors are:

1. From the CIS/E troubleshooting table, that says you should see a fluctuating .1 - .9V.

2. From a FI book that says a fluctuating .2 - .8V.

Two things may be happening. Your sensor is good, and your engine is running rich, except on a re-start and when it is going very lean. Or, your sensor is bad and sending a faulty signal. I would lean toward the former, but fi at 70K miles, the sensor has not been replaced, I would change it out with one of the generic Bosch units to eliminate the problem.

What is the on/off ratio doing - pin 3 on the diagnostic socket - at hot idle? Lots on posts on this.
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #3  
Old 11-12-2004, 11:24 AM
SL Owner
 
Join Date: Sep 2004
Location: East Coast U.S.
Posts: 131
excellent post

I agree that you should go ahead and swap-out the oxygen sensor simply because they are inexpensive, a big variable here, and I've never done it on your car but presume it isn't difficult.

Your car certainly sounds as though it is leaning-out and stalling. I assume it runs smoothly under acceleration. This points to an oxygen sensor, vacuum leak which may be anywhere to include around the fuel system, or a problem with fuel delivery at low RPM's (injector, distributor possible but less likely).

Check for vacuum leaks everywhere you can. You've probably already done that but if not, then you need to. You can have leaks around the fuel injector housings (the o-rings around the fuel injectors get old) and in other places around the car. My other constant suggestion for everyone is to drive the car until empty and put a big bottle (they make 2 sizes, use the big one) of Chevron Techron in there and drive the car hard. That will clean-out your fuel injection system and intake. I have found this to make a real difference in some cases. These old cars get gunked-up over time.

Also, I presume this problem cropped-up without you doing anything to the car. For example, I assume that you did not change spark plugs at one point after which this problem was created and then got worse. The use of spark plugs other than what is specified in the Mercedes owners manual can cause HUGE idle problems. Today if you go to a Mercedes dealership and get plugs for your car, they may give you a different plug and it may not work well in your car (my experience). No zero's on the end of the plug number, that can be bad unless your manual specifies it (large electrode model). No platinum plugs unless you really want to guess (many reported problems with them, not everyone has problems).

By the way, yours is one of the finest posts I've seen in terms of building-up information to troubleshoot a problem, I enjoyed reading it.
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  #4  
Old 11-12-2004, 11:28 AM
dtf dtf is offline
Registered User
 
Join Date: Jul 2003
Location: South East CT
Posts: 874
My '94 wagon with the IL6 M104 had an idle like you describe and would cut out and then catch again while driving just a few weeks ago. Turned out to be the Air Flow Meter. Just a suggestion.
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1994 E320 Wagon (Died @ 308,669 miles)
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  #5  
Old 11-15-2004, 09:00 AM
Neil ('92 300CE-24 cab)
 
Join Date: Sep 2004
Location: London, UK
Posts: 54
Thanks for your replies/thoughts:

Did think soon after my post that what I really needed to do was the tests with the sensor connected, so as to see if the CIS-E is controlling things properly. Therefore at the weekend I simply removed a small piece of plastic insulation from one of the wires (black wire just before the female connector on sensor side) and put the multi-meter's probe point on to the exposed wire (and earthed the other).

Checking how things looked while at idle after taking the car for a 15min spin: The values shown did indeed fluctuate 0.15V - 0.85V as I believe is to be expected from a healthy sensor. Also saw similar if I upped the revs to 1500rpm.

The only annoying thing is that no matter how many times I stopped / restarted / left the car to stand, I couldn't get it to behave badly (hesitations and/or stall). Am therefore going to leave it for a few days then redo the tests as I really want to see what the readings are when the car is misbehaving......

Thanks also for your other suggestions which I have taken on-board and will return to later. Will post any progress.
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  #6  
Old 11-17-2004, 11:08 AM
Neil ('92 300CE-24 cab)
 
Join Date: Sep 2004
Location: London, UK
Posts: 54
Finally got the idle hesitating last night, but when connected up to the multi-meter the readings where as expected - fluctuate around 0.15V - 0.85V.

My assumptions from this are: 1) The O2 sensor is working correctly, 2) the hesitation is not being caused by a too rich/lean mixture.

Things might be different when the car goes into its stall/non-start mode, but this happens so infrequently I'm going to tape up the wire, put the flooring back down and look elsewhere for now.

Next things I was thinking of doing/checking:
a) Hard drive with Techron Concentrate added to a tank of fuel (as suggested by 'ericgr') - only problem there though is I haven't yet found a supplier in the UK. Anyone know of one or a suitable alternative UK available product?
b) Fuel pump relay (was reading today about possible bad solder on the pins)
c) Idle control valve (check the hoses for a leak)

'dtf': did see your thoughts on Air Flow Meter (also called 'Mass Airflow Sensor'?), but am pretty sure you don't get one of these on a CIS-E (KE-Jetronic) injection system.

'ericgr': Plugs are as specified in the manual - I'm pretty happy these aren't the cause of my problems

If I get any further results I'll endeavour to update this thread so as to hopefully help others.

Cheers for now,
Neil
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  #7  
Old 11-17-2004, 01:58 PM
SL Owner
 
Join Date: Sep 2004
Location: East Coast U.S.
Posts: 131
follow-up

If you can't get techron, just get a nice high grade fuel injection cleaner from the auto parts store. Look for one that promises to clean both the fuel system and the intake valves. I've just noticed that others don't seem to keep the car as smooth when in the tank-- techron in the tank actually improves the driving, some of the others will be a little rough until they are run through the car (just a subjective unscientific observation.) It tried the MB luby stuff (it's in europe) and didn't like it-- get one from a high end supplier and pay them the extra 5 bucks/2 pounds/etc

Next, I never did check to see if your car had an idle control valve. Yes these are known to cause idle problems. I also recommend checking them for SL owners like myself and on those cars they can be cleaned. MB charges and arm-and-a-leg for the same idle control valve you can buy online for 1/4th the cost (the SL is made by VDO). Anyway, you should check the idle control valve and the vacuum connections to it. Try cleaning it.
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  #8  
Old 11-22-2004, 07:26 AM
Neil ('92 300CE-24 cab)
 
Join Date: Sep 2004
Location: London, UK
Posts: 54
Latest thoughts:

1) From the point of view of testing the O2 sensor, there is a great post from Arthur Dalton on this forum to the thread "Oxygen Sensor Voltage Question", where he gives a really good technique for checking if the sensor is working correctly. Will give it a go sometime soon.

2) Read up a bit more on fuel system cleaners and the one I'm keenest to try is Red Line's SI-1 (or the smaller bottle called SI-2). Seems to be as highly recommended in the US as Techron, plus is available in the UK - even if it's 4 times the US price! (i.e. 15oz bottle in US lists around $5 while the equivalent 443ml bottle is £12.50 in the UK).

3) Got the car misbehaving nicely last night (very rough idle at the end of a 15min trip after the car had been on the driveway for 3 days). So had a chance to try out a tip I read somewhere: Left the engine running; opened up the bonnet; took off the cover to the top of the leads/plugs; one at a time pulled off (then replaced) the plug leads and listens for any differences in the engine's idle. End result was that for 5 of the leads, the engine ran even rougher when they where 'popped' off, but for the other one (cylinder #1) it made no difference if it was connected or not. This I presume is quite significant and points to my problem being with the firing of this cylinder, so dizzy cap, lead, lead-end, plug and injector are the likely suspects......

4) 'ericgr', pretty sure car does have an ICV. Cleaning it plus checking its connections is on the list to try if my cylinder #1 investigations go nowhere.

5) Also on the list was to check out the fuel pump relay but initial investigations have failed to even find it! I thought the 3 places to look where behind the battery by the OVP, behind the main fuse box and under the rear seat - all checked and no joy! Did read somewhere though that for some cars around '92 its functionality is included within the Engine System (MAS) control unit that in my car sits behind the battery along side the OVP, CIS-E control unit and HGS control unit (for the 5-speed auto). Anyone know if this is true?


Last edited by NDavies4; 11-22-2004 at 08:08 AM.
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