The manual says replace the O2 sensor once at 60.000 miles but makes no mention of changing it out ever again. What's the general practice for a 1991 420 sel. I'm at 207,000 miles.

J.Chip

Change it out each and every 60K interval...

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Jim

1992 500E
2007 CLK63
2003 Audi RS6
2001 Audi allroad

Although the sensor maintenance interval is stated at 60k, most techs don't change it untill its performing poorly. This is easily monitored in standard servicing. They usually go 100-120k before noticably poorer response. They can be ruined quickly by a number of problems, so performance should be the criteria.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Can someone tell me if the O2 sensor can my the problem given the follow scenario? I have an '85 380 SE that starts perfectly when cold but hesitates severly. I have to "feather" the gas pedal to start out and during the first mile or so, then the car runs great. However, once the engine is hot and I have to start it again, I have to floor the accelerator and crank the starter for 10 to 15 seconds before the engine starts to fire. It doesn't start immediately but sputters to life. Once started (when hot) there is no hesitation as mentioned after the "cold start". This problem is consistant and happens EVERY time without fail. I bought the car used so I don't know if the O2 sensor has been changed but I suspect it was not not judging from what I'm hearing about the O2 sensor in this forum. I'd really appreciate some feedback as it's been this way for over 8 months and the closest MB repair shop is over 150 miles from my home and a four hour drive! Thanks.

While its possible that you do have some pattern failures, one thing is certain your O2 sensor is not the cause.

First why its not.... The O2 sensor doesn't do anything untill its hot and even if the input was created wrong it would have no part in starting enrichment.

The pattern failures that apply to your symptoms are: warm-up pressure regulator clogged or inactive leaving high control pressure. This accounts for the cold running that disappears when warm. The hot starting problem has another pattern failure: fuel pressure leak down with subsequent fuel boiling/vapor locking. Much on both in the archives.

Both of these problems are simply identified with proper fuel pressure testing. The gauges can be expensive so you might want to find a technician with the equipment and more importantly the knowledge.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve:
Thanks for your response to my question and sorry I'm late with my appreciation. I've spent the last month obtaining the CD Rom for my car and teaching myself about the fuel delivery system. What you say makes a lot more sense now. I've read through the fuel pressure check procedures. If I have it right, fuel pressure is maintained for about 30 minutes after engine is stopped at operating temperature thereby facilitating hot starts. I understand you to say that fuel is leaking back (pressure drops) during that period and a subsequent hot start requires the fuel pressure to be pumped back up during cranking (since that's when the fuel pump is activated). I guess this would this explain why I have to keep the starter motor engaged for so long, so the fuel is repressurized? I performed a hot start test and learned that the engine will start immediately within 5 minutes after the engine is stopped (at operating temp.), starts within 5 seconds of cranking after being stopped for 10 minutes, starts within 10 seconds of cranking after being stopped for 15 minutes, and starts within 15 seconds of cranking after being stopped for 20 minutes. To me this indicates the slow leak down of pressure over time as you mentioned. Knowing this additional info, can you further isolate/suggest the probable cause of the hot start problem? I understand the cold start hesitation problem is separate and I see where your suggestion that the warm-up compensator is the likely culprit. At this point, I only know and understand enough to be DANGEROUS !! I intend to have a mechanic look at the car but I would like to be educated enough to discuss the problem with him somewhat intelligently and be able to give him useful info to troubleshoot the problem effectively. I'll look you up next time I'm in Gainsville to repay your good karma and if you ever make it to Key West, please email me for a return favor. Thanks for the help. You're aces up.

The hot restart problem is not exactly as you are viewing it. The fuel pressure comes to peak in a second or two. It does this the same hot or cold. It is very likely that there will be no pre pressure when starting cold.

The difference is vapor locking. When the engine is hot the heat is transfered to the pressurized liquid gas. Whether it boils or not is dependent on the pressure. The fact that you take longer the longer you wait when hot, probably isn't an indication of pressure. The pressure probably leaves in the first minute or two. From that point the time interval increases the heat transfer and the boiling of the fuel. Typically the problem is the pressure accumulator but a proper leak down test should be done.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve, I replaced the fuel accumulator after verifying that it was bad. I removed the hose from the back end leading to the damper (after pinching it off). Fuel leaked with a rapid drip and squirted out upon cranking the engine. However, during the next few day after driving the car, it ran increasingly rough, started stalling, then eventually would not even start. It will make an occasional sputter when cranking but will not turn over. I'm really puzzled now. Any suggestions with this one? Thanks.

The accumulator plays no part in system operation while running. its purpose is to maintain pressure after the pump has stopped.

If the car is running badly testing must be done. As to fuel pressure, it's only requirement is that it stay constant and within bounds. This is really an either/or situation: if fuel pressure is maintained thats all thats necessary from that standpoint.

If a car is running bad the first question I ask is whether the condition is the result of single cylinder misfires or whether the problem is systematic. With experience a good tech will usually determine this by the seat of his pants.

Individual misfires are identified by scope analysis or cylinder balance testing. Systematic problems (mixture too lean/rich, timing too high/low, etc.) are usually dealt with by either substitution or evaluation. By substitution I mean by changing the status quo and seeing the results. The easy one is mixture. I remove the aircleaner and lightly press on the airflow plate. If the car runs better then the car is running too lean. One can usually grab the plate by the 10mm nut and gently pull up to lean up a mixture.

Much of this is done in a few minutes by a driveability tech to determine which tooling to use for real evaluation...

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve,

I've seen a few leaking accumulators cause IDLE HUNTING and SPORADIC MISFIRES on WARM engines that were PURE HELL to re-start.