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Discussion Starter #1
I have S420 1995 with 223,000. I am not an auto mechanic but pretty good with cars. this car I have for about 5 years. I don't drive it much now so simetimes after sitting for two weeks espasially in a rainy seson it would start good but a few minuts into driving it would start to misfire. sometimes it fills like the engine is chocking untill it gets to 3500 rpms then get power back but if I just push axelerator very lightly it would go very slow but smooth. restarting the car does not help. then after sitting a few hours it is back to normal. then again... I am totally lost between asr system, timing problem, camshaft positioner sensor or just sparkplug wires. It might be loosing contact with one of the sensors ocasionaly but which one? even when engine is not misfiring it feels a little like it is chocking when I am trying to speed up. any one resolved this problem or have any idea please help.
 

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it might be the coil, the spark plugs, or the spark wires. and if you let it sit for awhile the car will stay in one gear because when it sits, the transmission fluid leaks which causes your transmission to get bad if not topped up.
 

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Discussion Starter #3
update

Hi, sorry for taking too long to update. I have been reading this forum a lot and working on my car same time. here is what i did. I checked EGR valve that works with smog pump ( my terminology might be wrong) I disconnected vacuum tube and created vacuum on the inlet of the valve- it stall the engine, so I knew it was not that. Next I checked the vacuum tube going to ignition control, when I took it of the control engine worked o lot worse. so it was not that. next i disconnected crankshaft position sensor the engine dies. camshaft sensor i checked by installing it on another car t is working. next i checked fuel pressure by putting a gages on fuel monifold it was around 55- 54 psi ( not sure if gage was ziro ) so it dropped to around 50 when engine started. pressure also changed when I disconnected vacuum going to pressure regulator I think it went up 4-5 psi. It all looked normal to me. the only thing i worried was pressure drop ( about 2-3psi) when i push accelerator then it would go up with RPM rise. so i checked my voltage at the battery with car running it was above 14 volts ( high end fluke multimeter) looked normal. I disconnected the MAF sensor and felt that engine worked even better without it. the car would also go into limp mode when I patually blocked the air intake or pushed the gass without air filters, then normal after restarting the engine. I replaced MAF sensor with the one from another car (old but cleaned) and misfire and hesitation was GONE. the car would Idle normal but still a little power loss on low RPMs then gein power back above 2800-3000 RPMs. after driving for about a week my check engine light came on. built it code reader shows code #2 and 26. Oxygen sensor code I newer had before. but tranmision delay came up before. I hope code 26 would not lead to power loss. but O2 sensor I wonder why it came up now after MAF replacement? How many O2 sensors are there on s420 1995? i am going to reset the code and watch it come back but I got a feeling that it is going to come back. i really dont want to spend money replacing unnecessary parts and I found that is it possible with the help of this forum. any suggestions would be appreciated. thanks much.
 

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Discussion Starter #4
update

one more thing I did was checking resistance of the camshaft and crankshaft position sensors on both it went up about one tenth with the engine temperature rising for example from 900ohms to little above 1000 which I thought was OK.
 

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I have the same problem I have a 95 600 its raining here in so cal and this has happen to me several times what does the trick for me is i let the car warm up for about 10 min then I drive off with no problem. If I just go into the car and start it up after sitting overnight on cold and rainy day I have no power it runs but it only goes about 40mph this happens only in the morning to me but after I warm it up I go about my day and dont have issues. not much help but this works for me maybe I need to look into this.
 

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Discussion Starter #6
Thanks. mine is the other way I think it runs better when cold, may be higher
RPMs or richer mixture. It just feels like choking a little when I step on gas harder than normal. but when I step on it sharp enough to switch gears then RPMs jum up and it goes fine. could it be O2 sensor? or may be tranny does not step down when it should?
 

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SEC 600 V12 2dr COUPE (RHD) One of the chosen few.
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those symptoms are ones that i would connect to lambda sensor..O2 sensor it balances the air/fuel mix. if you had got a leak in the manifold area the air mix in the exhaust would be weak so the sensor calls for more fuel to be added the other way if you had blocked injectors or bad weak fuel .trouble is that if the fuel air mix goes to far to the rich/weak side it hits a stop in the ECU and stays there and you can not recover it without clearing the memory stacks. the clue is in the fact that when you take the maff out the engine runs better. So what I think you need to do is to re-set your ecu clear all the fault codes and try again. there is a sequence of key/pedal engine actions that enable the stacks to clear but they are no use to you if the problem still exists.There are people nearer to you that have this knowlege. You might try the leaks in the inlet-manifold as a first step make sure they are all fixed before you go further..
 

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Discussion Starter #8
Thank you very much. In my head I never connected lambda sensor with O2 sensor. it makes sense now. I have degree in Electrical Mechanical Technology, not auto but i am good with relays, sensors,... Does clearing codes in built in code reader does the job? could you please tell me the key/pedal sequence? I will check the O2 sensor. my car had 180,000 when I got it and now it is 223,000 . I am sure no one looked at it in a very long time. Thanks. What is the socket size to remove theO2 sensor?
 

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you need to eliminate all other possible causes before you go there so if your code reader has flashes you should write them down it will be an indicator of where the problems are .
 

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1997 S500V, 1999 S500 grand edition, 2006 s65 amg
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my 1997 s500 use to do this but mine turned out to be a clogged cat. i couldn't rev past 3500rpm it felt like a misfire and the car would take forever to get to 4000rpm. i just cut out the cat and bought a magnaflow cat and it runs perfect
 

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If you have a bad cat.. it is easy to tell from underneath the bad one will be very discoloured because of the heat build up.
 

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1997 S500V, 1999 S500 grand edition, 2006 s65 amg
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well not actually true about the discoloring. my cat looked brand new but was clogged. i have one cat that is discolored its brown but its clean on the inside. you just run rich when your cats are burned up. but you will hear a rattle if the ceramic base inside is damaged or clogged. also muffler shops have a vacuum tool that helps them diagnose it.
 

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Discussion Starter #13
I tried to check the O2 sensor when car was running and I couldn't read voltage that should be there. mine read below 0.2v all the time. so I ordered a new O2 sensor. Anyone knows if there is a way to really check MAF sensor without specialty tools??
 

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are your catalytic converters good? also the only way i have seen people clean their MAF sensor is by taking it out and and cleaning it with a towel and maf cleaner. do hear a rattle type noise from under your car?
 

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Discussion Starter #15
I think my cats still ok. at least not clogged. I feel that exhaust air come out freely. and the cor runs better on high rpms than low rpms. no tattling noise from under the car. when I press the gas pedal while driving it feels like chocking but pick up power if I let go the pedal a little.
 

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what about the air filters are they ok? on mine i felt heat from the exhaust but when i pressed the gas it would not move it felt like it had the power but it was moving up the rpm slowly. also how are your coils, spark plugs? also know this if you have bad spark it will cause bad cats. and i doubt its the timing because you will hear a rattle on top of the end.
 

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I keep thinking my cats are clogged. There is very little exhaust coming out compared to my 1987 SAAB SPG. I will take it to a muffler shop tomorrow and have it checked.
My 119 has an intermittent issue with power loss and it goes away after I get it up to 45mph.
 

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Cat outlet temp must be 100* cooler then the inlet. If not the cat is plugging up.

MAF + o2 + random or solid misfire codes = plugged Cat.
 

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Discussion Starter #19
For those who happen to have same problems I want to list the things I did to my car. Some people keep Mercedes for pleasure and some to put money into. for me it is not either of those. It is reliable and comfortable transportation for my (now) family of five. We just got back from camping. I almost fell asleep on the two hour drive, so nice and comfortable ride. I am still working in it this is what is already done: MAF sensor- took care of misfire which I had occasionally (cleaning did not help). Even without misfire it still would have power loss specially on low RPMs ( at least to me it felt like choking). I replaced O2 sensor- very little change. distributor caps almost no change although they were bad. my biggest mistake was depending on built in code reader which would not show anything. people would tell me that I wanted too much from an old car. but I was not satisfied. finished building my code reader and started checking every pin on 38 pin connector. i found a code for open transmission overload switch, which retards timing when tranny overheats this code does not make check engine light to come on that is why I could not find it . mine stayed open even when car was cold and not running. it is $150 part but I decided to just jump it and see what happen, it worked car runs grate. it has 228,000 miles.
 
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