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  #1  
Old 08-25-2009, 02:02 PM
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M103 small misfire / "pee shudders" at idle. Fuel injectors possibly?

Hey guys,

Got a slight issue here, but not a show stopper. Would like to hunt it down before it becomes anything major.

The best way I could explain the issue is the car seems to have "pee shudders" () at idle. The car seems like it shakes just a little at idle, and it's not in very quick succession, nearly 1 second between each little wobble. I have new engine and transmission mounts, and it's not the characteristic constant shaking with worn engine mounts. It's just a quick little shudder while the car is in gear and idling. When the car is moving it runs great.

The airflow sensor, ICV, EHA, plugs, wires, distributor, cap have been replaced and adjusted properly. What I have not done is go through and replace fuel injectors, their seals, etc. When I replaced the airflow sensor and fuel distributor (as 1 unit), I was tightening down the line to the injectors and one of the the injectors could "spin" in the head. Could it be a small vac leak there? I checked all the plugs, and they're looking uniform and well tanned. Fuel pump was replaced by previous owner about 40k miles ago. Not sure if the accumulator was done though, and not sure if that makes a difference here. I also have a 100k mile old fuel distributor available here that I could swap out, but I don't want to fix something that's not broken (yet).

Suggestions much appreciated.

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  #2  
Old 08-25-2009, 06:25 PM
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Is this with the A/C on or off? My A/C compressor adds a bit of shudder to be sure.
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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #3  
Old 08-25-2009, 07:44 PM
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If you find out, let me know. My M103 does the SAME thing. Kinda shakes/sputters at idle (in gear). I thought I once read it was the spark plugs, so I never thought it was that big of a deal.
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  #4  
Old 08-25-2009, 07:59 PM
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shudder at idle

Hello,

I had this, finally traced it to ignition leads.

If you havn't done this recently, do a tune up, Ign leads plugs and rotor/distributer cap etc. Make sure, very important, you get the non resistive plugs they are hard to find now. Don't let the shops tell you they have a replacement, they don't?
Good quality ign leads as well?

Good luck

Gavinbr
300ce/m103
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  #5  
Old 08-25-2009, 07:59 PM
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if this is like a 1990 300e sort of model I would suggest changing the spark plugs. My 1990 300TE did exactly that and I danced all around it, thinking it must be the fuel distributor, the O2 sensor, poor oil pressure and whatever...turned out to be a new set of Bosch Copper-Plus spark plugs. Threw $25 of plugs in it, tested the result, and as my wife said, she wouldn't even drive the thing at the time (hard warm starting, poor idle, occasional stalling at stop) and came back after I told her to try it again; It's like a whole different car...been that way ever since...it was the plugs....
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  #6  
Old 08-25-2009, 08:08 PM
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and I didn't even tune the thing up properly. i know it could use a new cap and rotor and maybe even wires (the guy i bought from said that had just been done but he was full of BS, he tried to sell the thing as a 4-Matic, not, so i know it needs a good tune eventually)..anyway, i suggest trying the old plug route first and see what happens. it's the cheapest way to go and if you need to keep going at least you can eliminate the plugs and proceed from there....good luck....
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  #7  
Old 08-25-2009, 08:10 PM
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Interestingly, with the A/C compressor on, the shudders become much LESS apparent. Like having a load on the engine reduces the amount of slight shaking.

I have non-resistor NGKs that I purchased from the UK, specifically because they were non-resistor replacements. I also do have new Bosch OEM plug wires. Plugs and wires are both about 12k miles / 8 months old. I did pull the plugs to look and they did look fine too, all 6 of them. I would like to rule them out now, but I do have a spare set of plugs I could try out...

EDIT: Yes, full tune up in terms of plugs / wires / distributor / cap / fuel filter / air filter / all new vac lines in the engine bay / checked for all vac leaks @ intake manifold. And several things like the airflow sensor, ICV, EHA have been replaced as well.
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  #8  
Old 08-25-2009, 09:09 PM
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you just gave the answer...dump the junk NGK's and put the proper Bosch Coppers in...NGK's are Japanese plugs and NEVER work in German engines..they just don't.
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  #9  
Old 08-25-2009, 09:11 PM
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Any data to support that?

I can't imagine just because it's a Japanese plug means it doesn't work in a German car. It also doesn't explain why the car ran fine for 12k miles since I changed them. FWIW, I had Bosch resistor plugs in the car for a while too, and it ran no different by feel, mileage, smoothness, and power. But, I did get non-resistor ones just to be sure.
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  #10  
Old 08-25-2009, 09:12 PM
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Y'know, I have never done my injectors either and I often wonder about their effect on my idle smoothness. I have noticed that I have to give it a little gas to make the car start, which I attribute to leaky injectors with no evidence other than gut feeling that I'm a tad flooded at startup. Do you have this symptom too?

If you are wondering if your injector seal is leaking enough to make a vacuum leak, you can test by spraying starting fluid around the injector and see if your idle surges up. However, I don't know what effect the ether would have on the seal, so attempt with care....

If you do go and replace your injectors, I'd be very keen to know what your results are. Please post back if you do! I've been considering it for a while, but I really think I need to do my caster joints first, not to mention my rather ran-out front discs. Also, that stupid air hose that connects to the cold-start injector and the idle air valve likes to get brittle and leak, that gave me a terrible idle for a while.
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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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  #11  
Old 08-25-2009, 09:26 PM
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Quote:
Originally Posted by tinypanzer View Post
Y'know, I have never done my injectors either and I often wonder about their effect on my idle smoothness. I have noticed that I have to give it a little gas to make the car start, which I attribute to leaky injectors with no evidence other than gut feeling that I'm a tad flooded at startup. Do you have this symptom too?

If you are wondering if your injector seal is leaking enough to make a vacuum leak, you can test by spraying starting fluid around the injector and see if your idle surges up. However, I don't know what effect the ether would have on the seal, so attempt with care....

If you do go and replace your injectors, I'd be very keen to know what your results are. Please post back if you do! I've been considering it for a while, but I really think I need to do my caster joints first, not to mention my rather ran-out front discs. Also, that stupid air hose that connects to the cold-start injector and the idle air valve likes to get brittle and leak, that gave me a terrible idle for a while.
Will do. It's kinda been on my list of "to do" for a while, but never critical to the running of the vehicle.

I do have some slight starting issues. It doesn't always go first crank, kinda like the fuel is either a) leaking into cylinders, or b) it's siphoning back to the fuel tank. I did buy a new fuel pump check valve but haven't installed it yet. I'm going to guess that it won't help anything with the idle though, but it's a cheap part that I don't mind replacing. The fuel filter and fuel pump are both decently new. Accumulator is old though, and injectors are original.

I'm thinking about trying the starter fluid on each injector seal, but that would only tell me if the seal is leaking, and not if the actual injector itself may be faulty. Might still be something to check out though.

And I have replaced all 4 of the hoses which typically crack in that area. The 2 to the ICV, the one from valve cover to head, and the one from air box to valve cover.
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  #12  
Old 08-25-2009, 10:04 PM
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I've always been curious as to how these engines are supposed to idle. With new engine and transmission mounts installed, I can barely feel the car shudder, but I can clearly see the engine shaking around in the engine bay. Is this normal?
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  #13  
Old 08-25-2009, 10:08 PM
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I bet it's not injectors...on sparks plugs there's this thing called heat rise and disappation...the rate spark plugs recieve heat and then cast it off...very important stuff....very important. If you don't have the proper sparks installed things like you say can happen. I had a BMW 633 that required silver tip spark plugs due to heat disappation matters...believe me, the damn would not run right until I put the Silbers in, as the box said.....$6 each...don't screw around with the spark plugs.... they seem simple and stupid, but they are as important as your tires....
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  #14  
Old 08-25-2009, 10:12 PM
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the answer to the idle question is; you shouldn't know that the damn engine is even running on a 3 litre Mercedes...after doing the above I have to look at the tach at stop lights to make sure the engine is still running...it's that smooth....absolutely silent and motionless from the drivers/passengers seat....
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  #15  
Old 08-25-2009, 10:57 PM
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Quote:
Originally Posted by alienman View Post
I've always been curious as to how these engines are supposed to idle. With new engine and transmission mounts installed, I can barely feel the car shudder, but I can clearly see the engine shaking around in the engine bay. Is this normal?
Did you do the engine shocks too? Don't really know if that makes a difference, but nobody ever replaces them.......

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1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition
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