M103 Rough idle and vacuum leak - Mercedes-Benz Forum
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post #1 of 176 (permalink) Old 09-07-2015, 05:59 PM Thread Starter
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M103 Rough idle and vacuum leak

My 89 300 SE with 262k miles has developed a rough idle. When warmed up it idles at around 550-600 rpm. The engine seems to be hunting as the idle will go very slightly up and down. Occasionally when coasting my engine will speed up and stay there for a few seconds before returning to normal speed. I need to depress the gas pedal to start the car, which I understand should be unnecessary. After starting, If I don't keep my foot in the gas for a few seconds the engine will usually die immediately but after restarting will usually stay running without depressing the gas pedal.

About 6 months ago I replaced the fuel filter and spark plugs due to the engine failing to start. Around 3 months ago I replaced the fuel pump relay due to the engine dying and failing to restart. I replaced the o2 sensor about 6 weeks ago after reading the check engine light codes.

I have tested the OVP, ICU, ICV, coolant temperature sensors, air flow sensor plate, throttle position sensor, and a variety of other components ( been working on it a while and I don't remember all I've tested) which have all tested to be working.

The economy gauge does not peg to the left (in the black) when warmed up and idling in drive with the AC off which I understand indicates a vaccuum leak. Using WD-40 I diagnosed a leak in an injector seal and replaced all 6 injector-to-housing seals. The car runs better now, but is not running perfectly. The economy guage still does not peg to the left, so I believe there is still a vacuum leak.

Before replacing the seals I would occasionally get a knocking sound in the engine while idling, but since replacing them about a week ago I have not heard it.

I suspect that the vacuum leak is not my only problem but likely is the largest problem. When testing for leaks I followed this wiki and did not find any other leaks. What are other locations in the vacuum that are likely to leak? I've searched the forum for a diagram but haven't found one for my chassis and engine, does anyone have a diagram?

Any information about a vacuum leak or other problems I have described will be greatly appreciated

Thank you,
Codycool55
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post #2 of 176 (permalink) Old 09-07-2015, 08:29 PM
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post #3 of 176 (permalink) Old 09-08-2015, 02:14 PM
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You need to go through and check ALL the vacuum lines and rubber connections (I need to do this with the Coupe' too), don't rule out the plastic hard lines as I had one or two of those that were leaking as well. Rubber can be replaced with proper sized line found locally. On my former SE there were several leaks found and fixed. How is the cap and rotor ?
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post #4 of 176 (permalink) Old 07-10-2017, 01:03 AM
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Originally Posted by codycool55 View Post
My 89 300 SE with 262k miles has developed a rough idle. When warmed up it idles at around 550-600 rpm. The engine seems to be hunting as the idle will go very slightly up and down. Occasionally when coasting my engine will speed up and stay there for a few seconds before returning to normal speed. I need to depress the gas pedal to start the car, which I understand should be unnecessary. After starting, If I don't keep my foot in the gas for a few seconds the engine will usually die immediately but after restarting will usually stay running without depressing the gas pedal.

About 6 months ago I replaced the fuel filter and spark plugs due to the engine failing to start. Around 3 months ago I replaced the fuel pump relay due to the engine dying and failing to restart. I replaced the o2 sensor about 6 weeks ago after reading the check engine light codes.

I have tested the OVP, ICU, ICV, coolant temperature sensors, air flow sensor plate, throttle position sensor, and a variety of other components ( been working on it a while and I don't remember all I've tested) which have all tested to be working.

The economy gauge does not peg to the left (in the black) when warmed up and idling in drive with the AC off which I understand indicates a vaccuum leak. Using WD-40 I diagnosed a leak in an injector seal and replaced all 6 injector-to-housing seals. The car runs better now, but is not running perfectly. The economy guage still does not peg to the left, so I believe there is still a vacuum leak.

Before replacing the seals I would occasionally get a knocking sound in the engine while idling, but since replacing them about a week ago I have not heard it.

I suspect that the vacuum leak is not my only problem but likely is the largest problem. When testing for leaks I followed this wiki and did not find any other leaks. What are other locations in the vacuum that are likely to leak? I've searched the forum for a diagram but haven't found one for my chassis and engine, does anyone have a diagram?

Any information about a vacuum leak or other problems I have described will be greatly appreciated

Thank you,
Codycool55
I replied to your PM, but it looks as if good advice is already being provided.

It would help to click 'User CP' on the top toolbar and complete your profile with your car's year and mod (300SE or SEL) and at least a general location, like State if within US.
Daimler Benz made different versions of same model cars for different parts of the world, so to get the correct advice on a international forum, location can be quite important.
Mercedes Benz USA = MBUSA used to host these manuals free of charge for US resident members, unfortunately, that's no longer the case. Alternatively, as US resident, you can navigate to technical literature on MBUSA and see what is still available for your car.
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post #5 of 176 (permalink) Old 07-10-2017, 08:54 AM
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I replied to your PM...

And it only took you two years.
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post #6 of 176 (permalink) Old 07-10-2017, 07:43 PM
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Not sure why this old thread was revived, but am curious if the rough idle issue was ever resolved and what the solution was.
Seems like an unfinished tread.
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post #7 of 176 (permalink) Old 07-10-2017, 11:06 PM Thread Starter
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Teutones reply came at a great time! I actually just came back home from college for the summer and I'm looking to repair my car more. I was never able to replace the vacuum lines, and the rough idle never ceased. Lately I've been needing to replace the spark plugs excessively frequently (roughly every 6 months) as they are becoming fouled to the point where the car will not start. At times my engine would stall at a stop, if I accelerated too quickly from a stand still, and even when accelerating out of rolling corners. During the winter there were times the engine wouldn't start at all, and some of those times when I ceased cranking the engine the air flow meter plate would flutter and eject black smoke. These problems have went away since the weather has gotten warmer, but when coasting or stopped the idle is still low and feels as if it's dying and still a hard start. Once it's reved up to 1500 RPM plus it runs fine although it's lacking in power. Smells like its running very rich. I still plan to replace all the vacuum lines, but if these symptoms seem like something else let me know too!

Cody
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post #8 of 176 (permalink) Old 07-10-2017, 11:52 PM
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You have a multitude of problems but I can safely say that your vacuum lines might not be the root cause of your problems.
So I guess i'm saying the symptoms seem like it is something else.

Have you determined if your Lambda system is working properly and taken some duty cycle measurements, especially when it smells like its running very rich? I suspect the air/fuel mixture is not being maintained/regulated in closed loop fashion.

What is the date code on your OVP? Check that please as a first step. You said you checked it for functionality, how did you do that?

You can also video your idle by recording your instrument cluster during these episodes of low idle, unstable idle. That may present us with a clue.
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post #9 of 176 (permalink) Old 07-11-2017, 10:44 PM Thread Starter
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Originally Posted by dolucasi View Post
You have a multitude of problems but I can safely say that your vacuum lines might not be the root cause of your problems.
So I guess i'm saying the symptoms seem like it is something else.

Have you determined if your Lambda system is working properly and taken some duty cycle measurements, especially when it smells like its running very rich? I suspect the air/fuel mixture is not being maintained/regulated in closed loop fashion.

What is the date code on your OVP? Check that please as a first step. You said you checked it for functionality, how did you do that?

You can also video your idle by recording your instrument cluster during these episodes of low idle, unstable idle. That may present us with a clue.
I figured it was more than just the vacuum lines.. But seeing as it's a 25 year old car couldn't hurt to switch out any rubber/ plastic parts I can get my hangs on

I checked the duty cycle at one point and it was acceptable, however I believe that was 2 years ago. I will check it again this weekend and get back to you!

The date code is 23/01/1998. Is this an expiration or a date of manufacture? Basically removed the OVP, started the engine, and noticed it was running much worse than with the OVP. Again, that was 2 years ago. Picture of date code.

Here's a video! This is stopped, in drive, with the AC off, warmed up after a few miles of driving. At about 20, 25, 30, and 35 seconds in (when the RPM dropped to nearly 0) I had to throttle up the engine as it was going to stall. At about 40 seconds (when the RPM moves up to around 700) in I shifted into park for the duration of the video.

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Originally Posted by Teutone View Post
I replied to your PM, but it looks as if good advice is already being provided.

It would help to click 'User CP' on the top toolbar and complete your profile with your car's year and mod (300SE or SEL) and at least a general location, like State if within US.
Daimler Benz made different versions of same model cars for different parts of the world, so to get the correct advice on a international forum, location can be quite important.
Mercedes Benz USA = MBUSA used to host these manuals free of charge for US resident members, unfortunately, that's no longer the case. Alternatively, as US resident, you can navigate to technical literature on MBUSA and see what is still available for your car.
I have updated my information!

I looked for technical literature on that site, but I think it only goes back to 2000 I (believe) I have found a download elsewhere though.
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post #10 of 176 (permalink) Old 07-11-2017, 11:15 PM
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For some reason the links to your google photo album is not working for me.
But if your OVP is truly 19 years old, like you believe it is, it has to be changed. A KAE unit is about $45 I believe. Mercedes Benz brand might be slightly higher.
I would recommend KAE, that's what i used and was was in the car for 12 years.
I have tested 2 OVP's on my M103 in the last 2 months, one was 12 years old the other was 18 years old.
Both appear to be OK but both were having intermittent idle problems because the ECU kept switching on and off, most likely because of the internal relay circuitry being defective/worn in the OVP.

Whether the OVP is your main problem remains to be seen but it is quite likely. However I would like to see your idle first to see if it is similar to mine when I was having similar issues at a lesser extent.
Can you put it up on youtube instead and post a link?
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