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Discussion Starter #1
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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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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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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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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Discussion Starter #7
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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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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Discussion Starter #9
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.

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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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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I don't know how much help this will be and I'm doing it from memory here, the car was sold over two years ago. The 91 300SE I had for 12 years slowly over time got to running not as well as when I bought it. Over a period of trial and error I replaced all the rubber vacuum and two hard lines, got a duty cycle meter and adjusted the lamda/fuel mix, EHA was leaking a little and was replaced. It was about this point that sometimes I'd have to ride the shoulder uphill due to absolutely no power, downhill was better and this was erratic as hell. New cap, rotor, wires, and plugs (believe it or not 100K miles on the plugs that were in it when I bought it :eek I had read that aluminum heads sometimes had stripped plug holes, these didn't). She ran some better but still sometimes no power and when she did and I floored the gas it looked like diesel exhaust out the back. Around this point someone mentioned the cats may be plugged, I had a set of pipes made and dropped the cat, problem solved and she was still running quite well a year later when I sold her. The new owners are my neighbors in-laws and I still see the car from time to time
 

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After seeing that video my money would be your OVP. It is similar to my '89 M103's behavior although not exact. You can try to measure the output of the OVP when the idle is cycling to see if it is cutting off at that exact time and coming back on.

But your OVP is so old it needs changing anyway, so you may as well just change it and see if your idle cycling issue goes away.

Your vacuum problem is separate in my opinion and should be tackled separately. I assume the AC was not running during the video.
 

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Discussion Starter #14
Last night I checked my duty cycle by checking the voltage across pins 2 and 3 of the x11 using a multimeter. It read out a constant roughly 14.1 to 14.2 volts. So if my assumptions are correct this would indicate 0% duty cycle, meaning the system isn't attempting to adjust the mixture at all. Does that sound right?
 

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... So if my assumptions are correct this would indicate 0% duty cycle, meaning the system isn't attempting to adjust the mixture at all. Does that sound right?
The voltage you read out does indicate 0% duty cycle … but that does not mean that the system isn't attempting to adjust the mixture at all … quite the contrary … it‘s trying its best to adjust the mixture … which seems to be rich beyond the system‘s capability to lean it out via EHA.

For details see: http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/2720049-ke-jetronic-lambda-control-duty-cycle.html

Btw … your entry under occupation in your profile takes me back (35 years) … :wink_2:

H.D.

P.S.: Since, unfortunately, I’m not receiving notifications on threads I’m subscribed to from Benzworld, I might not notice further questions in this or in the above-mentioned thread. … Sorry if that happens.
 

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Discussion Starter #16
So should I adjust my Lambda control adjustment? I've previously been instructed by a mechanic to never adjust it as he has set it where it needs to be.. Does a 0% duty cycle indicate any potential issues other than running very rich?
Once I get paid I will purchase a new OVP and let you all know how that turns out!
 

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... Does a 0% duty cycle indicate any potential issues other than running very rich? ...
If you checked the Lambda control adjustment (at X11 across pins 2 & 3) with a voltmeter and read out “a constant roughly 14.1 to 14.2 volts“ (instead of measuring a duty cycle of 0% with a duty cycle meter), then:
- either the engine is running very rich
- or there‘s a problem with the ECU
- or there‘s a short circuit on the wire to pin 3 of X11

So should I adjust my Lambda control adjustment? I've previously been instructed by a mechanic to never adjust it as he has set it where it needs to be ...
Very good question … much better than rashly changing the Lambda control adjustment when the meter shows a too low/high duty cycle. … And I hope that the mechanic made sure that everything else was in order before he set it.

I suggest to check a few things that might cause the 14 volts (0% duty cycle) reading … starting with measuring voltage, with “igniton on“ (engine not running), between X11 pin 3 & ground … and between X11 pin 6 & ground. The quotient of the pin 3 voltage and the pin 6 voltage should be about 0.3. … If that's the case, the third of the above-mentioned possible causes can be excluded (and the second one is less probable).

The next thing I‘d suggest, is to measure the o2 sensor voltage at the (connected) o2 sensor connector under the passenger side‘s floor panel (green cable). ... 0.8 volts or higher indicate “rich mixture“.
If that‘s the case, I suggest to check the CSV (cold start valve) for tightness.
If the CSV is tight, I suggest to check the fuel pressure.
With your engine running at idle, SP (system pressure) should be about 5.3 - 5.5 bar (77 - 80 psi) and LCP (lower chamber pressure) should be about 0.4 bar (5.8 psi) below SP ! … SP higher than 5.5 bar (80 psi), or LCP more than 0.4 bar (5.8 psi) below SP are possible causes for too rich air/fuel mixture.

Let‘s see the results of these test prior to further suggestions.

H.D.
 

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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?
When you say "OVP", do you refer to the over-voltage protection relay behind the engine on the firewall of the 126? If so, yes, I agree: I replaced mine, too, when I was having all sorts of fuel delivery issues—as part of a fuel pump, fuel pump relay and overflow relay solution.

If not, what does "OVP" refer to, as I have a 300SE and mine probably needs changing as well. I have rough idle at cold start and even when accelerating from a standstill when cold. Once the engine's fully warmed, the problem seems to disappear.
 

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When you say "OVP", do you refer to the over-voltage protection relay behind the engine on the firewall of the 126? If so, yes, I agree: I replaced mine, too, when I was having all sorts of fuel delivery issues—as part of a fuel pump, fuel pump relay and overflow relay solution.

If not, what does "OVP" refer to, as I have a 300SE and mine probably needs changing as well. I have rough idle at cold start and even when accelerating from a standstill when cold. Once the engine's fully warmed, the problem seems to disappear.
Yes OVP stands for Over Voltage Protection relay. I now recommend replacing OVP's now older than 10 years old (newly converted believer) no matter what to avoid future headaches.

I am skeptical that your cold issues are OVP related. AS the OVP matters the least during cold as the lambda control is actually in open loop mode so some of the sensors are ignored during cold (like O2 sensor, etc). OVP problems are amplified during Hot (not cold).

I would first look into things that effect operation during cold (like fuel mixture screw, EHA, coolant temp sensor, etc)
It sounds like your fuel/air mixture is not rich enough during the warm up period and once the engine warms up the ECU and actuators compensate for the problem.
 
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