Mercedes 300CE rough idle

Maintenance/Repairs
61

I’d be happy to. Having had 23 years of experience in the manufacturing industry at all levels including senior management, the college hired me to develop and manage corporate training programs. I was the “Director of the Center For Training and Business Development”. I’d work with senior managers and/or HR managers at the companies, review their operations, perform a “gap analysis” of where they were contrasted with where they wanted to be, design an educational program to fill the gap which often began with basic skills in math and physics and sometimes ended with 2-year degrees in technology or engineering. From there I’d hire faculty on adjunct contracts, oversee the program including cost analysis, problem resolution, scheduling, all negotiations and contracting, and in many cases obtaining grant funding. The manufacturing industry is the primary contract-education market segment in my corner of the world.

I loved the work. However, in 2000 the bottom fell out of the market. Between 2000 and 2005 alone the manufacturing industry lost over 25,000 manufacturing jobs just in NH (Department Of Labor statistics). The rate of loss slowed, but the loss continued. Eventually the business was no longer financially viable for the college and the administration offered me positions in the Continuing Education Department, and then Admissions. I was a square peg in a round hole there. I’m not much of an academic. So, after 17 years total with the college and two heart attacks, I called it a day and retired. I was offered teaching positions, but I have never in my life had any desire to teach. I have the utmost respect for the profs, and many are good friends, but it just ain’t me.

62

@the same mountainbike

Thank you very much for the detailed description of your activity at college!

I take my hat off to you - that’s a good example for an impressively extensive career in the manufactoring world! Equiped with enthusiasm and a good basis in math and physics, practical experience and knowledge gained through all levels (best starting at the workbench) and then let other people learn from your experience and knowledge by teaching and showing them how things are done in the manufacturing business.

Besides managing my own one-man engineering office, I never climbed onto the management level.
If a project exceeds my own working hours, there are several other freelance engineers who cooperate with me then.
Sometimes I coach mechanical engineering students in the subject of “engineering mechanics”. My hometown “Aachen” is home to a very popular university for technical courses (RWTH Aachen University). For mechanical engineering it’s the number one among universities in Germany.

I hope you’re OK after two heart attacks!?

63

Thanks for the compliment. That was only the college experience. The engineering comes from the manufacturing side. And, of course, school. And learning from more experienced engineers. And countless failed experiments.

The only reason I even ended up with the college is because of a company bankruptcy and resultant personal problems. Divorce, lawsuits, support payments (IMHO excessive), the usual drill. I needed a job and they needed a corporate training center director.

Re: the heart attacks, I’m great. Thanks for asking. It took a few years to get all the meds etc. squared away, and I never really got my energy back (they were both myocardial infarctions, some of my heart muscle died each time), but if I wake up in the morning and make it 'til bedtime I’ve won for another day, and if I don’t that’s okay too. We live, we die, and if we can make the best of everything in between we’re fortunate indeed. Of course, if I could find a rich hot widow it would be even better… {:slight_smile:

64

@the same mountainbike

You may not have the healthiest body, but you show a very healthy spirit – after all you went through!
I keep my fingers crossed for you to attract a nice and wealthy lady, but I’m not sure whether I should keep ‘em crossed for you to attract a “hot” lady – remember your heart!

65

LOL, thanks HD. It gives me something to hope for. It’d be a great way to check out.
Dreaming is healthy as long as one doesn’t get it confused with reality.

66

@db4690 + everybody else of course:

I checked the o2-sensor today as following:

  • First I let the engine reach to 185°F.
  • Then disconnected the coaxial plug of the o2-sensor under the carpet on the passenger’s side.
  • Then connected a voltmeter between ground and the o2-sensor plug (male part).
  • Then connected a cable with a cord switch between ground and the CIS-ECU plug (female part).

That way, up from the moment when the cord switch is switched on, while the engine is running, the ECU is tricked into “thinking” that the mixture is too lean, thus giving order to the EHA to enrich the mixture, which should cause the o2-sensor to react by supplying a voltage above 450 mV.

  • Then I started the warm engine and let it run at 2500 rpm for about 30 seconds, to make sure that the o2-sensor has it’s operating temperature.

  • Then I reduced the engine speed to 1500 rpm, where I kept it.

  • At that moment the voltmeter was reading 600-650 mV, which means that the CIS base setting is a little rich. That is in accordance with what I did last Saturday, when I changed the base setting of the CIS a little to rich.

  • Then I connected the ECU to ground by switching “on” the cord switch.

  • engine still running at 1500 rpm.

  • Then the voltmeter quickly rose to 730 mV and within 25 seconds it rose further to 840 mV.

I repeated this procedure twice with more or less the same result.

A brand new o2-sensor would deliver up to 900 mV on a too rich mixture, but I think the top amplitude of my o2-sensor (840 mV) is still acceptable. But maybe it does not react fast enough for a good idle quality.
At 2500 rpm the o2-sensor cycle time is 1.2 seconds, at idle it is 4.5 seconds.
What do you think?

Maybe I should also feed an external voltage of 900 mV to the ECU with the unplugged o2-sensor and check the voltage which the o2-sensor delivers then!? Then it should react with a voltage dropping below 200 mV (new o2-sensor: 100 mV), because with 900 mV input to the ECU the mixture would be changed to lean.
Or do you think I can also connect 1.5 V to the ECU without putting it at risk? I mention 1.5 V, because I have some 1.5 V batteries at hand.

67

@‌HD

Regarding that oxygen sensor . . .

I think you should backprobe the sensor signal wire, rather than unplugging anything at this point

I’m going to give some generic advice, as far as oxygen sensors go

The sensor should fluctuate rapidly, below 200 millivolts to above 800 millivolts, and the average should be about 450 millivolts

Even though old sensors may have the range, they may slow down, just as we do when we get older

As I said before, the adjustability of this system has always bothered me. By messing around with the mixture, you might create a condition in which the sensor is switching very slowly, or doesn’t have full range

Here’s an idea . . . once you’ve backprobed the signal wire, create a large vacuum leak. The signal voltage should drop below 200 millivolts very quickly. It should actually drop far below that, perhaps even below 100 millivolts. A very good sensor will even drop below 50 millivolts. If you can find some way to create a very rich mixture . . . propane, perhaps . . . the sensor voltage should very quickly go above 800 millivolts.

To be honest, I’m not even sure the oxygen sensor has anything to do with the rough idle. It’s hard to say when I’m imagining this car in cyberspace

68

Okay – first I have a confession to make:

Today I wondered, why the duty cycle readings were so different between 2500 rpm and idle after I had changed the CIS base setting on Saturday. Then it crossed my mind: I had forgotten to pull off the small vacuum hose from the fuel evaporation system and to block it first.

So today I repeated the adjustment of the CIS base setting (with the disconnected and blocked vacuum hose) with the following results:

  • at 2.500 rpm: ECU duty cycle output fluctuating between 45.5 % and 49.5 %
  • at idle: ECU duty cycle output fluctuating between 44% and 48%
  • difference between both center values: 3.15 % (target according to Mercedes: maximum 10 %)
    The Mercedes people always say that experience has shown, that the M103 engine runs best with an ECU duty cycle output between 40 and 50 %.

After that I repeated the o2-sensor test procedure that I conducted yesterday with the following results:

  • without connection between the ECU and ground the voltage dropped to 7 mV ! within 45 seconds
  • with the ECU connected to ground the voltmeter rose within 10 seconds from 7 mV to 730 mV and within the next 10 seconds it rose further to 840 mV.
    That shoes that the sensor still has almost full range (0 – 900 mV), when continuously exposed to a too lean or too rich mixture for a long time.

After that I backprobed the o2-sensor signal wire (with the sensor connected to the ECU) – as you suggested db4690.
To connect the voltmeter to the o2-sensor cable without damaging the cable, I opened the connector of the sensor and connected a selfmade adapter between the male and the female plug of the connector (see picture).
Unfortunately I don’t have an analog voltmeter at hand on which you could sufficiently see voltage flucuation below 1 V. And you know how it is with the digital voltmeters – they can’t follow the voltage fluctuation in real-time.
But it was rather well recognizable that the sensor signal fluctuates nicely around the ideal value of 450 mV (approximately between 300 mV and 600 mV). And the voltage range hardly changes at various engine speeds.
So from my point of view, if the voltage output of the sensor corresponds correctly with the real oxygen level of the exhaust gas, the oxygen level is being kept around it’s ideal level.

A drive after the new CIS base setting was pleasantly showing a slight improovement of the idle quality too. Even my wife – who is not very sensitive about cars – said that the kicks have become less frequent.
But idle quality is not quite what I want it to be yet.

I already mentioned, that the cycle time at idle is probably to long (4 - 5 seconds, at around 2500 rpm it’s 1.2 seconds), probably – as you say db4690 – because of the sensors age (26 years) and milage (120000 miles).
But under normal operating conditions (connected with the ECU, which constantly changes the mixture), the amplitudes (+/- 150 mV) seem too small to me too.
What do you think?

69

@‌HD

The sensor should be fluctuating from less than 100 millivolts to above 800 millivolts in closed loop . . . with a warmed up engine, and everything connected.

That doesn’t seem to be the case

4-5 seconds is too slow, in my opinion

It sounds like your idle is 10 x better than when you started this thread . . . !

70

I apologize, @HD, I’ve lost track; have you installed the new injectors?

71

At the risk of sounding like a stuck vinyl record, verifying beyond all doubt whether there are any vacuum leaks at all would still be a first step in my opinion.
Overanalyzing things can be an easy trap to fall into.

72

@db4690:

In closed loop like yesterday, with everything connected and the engine so warm that at times the cooling fan started to run, the digital voltmeter never displayed voltages close to 100 mV or 800 mV.
At times you could see voltages in the area of 180 mV or 700 mV. But in 90 % of the cycles the display fluctuated between roughly 300 mV and 600 mV.

Yes, you can say, that idle feels almost 10 times better now.
The new transmission and engine mounts brought a huge improvement. Yesterday’s CIS base setting adjustment definitely added a little improvement too.

It looks like the CIS base setting adjustment became necessary with the decreasing quality of the injectors and the o2-sensor.

I will check this setting again after renewing the o2-sensor or the injectors. I will certainly have to adjust the setting again then.

@insightful:

I haven’t installed the new injectors yet, they’re still laying on my desk, together with new guide- and seal- sleeves.

Since idle quality is so much better now, I’m not so much in a hurry anymore. Now it’s more about understanding the car in more detail. Maybe also other readers, who own a car equiped with a continuous injection system, may benefit from what we guys find out about it.

I may install the new injectors after all, even if the renewal of the o2-sensor should unexpectedly solve the rest of the idle problem completely.

Besides other reasons, the injectors could at least still be the reason for problem No. 2 in my initial posting (engine stalls at idle and has to sit for about 5-10 minutes before it starts again - each time in the city, when the engine was rather hot).
Maybe one or several injectors are spraying too much fuel at a certain moment during idle, causing:
a) engine flooding and stalling
b) a vapor lock
The vapor lock could occur due to a sudden drop of pressure in the fuel lines at the moment when the injector(s) spray(s) to much fuel. The higher the heat from the engine and the lower the fuel pressure in the fuel lines close to the hot engine, the more likely the vaporization of the fuel.
But since that happened only 5 or 6 times during the last 15 months, there’s no urgency either.

First I want to renew the o2-sensor and see the result of that.
That may take a little while, I have to synchronize with the workshop. I don’t expect the old sensor to come out easily after 26 years, so I don’t want to jack up the car and crawl under it to try and get this done.

@ok4450:

I’m quite sure that there are no vacuum leaks anymore. There was one when I tested the whole intake system last fall, it was the injector seal of cylinder No. 5. However after renewing all 6 injector guide- and seal-sleeves when I tested the old injectors, there was no noticable improvement to the idle quality.

But - since you suspect false air so emphatically, and since I don’t have a smoke machine, which db4690 suggested, I’m considering to buy a package of cigarettes. As a nonsmoker I just hope I’ll not become sick!

73

This is in regards to vacuum leaks and idle quality

I have worked on numerous cars that had vacuum leaks, yet had an excellent idle

After fixing the vacuum leaks . . . to correct maxed out positive fuel trim . . . the idle was the same

74

I’ve also worked on cars with vacuum leaks that had an excellent idle and also ones with leaks and bad idles.
My CIS experience has been mostly with VW along with a few Volvos and BMWs; not Benzes. A leak on a VW would mean a rough idle to some degree. The usual suspect was injector seals.

I’m not saying that the OP should spend big bucks on a smoke machine that will be seldom used. A vacuum gauge is dirt cheap, easy to use, and may reveal any one of a number of faults that could cause a rough idle.
I’ve seen a few rough idles caused by one weak cylinder head valve spring although I would tend to think that is not the case here. A gauge would show it though.

75

I’m not suggesting OP buy his own smoke machine

All I suggested was that he consider bringing it to a shop and asking them to smoke the car

76

@db4690

“…I’m not suggesting OP buy his own smoke machine …”

I also understood your first posting to my thread to the effect that it could be useful to just use someone’s smoke machine or have someone use his smoke machine on my car, and not to the effect that I should go and buy one.

In my last posting I should have said more properly:
“…since I don’t have a smoke machine, something which db4690 suggested to use, …”
instead of:
“…since I don’t have a smoke machine, which db4690 suggested, …”,
Sorry for that, db4690 !

@ok4450

I agree with you, if you would use a vacuum gauge to check whether there is a leak somewhere.
But then you would still have to locate it, for which smoke would certainly be helpful.

Before I would test the intake system with a vacuum gauge, I better wait until after I renewed my o2-sensor first, because with the slow reaction of the current sensor at idle there’s to much flactuation in the idle speed in “P” or “N” now (625 - 725 rpm), causing a flactuation of the vacuum level too.
Unless I use the parking brake and put the transmission in “D”, in which idle is very stable at around 540 rpm. Of course the vacuum level would be lower than too.

I never used a vacuum gauge before. Where do you usually connect it?

77

it sounds like you have it figured out. good work.
i need to spend some quality time with my vehicles too.

78

A vacuum gauge can be connected to any vacuum source from the intake manifold as long as it’s below the throttle plate; meaning between the throttle plate and the cylinder head.

If a leak is suspected based on the gauge reading then one can start gently pinching off vacuum hoses with a pair of needle-nose pliers. That will at least steer you in the direction of the leak; assuming it’s not related to injector seals, intake gasket, etc.

79

OK4450, you’ve been helping countless people with your own high level technical expertise for years here. I seem to recall a SAAB carburetor that did not seem to respond to vacuum the way it should. If that one never got resolved, HD might be just the guy to offer some ideas. Like your own, his level of knowledge and expertise seems far higher than many of us, and being in Europe he may have more exposure to SAAB eccentricities than most. After his challenge is resolved, perhaps he’ll stay with us long enough to take a look at that one of yours that “got away”. I still think about that one occasionally, but it’s admittedly over my head.

For the record, HD, while I’m unable to help I still follow this thread regularly to learn from it. And you have my gratitude for providing such a great learning experience.

80

@ok4450
I think buying a vacuum gauge wouldn’t hurt. Not only to check the intake system, but also for other things, like problems with the vacuum operated central locking system of my Mercedes.

@the same mountainbike
Of course you can see SAAB frequently on the road here in Europe. But I don’t know much about these cars. Come to think of it – so far as I recall, I have never sat in one either.

@wesw
“…it sounds like you have it figured out…”
Well, I’m not 100% happy with the idle quality yet. The KE-Jetronic is quite comlex - and to be honest, I’m not an expert on this system. I keep finding out more details about the synergy of it’s components, which leads me to ask a question:

@db4690 or any other (Mercedes) expert:

Yesterday I did another fuel pressure test:

  • upper chamber pressure (“control pressure”)
  • lower chamber pressure (“primary pressure”)

I did that according to the Mercedes workshop manual:
In there it says under “07.3-1603” that, in order to measure the lower chamber pressure, the engine must be warm and the fuel pump relay should be pulled out and a bridging wire should be used to let the fuel pump run.

It says further:

  • Switch on the ignition.
  • Then pull off the EHA plug.
  • Then read the pressure gauge.
  • Then reattach the EHA plug. Pressure must not change !!!

But the pressure dropped promptly by 3 psi !
I pulled the plug off again – pressure promptly 3 psi higher again.
I reattached the plug again – pressure promtly 3 psi lowerer again.

First I thought there’s probably something wrong with the EHA (a component which Bosch charges 360 Euro for)
I checked the current delivered by the ECU to the EHA. The ampere-meter read 19.82 mAmp (close enough to the target with ignition switched on: 20 mAmp).
And then it hit me: besides load or overrun – for instance at idle - the current delivered by the ECU to the EHA with the engine at 185°F, fluctuates with maximum +/- 3 mAmp around 0 mAmp after the o2-sensor is hot enough to start working. Before the o2-sensor starts working it’s 0 Amp. But without the engine running and only ignition switched on, the ECU delivers + 20 mAmp to the EHA. And + 20 mAmp brings the valve plate of the EHA significantly closer to the EHA’s inlet hole, thus narrowing the gap which the fuel has to pass from the upper to the lower chamber, which results in a drop of pressure in the lower chamber. And that’s what I read on the gauge!

Then I brought the engine back to 185°F and meassured the lower chamber pressure again – this time with the engine RUNNING (at idle).
I read the gauge: pressure at target.
I pulled off the EHA plug – no pressure difference!
I reattached the EHA plug – no pressure difference!
That’s what I wanted to see!

So what do you think?
I think there is a mistake in the Mercedes workshop manual:
Instead of only ignition switched on, the engine must be warm and running while meassuring the lower chamber pressure !