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Old 02-17-2006, 11:22 AM
Registered User
Join Date: Nov 2004
Posts: 121
300ce running rough


I have read most of the posts on this site pertaining to “Rough Idling” and have applied them where appropriate to my specific problem.
I am now getting to the end of the line with this issue, and my wife is getting upset with me.
Here is where I’m at……………..

I have a 300ce/1989 Calif. model , with 200k miles. This has been a great car with the usual on going maintenance. The oil and the filter have been changed every 3k miles and basically this car has been well looked after. Just recently the car started to run rough at idle to the point that my wife was scared to drive it. Before it was as smooth as silk.

When we first bought the car [100k miles] it would randomly stall usually on de-acceleration.
This was fixed by me with a combination of, fuel pump relay, OVP relay, Idle control valve, De-acceleration/idle micro-switch and the 4pole temp sensor, the last one on the head.

So here’s where I’m at with this newer problem to-date…………….

1] Replaced the EHA valve it was leaking slightly though the valve body. However leaked from the “O” seals at first.

2] Replaced the spark plugs, it was time. These are the Bosch ones that are recommended for the car, with the .035 gap.

3] Replaced the injectors with seals and holders, again it was time, these were the original ones with 200k miles. Seemed to run smoother when I got going.

4] Replaced the O2/Lamda sensor with a universal type, again this was time, 200k miles.

5] Adjusted the air/fuel mixture using the Lamda sensor as my input guide.

6] Checked the fuel pumps[2 in series] for operation appeared to be ok. When you put your hand on them you can feel the operation and here the wine.

7] Checked and cleaned the Idle control valve appeared to be Ok. This is relatively new item.

8] Checked the Air Flow Sensor appeared to be OK and was within specs. This also is a newer item.

9] Checked the air throttle control flap switch for idle and full throttle again worked OK

10] Cleaned the intake body around the air throttle flap.

11] Replaced both the new spark plug and ignition leads with an older good set, no change. Also checked the spark, appeared to be OK.

12] Checked for leaks using propane gas then again with soapy water, disconnecting feeds and plugging them. The feed that goes to the air conditioning appears to be open. So did the Charcoal emission circuit . I tried using my Mitivac on these but could never get any pressure? Also disconnected the Brake Booster feed and plugged. However this didn’t change anything on my inlet manifold when plugged.

13] Checked the vacuum pressure on the Fuel Pressure Regulator with my Mitivac seemed to be OK. I don’t think this the correct test for this unit. At least it held the vacuum.

14] Replaced fuel filter around 5k miles ago

15] Checked the car at night[pitch black] to see if I had any arching.

16] Checked the inlet manifold with a Vacuum gauge, no apparent problems her thought the rpm range. A manifold leak would show a low steady reading.

17] Disconnected the start valve electrically to see if it was leaking. However it could be stuck open very slighty. But the Lamda readings didn’t show this.

What needs to be checked is as follows………..
1] Cylinder Compression. This hasn’t been done yet because the car is so strong in acceleration and has lots of power.
2] Fuel pressure and double check the FPR as indicated in 13 above
3] TDC sensor
4] Hall effect sensor
5] Altitude correction sensor
6] Double check 4 pole temp sender.
7] Cylinder Leak down test
8] revisit the ignition.

Well that’s about it for now, I’m still working this problem and will go back and revisit some items.

I have one question regarding the valve adjustment. I know on the diesels, I have three that the valves tighten up over time. Does anybody know if this is the case with hydraulic ones. Occasionally I get a loose one for a few seconds on start up and then it fills up and goes away.

Any ideas would be appreciated

300CE, 240D, 300D Euro, and a 300SD

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Old 02-17-2006, 03:13 PM
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Join Date: Apr 2005
Posts: 51
poor idle

Make sure you used the correct plug, copper core design only. (read that you already have)Since you have changed almost every possible thing that supports the engine management system except the CIS-E unit, try a new coil.... Also get a fuel quage with a "T" fitting, between the fuel delivery line and FPR. You need around 80PSI there constant. You might have carbonization and flaking on top of the pistons causing a rough idle.(They remain glowing after a power stroke and cause uneven burning plus pre ignition problems) Try running a one pint can of Marvel Mystery oil mixed with gumout, (one quart) slowly fed thru fuel plate opening at 3500 rpm. Take your time doing this, maybe 15 mins, do it slowly and dribble it in, change the plugs after set @ .034 and see....

Last edited by dave_rose69; 02-17-2006 at 03:26 PM.
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Old 02-17-2006, 03:16 PM
yal's Avatar
yal yal is offline
Join Date: Oct 1999
Location: New York, Long Island
Posts: 2,707
How bad is the idle? Is it worse in drive? I may have missed it on your list, or you've probably checked it, but how about the motor mounts?
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Old 02-17-2006, 04:15 PM
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Join Date: Jul 2001
Location: SoCal
Posts: 1,303
How is your idle rough? E.g., idle speed varies, speed constant but appears to be missing, car vibrates, engine randomly speed way up then drops back down, etc.

Also, item #13 - the fitting I think you are testing is not a vacuum port - it goes to the breather side of the pcv orifice. It is supposed to keep your engine from catching on fire the FPR fails.

'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7
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Old 02-17-2006, 05:29 PM
Bigpete123's Avatar
Join Date: May 2005
Location: Seattle
Posts: 130
I'm not a mechanic at all... But sometimes the solution is extremely simple. My 1990 300ce was having lots of idling problems and I had a million things replaced. I spent over $7k on a laundry list of stuff. The idle problem went away for a bit, but came right back after 1,000 miles.

I sent some injector cleaner through and the car actually ran much much better after that. So the next time I went in for an oil change at Jiffy Lube and they tried to sell me their fuel-line cleaning service, I took it. My car has 190k miles or so... Apparently everything the fuel runs through gets cleaned. I changed my fuel filter, too. When the ran their cleaning mechanism, my car sent white smoke everywhere, for a couple of minutes. It was crazy! Their whole service bay was smoked out and the employees had to step outside. Apparently, my combustion chamber, etc was caked up with crud. Now, when I start my car, I can't even tell that it's running. When I accellerate, it doesn't hesitate at all... The improvement was far greater than what I noticed when I had the mechanic tear everything apart and replace nearly every damn thing under the hood!

Give it a try! I've also heard of a similar thing called BG 44k from these forums. Good luck, I know how frustrating this sort of thing can be!!
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Old 02-21-2006, 10:29 PM
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Join Date: Nov 2004
Posts: 121
300ce rough idle

Hello again,

Thanks for the input from fellow DIY engineers

I decided to do the BG44k treatment as suggested by two of the respondents. See their site…………..

This process is a three part application 1] Inline fuel injection through the gas line, 2] Spray product through the distributor air inlet, and 3] The tank supplement. This process is done via approved garages using special injection process is and is supposed to remove combustion crud, distributor build up and intake crude. This was not cheap, it cost me $250 for the labor and $80 for the stuff. They said Mercedes was more complicated then other auto’s to implement. My initial concern was what would it do to my Kat and o2 sensor. I was assured NO problem!

Anyhow, it’s done and the car runs a lot smoother then before. Lets face it after 200K miles there has to be some gunk in there someplace. This has given me the basis to start again. I adjusted the Lamda settings again for good measure.

However, now it’s intermittent, idling roughly. Not as bad before.
When you come to a stop in drive it will slowly drop RPM until around 500 rpm. Then it will start to what appears to be missing. Give it the gas and the car runs great. Overall performance is excellent.

It almost makes you think the plugs are going. I have new Bosch H9DCO . Anybody think otherwise let me know. These are recommended and listed in the owner’s manual.

I also had another go at my fuel regulator. I actually removed it this time and gave it the air test. I applied air pressure to the pressure side and it held its ground. Nothing coming out of the return line. Incidentally there are two none return valve at the fuel pumps, one for each, which can/could cause issues. I addition to the fuel accumulator at the pumps.

I also had another go at the cold start valve and low and behold the pressure on the input side is not holding. I applied my trusty Mittvac and it wouldn’t hold up. I have decided to order a new one anyway [200k miles?]. This could cause dribbling fuel into the intake manifold if not completely closed, regardless of the computer [CIS] telling it no. It met the electrical specs has previously mentioned.

I also looked at the relationship between the idle stop switch [deceleration stop switch], the throttle position stop and the kick down cable adjustment. Per the specs I readjusted. My wife didn’t tell me that the kick down was not working the way it should have. Now I have a nice gear change if I need it. I was reviewing the way idle is set with this relationship hoping I could get some extra RPM.

I will update you all when I have installed my Cold Start valve.

Anybody have any thoughts on the grounding of the o2 sensor in the exhaust. Did a digital ohm check between the exhaust and the battery ground and I read some 3-4 ohms. The exhaust is the ground for o2 sensor. It seems to me with such small voltages we are dealing with here that a solid ground back to the source would appropriate?
I guess the o2 sensor engineers no what they doing??

300ce, 240D, 300D Euro and 300SD
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Old 03-05-2006, 01:16 AM
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Join Date: May 2005
Location: Seattle
Posts: 130
Any updates?

Have you figured out the problem yet?

EDIT: or should I say, have you figured out the cause??
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Old 03-05-2006, 04:08 AM
Posts: n/a
Rough Idle

Try this. Use a meter with duty cycle. Check at x11 test pin 3 at idle.Engine temp 180. Idle duty cycle adj to 50% +- 5%. Run engine to 2000 rpm. Duty cycle 30 to 40%. If not. Take The EHA out. Take the plug out at bottom use a 2 mm alan key to adj turn L to lean R to rich.High no lean loe no rich. Put back.Re test again.You may need to do 2to3 time to get it right. On the left side of air flow sensor housing. This is the air flow meter with A plug have 3 wire. Pull out about 1/8". Use jump wire at centre wire. Use a volt meter. You need to have .65v to .85v. If not. Loose the screw 1/4 to 1/2 turn. Adj to .65 to .85v at idle engine temp at 180. Take car out for a test drive. Re test & re set to get it right. Hope this help you rough idle.
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Old 03-05-2006, 05:17 PM
Registered User
Join Date: Nov 2004
Posts: 121
rough idle

Hello again,

Thanks for all the comments and suggestions.

To update this thread, I have now got the car to a point where in Nuetral or Park it idles great. Around 550~600. I believe thats close to the spec [600~700 rpm].

However, when Drive or Reverse is selected the idle drops slowly to around 500 rpm. Then a bit lower and then starts to shake slightly.

This car is my wifes and she has put around 150 k miles on it. It never had engine vibrations before, sometimes slightly but would always go away. She is a bit nervous driving with it now the way it is.

To summarize items replaced so far were 1] EHA valve, 2] O2 sensor, 3] Fuel injectors, 4] Spark plugs[ Bosch 9DCO], CIS temp sender. These were all replaced based on analasis of the problem and the high mileage on the specific items. In excess of 100K miles.

Items replaced before this, [approx 100K miles ago ]were the Idle control valve, Fuel pump Relay, OVP relay, Idle position micro switch, plugs, points, HT wires and rotor.
This was to solve a stalling problem which worked.

I would love to squeeze anothe 50~100 rpm out of the CIS. This would solve my problem now.

I have some questions for "Andengines".............

Please bear with me on the Tech Lingo talk you have used, I'm not a mechanic!
The meter you talk of that has "duty cycle" is this a Multimeter? I have at my disposal a analogue Multimeter and Digital meter. What are we adjusting, the Rich/Lean screw in the Filter housing? or the EHA screw.
With the air flow meter test, are we adjusting EHA screw or the Rich/lean screw.

Please advise

PS: I'm at a point now where I would like to fix this thing or go to alternatives like this one.....
This note from the Mercedes Workshop Manual that discusses "Vibrates at idle", in effect says that you can glean another 50 rpm out the CIS by splitting a wire that runs from the Starterlockout switch to the CIS, and connect the CIS end to ground
It seems to me that Mercedes recongised some time ago that idle adjustment is a complex problem and even if all the parts are working you may still have a problem with idling per specs. Sooooo, they came up with a couple of mods? This one and the resistor in the CIS temp sendor circuit.
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Old 03-05-2006, 06:25 PM
Posts: n/a
Red face Rough Idle

Brian. sorry I am no good on writing. You need the MULTIMETER with duty cycle. The rich/lean screw in the filter housing only adj idle rich/lean.The EHA screw adj off idle. You need that adj to 30% to 40% at 2000 RPM. Air flow meter test is for how many air go in to engine. At idle if over .9v. When idle down the RPM will down to 4-500 rpm then go back up or stay there or stall. Check the air flow meter volt first. The EHA is for fine tune the engine. All so you need check the KLIMA RELAY .The one beside the fuel pump relay. This relay control the idleing, Open the couver check inside see is any solder is crack. If you need to adj the air flow meter volt. Pop both side small couver out at air flow meter. You will see 2 screw each side. Loose it about 1/4 to 1/2 turn. tap the meter back & fore to adj to .65v to .85v. And check the fuel pump and OVP relay. Hope this help. Let me know.

Last edited by andengines; 03-05-2006 at 09:44 PM.
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Old 03-08-2006, 04:47 PM
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Join Date: Nov 2004
Posts: 121
Air flow meter


Thanks for your reply.

I have tested the air flow meter and have around 1.4 volts between pin#2 [center pin] and ground.

Andengines......."you said it should be around o.55 volts to o.95 volts", correct?

The resistance of AFM appeared to be within the specs of 1 ohm [ign off] It rose in resistance evenly when the throttle/air plate was depressed.

My question is with 210K miles clocked todate, it might be time for a replacement.
What do you think?

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Old 03-08-2006, 07:18 PM
Posts: n/a
Adj Afm

You need the volt to tell the comp todo compensation. Not the OHM. When you push down the AFM plate down the volt go up evenly. The afm is ok. Do not need to change. 1.4v will give you rougn idle. My 87 16v engine cuts out and intermittent idle surge when stop or try to parking. I adj from 1.35v down to .8v. Run just like new. Give it a try. My wife 85 8v have 320000km same afm. Run better than my. So some time change things is not go by how old or miles.
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Old 03-08-2006, 09:52 PM
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Join Date: Nov 2004
Posts: 121
Air Flow Meter


Thanks for your reply and suggestions.

I had already adjusted the Air Flow Meter per your comments.

I have now around 0.65/0.70 volts.

I double checked the Mercedes specs in the book/CD. They call for .55v to .95v, for engines that are 103.980 .75 volts to 1.05V.

How critical is this? What do you think of changing the AFM anyway, or as it meet specs it should be OK. A lot of the time the engine is sitting at idle and in this position the Air Flow Meter wipers will get its max wear in the same spot? Even though it now meets the specs I still got the idle around 500 rpm in drive. I tried swapping to an old good Idle Control Valve, but the result was the same.

In a separate issue, I had in an effort to get rid of the rough idle, changed the spark plugs gap to .030 from the spec of .035. Today I changed it back to .030. Wow, it made it worse! Is this trying to tell me that I may have a HT voltage problem.

Currently in Park or neutral its almost a perfect idle around 500 rpm. As we know this is not enough for reverse or drive. Once you get going this car drives like it was new!

Sooooo, in frustration today I did a CIS mod. Suggested by another lister under "Ripples in my Coffee". This grounds one of the wires that comes from the Ign switch via the starter lockout. Guess what this does work! For my wife to start using the car its great. However, I'm not happy with the solution.

There has to be something that we can do to solve this problem.

Unless because of the age, and the miles 210k, I have this has an ongoing problem. The biggest overall solution for me was to clean out the combustion chambers and the intake manifold with the BG44 professional cleaning. I have mentioned this earlier in my postings. Not cheap though!

Once you get going with the trouble shooting of the Bosch KE system you realize that the complexity of it all is enormous and there so many factors envolved. I can see why some listers have clocked up thousands of $$$ in repairs by going to the professional shops and they don't have the expertise either.

Incidently I didn't touch the EHA valve, this is brand new and didn't want to mess with it!

Any thoughts

1989 300ce/210K miles
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Old 03-08-2006, 11:07 PM
Posts: n/a
If you can adj to .7v. The afm is ok. If afm no good. You can not adj to that volt at all. If you have 500rpm in gear. Engine not stall. ok. Time to fine tune the system. Check the duty cycle at 2000 rpm adj to 30%to40% from eha. re set idle duty cycle to 50% +- 5%. You need set the system to engine cond. You put new part in older engine. Some time you need to fine tune it. When I change the leaking eha on my wife. Engine run rough till I fine tune eha and idle co.
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Old 03-09-2006, 08:04 PM
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Join Date: Nov 2004
Posts: 121
Air Flow Meter[AFM]


Thanks for the continued support.

When I was adjusting the AFM, i had left the screws loose and some air was getting in. This increased the idle. Surelythen we need to tell the Idle Control Valve to let more air in at idle?

Whats your take on that.

Please advise


PS: I havn't played with the EHA valve yet. Do you know if the adjustment is the same as the lean/rich screw. e.g. Clockwise for enrichment, and anticlockwise for lean. I had played with the old EHA valve before replacing it and I got all out of wack.

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