'86 300E rough idle when warmed up - PeachParts Mercedes-Benz Forum

can-do · #1 09-02-2003, 07:20 PM

Greetings All,

I know your first response after reading this is to tell me to use the search to find my answer. I've been here three days using the search to get some idea as to the problem and troubleshooting plans and have tried quite a few of them today without any luck.
Here's the problem. The engine starts up fine and idles great until operating temps are achieved. When bringing the car to a stop and letting it sit for perhaps 3-4 minutes in drive it starts what I call missing, or a bumping action felt from the engine and the rpm in most cases drops from around 750 to about 500-600rpm. As you start to accelerate there is a chugging action of the engine until about 2000rpm where it then feels as if nothing at all is wrong. It being a cool day today I wondered if engine temp had any relation to the missing of the engine at idle so I kept the a/c engaged when driving which elevated the engine temp at a stand still to about 90-100c if left there in drive not moving for five or more minutes. This did indeed accelerate the missing of the engine. The things I have done today to see if perhaps these were the problem areas are as follows. I checked what I think were all the engine vacuum lines for breakage and vac most of them down to ensure the did hold a vacuum. I removed and cleaned the idle control valve. I replaced the fuse on the ovp, not bad anyway, and cleaned plug-in pins. Removed the distributor cap to check the condition, and cleaned the contact points on it. The cap didn't look to awfully bad, no carbon trails but I polished the leads with sandpaper anyways as well as the rotor edge. Seeing as this is my wife's daily driver, the only sympton of this happening wasn't gradual, but all of a sudden at a stop light that she was waiting at to change to green. Within the last 20,000 miles it has had the fuel filter changed, oil of course, several times, and techron run through it less than 3000 miles ago. Any logical ideas would be well appreciated. I'd like to troubleshoot this car again on Friday, so your input is most welcomed as well as any testing with reasonable tool means. If you think I have overlooked something, please feel free to ask as I tried to pay attention to detail as this has all occured.

Thanks,

Can-Do

can-do · #2 09-03-2003, 07:48 PM

Greetings,

I know everyone is busy out there with not only their own lives and auto problems, but I've run out of ideas of things to troubleshoot to fix this problem.

Help,

Charles

ctaylor738 · #3 09-03-2003, 08:04 PM

Everything I have tried or planned is in my posts, especially the latest one on "Idle Control Valve." It is a very frustrating problem to try and solve.

mike690003 · #4 09-03-2003, 08:09 PM

I am having the exact same problem. I just posted basically the same problems. I have replaced numerous parts and nothing has changed. It still misses when it gets fully warmed up.

New:

OVP relay
02 sensor
fuel filter
cap and rotor
plugs and wires
fuel filter

cleaned idle control
numerous bottles of techron
replaced all vacuum hoses

and it still misses when warmed up....I have to unplug my new 02 sensor, because the missing gets even worse. My air/fuelmixture is set to spec.

HELP!!!

zorin · #5 09-03-2003, 09:15 PM

Connect a voltmeter to the battery while the problem is happening. If you see the voltage fluctuating then you have a bad regulator/rectifier on your Alternator.
Otherwise it could be a bad ignition coil.

can-do · #6 09-03-2003, 09:36 PM

Greetings and Thanks for the replies,

First for chuck Taylor, you are definitely dedicated to keeping your Benz running strong with the research and troubleshooting you do to keep it running. My question to you is whether or not your car actually acted up at idle but never died as mine does. Mine goes into this bumping, missing fit if left to idle to long after warm up and chugs it's way out of it at 2000 rpm after you start accelerating. It has never stalled yet in this condition, even with the a/c on. Also wanted to ask you if the air flow position sensor is a part that can be bought seperate from the air flow meter, or not? I wish I had the high tech equipment to test this out but all I have at this point is a vom for electrical troubleshooting which is basically all I needed as my other Benz is a diesel, wish the wife could stand the racket of the engine and switch, but that's another story.

To Zorin, I'm not quite sure where the relationship is between the alternator and the fuel/air system of this car are, but if you have hidden info, please reveal it.

To Mike, I can only feel for you at this point in time and if you don't recieve any answers to your post please stay tuned to mine. I noticed your post first thing after signing on, so it must be pretty common from all the search responses I've read this week, and various fixes have help several, so who knows at this point. I'd just like to not have to spend $1000.00 to find out the problem was a $50.00 fix, not good economics and it would actually piss me off in the process when the kids need clothes and other supplies and I spent the money on replacable troubleshooting.

Keep the ideas coming,

Charles

zorin · #7 09-03-2003, 09:54 PM

What I'm saying is that your problem maybe electrical rather than fuel/air-related. I had the same symptoms on my 300E , and that's how I fixed it. But in your case might not be the same.
Just one more thing to check.

can-do · #8 09-03-2003, 10:46 PM

Greetings Zorin,

What engine symptons did you have? I don't currently have any electrical drain on the engine, IE. dimming lights while idling, overloaded alternator running or at idle. Nothing points in the direction that the alternator isn't keeping up with the output of eletrical needs. I'd think the demand at a higher RPM would be greater on a coil, and that's where the engine actually smooths back out. I have no problems whatsoever at highway speeds with any aspects of this car.

Charles

ctaylor738 · #9 09-04-2003, 12:48 AM

Mine started out as a stumble, then worsened to a stall at idle plus rough running at low RPM. With the OVP and new coil it has gone back to stumbling but not stalling.

Everything thing I have done has made it better for a while but has not seemed to address the basic problem.

My next suspects are the idle control valve and hoses, fuel injectors, and (shudder) the ECU.

My tools are a DMM and a dwell meter - not exactly sophisticated.

You can buy the airflow meter from Bosch separately or the complete pre-calibrated air/throttle plate assembly. There are lots of posts either way. "Airflow meter pot source" is a good one.

zorin · #10 09-04-2003, 01:23 AM

I had two separate problems with two different sets of symptoms and causes :

On 260E:
1. a) Engine was OK at high speed but would stall at traffic lights.
b) Engine was ok when cold but once it got to the operating temperature rpm would drop and eventually stop. However when I kept the RPM at about 2500 it would run smooth.
c) Hard start after stalling

These problems disappeared when I took the Ignition Coil out of my 300E and put it on the 260E. (of course all these symptoms then appeared on the 300E)

On 300E:
2. a) dimming lights at low RPM
b) Seemingly rough idle - (as if jumping a cylinder)
c) hunting engine
d) fluctuating Voltage at the battery

These symptoms went away when I replaced the rectifier/regulator on top of the alternator.

Benz300 · #11 09-04-2003, 11:13 AM

Can-do,
my 260E has the same problem as well. After spending loads of cash on the problem here's my sincere advice to you. If the car doesn't die when the idle is rough just run it till it's time to junk it. otherwise these cars can be a black hole for your hard earned money especially in problems where it's almost impossible to pin point the problem.
good luck !

#12 09-04-2003, 12:53 PM

I also have a (sporatic) rough idle. It usually starts 15-30 seconds after coming to a stop and only when the engine is at operating temperature. In fact after a cold start you can feel the roughness start as the engine comes to operating temp. I'm thinking its a non-electrical problem (ie:vacuum or fuel). At this point I'm taking BENZ300's advise.

Mark
1989 300se
69,000 mi

can-do · #13 09-04-2003, 08:17 PM

Greetings All,

I realize some folks would let the situation continue on without fixing it, but if you bring up all my previous posts you'll probably come across my first 50-75 posts and questions concerning a non running diesel 300TD that through troubleshooting and lots of patience it became a great addition to the family, running and with very little outlay of cash because I didn't do the replacement method of troubleshooting to make it happen. I know this is a different animal, but the game is still the same. I wouldn't expect the wife to drive this car daily with it's current running condition so yeah, it's gonna get fixed without using the total part replacement method of troubleshooting.

Chuck, after reading your previous threads on your car condition, it appeared that you had fixed it with the air flow position sensor. This isn't correct then, I take it? I am also wondering what the corrolation would be between the OVP and the rough idle. I have had the antilock light come on a few times and not go off during the duration of the drive and have either replaced the fuse that wasn't blown and wipe the plug in pins off then reinsert it and everything seems fine. Can the OVP cause this rough idle situation? The other question was whether your rough idle situation was gradual or all of a sudden like something stopped working right then? Which is the case on this car. I'd say some of the techs out there can probably narrow it down for us, but still waiting for them to chime in if they will.

Any more ideas, please chime right in

Charles

mike690003 · #14 09-04-2003, 08:22 PM

I really hope someone comes up with a solution for this irritating problem. I have spent a lot of $$$ and I am still experiencing the same symptoms as "can-do".

Help us please!!!

ctaylor738 · #15 09-04-2003, 09:25 PM

The OVP relay supplies power to the ECU as well as the ABS. So a flakey ABS can shut the ECU down which shuts the idle valve. That is why an ABS light was a what I thought was a major clue.

The night that it happened, Senior Management and I were on our way to dinner. The really bad stalling episode happened and we sort of rolled into the parking place. After dinner I started the car and it went into sort of "limp home" mode, with a high idle but running OK.

If you are getting an intermittent ABS light, I would definitely replace or re-solder the OVP relay.

About all you can do with this problem is test the wiring, and then throw parts at it because there are really very few diagnostic tests.

This is not so unreasonable as it sounds. All of our cars are old with lots of miles. All of the components have been hot, cold, and vibrating, for at least 15 years. If you need to replace the air flow meter, then the EHA cannot be in very good shape and will probably be the next to go. Same for the idle control valve. Same for the OVP. And the injectors have been subject to constant abuse.

Thee is no magic bullet - if we want good-running high-mileage 103's that will stay that way for a while, we are probably going to have to suck it up and replace all the stuff that is at or near the end of its useful life.

FWIW, when Herr Fuchs gets a car with this problem, his advice is to check the coil and replace the following:

- decel micro switch
- throttle position sensor
- air flow sensor
- coolant sensor
- OVP relay
- all vacuum fittings
- idle air hoses