300E Random stalling and rough idle - PeachParts Mercedes-Benz Forum

Brian300E · #1 05-08-2003, 07:06 PM

Hello all -

It has been a while since I've last posted here, but I've ran into a problem that I haven't been able to solve. So here it goes:

My 1987 300E will randomly stall. It has stalled on the freeway, backing out of parking spots, etc. It takes about 7 or 8 times times for the car to crank and re-start, but it eventually does. I am thinking it is something related to the fuel system, but I'm not sure how to even diagnos such a problem. Also, could this problem be related to the extremely rough idle and surging that happens when my car is stopped at a light? Any help would be GREATLY appreciated, because I haven't been able to find any solutions in previous posts.

I'm also looking for a good mechanic in the San Gabriel Valley to help fix this problem. Any recommendations would be really helpful. Feel free to e-mail me if it is easier (mu_brian@yahoo.com) Thanks so much.

Brian

#2 05-09-2003, 10:31 AM

Brian300E,
I believe you have a severe vacuum leak. You could also have a bad fuel pump relay.
The fuel pump relay in my '85 500SEC, is in the fuse box. It looks like black 1/2 pack of cigarettes. The newer ones have a small white filter disk on the front side.
It pulls straight up and out.
Pop the black cover off of it. It may be sealed but you won't harm anything.
You'll find two solenoids(coils inside brackets), the larger of the two is the fuel pump. The smaller is the over rev protection.
Turn the circuit board over and check the support bracket posts on the larger of the two. Mine were corroded and a hairline crack ran around each of the protruding posts. As the vibration of the running engine rattles this crack, the fuel pump shuts off and can re-connect from more vibes. Just re-solder the supports with a little flux and solder.
Also, as the engine's heat and age go up, the vacuum lines become brittle. Check your vacuum lines carefully.
This will help the surging.
Good luck, John F.

Brian300E · #3 05-09-2003, 12:19 PM

Thanks John for the help. I'll check out the fuel pump relay to see if there are any visible signs of damage. I think it is in a different location in my car - behind the battery. I'll let you know what I come up with.

Brian

WANT '71 280SEL · #4 05-09-2003, 03:33 PM

The rough idle and surging is normally the "idle control valve" I talked to a mechanic and he said to remove it, clean it out with carb cleaner, then spray inside with penetrating oil.
To get to the idle control valve, remove the air cleaner. You will see a cylinder about 4 inches long? It is shaped like a "T". It is in a rubber mount that is help by 2, 10mm bolts. On the front of it are two wires that join into 1 socket on the valve. It has one hose going off each side of the "T". Hose on the right goes to the fuel distributor, I believe. I am not sure where the one on the left goes. It is hard to see where it goes to. Once it is out of the car, spray the inside of the valve with carb cleaner and then try to get out all the excess out, maybe after it has set for a minute or so? Then spray with penetrating lube and reattach.
I know I am kind of rambling but the surge is common on 300E's and this is how I partially fixed my fathers. I also tried to be really simple as to the fact that I do not know about any mechanical background which you may have.
I am sure though that this will help or solve your problem.
David

#5 05-10-2003, 12:50 AM

Brian,

I would do all of the above. You might consider replacing all the rubber parts of the intake system. If is rubber and has been exposed to 16 years of engine heat, it is a potential vacum leak.

You might also resolder or replace the OVP relay.

If you still have the problem after all this, test (or have tested) the airflow sensor position indicator. Mine caused a problem exactly like you describe. They are now available by special order as a separate item so you do not have to buy the entire airflow meter.

Good luck and let us know.

WANT '71 280SEL · #6 05-10-2003, 12:53 AM

What is the OVP relay? Where is it and is it easy to repair?
Thanks
David

Brian300E · #7 05-10-2003, 01:27 AM

Thanks for all the replies. The car goes into the shop and is going to get looked at by Enrique. I'll make sure to post the results.

BENZ-LGB · #8 05-10-2003, 03:22 AM

Do a search on "hall effect sensor." I think that you will find that a defective HES is sometimes responsible for the symptons that you describe.

You also may want to, if you have not done so already, check your distributor cap.

good luck

Brian300E · #9 05-10-2003, 01:35 PM

I'll have the hall effect sensor checked too. My cruise control is erratic, but I thought that the HES only dealt with speed maintenance and control, not stalling and idle. I'll be sure to let everybody know what happens.

Brian

haasman · #10 05-10-2003, 01:55 PM

Brian

On a more check the basics question, the 300E did similiar things and it turned out to be a very bad rotor and cap? Have you checked these recently?

Haasman

UKEvo · #11 05-10-2003, 04:34 PM

Check for vacumn leaks, remove and clean idle regulator, clean air flow meter and throttle body housing. Check your ignition components - rotor/cap/spark plugs. Check the duty cycle to see if the mixture is within spec, adjust if necessary. If it's still a problem start looking at fuel injection setup and sensors.

#12 05-10-2003, 05:49 PM

I just connected with your stated location. If this is an original Calif car, you have stored fault codes accessable from the connector by the battery. This was only available for Calif 300Es in the 80's. This would seem to be a good starting point.

Brian300E · #13 05-14-2003, 01:21 AM

Here's the update:

I just got my car back from Enrique at Mr. MB motors. It turned out that the harsh vibrations in the engine were caused by bad motor mounts. He replaced the motor mounts, and the car now drives like it is brand new. I became so acustomed to driving the car with a rough idle that I didn't really realize how bad the problem really was.

As for the intermitent stalling, the fuel pump relay was the culprit. When I first took the car in I told him that I thought it could possibly be the hall effect sensor, but he said that since the stalling occurred when I was accelerating it could not be the problem. He said that the fuel pump relay was the most likely cause of the stalling, and he was 95% sure that this was the case. To date, the car hasn't stalled, so hopefully the problem has been fixed.

This was my first time taking my car to Enrique, and I would highly recommend him to anybody in Southern California. As posted in previous posts, he was extremely friendly and efficient. He had my car for only a day and even called to follow up and see how the car was running. He is the best mechanic I've encountered, and I'll definitely be going back to him for future problems. His prices were fair, and his diagnosis was spot on. If anybody has any other questions, feel free to e-mail me.

Brian

Mick J · #14 05-16-2003, 03:26 PM

If ABS light is not on then OVP relay is OK

hihosilver · #15 05-18-2003, 04:27 AM

my 86 is smooth running but if I use the cruise it won't stay at a speed unless I am on flat ground even then it willkind of surge like my old diesel cruise, but on any hills it is all over the place then drops out, any ideas