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PLEASE DO NOT ANSWER unless you work on Mercedes vehicles all day long. I'd appreciate it. I am working on a 1987 420SEL with only 60,000 miles. It was running perfectly when parked in a hay barn for the winter on trickle charge. Upon starting it started instantly. The problem is this. Stopped at idle, when just starting to accelerate it stumbles badly. As if it were a carbureator car with a dead accellerator pump. But it is fuel injected. It will stumble for about 3 seconds and then rev up normally. It runs perfectly with lots of power. It will stumble wether hot or cold. It does not matter if I gently accelerate or quickly accelerate. Same severity of stumble. No backfiring. I did an Italian Tuneup and red lined it while driving 6 times in a row to clear out anything that might be in the exhaust system. No change. I have ALL the factory original manuals. I am an Electronics Technician so I can follow trouble shooting on any component in most cases. It is that there are SO many components I don't know where to start.
Year: 1987 Make: Mercedes-Benz Model: 420SEL
Hi, it sounds to me as though you may have some clogged injectors or a clogged orifice in the fuel distributor. I see this problem a lot when a CIS fuel injected car sits for a long time. I would first make sure that the ignition is all in good order, plugs, cap, rotor and wires. Once you've eliminated any possibility of an ignition misfire I would be focusing on the injection. If I were you I would add some BG 44K fuel additive to the fuel tank and run one or two tanks with the additive and see if it clears up. If the fuel distributor is clogged or the injectors are dirty the 44K will usually help. You could also try tweaking the fuel mixture a tiny bit to the rich side and see if it helps. The mixture adjustment tower is between the air flow sensor plate and the fuel distributor. You insert a 3mm allen into the adjuster, press down to engage the mixture screw below and then turn the screw about 1 mm in the clockwise direction. It is a very sensitive adjustment. It only takes a little nudge in the clockwise direction to make a difference. Normally I would perform a CIS fuel pressure test to make sure that my system pressures are good but it requires a special gauge for the test. You could perform a fuel volume test and get a good idea if you have enough fuel. You need to remove the fuel return line at the fuel distributor and bypass the fuel pump relay. You should get approximately 1 liter of fuel in a container in 40 seconds. Let me know if you have any questions.
Hi. Thanks so much for responding. I have several points that I want to clear up and more information to give to you. First of all I am 250 miles from a dealer. That makes it rough. Here is what I have done since I asked the question. I performed the diagnostic tests on the EGR valve. I actually had to remove it to absolutely confirm that it was functioning correctly. It is. I tested the 50 degrees C water temp sensor/switch that is connected to the vacuum amplifier. All ok there. However! Leave it to me to confuse things. There are three hoses connected to the vacuum amplifier. One runs from the single outlet on the top of the vac. amp. And two on the bottom that go left toward the water temp 50C sensor. I am not sure that I have the three hoses connected correctly. ( Yeah, I know...dummy me.) Could you please verify which hose goes where?Next: do you recommend, or is it better to put a can of the 44K in with only 1/2 tank so that it is more concentrated? I drive the car only about 500 miles a year now. The tank was only 1/4 full at most all winter. I put in 5 gallons of fresh fuel. No difference. There IS a possibility that mice built a nest in the exhaust system! I took the car out and ran it hard for a few minutes at high rpm to attempt to dislodge any possible nest. No change. I do not have the equipment to perform the fuel flow tests. The car was running PERFECTLY when put into storage. A real puzzle. This question might take a few interchanges to solve. Let me get back to you after I run the 44K through and let you know the outcome. Please address the question of how to determing if I did or didn't hook the hoses up correctly to the vac amp. Thanks!!
Hi, I am away until tomorrow. Send me a note tomorrow in case I get busy.
Good morning, I would fill the tank half full and run the 44k and then fill it full again and run another can. I have attached a vacuum schematic below. #90 is the amplifier. Let me know if you need anything else.
UPDATE: I now have the 44k in there. I did move the mixture just a tiny nudge. I checked all the vacuum lines that were easily accessible. All ok. Next I am going to check for vacuum level at idle. I have a feeling it is going to be low. When I drive in 2nd and the rpm's are up there, I have NO problem with instant accelleration. Seems ONLY to be upon giving gas. I ran the poop out it today. Many "floor it" to 6k redline. I thought it should help, but no. I "think" that I should get a minimum of 15 vacuum at normal idle on this car. Right? I truly hope there is an unknown location vacuum leak. I'll keep you posted. Thanks a HEAP!!
Yes 15" or better for engine vacuum at idle. Let me know if you need anything.
Hi. I found the problem. At idle I am getting only 12-13 pounds. Using carb spray cleaner with a fire extinguisher handy, I located two cracked hoses. After you answer the following questions I will hit ACCEPT
Continuing: and hit max stars. Along the inner side of both valve covers, these is a hose that supplies each cylinder. One hose per side with 4 small connectors and 1 large connector. I need a diagram showing the part number or callout number or "something" that I can use to speak the Parts Department's language. It should include the four hoses that are all linked together. One on each side, One that connects the two under the fuel. distributor and on that connects those three to a hose that runs up to the front of the engine near the distributor. There are also two small vacuum hoses that are just a J or an L shape that connect to the Vacuum Amplifier. AND, lastly there is a hose from the rear of the engine (vacuum) that goes to the left side of the car. It then goes to a "Y" rubber piece that I also need. That is it. I am 90% sure it will solve my problems. Thanks for your help. If I need help with other benz items I'll ask for you. Len in Colorado
Hi Len, those are your air distribution hoses that run down the left and right side. The part number is XXXXX 094 25 82, Air Distribution Hose. You need 2 of those hoses. The one under the fuel distributor is a 116 090 08 82, Air Distribution Hose. Your little vacuum hoses at the amplifier are not identified separately in the catalog. The parts man will need to go to group 14 in the catalog and match them up. The Y connector is part number 117 078 01 45, Vacuum Hose Connector.
HUH? You mean that my membership does not include free parts??? ( Kidding) Thanks for the part numbers and all your assistance. I am positive this will cure the problem since I can see the "through the rubber" cracks. Len
I'm looking for the stars and the accept button. None here. Can you cause one to be sent to me so that you can get paid?
Hi Len, we are introducing a new system and there may be a problem on your page. If you cannot see the stars or smiley faces just reply back and I'll ask customer service to put it through.
Experience: Bosch Certified Master Tech. Over 35 years experience.