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Discussion Starter #1
Hi Guys,

I’ve been trolling the forums here and have looked at essentially every thread pertaining to these issues. I just brought home my first Mercedes, a surprisingly clean 1987 420SEL with 92k miles for a cool $700 and a nearly perfect interior save for a locked radio. The previous owner had it for a year and never got it running quite right. He only ever replaced a hose and drained the fuel tank. I jumped it and added a gallon and after priming the fuel pumps 2x and it fired right up after only a crank or two, but stumbled and died pretty quickly when cold. When warmed up, it seems to stall after a longer period of time while the idle bounces between 400-1000rpms and runs quite rough, shaking a bit when at lower RPM. I cleaned the idle control valve by soaking it in carb cleaner and when shaken you can hear the valve flap. I also took out the ovp relay, checked the fuse which seemed to be good, and tried to start with it unplugged and it just wouldn’t start at all.

I assume maybe a fuel pump? they look to be the smaller, newer style models but they buzz pretty loudly when standing at the rear of the car. The exhaust was cut off at the cat as it was clogged so it’s otherwise very loud. I have the cat back section and am will reattach when and if I can get it running right. Another odd symptom is when I take off the fuel cap, there seems to be quite a bit of pressure. This is worse if I let it sit for a day. One mechanic friend told me there should be some pressure, others tell me there should not. I found thread posts pertaining to this and one said maybe a valve in the fuel tank? I understand in order to save this car it will take some time and money, but I’m doing my best to keep this a low budget, fun project, and it looks like you guys have a MUCH better grasp on this than I. Any direction that will help me find a place to start diagnosing this without just blindly throwing parts at it would be greatly appreciated. Also, if this is helpful, I ran 5 gallons of gas and fuel system cleaner through it and let it idle. It did not seem to improve the condition much if at all. No lights on the dash save for a dead light indicator.

Rocco
 

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1986/1990 W126
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14,132 Posts
That's a great price. Welcome to the forum.

Yeah do check the tank pressure valve, it's a very cheap item from MB and just plugs in the vent hose. Above the rear axle area on my cars.
 

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1986/1990 W126
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On one of mine the lines to the charcoal cannister were gummed up so I have the vent valve on the vent line to maintain correct tank pressure, but it vents to fresh air instead of the line to the cannister.
 

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1990 420 SEC
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337 Posts
Have you already noticed thread SERVICE MANUAL&INDEX at stickies of W126 section? It has a link to a proper service manual.
 

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Discussion Starter #5
You guys rock... little update. Changed the plugs because the old ones looked pretty fouled, maybe a little smoother but same issues today. It sounds almost as if it may be lightly misfiring, though there is a rusted screw on the distributor cap that I’m not quite ready to drill out and check. Another note: the Economy gauge sits at about 25% when idling, which goes back to 0% (the left) when lightly applying gas or coming off of a rev. Indication of a vacuum leak possibly? This is a much different world than 2014 Honda’s haha, the car is too nice to not be saved however. That said, how should I prioritize and diagnose? I posted 2 fuel tank valves, anyone know which one is correct? Any chance it can be a line, fuel pump, or charcoal canister? Would a bad coil/rotor,distributor cause the car to stall out? Could it be an injector, cold start or otherwise? EZL? The optimist in me said new plugs and fuel system cleaner would solve the issue, though the optimist appears to be wrong. Cheers guys

Rocco
 

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Discussion Starter #6
On one of mine the lines to the charcoal cannister were gummed up so I have the vent valve on the vent line to maintain correct tank pressure, but it vents to fresh air instead of the line to the cannister.
How do I go about this? It seems like a solid way to rule the canister out
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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The A 123 470 03 93 is proper for your car.

For reference:

By the way.. on the above site, one can input one's VIN and it will display only parts appropriate to that particular model. It has proved invaluable to me time & time again.
 

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1990 420 SEC
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337 Posts
Probably not EZL. My money would be on vacuum leak. Any idea is it running lean or rich at idle? Maybe reading H.D. s thread of lambda adjustment would help you in diagnose.
 

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Discussion Starter #11
420SEL running rough
Here is a video of a warm start and then when the engine fails. When cold, it won’t idle for more than 15 seconds, when warm, between a minute up to 20. What are your guys’ thoughts?
 

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Discussion Starter #12
Probably not EZL. My money would be on vacuum leak. Any idea is it running lean or rich at idle? Maybe reading H.D. s thread of lambda adjustment would help you in diagnose.
I would assume it’s rich based on the plugs. Are they the originals? The exhaust is currently cut off just before the cat so I don’t have the best view but no smoke coming from exhaust
 

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How much of pipe you have after the O2 sensor? If air is mixed to exhaust gas at O2 sensor it will cause a problem when warm.
 

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Discussion Starter #15
After spending some more time shaking it down, I cleaned a few contacts from the distributor to the plug wire and it seems to keep its idle at around 600rpm. It bogs slightly when put into gear but doesn’t die. If you try to abruptly accelerate, it will stall. This is when warm. On a cold start, you must keep blipping the accelerator to keep it from stalling
 

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1983 380-SEL, '67 220D (sold), '65 200D (sold), '75 L-206D (sold)
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Similar problem in an older post with a lot of tips to try.

My fix on a '83 380SEL is included on this page I managed to get everything starting up like new.

Hope that helps.
 

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Discussion Starter #18
Thanks for your help guys, I ended up trying to get the rotor cap off and realizing it was cracked. I noticed quite a bit of play in the rotor. The picture below speaks for itself. Feel like an idiot for not checking this in the first place, I was too fixated on it being a fuel delivery issue. Learn and move forward, I suppose. New cap and rotor on order along with a fuel tank vent valve. Still probably going to drop the tank and thoroughly clean out the fuel system but Im glad we’re getting somewhere!
 

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Discussion Starter #19
I'm not a MB expert by any means! That said, I'd start with the basics, give it a complete tune up. etc

Did you clean the idle air control valve?

Pete
That was my first suspect. Soaked it over night in carb cleaner and I can hear the valve flap. Haven’t tested it but seems to be working
 
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