Hot Stall

Matt Owner North Carolina Posted   Latest  
Unsolved
Driveability
1990 Mercedes-Benz 500SL 5.0L (119.960) 4-spd (722.3)
Stall

Hello everyone. I have 1990 mercedes 500sl with a customer complaint of a stall condition under load when the vehicle is hot. When it stalls the vehicle will run poorly until it cold soaks and then will start and run with no issues until hot again. I do not have scan tool access on this vehicle so I have no info to share other than there is no illuminated check engine light on the dash. The customer provided prior service records for the last 12 years of maintenance on the vehicle and the list is long. Prior repairs potentially related include, ckp sensor, distributor caps and rotors, both in tank fuel pumps and the accumulator, the injectors, spark plugs, ignition switch, and the ezl module. The customer stated that each one of the prior repairs appeared to him to be separate issues and not related to his current complaint. I decided to start with secondary ignition due to the lack of scan tool data I figured this would be the easiest way to get the most information. I connected to the coil feed to each distributor to see all of the ignition events on 2 channels. I set a long time base and let the vehicle warm up and then performed a power brake test. While power braking, the customer complaint was verified as the vehicle shut off. I performed an under-hood vehicle inspection and then reviewed the scope capture. During the under hood inspection I could see that fuel distributor pressure regulator on the high pressure pump was leaking, and the distributor caps were holding moisture. The plugs also were very dark as if the vehicle has been running rich and there is a strong fuel smell to the exhaust. After reviewing the scope capture I (1st attachment) I could see that appears as if there is an open in the wire or coil when the vehicle stumbles and ultimately stalls. I did another ignition test to include the primary coil current with the secondary voltage to the coil wire to the distributor and a sync to number 8 plug wire. This test shows a lot of feedback on the coil turnoff. My questions are, mainly regarding input into the high kv during the burn lines. It looks like there are some lean conditions in the burn line (shape and KV) but the burn time does not appear to show lean(over 1.5ms in most cases) and the plugs and exhaust act like it might be running rich. There seems to be some EGR disturbance maybe during the snap throttle? Does anyone have some suggestions as to how I could verify fuel delivery on this vehicle with no access to injector patterns, or pid data? He has spent over 14K in the repairs he has shared and I just dont feel that I have enough info to confidently say that we have everything nailed down for a recommendation. Any help would be appreciated. 

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Eric Owner/Technician
Michigan
Eric Default
 

When you say it shuts off, as in immediately or stumbles and stalls. Does it lose primary voltage? Trying to determine if power is being cut to coils or if poor running is causing stall.

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Matt Owner
North Carolina
Matt Default
 

Eric, it stumbles and stalls. Does not lose primary voltage. When it starts to stumble, the spark kv jumps off the chart and there is no burn line present. It looks to me like an open in the secondary.

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Mike Owner/Technician
Illinois
Mike Default
 

I had one of these it kicked my but cap caps and rotors were bad. If you have to replace them I would only purchase oem ones .

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Agree
Jan Owner/Technician
Ohio
Jan Default
 

You might go to a local service center, such as AZ, AAP, NAPA, Oreillys for a free diagnosis for any codes.

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Matt Owner
North Carolina
Matt Default
 

Mike, Im with you. All of the research that I have done says that is the most common failure for this issue. The customer had this replaced with OE parts 20k miles ago. This car is a garage queen about 5k miles a year. When I did the visual there was some condensation and a little oil in there. It's on my recommendations list to clean the caps and points and reseal the distributor.

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Robert Engineer
North Carolina
Robert Default
 

Take a pitcher of ICE water. Let the car get as hot as you can until it stalls. Try to start it to make sure it does not. Open the hood and dump the pitcher on the EZL (ignition switch module on the fender). Try to start it. Repeat... to be sure. There is not any easy way to read codes on a 1990 Mercedes ignition system. You can get some blink codes but it is a waste of time. Tell Matt hey for…

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Matt Owner
North Carolina
Matt Default
 

Robert, hope you're good it's been a while. If it is the ezl are you still refurbishing them? I know this one was replaced by a shop in Raleigh. I actually thought of you when working on this car. Thanks for your help. I am assuming if it starts up after cooling the ezl then it's safe to call the module faulty.

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Robert Engineer
North Carolina
Robert Default
 

Matt, EZL can't be fixed. Try to take one apart and you will see why. Hope your problem is something else because if it is the EZL it is often a death sentence for the cars. Very hard to find parts. Sometimes can get lucky. That car also has two coils i believe. Right and left bank.

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Paul Mechanic
Illinois
Paul Default
 

One other possibility may be cat restriction to consider. Not sure how easy it is to pull an O2 sensor, but if you have a scope and pressure transducer, you can pull a plug and do an in cylinder pressure test and measure back pressure that way. Check cold then when symptoms present.

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Mihail Technician
Maryland
Mihail Default
 

These old mercedes vehicles have a well known problem with the engine harness - the wire insulation is falling apart and that sets off all sorts of problems. I had one of these S class mercedes cars of that era that was doing the exact same thing - running fine fo 15-20 minutes and then shuts off. The wires from the Crank position sensor had hardly any insulation left. The heat was melting the…

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Michael Mechanic
Texas
Michael Default
 

I don't have a lot of experience on a Mercedes, but the symptoms you describe remind me of a weak coil. A quick check is to recreate the circumstance and feel the coil with your hand if you can hold it on there it's not too hot if it's really uncomfortable, or too hot to hold, probably the coil is going bad.

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Raymond Mobile Technician
New York
Raymond Default
 

I know this may sound far fetched. Try spraying both distributor caps internally with WD40. Only WD40. Try this and see if symptoms go away. Caps and rotors on these are famous for failure. Just a thought. If this cure your problem, order new caps and rotors from dealer only.

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Raymond Mobile Technician
New York
Raymond Default
 

Any updates as to what you found, just curious

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Aaron Owner/Technician
Wisconsin
Aaron Default
 

Your symptom to me sound more like it’s getting too rich at operating temp. First look down in the airflow chamber and make sure it’s not pouring fuel all over the throttle plate. If so, the fuel distributor is leaking into the chamber and that is bad. Also check line and control pressures at the fuel distributor. The EHA valve on the side adjusts control pressure. You can see what the ecu is…

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Matt Owner
North Carolina
Matt Default
 

Aaron, thanks for the info. I definitely wished I had a gas analyzer for this car. Every ones help was much appreciated. After following the tips from guys here's what we found. We found that the secondary side of the ignition would breakdown after the vehicle was hot. The car still had original wires and coils. The cap and rotor had the wrong o rings installed and moisture/oil was in the cap…

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