Intermittent stalling with rich condition - 1981 380SL

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ABin65809

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Sep 5, 2024
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Greetings everyone,
I own a 1981 380SL (USA) that has been stationary for some years. This summer, I resolved to restore the vehicle, which exhibited various issues such as rough idling, misfires, and elevated idle speed.

Commenced with a smoke test and identified leaks around the fuel injectors and valve cover gaskets.
I changed many components, beginning with the most prevalent ones:

-New Bosch distributor cap -New Bosch rotor -New Bremi ignition wires -New NGK BP6ES spark plugs
-New Bosch fuel injectors with seaters and seals -New injector breather hoses
-New valve cover gaskets
-New air filter -New air pump-To-cleaner box hose -Adjusted C/O for optimal system suction

The vehicle exhibited significant improvement; nonetheless, it continued to falter during acceleration and maintained a high idle. I have replaced more components:

-Repaired the damaged fuse on the OVP relay -Installed a new Bosch O2 sensor -Replaced the WUR with a remanufactured unit -Cleansed the fuel return line using compressed air -Installed a new fuel filter -Installed a new fuel accumulator -Replaced both ceramic resistors regulating the ICM and ignition coil -Readjusted the C/O for optimal system vacuum

The vehicle now operates far more efficiently. There is no longer any stumbling, and it operates really well, particularly when cold. The issue arises when the vehicle reaches operational temperature, particularly after extended drives or prolonged idling in the driveway, resulting in sporadic shutdowns. The vehicle will resume if allowed to idle for a minute, and sometimes instantly upon fully depressing the gas pedal. However, it restarts under a poor rich state, emitting significant black smoke, the economy gauge fluctuates to the midpoint, operates poorly, and is likely to stall again shortly thereafter. I restart the vehicle many times, about four or five, while stationary in the driveway, until an abrupt change occurs, almost by chance, restoring normalcy: the vacuum increases, black smoke ceases, and the engine idles smoothly. I observed that when the rich state occurs, feathering the throttle and shifting the vehicle into Drive improves the situation more rapidly, suggesting that something may be overheating and that improved air circulation aids in alleviating the issue. I am now experiencing an erratic idle, like a misfire.

All indications suggest a malfunctioning ICM (mine is 0227100001), but I like to avoid unnecessary expenditures and seek confirmation before investing much in a replacement.

Additionally, it is important to mention that I acquired a new UROparts fuel pump relay some time ago, and upon installation, the vehicle either failed to start or would only run for one second before shutting off. Insert the original component, and the vehicle starts immediately. I saw it as worthless and returned it. I bought another unit of superior quality (KAE), nevertheless the same issue persists; the vehicle fails to start despite the new fuel pump relay. My vehicle is equipped with a 6-pin relay (0015450605). I examined the original relay and saw that one of the solder spots on the circuit board is entirely absent, appearing very clean as if it were intentionally omitted by the manufacturer.

I am now perplexed. I am uncertain about the next steps to take. Any assistance or suggestions would be highly appreciated.

Some inquiries I need clarification on: Does a malfunctioning ICM result in a high fuel mixture?
-What is the reason a new fuel pump relay can result in a failure to start?
-Does the fuel pump relay influence the air/fuel combination in any manner? I am unable to locate a 6-pin wiring schematic anywhere.
 
If the vehicle has been inactive for many years, the gasoline distributor may be compromised due to the accumulation of rust, dirt, and varnish from the fuel. Verify fuel pressures with a tester. CIS Flowtech for the refurbishment of the fuel distributor.

Contact Us – CIS Flowtech, LLC

cisflowtech.com cisflowtech.com
 
The service manual is available here: Maintenance Manuals for the Mercedes-Benz Model 107 - M116 3.8 Liter

To the best of my knowledge, the ICM functions well or it does not operate at all.

I have heard similar criticisms about the Uro Parts FP relay's quality. I possess little knowledge on the KAE, other that I have located a functional spare, which works well in my vehicle during testing. The relay utilizes signals from the starter and the ignition module to first activate and thereafter maintain the activation of the fuel pump. The accurate wiring diagram is included in section 24 of the handbook for the 1981 vehicle.

A fuel pressure test is essential to verify that the fuel pressures are accurate prior to diagnosing the injection system. If these are incorrect, no efforts will be effective. The installation of a new warm-up regulator does not guarantee that the control pressure will be accurate, which is essential for achieving the optimal combination.

Did you mean to modify the air/fuel ratio using the screw on the air intake flap when you referred to "adjusted the C/O"? This vehicle is equipped with the "lambda" system, which dynamically regulates the fuel mixture according to O2 sensor readings by using a valve to release fuel pressure from the lower chamber of the fuel distributor. If this system is functional, changing the air/fuel screw does not influence the final mixture, as the lambda system will adjust to restore it to the required levels based on the O2 sensor readings. The actual mixture will be modified only if the modification exceeds the lambda's adjustment range. In this instance, it will be significantly imbalanced, either too rich or excessively lean. The only method for accurate adjustment is using a tiny oscilloscope on pin 3 of the diagnostic connector, and modifying the mixing screw until the waveform indicates that the system operates at about a 50% duty cycle, with the frequency valve toggling on and off at 50%. When the system is operational, the valve emits a buzzing sound.

The system has two fixed modes: a set 60% operation for warming and a fixed 50% operation when the O2 sensor values are out of range.

The subsequent step should be a fuel pressure test, as previously indicated.
 
By C/O adjustment, I refer to the air/fuel adjustment screw located on the throttle body.

I just discovered that my automobile is equipped with a lambda system that automatically adjusts the mixture; nevertheless, while the engine is operating, any modification to the air/fuel combination yields instant feedback, irrespective of the temperature. Turning clockwise will enrich the mixture, and I instantly see the change in the idle, exhaust, and economy gauge, whereas counterclockwise has the reverse effect. This leads me to doubt the functionality of the lambda mechanism.

I continually ponder a significant question: why is a new FP relay causing the vehicle to malfunction?
I propose a theory: could it be that my original fuel pump relay is defective, supplying power to the fuel pump while malfunctioning in its Lambda control function, and that I have adjusted the fuel-air mixture enough to compensate for this defect? Currently, after installing the new fuel pump relay, the vehicle fails to start owing to either a low or rich fuel mixture. Is it a possibility?

I really appreciate all those who have answered so far.
 
The fuel pump relay output activates the fuel pump, the WUR heater, and the power supply to the frequency valve. All three originate from the same output terminal of the relay. The most probable issue with the Uro relay is its defectiveness. Measure the voltage at the fuel pump positive terminal when cranking. Additionally, you may measure the voltage on the black/white/red wire connected to the FV, or the black/green/white wire linked to the WUR socket. All connected to the same circuit.

When the lambda system operates in one of its two "fixed" modes, the mixture screw will directly influence the mixture. The valve is regulated by the lambda ECU on the ground side. If the valve is nonfunctional (closed), the mixture will be lean (according to the initial mixture adjustment established when everything was operational). To determine its functionality, disconnect it and observe the impact on engine performance.
 
Concerning the malfunctioning Uro relay, please be informed that I acquired an alternative KAE (KAEHLER, German-manufactured) relay, which exhibited same behavior, resulting in no ignition. I find it improbable that I received two distinct faulty units.

I applied direct power to the pin supplying the fuel pump, however this did not enhance the situation. It seems that the FV may be experiencing sporadic failures. Would it result in such an issue?
My understanding is that the FV valve regulates the volume of gasoline delivered to the injectors; hence, excessive fuel flow may be the underlying problem.

In the latter portion of your statement, you reference "the valve." Are you referring to the Frequency Valve?
whether so, I may attempt to disconnect it while the vehicle is operating properly to see whether that replicates the rich state or rough idle I am experiencing intermittently.
 
I am uncertain; does the vehicle fail to start with the Uro/KAE relays, or do these relays not enhance your suboptimal operating condition? The relay will either activate the pump or not; it has no further impact on engine function.

The lambda system via the FV influences the mixture, but within a restricted range. Your remarks on the outcomes of modifying the mixture screw suggest that it is either functioning in a fixed mode or not at all, or that the mixture has been manually altered beyond its compensatory range, resulting in the fuel valve being driven to either fully rich (open) or fully lean (closed). If operational, it will emit a buzzing sound. To accurately configure it, an oscilloscope is required; a duty cycle meter or dwell meter will provide incorrect results. It is difficult to ascertain the extent of its contribution to the issues, but it must be appropriately configured.

The control pressure significantly affects the mixture, so it is crucial to ensure its accuracy. Additionally, it is essential to inspect the vacuum systems for the timing advance/retard and the cold enrichment mechanism on the WUR. If they are not correctly linked or contain damaged components, they may also create issues.
 
Indeed, the perplexing aspect is that using a new fuel pump relay renders the vehicle inoperable; it fails to start entirely, even attempting with two distinct replacement relays, Uro and KAE. I reinstalled the original OEM relay, which has been in the vehicle longer than my ownership, and the car starts without issue. However, as indicated in my first article, the OEM relay has a missing solder joint on one of the IC's legs.
It remains incomprehensible to me why a new relay would prevent the vehicle from starting altogether.

I own an oscilloscope that indicates 60% at operational temperature when the vehicle is functioning optimally. Although I can assert with certainty that I am using and comprehending it accurately.
 
I just purchased their refurbished WUR and FD; the improvement is remarkable. It was the last abnormality of my MB.
 
I can also affirm that the URo fuel relay is utterly deficient. It seems you may be experiencing many troubles. My first action would be to remove the gasoline relay and connect the pins that activate the pump to see whether that maintains its operation. From that point, you may go further into troubleshooting to see if the appropriate signals are being sent to the gasoline relay. The ignition control module is also involved in this process. It caused me several issues, but with the assistance of many kind individuals here, I was able to resolve it. The aforementioned links should guide you appropriately.
 
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