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Greetings, everyone.

I want assistance with a persistent hot start issue on my 1988 300E, W124, equipped with the M103 gasoline engine.

I have had my automobile for almost ten years. It has 120,000 km on it. It is a German model including the adjustable EZL-KAT resistor. The automobile was manufactured in late 1987. Since acquiring it, I have had difficulties in starting the vehicle whether it is left in direct sunlight or parked in warm subterranean parking. Initially, it was necessary for the temperature to be very high, either outside or underground, for around 2 to 3 hours. Recently, the situation has deteriorated. Initiates without difficulty when heated for 15 to 20 minutes; nevertheless, after many hours, it becomes challenging to start.

The vehicle starts well when cold, operates smoothly, exhibits little hesitation, and achieves satisfactory fuel efficiency. It exhibits a little occasional misfire while idle. When heated, it idles at around 600 RPM when in gear. Upon starting the ignition without using the accelerator, the vehicle accelerates rapidly for a few seconds before decelerating to around 1000 rpm (in neutral) until it reaches optimal operating temperature.

Throughout the years, I have changed and tested several components, not alone due to challenges in starting while hot, but also as part of routine maintenance. The one difference was that when replacing the injectors, the issue exacerbated, albeit the engine exhibited improved low RPM performance.

I installed a light and a switch on the cold start valve to identify the issue. I believed it may be stimulating when the engine gets overheated and inundated. The light activates at low temperatures but not at elevated temperatures. Currently, the only method to begin the automobile while it is heated is to provide electricity to the cold start valve. This is somewhat counterintuitive; it resembles suffocating a carbureted vehicle when it is heated. This action will initiate ignition, and often, due to my little depression of the gas pedal, the engine will ascend to 2500 rpm smoothly, but it will stall if I release the pedal. In the absence of activating the cold start valve, the vehicle may briefly ignite while cranking, however insufficiently to achieve ignition.


This is a summary of completed actions regarding fuel:

Replaced the oxygen sensor.
Establish a mixture using duty cycle measurement on an oscilloscope.
Substituted the fuel filter.
Substituted the change-over valve.
Replaced the cold start valve.
Replaced the hose of the idle speed control valve.
Evaluated the idle speed valve for smooth operation and adherence utilising a variable power source.
New Bosch fuel injectors, O-rings, and holders.
The temperature sensor's profile was assessed at different engine temperatures, yielding resistance values that closely align with those specified in the Mercedes handbook. My temperature sensor is of the two-pin kind.
Substitute the fuel pressure regulator.
Both gasoline pumps are operational and rather silent.
Attempted to exchange Jetronic engine control units, despite the exchanged unit having a slightly different designation. The automobile exhibited no differences in performance or starting, save that it idled too quickly with the alternative computer.


Ignition and electrical systems:

New Coil.
New spark plugs (Denso non-resistor).
New distributor cap and rotor.
Substituted the OVP relay.
Replaced the EZL ignition control module.
The crank position sensor exhibited a resistance of 800 ohms when cold.
The measured resistances of the EZL-KAT plug are all within specifications.
Employed a spark tester to assess spark voltage. It exceeded the meter's measurement capacity.


Several years before, a local establishment assessed the fuel pressure and conducted a smoke test to identify potential leaks. Both were satisfactory.


I am uncertain where to search further. Assistance would be appreciated.

I have an additional inquiry: does anybody understand the distinction between various jetronic engine computer models?
My vehicle is equipped with a Bosch “0 280 800 206,” also known as Mercedes “005 545 57 32.”
All vehicles in Canada seem to possess a "0 280 800 256."

Upon replacing the 0 280 800 256, the vehicle operates similarly, but with an elevated idle speed of around 1200 rpm or 900 rpm in gear, compared to the 600 rpm achieved with my computer. There is no indication of misfire when idling at 900 rpm.

Thank you very much.

Degradation of the wire loom?

Initiate the system in darkness and observe for electrical arcing inside and around the wiring harness.It may not be that, but it serves as a means to ascertain whether it is.What is the make of the rotor?

The distributor cap and rotor are both of the Bosch brand. I neglected to add that I also inspected the coil and spark plug wires for corrosion. All are really clean.

Where should I check for flashing on the loon? I do not detect any arcing while inspecting the engine compartment. I can attend to it after dusk, which occurs very late in the northern regions at now.

Throttle valve or throttle position switches?

Our family has had M103-engined Mercedes-Benz vehicles from their inception to the present day. When you mention that your engine is difficult to start when hot, how many cranks are required for it to ignite and operate smoothly?

I have never had this difficulty, even in elevated ambient temps during my previous tours to Southern Europe.

The only remaining component to be replaced is the EHA valve located on the side of the gasoline metering head.

Thank you everyone for the responses and recommendations. I conducted a thorough inspection and auditory examination beneath the hood for any arcing this evening, and everything seems to be in order.

In response to the duration of cranking before to ignition, I would estimate around 30 seconds. I often engage the mechanism for no more than 10 seconds, then pause briefly before attempting again. After about five seconds of cranking, one cylinder will weakly ignite, but the engine will not start. Adjusting the throttle has little or negligible impact.

I examined the throttle position switch, which exhibits continuity when the throttle is closed and opens upon little movement of the throttle. However, I saw that one of the two little female connections inside the plug was not properly seated in its slot. It is uncertain if contact was made. The temperature has dropped again, so it may take some time before I can ascertain if the repair has had any effect.

Rather than a replacement, an adjustment may be necessary. The EHA contains a slider track that serves as the electrical contact. Over time, this contact may experience slight wear at its resting position during engine idling, necessitating an adjustment. To achieve this, loosen the two small Torx screws slightly and then adjust the slider up or down until the desired voltmeter reading is obtained. From recollection, the appropriate voltage should be 0.70V at hot idle. This adjustment resolved my uneven idle and inadequate hot starts on my 300E.

I contend that wiring loom degradation is improbable with this engine; the M103 is a robust and resilient unit, and in comparison to the subsequent DOHC engines, there is minimal wiring situated near high-temperature areas. Upon replacing the injectors, did you also substitute the clip-on collars in addition to the O-rings that seal around the white plastic injector holder tubes?