1972 280se 4.5 fuel fouling spark plugs - Mercedes-Benz Forum
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post #1 of 2 Old 08-07-2019, 06:35 PM Thread Starter
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1972 280se 4.5 fuel fouling spark plugs

Hi everyone, I have an problem with my 4.5. The spark plugs with become saturated with fuel after about 20 minutes of running the engine that the car will stall and not restart. If I change the plugs that it will run fine. I checked for spark at the plug and found good spark there, so that tells me that it's a fuel problem. I checked the fuel pressure, it was within spec at 28psi. I checked the points and even changed them and set the dwell- ok. Next I replaced the trigger contacts, cleaning them and reinstalling them. I replaced both cylinder heads and did a complete valve job. I replaced the MAP sensor with a new one. The fuel pump and filter have been replaced also. Distributer cap and rotors have also been replaced. The fuel injection wiring harness was completely redone. The cam timing is correct and the distributor was installed correctly. The ignition timing was set and the valves were adjusted to spec when the cylinder heads went back on. The ECU was remanufactured by someone I don't think was comfortable rebuilding it but he says it's fine now. I've come to the conclusion that the injectors are dumping fuel into the cylinders. Here's my question, is it possible that the ecu could be receiving the correct data from the MAP sensor but telling the injectors to stay open too long? This would mean the ECU is no good. Any other ideas? Thanks.
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post #2 of 2 Old 08-08-2019, 12:06 PM
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I do not think the ECU is bad. There were still some air fuel mixture adjustments in those D Jetronic injected engines.

The best thing to do is to get the emissions checked. There is a CO limit that it should be running at, it should be there at idle and at about 3K RPMs under load. You set the load first on the MAP, then the idle on the ECU.

If you want a quick fix, turn the air speed screw (by the aux air valve) counterclockwise about half a turn. Or, turn the ECU screw (large plastic screw under the rubber protective cap on the ECU, which is to the left of the rad). Turn it about 3-5 clicks counterclockwise to lean it out. This changes ONLY the idle. The MAP changes both, but if you lean it out at the MAP, you can burn a valve if you're lean at highway speeds.
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