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post #1 of (permalink) Old 06-17-2019, 06:21 AM Thread Starter
Neil W. Lareau
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Date registered: May 2017
Vehicle: 1974 450SL
Location: Duluth, GA (Atlanta)
Posts: 62
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1974 450sl drowning in fuel

I'm at my wit's end. I'm restoring a 74 450sl and I can't fix a fuel problem. I'll explain the problem, then tell you what I've done.

Engine starts right up and idles initially. I've got a fuel pressure gauge on the fuel line. It takes about 5 seconds to start to build pressure, and over about 10 seconds it builds to 30 PSI. The engine starts immediately and idles. The RPM drop in direct correlation to the pressure building. At 30 PSI, it chokes itself out. My assumption is that the system is getting so rich, it can't run. Lots of smoke as it chokes out.

Too much fuel is either TPS, MAP, injectors, or ECU. I've removed the 9th injector (startup injector). Half injectors are new. All the injectors have been leak tested (using water so that I didn't kill myself). They didn't leak water and the cone was acceptable considering that water is a lot heavier than gas.

TPS: The TPS has three modes: IDLE, 'in-between', and FULL. It is clean and adjusted. At idle (no pressure on the gas pedal), the output pin is GND - movement of the pedal causes it to OPEN. The 'in-between' mode alternates between GND and OPEN ten times as the pedal it pushed. The FULL output is OPEN all the time. It should go to GND at full gas pedal travel, but it doesn't. That shouldn't be my problem (so says the guy who can't figure out the problem). The most logical thing that WOULD cause the problem is that if the TPS GNDed the FULL pin. That would override the MAP sensor and have the ECU dump wide-open fuel all the time. The FULL pin is OPEN, not GNDed.

MAP: I can put a vacuum on it and hold a vacuum (implies diaphragm is OK). Open circuit resistance is correct for both coils. Using an impedance meter, I measure the inductance of the coils and they vary linearly as I increase/decrease vacuum. I don't remember the reading (typing from memory here), but it was something line 0.7H - 0.9H on the small coil and something like 1.2H to 1.5H on the larger coil.

ECU: wiring to the ECU appears to be good. I've opened the ECU and everything looks OK, no obvious corrosion, smoked parts, or leaking caps (I'm an electrical engineer - without having schematics to check it with, it appears to be just fine).

Fuel pump is new, fuel lines are new, fuel filter is new.

What is causing this system to dump so much fuel???? I'm about ready to rip out this motor and put in a modern engine. I'm getting desperate. What am I missing?
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