1974 450sl drowning in fuel - Mercedes-Benz Forum
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post #1 of 30 (permalink) Old 06-17-2019, 06:21 AM Thread Starter
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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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post #2 of 30 (permalink) Old 06-17-2019, 08:21 AM
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Have you checked timing as well as looked to see that both cams are aligned at TDC to check for stretch of the timing chain?

Have you done a pressure test on each cylinder?

If you feel like it is fuel pressure related, which im not convinced it totally is (but by no means am I an expert), have you tried turning down the fuel pressure regulator?

Just a few thoughts, I restored a 75 and had a ton of trouble figuring out why it wouldn't run and it had similar symptoms, turned out the timing chain was stretched.
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post #3 of 30 (permalink) Old 06-17-2019, 08:30 AM Thread Starter
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Originally Posted by RobbyH View Post
Have you checked timing as well as looked to see that both cams are aligned at TDC to check for stretch of the timing chain?

Have you done a pressure test on each cylinder?

If you feel like it is fuel pressure related, which im not convinced it totally is (but by no means am I an expert), have you tried turning down the fuel pressure regulator?

Just a few thoughts, I restored a 75 and had a ton of trouble figuring out why it wouldn't run and it had similar symptoms, turned out the timing chain was stretched.
First and foremost: Clearly something I believe to be OK is not. So I'm no longer positive about anything. Here is what I believe to be the answer to your questions.

I did do a pressure test on all the cylinders and (again from memory) were all about 120 PSI.

Forget turning the fuel pressure regulator down, I can disconnect the fuel pump and the car will idle beautifully at 0 PSI. But just turning down the fuel pressure is a "cheat" that will only mask a bigger problem, right?

I checked the timing chain for stretch and replaced the timing chain guides. The timing is good (my neighbor is a Lexus mechanic and I got him to look over my shoulder to verify I had it right).
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post #4 of 30 (permalink) Old 06-17-2019, 08:39 AM
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Remind me, the 74 is D-jet? Im not sure how the care will idle if you disconnect the fuel pump?
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post #5 of 30 (permalink) Old 06-17-2019, 09:32 AM
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The trigger points inside the ignition distributor tell the ecu when shoot fuel I believe.

The throttle position sensor sends a throttle position.

The manifold pressure sensor senses the level of vacuum, and I believe is only in play when not at idle.

Thereís a knob on the ecu for idle mixture. Righty richy, lefty leany I believe.

There are coolant and air temperature temperature sensors too.

The Aux Air valve should cutoff additional startup air as the engine warms up, as the coolant expands the wax in the bulb.

Are you sure itís running rich?

Are you using the correct pump?




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post #6 of 30 (permalink) Old 06-17-2019, 09:45 AM Thread Starter
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Remind me, the 74 is D-jet? Im not sure how the care will idle if you disconnect the fuel pump?
Yes, it is D-jet. I was as surprised as you are that it ran with the pump off. Apparently it siphons enough fuel to run. I've got a video of the idle with the FP gauge at 0.

https://www.facebook.com/neil.w.lare...8579746313711/
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post #7 of 30 (permalink) Old 06-17-2019, 09:52 AM Thread Starter
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Originally Posted by Fonzi View Post
The trigger points inside the ignition distributor tell the ecu when shoot fuel I believe.

The throttle position sensor sends a throttle position.

The manifold pressure sensor senses the level of vacuum, and I believe is only in play when not at idle.

Thereís a knob on the ecu for idle mixture. Righty richy, lefty leany I believe.

There are coolant and air temperature temperature sensors too.

The Aux Air valve should cutoff additional startup air as the engine warms up, as the coolant expands the wax in the bulb.

Are you sure itís running rich?

Are you using the correct pump?




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1. Yes, but the ECU looks at 3 things on the TPS. IDLE, FULL THROTTLE, and accel/decel. A FULL THROTTLE TPS signal should "over-ride" the MAP sensor and cause the ECU to send max fuel.
2. MAP is in play when not IDLE or FULL
3. Yes, I've gone full rich to full lean and there is no change.
4. Coolant sensor is in place; air sensor is connected and just lying in the engine bay while I have the air housing off
5. The aux air valve was broken - seized in place and I removed it. It really is only needed when the car is cold and that's not a problem for me. I put an aftermarket water temperature switch to control an aftermarket electric fan on the radiator
6. I say it is rich because it shuts down as the FP rises, smells of gas, runs rough/misfires, smokes, and eventually I'll get enough gas in the oil to require a change.
7. Fuel pump is an aftermarket airtex pump, but the problem existed with a the OEM pump.
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post #8 of 30 (permalink) Old 06-17-2019, 10:33 AM
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I would try decreasing the fuel pressure regulator and ensure that the pump you have on it is correct. How long does it run, does it drive? If so for how long? When you say misfire does that mean backfire? Does it start when it is cold and hot? It really sounds like timing if you are getting misfires and gas in the oil
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post #9 of 30 (permalink) Old 06-17-2019, 11:16 AM Thread Starter
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I would try decreasing the fuel pressure regulator and ensure that the pump you have on it is correct. How long does it run, does it drive? If so for how long? When you say misfire does that mean backfire? Does it start when it is cold and hot? It really sounds like timing if you are getting misfires and gas in the oil

Misfire does not mean backfire, just the occasional 'bump and miss'. It starts just fine when cold or warm. See the video I posted.

The timing is dead-on the marks at 5deg BTDC.

Again, note that I'm responding with a bit of humility: Something I "know" is clearly not true or else I'd have figured this out by now.
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post #10 of 30 (permalink) Old 06-17-2019, 12:06 PM
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Video link does not work.

I would recheck your cam alignment if it is misfiring but the ignition is timed correctly. Even being off a little caused me weeks of headache.
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