1972 280SE 4.5 Won't start! - Mercedes-Benz Forum

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post #1 of 11 (permalink) Old 10-01-2016, 07:47 PM Thread Starter
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Vehicle: 1972 MB 280SE 4.5
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Unhappy 1972 280SE 4.5 Won't start!

I really need some help. I have been trying to diagnose myself (limited auto repair knowledge) for about 6 months. Here is what is happening:
The car cranks but won't fire up;
1. fuel pump works ( I held with it in my hand to feel it start when I had someone crank;
2. squirted starting fluid into intake manifold and crank, did not fire;
3. connected a jumper wire to coil positive side and grounded with the ignition key with ON position without the high tension wire on coil, the light on my jumper wire was bright.
What should I do next to diagnose? Any help will be much appreciated.
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post #2 of 11 (permalink) Old 10-01-2016, 08:59 PM
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Do you have the original style points? Often the rider on the points wears down while riding on the cam lobes (under the rotor in the distributor), and the points need adjustment so they open at the cam lobes to create spark. The points opening is the trigger for the coil to fire and send spark to the plug.

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post #3 of 11 (permalink) Old 10-01-2016, 09:08 PM
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pull a plug. Attach it to its respective plug wire. Lay it on the valve cover so it's grounded. Crank the engine and look for spark.

What is the history of this car? When did it last run?
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post #4 of 11 (permalink) Old 10-02-2016, 06:23 PM Thread Starter
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pull a plug. Attach it to its respective plug wire. Lay it on the valve cover so it's grounded. Crank the engine and look for spark.

What is the history of this car? When did it last run?
Just tried it, there is a spark. What else can I check?
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post #5 of 11 (permalink) Old 10-02-2016, 06:55 PM Thread Starter
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The car was running fine, about 6 months ago it wouldn't start. I have been trying to diagnose ever since on weekends. It has new plug wires and coil.
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post #6 of 11 (permalink) Old 10-02-2016, 06:57 PM Thread Starter
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Originally Posted by Fonzi View Post
Do you have the original style points? Often the rider on the points wears down while riding on the cam lobes (under the rotor in the distributor), and the points need adjustment so they open at the cam lobes to create spark. The points opening is the trigger for the coil to fire and send spark to the plug.
It has the original style points.
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post #7 of 11 (permalink) Old 10-02-2016, 08:07 PM
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1972 280SE 4.5 Won't start!

If you are getting a good strong spark, then you should get combustion when you dump fuel in the throttle body.

But if you have a bad or wrong coil and a weak spark, it might not spark well under compression in the cylinder.

If you've got good spark, it should fire when you dump volatile fuel in the intake.

Do you have a points gap (about the thickness of a business or playing card) at the cam lobes? (I assume you do if you are getting spark when laying the plug on the valve cover.)

Edit:
At this point, if I were you, I would probably try dumping some more fuel into the intake. The plenum at the bottom of the manifold is the full length of the engine and about six inches wide. So it can hold a significant amount of fuel to make the vapors you want to burn. Here are a few pictures that might make you feel a little more comfortable about dumping more significant amounts of fuel in the throttle body.

Assembled:


Just the top half with intake ports:


Bottom piece connected to top with 8 rubber gaskets, showing the large throttle body hole in the center. You can see how this can hold plenty of fuel:

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Last edited by Fonzi; 10-03-2016 at 06:13 AM.
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post #8 of 11 (permalink) Old 10-08-2016, 09:04 PM Thread Starter
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Originally Posted by Fonzi View Post
If you are getting a good strong spark, then you should get combustion when you dump fuel in the throttle body.

But if you have a bad or wrong coil and a weak spark, it might not spark well under compression in the cylinder.

If you've got good spark, it should fire when you dump volatile fuel in the intake.

Do you have a points gap (about the thickness of a business or playing card) at the cam lobes? (I assume you do if you are getting spark when laying the plug on the valve cover.)

Edit:
At this point, if I were you, I would probably try dumping some more fuel into the intake. The plenum at the bottom of the manifold is the full length of the engine and about six inches wide. So it can hold a significant amount of fuel to make the vapors you want to burn. Here are a few pictures that might make you feel a little more comfortable about dumping more significant amounts of fuel in the throttle body.

Assembled:


Just the top half with intake ports:


Bottom piece connected to top with 8 rubber gaskets, showing the large throttle body hole in the center. You can see how this can hold plenty of fuel:
I cleaned the connections to the coil and measured the ohms on the coil. The coil resistance is within the range. Squirted starting fluid into the manifold with the throttle flap open, about 5 or 6 seconds. The car appear to fire up but for a few seconds only and won't continue. This makes me think the ignition is ok, maybe the fuel pressure sensor is bad. Any ideas? And how do I test the fuel pressure sensor? Is the something that triggers the fuel injection? I suspect the fuel injection is not working. Thank you in advance.
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post #9 of 11 (permalink) Old 10-09-2016, 08:12 AM
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There is no fuel pressure "sensor". but there is a fuel pressure regulator.

Fuel is supplied to the fuel "rails" (distribution tubing that feed the injectors) from the fuel pump. It then returns to the tank via a fuel pressure regulator and another device called a damper. The fuel pressure regulator maintains a pressure of about 30psig in the fuel rails.

To check if you have 30psig, disconnect the tube that goes to the start up valve (This is on driver side of engine (in NA) front side of engine - 95mm line in pic below that tees off main rail). Install a pressure gauge on that line. Any inexpensive gauge plus a few adapters will do, such as they sell at plumbing stores for water pumps.

Then turn ignition key on and off a few times. You should hear the fuel pump run and you should see pressure reading on gauge. If you don't hear pump run, problem could be pump itself or the fuel pump relay or just a fuse.


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post #10 of 11 (permalink) Old 10-14-2016, 07:16 AM
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The fact that it starts using starting fluid tells me it's not spark. You mentioned you can feel the pump run with someone cranks it, so I'm led to believe you have a fuel delivery issue or a fuel control issue. By taking a successful pressure reading at the cold start valve, you rule out clogged screens and filters leading to the engine itself.

What state are the injector points in? Those are the assembly in the front of your distributor - you remove two screws and the whole thing comes out with care. I'd clean the heck out of those and the associated plug and try again.

When you first turn the key on, without cranking the engine, you should hear the fuel pump run for a brief period. Can you confirm this is happening?
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