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  #1  
Old 01-26-2007, 07:33 PM
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Question 1978 450SE fuel Injection problem

I have this mercedes that has been sitting for years. I Cleaned out the entire fuel system and run it every other day to clean it out further. When it runs it runs Great. My question is, everytime I go to start it, I have to pour a little gas in the throttle body. It will run good until it warms up. Then as I am watching it, the throttle valve falls(opens) a little bit and the car dies. Then (sometimes) I have to put a little more gas in the Throttle body to start again. not a mechanic. hope this makes sence. thanks.
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Old 02-25-2007, 07:25 PM
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I'm not expert but if you need to "prime the engine" then your fuel pump sounds like its dead. When you turn the key to position 2 can you hear it engage (the pump is located at the rear of car by the LH wheel)?

Mike.
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Old 02-27-2007, 12:06 AM
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Mounted on the front of the motor is small metal box about the size and shape of half a brick. That's your warmup/enrichment compensator, usually just called a WUR. It's gunked up. Find a replacement at a junk yard or on ebay and switch it.

If you want to actually diagnose it, you need a fuel pressure guage between it and the fuel distributor (round thing with 8 fuel lines on it).

Once the car does run, does it accelerate well when you stomp on the gas or does it stumble?

One other component that may be baulky is the fuel accumulator. It's the small metal can that connects between the WUR and the fuel tank. It's on the motor someplace; not sure where.

Nice thing about CIS is that there aren't a lot of parts to worry about.

-CTH
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Old 03-06-2007, 12:33 AM
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Clarifications

on the 450 SEL, the fuel accumulator is part of the fuel pump assembly new the gas tank. I am parting a 77 450 sel and have both a warm up regulator and accumulator. You might also try chceking the warm up injector on the intake manifold. PM me if you are interested in the parts.
Thanks,
Lee
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Old 03-07-2007, 12:51 AM
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Most likely you have a vacuum leak. You cannot see the throttle plate in that car unless you have the fuel distributor removed, so what you are looking at is the fuel metering plate in the fuel distributor air horn.

You probably have a big vacuum leak, most likely the idle control valve hoses or the boot between fuel distributor air horn and throttle body. It's also possible you have high fuel pressure from a plugged return line or defective pressure regulator, or a stuck "frequency valve" -- the feedback control for the oxygen sensor. It runs cold because it has richer fuel mixture cold, once it warms up and the computer leans it out, the stalls.

You may also have a bad oxygen sensor, reading rich all the time, so that it leans out too far warm.

You may also simple have it set too lean -- but you need a CO meter to check it properly.

Check for vacuum leaks -- a can of carb or brake cleaner spray works great -- simply spray a small amount on suspected areas with the engine running -- if the idle changes, you have a vac leak. Check idle control valve (don't know the location off the top of my head, but it will have a hose from under the fuel distributor air horn and one to the intake manifold), boot between fuel distributor air horn and throttle body, injector seals, and the seals between the two part of the manifold. All, alas, are probably shot.

I'd remove the brake booster line and plug that fitting temporarily, too -- a split diaprham in the brake booster can give you a big vac leak, as will a cracked line or bad check valve, or a rotted seal at the booster.

Peter
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Old 03-07-2007, 06:39 PM
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As I recall, O2 sensors were not part of the mechanical injection system that these cars used. The system relys upon fuel pressure and pump volume to assure proper fuel supply. As the air flow sensor opens, the amount of fuel flow to the injectors is increased.

Quote:
Originally Posted by psfred View Post
...It's also possible you have high fuel pressure from a plugged return line or defective pressure regulator, or a stuck "frequency valve" -- the feedback control for the oxygen sensor. It runs cold because it has richer fuel mixture cold, once it warms up and the computer leans it out, the stalls.

You may also have a bad oxygen sensor, reading rich all the time, so that it leans out too far warm.
....
Peter
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Old 03-07-2007, 09:32 PM
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Quote:
Originally Posted by zooklm View Post
As I recall, O2 sensors were not part of the mechanical injection system that these cars used. The system relys upon fuel pressure and pump volume to assure proper fuel supply. As the air flow sensor opens, the amount of fuel flow to the injectors is increased.
The last of the CIS cars did have a O2 sensor. It drives the frequency valve which modifies control pressure in an attempt to close the loop on the fuel injection.

I don't know if it was late '78 or '79 that it was added.

Michael
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