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  #1  
Old 08-05-2001, 10:51 AM
aplan4you
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Unhappy '72 350SL (4.5L) Rough Idle

HEy techies,

I have a '72 350SL 4.5 ( Yes it really is a us 450sl) same engine as the 450sl. 140,000 miles

It has started to have a rough idle ALL THE TIME.

All 8 Spark plugs foul up with a Black Soot, Like Black Smoke.
I have been told this is an indication of a Rich Fuel Mixture.

I have replaced most of the hoses, I have replaced the Fuel Pressure regulator, Spark plugs are new, Spark plug wires are new, I replaced the Fuel Filter, The Heater Valve Sensor, the Engine Temp Sensor.

What should I be looking for? The mechanic that I had look t it in Lubbock, TX suggested an engine overhaul at $4,500. I don't really think I need that. So I am having my father work on it, He has repaired cars for 40 years, this one has him stumpded, and I can't get this to my regular mechanic in Dallas.
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  #2  
Old 08-05-2001, 11:56 AM
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These systems are very hard to deal with without proper test equipment.

Your mixture is worse than just rich if your plugs are all sooted up. With the D-jet system this probably means fuel pressure or manifold pressure sensor although open circuits on the temp sensors (air - in air filter and coolant - at front of intake manifold) can also send it that far rich.

You should verify manifold vacuum. I would do it by teeing to the manifold pressure sensor vacuum hose (7.5mm id rubber hose). The vacuum should be as close to 17in as possible. It will take good timing and proper running to get this much vacuum. To test the manifold pressure sensor try pulling a vacuum on the MAP sensor (manifold absolute pressure) higher than the engines vacuum and see if it leans out (to do this plug the cars hose and suck directly on the sensor). A gas analyzer sure helps at this point or a scope to evaluate the mixture by measuring injector pulse width. It you have either of these available I can give better instructions.

The MAP sensor is adjustable with an Allen screw under the black plastic cap (opposite end from the vacuum fitting. You probably can get idle mixture correct no matter what the problem with this adjustment. The problem is that you can't sit there and adjust for every situation so these are just tests.

To evaluate the temp sensors first make sure they are connected and both wires intact. Measure ohm value if you like, but the real test is to backprobe the sensor input at the controller. The signal comes out at 5v disconnected. This is brought lower by passing through the resistance of the variable temp sensor resistance. By testing at this point the voltage a true measure of what the computer sees is at hand.
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  #3  
Old 08-05-2001, 01:39 PM
aplan4you
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Thanks, that is the direction I was heading, Just so I don't sound like an Idiot to my dad, where is the MAP located on the MB 350SL(4.5L) Also, the other two sensors you were talking about, can I just replace them and not worry about it< Thay couldn't do to expensive? HWere are they located, I will Fax him the Answer to test.

Where are the Water Temp Sensor and the Temp sensor located, actually is there a diagram I can see online somewhere,

I like this forumn, I can Call my Dad and he is amazed how much I really do know about cars, evn though I can't even change the oil on any of my cars. THANKS for th ehelp.
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  #4  
Old 08-05-2001, 02:27 PM
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The MAP sensor is located forward and below the power brake booster. It is about the size of a grenade or an orange; shaped more like the grenade. One four wire connector and one vacuum line the size of a finger.

As I said before the air temp sensor is in the aircleaner airhorn (two wires). The coolant sensor is at the front of the motor along with the thermal time switch. The coolant sensor will have the same type connector at the air temp sensor. The thermal time switch is also two wires but has a different style connector.
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  #5  
Old 08-05-2001, 10:16 PM
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Which fuel pressure regulator did you change, the primary or the secndary? Did this problem come on all of a sudden or was it gradual? I had to adjust mine but was lucky enough to have a exhaust gas analyzer at a friends shop. The same symptons occured a year later when there was a dead cell in my battery. I could start it but only had 12 volts at a fast idle, swapped batteries and it ran great with 14 volts.
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  #6  
Old 08-06-2001, 12:04 PM
aplan4you
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I am learning so much about this mercedes, My guess is that I changed out the primary, i wasn't aware of a secondary.

Part # 000-078-03-89 Fuel Pressure Regulator.

Part # 005-542-26-17 Temperature Sensor
Part# 002-542-21-17 Water Temp Sensor
Where on the car those two sensors go I have no idea, But I am sure my father took care of those.

The Battery is one month old, so that should be ok.

I think this occured all of a sudden, The Car was sitting in My Garage for 4 months, ( I bought a new BMW 328i Convertible)
Thn I asked my Father to come pick the mercedes up in dallas so that we could finish restoring it. Made the 400 mile trip no problems,

Then this rough idleing, and the spark plugs having that black carbon soot on them, we clean them, the they get sooted up again.

I will check on a secondary Fuel Pressure regulator,
I will order today a new Manifold pressure sensor
and a fuel pressure sensor today and have those sent to him

You guys are alot of help THANK YOU SO MUCH, I will let you know what finally fixes it.
NO wires or anything have been replaced on this car it seems like for 30 years.
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  #7  
Old 08-06-2001, 12:18 PM
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There is only one pressure regulator. The other device that looks similar (but with only two hose connections) is a dampener (for noise).

I like your open checkbook approach to diagnostics. To bad the thought of paying a competent technician usually never comes to the mind of those using such diagnostic techniques. This work is so easy.
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  #8  
Old 08-06-2001, 12:32 PM
aplan4you
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Steve,

Trust me, If I could find a Competent Tech In Lubbock Texas, I would use them in an instant.
The closest Tech is in Midland Texas and they get parts from here in Dallas, Texas. He is the one that said rebuid the entire engine and that would PROBABLY fix it. (Doesn't sound Competent to me)
The tech I do trust, is here in Dallas, I wish this problem had occured while it was here, But I don't want to risk damage having to drive the car to Dallas, then fixing a $30.00 sensor ( I can only wish it's that easy, but with my luck thats all that it is)

I think that once I fax all this info to my Father, Whom I do Trust on my BMW and any of my other cars. I think the suggestions that you have given will put him in the right direction.

I do apprecaite all the input on this forum, and by NO MEANS do I want to imply that having a Mercedes Professional look at it is not worthwhile, It's just not an option I have unless I have the car brought back to Dallas.

Thanks Again,

Al
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