450SL Odd Behavior - PeachParts Mercedes-Benz Forum

350SL4spd · #1 06-22-2006, 05:13 PM

I'm tuning up the old and she's been sitting for a while ('75 450SL).

When she gets up to operating temp the idle drops to about 500rpms & she starts to smoke. I didn't have her running that long hot, so I don't know if it was oil or fuel smoke, but it was definitely darker & not H2O smoke.

Any idea what would be the root ot of such a system after warm up? My old 380SL used to smoke UNTIL it warmed up due to bad valve guide seals, but AFTER warm up?

Thanks!

dima4855 · #2 06-22-2006, 09:07 PM

How long has the fuel been in the gas tank?
How are the spark plugs?

#3 06-22-2006, 11:36 PM

I would suspect the coolant temp sensor for the fuel injection. If it is telling the computer the engine is cold the computer will run a rich mixture causing black smoke and rough running once the car is up to operating temp. It usually has a blue top and brass body with a two pronged plug and is screwed into the intake manifold near the distributer or upper rad hose outlet. if I recall. You can check resistance across the two prongs with a ohm meter and compare your reading to the specs listed in the shop manual. They are cheap and easy to replace.

350SL4spd · #4 06-23-2006, 11:49 AM

Thanks Guys,

I'm replacing the plugs & wires today. I'm sure the fuel is a bit old, but I'd be surprised if it only created a problem after the car was warm...

Interesting idea about the temp switchover valve. I know the one you are refering to, it's on top of the thermostat housing. It's interesting that you mention it, because I replaced the coolant temp sensor for the gauge in the dash yesterday, as it was dead.

Does anyone have the specs for the OHM readings over the switch over valve by the distributor.

Thanks!

oxymoron · #5 06-23-2006, 02:12 PM

I am not aware of a computer on a 450 SL. This is Kjet CIS, right?
I think you might want to give a thorough cleaning to the WUR.

Is the smoke black (indicating rich condition)?
If the WUR isn't switching from running rich at cold to leaner when warm, you will get symptoms you describe.

Will you pull the plugs to check. Wet black plugs can mean rich.
Also the plunger in the FD, if it is stuck to rich (low control pressure) will blow black smoke.

The fuel passages in the Fuel Distributor and the Warm Up Regulator in CIS cars are tiny and the least bit of dirt and/or rust will affect flow and pressure.

Don't let it run too rich too long. It will mess up the cylinders when the oil washes down.

#6 06-24-2006, 09:55 AM

Through 1975, MB used D-jet fuel injection on the 4.5 v8 models. It is a early comuterized fuel injection, and uses a analog computer. The first model to use D-jet were 1969 3.5 v8 models(230 hp).

It was a expensive and complicated system, so MB switched over to CIS for the 1976 model year for all v8 motors, including the 6.9 M100 motor used in the 450SEL 6.9.

I would be very aware of rich running on 1975 model 450SL's beacause of the thermal reactors mounted on each exhaust manifold. They can melt down.

oxymoron · #7 06-24-2006, 09:58 AM

Quote:

Originally Posted by John Holmes III

Through 1975, MB used D-jet fuel injection on the 4.5 v8 models. It is a early comuterized fuel injection, and uses a analog computer. The first model to use D-jet were 1969 3.5 v8 models(230 hp).

It was a expensive and complicated system, so MB switched over to CIS for the 1976 model year for all v8 motors, including the 6.9 M100 motor used in the 450SEL 6.9.

I would be very aware of rich running on 1975 model 450SL's beacause of the thermal reactors mounted on each exhaust manifold. They can melt down.

Thanks for that clarification. I stand corrected. I thought CIS Kjet came in in 1975.

Tomguy · #8 06-24-2006, 12:46 PM

Quote:

Originally Posted by wdba123190

Does anyone have the specs for the OHM readings over the switch over valve by the distributor.

There should be 3 temp sensors: A thermo-time sensor for the cold start valve, a 212°F switch for the cooling system, and a water temp sensor for the EFI. The latter should have readings close to these (specified as "Heavily temperature dependent):

5.9 kohm at 0°C
2.5 kohm at 20°C
1.2 kohm at 40°C
600 ohms at 60°C
325 ohms at 80°C
190 ohms at 100°C

Quite a broad range indeed. Also, while it's idling at operating temp, pull the cold start valve's plug to make sure it's not still on, if you're rich enough to smoke at idle this could be a cause. Also, pull the air temp sensor off the intake manifold. This will make your mixture richer. Your idle would normally go up but if you're too rich it might cause it to stall.

The D-Jet system is VERY sensitive to vacuum leaks. First and foremost, make sure the hose going to the MAP sensor is new and leakproof. ANY holes in this hose and you WILL run rich. Especially if it's the original, braided-fabric line.

350SL4spd · #9 06-24-2006, 04:35 PM

Thanks for all the input!

This girl has not been getting her exercise. Upon inspection the plus/plug wires were orginal. 94k mi & 31yrs on the original plugs & wires. Replace those and a nice italian tune up/hill climb and she's better, but still a little rich.

Can anyone point me to where the three temp sensors are? I replaced the one at the rear of the right head to get the temp gauge operational & I know there's one single-pronger on top of the thermostat housing. Which one is that & where's the other one?

Thanks.

#10 06-24-2006, 11:18 PM

The single prong switch on the thermostat housing is the "212 switch". It grounds the wire at 212 deg/f and turns on the electric fan, and on some models advances the ignition timing to prevent overheating.

Tomguy · #11 06-25-2006, 04:03 AM

The thermo-time one is the one with a tab on it so the plug goes on the right way (it might be cracked with age) See: http://img.eautopartscatalog.com/live/C401012256VDO.JPG

The water temp sensor has a female plug end. See: http://img.eautopartscatalog.com/live/C602012894BOS.JPG
The 2nd one is the one you want to check resistance on. Make sure it's within a reasonable range of what I posted. If your engine is cold and it's reading 0 ohms (short), replace it. If your engine is hot and it's reading 1 kohm (way too high), replace it. These USUALLY don't fail. I'd suspect a vacuum leak or malfunctioning thermo-time switch before this.

You say it hasn't been run for a while. Clean the points (if you didn't replace them). Regap if needed. Set the timing to 5° ATDC with the vac line connected to the distributor - if there is no vac at this line, plug it for the time being and set your timing to 7° BTDC. Take it out and RUN THE CRAP OUT OF IT. For every time you floor it on the highway, kicking it into 2nd gear at 75-80MPH, you will blow more crud out of the engine.

350SL4spd · #12 06-25-2006, 03:07 PM

Great info! One question: Where's the water temp sensor for checking the resistance?

We're definitely making progress. Plugs, wires, timing set to 5 ATDC & an italian tune up and the warm up smoke is gone. I still need to check the point gap; anyone have the gap size handy? I can't find it in my books...

I do have a new problem, however: On cold start (sitting all night) she stumbles and will stall if you don't keep the idle up by feathering the gas pedal. After 10, maybe 15 seconds of this she's running perfectly w/o so much as a stumble. Cold start valve? The smog pump is toast, but that shouldn't effect cold start that bad. Suggestions?

Thanks!

PS: Where's the MAP sensor on this car? I need to replace that hose too...

Tomguy · #13 06-25-2006, 03:23 PM

The water temp sensor is on the intake manifold. It's probably hidden by the air filter assembly. Literally RIGHT NEXT to the thermo-time switch. See image.

As for the MAP sensor, I believe on the 107 chassis that it's on the driver's side fender. On the 108 and 109 it's on the firewall, on the 116 it's on the driver's side fender (right above the wheel)

Jetjason · #14 04-19-2012, 01:43 AM

HOW DO YOU CLEAN THE WUR? REMOVE FROM MANIFOLD AND USE BRAKE CLEANER? THANKS FOR ANY INFO

I am not aware of a computer on a 450 SL. This is Kjet CIS, right?
I think you might want to give a thorough cleaning to the WUR.

Is the smoke black (indicating rich condition)?
If the WUR isn't switching from running rich at cold to leaner when warm, you will get symptoms you describe.

Will you pull the plugs to check. Wet black plugs can mean rich.
Also the plunger in the FD, if it is stuck to rich (low control pressure) will blow black smoke.

The fuel passages in the Fuel Distributor and the Warm Up Regulator in CIS cars are tiny and the least bit of dirt and/or rust will affect flow and pressure.

Don't let it run too rich too long. It will mess up the cylinders when the oil washes down.[/QUOTE]