Cold start issue - PeachParts Mercedes-Benz Forum

indisguise · #1 10-26-2013, 11:29 AM

Back again

FYI...I recently changed to pertronix and this may be timing... But I doubt it.

Issue..........

1. On cold mornings (50ish...spot the Californian) the car starts fine.
2 .for the first mile the car drives fine.
3 Suddenly the idle drops then the car idles very erratically.
4 if the accelerator is pressed the car runs fine.
5 this symptom lasts for about 30 seconds and then is never seen again!
6. This never happens on afternoon cold starts
7. This only happens on cooler mornings

Any ideas?

I am thinking,

Temp sensor issues.

TnBob · #2 10-26-2013, 12:30 PM

Soo symptomatic of fuel starvation. Clogged filter or a very weak fuel pump. The fact it goes away aims me more towards the filter

Jub · #3 10-27-2013, 07:29 AM

Did you install a fuel pressure regulator (if you have weber carbs)?

Also, did you check for a vac leak (I remember this happened to me; turns out the gasket that sits between the manifold and the carb's had a hair line fracture: when car was cold and metal sort of shrank it would be problematic; but when car warmed up and metal expanded the problem would go away).

meltedpanda · #4 10-27-2013, 10:53 AM

what car are we talking about?

indisguise · #5 10-27-2013, 11:12 AM

I should really mention the model hey.....

1973 280 SEL

4.5 V8 D-jet.....

I have a new fuel filter and will try that

Graham · #6 10-27-2013, 11:27 AM

Quote:

Originally Posted by indisguise

Back again

FYI...I recently changed to pertronix and this may be timing... But I doubt it.

Issue..........

1. On cold mornings (50ish...spot the Californian) the car starts fine.
2 .for the first mile the car drives fine.
3 Suddenly the idle drops then the car idles very erratically.
4 if the accelerator is pressed the car runs fine.
5 this symptom lasts for about 30 seconds and then is never seen again!
6. This never happens on afternoon cold starts
7. This only happens on cooler mornings

Any ideas?

I am thinking,

Temp sensor issues.

OK, I am assuming you are talking about your V-8 D-Jet.

The D-Jet has two mixture control circuits. One for idle and one for when pedal has only slightly been moved. So when you take your foot off pedal, you are changing to the idle mixture control circuit in ECU (which can be adjusted using the screw on the ECU).

On the D-Jet, the AAV (aux air valve) gradually reduces the amount of air bypassing the throttle plate and entering the intake manifold as the engine warms up. This in turn reduces the idle speed. AAVs can be troublesome and sticky. If you haven't cleaned it, do so. Once engine gets good and warm, the AAV should be closed (it should actually be closed at 150F and move smoothly as it warms up). Try getting engine fully warmed up and then set idle speed to about 750rpm. That might avoid your cut out problem.

The other temperature inputs to ECU are from the two temperature sensors. The manual provides the resistance vs temperature curves for the sensors. Maybe pull them, clean up the ends and measure resistance at various temperatures. The curves are also in this link:
D Jetronic

meltedpanda · #7 10-27-2013, 04:10 PM

here you go , second Grahams thoughts

and a little more ....Hey Graham how ya been ??

D Jet explained

Trigger Points

The trigger points are located in the base of the distributor below the ignition point breaker plate.These points are used to produce the signal that synchronizes the injector to the crankshaft.They consist of two low voltage,low current contact points which are driven by a cam located on the distributor shaft.Each set of points initiates the opening of one or two groups of injectors.

Unlike ignition points,the trigger points can last 100,000 miles or more.As the distributor rotates,a pulse is created by the opening and closing of the trigger points.This pulse is sent to the ECU.The ECU uses this signal to open the injectors and will use the inputs from the other sensors to determine when to close them.

Temperature Sensor I

Temperature Sensor I is an ambient air temperature sensor.As the ambient air temperature decreases,the density of the air increases.As a result,the ECU must inject more fuel on a cold day than on a warm day.

Temperature Sensor I is a temperature sensitive resistor known as negative temperature coefficient thermistor (NTC). This sensor has a resistance of between 400 and 500 ohms at 50 degrees F. At 100 degrees F.,the resistance is between 150 to 200 ohms.

Temperature Sensor I actually has little effect on the operation on most D Jetronic equipped vehicles.This is because most of these cars have tens of thousands of miles on the engine and are running very rich due to this wear.The troubleshooting consequence of this is that disconnecting the air temperature sensor during the diagnostic procedure may have little effect on the way the engine runs,and in some cases it may actually improve the way it runs.

Temperature Sensor II

Temperature Sensor II is the coolant temperature sensor on water cooled engines.Like temperature sensor I, it is a NTC thermistor.The temperature of the engine is important because the intake manifold design of fuel injected engine does not permit the use of an air restictive choke.Additionally,restricting the air to enrich the engine during warm up would cause inaccurate readings from the manifold pressure sensor.The warm up choke function is therefore performed by Temperature Sensor II.

When the coolant temperature is about 50 degrees F.,the resistance of Temperature Sensor II is between 3000 and 4500 ohms.As the temperature increases to more that 120 degrees F.,the resistance drops to less than 1000 ohms.Thus the sensors work is done once the engine is warmed up.Also it must be continuously remind the ECU that the engine has warmed up.If damaged,Temperature Sensor II will not continue to partially function( such as the resistance values shifting). It will completely fail,creating an open short or ground.

An open circuit in Temperature Sensor II or the wiring leading to it will cause the engine to run extremely rich once warmed up.Symptoms would be dark smoke from the tailpipe (most noticable at idle),rough idle and poor power.Keep in mind that these same symptoms could also be caused by engine compression problems and ignition.

Should the sensor become shorted or the wiring harness grounded,the effects may not be noticable at all when the engine is warmed up.The symptoms would be more like a carburetor with the choke stuck open-rough or erratic idle,stalling or hesitation when the engine is cold and progressively running better as the engine warms up.

Throttle Switch

The throttle switch tells the ECU when the throttle is closed,when the throttle is wide open and when the throttle is moving toward the open position.The switch consist of twenty two contacts,with a set of wiping contacts that move across them as the throttle progresses from the closed position to the wide open position.

One wiping contact is used to inform the ECU that the throttle is closed.Another makes contact only when the throttle is wide open and a third makes and breaks contact twenty times as the throttle opens.The electrical pulses created by the making and breaking of the contacts signals the ECU to open the injectors more frequently,thereby enriching the mixture for acceleration.This feature behaves much like an accelerator pump on a carburetor.

Symptoms associated with a defective throttle switch include a rich(smoky)idle and hesitation.An intermittent condition at cruise-which feels like you shut off the engine and immediately turned the key back on,can also be caused by the throttle switch.

To test the throttle switch,open the throttle with the key on but the engine not running.The injectors should open exactly twenty times,evidenced by twenty evenly spaced clicks.

Pressure Sensor

The D Jetronic pressure sensor is known as a linear variable displacement transducer (LVDT).It consist of a pair of coils,one with about 150 ohms of resistance and the other with about 85 ohms of resistance.An iron core attached to a diaphragm runs through the center of these coils.As changes in manifold pressure moves the diaphragm,the iron core moves inside the coils,causing ripples in the current flowing through these coils.This signal is used by the ECU to monitor the relationship between barometric pressure and manifold pressure.

The most common symptom from a defective pressure sensor is a rich running condition.Of course rich running can be caused by several other defects as well.

Of all the sensors used on D Jetronic,this one both the easiest and the most difficult to test.Usually a simple resistance test of the coils is enough to determine if the unit is good or bad.On the other hand,the only way to be sure is to replace it with a known good unit.

Electronic Control Unit (ECU)

The ECU receives input signals from the pressure sensor,Temp Sensor I,Temp Sensor II and the throttle switch to determine how long to leave the injectors open.It is only able to respond to air-fuel ratio request from one sensor ata time.As a result,whenever a sensor fails,the tendency will be for the ECU to send the injectors a full rich supply of fuel.

The electronic unit has no servicable components.In the event of a failure,the ECU is replaced as a unit.Failures are extremely rare and usually result in a no start.

D Jetronic that have an adjustable air fuel ratio have a detent potentiometer on the side of the ECU.This potentiometer can be used to fine tune the air-fuel ratio during a tune up.

Pin # 19 and 25 of the ECU are connected to the fuel pump relay.When the ignition switch is turned to the on position,the ECU energizes the fuel pump to ensure that the fuel system is filled for ease of starting.If the engine is not cranked,the ECU will shut off the fuel pump after one or two seconds.If the engine is started,the fuel pump runs continuously until the engine is shut off.

D Jet Components

D Jetronic Fuel Components

In Tank Filter

Located inside the fuel tank is a screen or filter designed to protect the fuel pump from rust,dirt and debris.Although seldom the cause of a drivability problem,the in tank filter should be high on the list of items to check.In many cases these filters have been ignored,even on well maintained vehicles.

Fuel Pump

The fuel pump is a high speed roller vane pump.It is capable of pumping fuel at pressures and volumes much higher than the engine or injection system would ever require.This type of pump is very efficient at pushing fuel bur does not do a good job of pulling fuel.As a rsult,these pumps are located very close to the fuel tank or even inside the fuel tank to reduce the chance to vapor lock.

The fuel pump can react adversely and even fail as a result of using some fuel additives.Use extreme when selecting them to ensure that they do not contain methanol or other corrosive substances.

A defective fuel pump can cause low fuel pressure which would result in symptoms such as hesitation,stalling and poor power.Often a defective fuel pump bypasses the poor running stage and simply stops operating.This causes the engine to die or keeps the engine from starting.

Fuel Filter

The fuel filter is the only real protection the injection system has from internal contamination from dirty fuel.Therefore the filter should be changed every time the spark plugs are changed.

Fuel Pressure Regulator

The fuel pressure regulator used on D Jetronic systems consist of a valve connected to a spring loaded diaphragm.The regulator controls the fuel pressure at 28-32 psi and is adjustable so it can ensure the proper fuel pressure throughout the life of the vehicle.Incorrect fuel pressure can cause a lean running engine if the fuel pressure is too low and a rich running engine if the fuel pressure is too high.

A defective fuel pressure regulator can result in high fuel consumption,rough or erratic idle and poor power.

Injectors

The injectors are solenoid operated,normally closed valves controlled by the ECU.Grounded to the engine block or chassis,each injector is opened by a 3 volt pulse from the ECU.The lenghth of the pulse is only a few milliseconds(2 to 5),and it takes time for the injector to close from the spring tension.Thus the injector is open for a total of about 3 to 6 milliseconds.

Very little goes wrong with the D Jetronic injectors themselves.An occasional burned out solenoid winding or restriction from contamination is the most common problem.Another problem is leaking from the hoses that attach the injector to the fuel rail.

Typical symptoms of injector problems include rough idle and poor power.

Fuel Rail

The injectors and the fuel pressure regulator are attached to steel tubing known as the fuel rail.The inbound fuel lines from the fuel pump and filter feeds fuel to the injectors and the fuel pressure regulator through the fuel rail.

As a passive component of the system,very little can go wrong with it except for leaks and restrictions.

Cold Start Injector

Also attached to the fuel rail is a solenoid operated valve known as the cold start injector.Since the earliest applications of the D Jetronic system were four cylinders,the cold start valve picked up the moniker "fifth injector." It stuck in some circles,even for six and eight cylinder applications.

The cold start injector receives battery voltage whenever the starter is engaged and is grounded through a device known as a thermo-time switch.The thermo-time switch is a temperature sensitive bimetal switch designed to provide a ground for the cold start injector when the temperature of the engine is less than 95 degrees F. A second circuit in the switch is an electric heating element intended to heat the bimetal as the engine is being cranked.

Consequently,the cold start injector should operate only when the engine is being cranked,the temperature of the engine is less than 95 degrees F. and for a maximum of five to twelve seconds.

Two of the most common symptoms of a cold start injector problem are hard starting when cold becuse the cold start valve is not operating and a leaking cold start injector which can cause an extremely rich running condition.

indisguise · #8 10-27-2013, 06:01 PM

Great info all.

I will try cleaning the AAV followed by the fuel filter.

Thanks all.

Graham · #9 10-27-2013, 07:40 PM

Quote:

Originally Posted by meltedpanda

here you go , second Grahams thoughts

and a little more ....Hey Graham how ya been ??

Just fine Ron. Thanks for asking.

I spent this summer working on my 300D which had hidden rust

. 350SL became my daily driver. 107 still amazes me - feels and runs like a new car and no issues.

We sold our diesel smart car and have a new GLK250 coming. Not sure why. Our 98 E320 is running great.