Where to go from here?

[gtwendt]

I have a 1976 450SL with 140K on the clock. I have posted a few times to this site and have received some good advice. Here's my problem. The car is hard to start, but eventually does. It chugs, coughs, and backfires through the throttle plate. Based on advice, here is what I've done so far:
1. New fuel filter, fuel pump, and fuel relay.
2. Cleaned all injectors and fuel lines.
3. New spark plugs, plug wires, and distributor cap.
4. Set timing as per specs.

Result: Same issues. However, today I turned the ignition on and pressed down on the throttle plate. I could hear fuel entering the fuel distributor. Car started right away, then went into the chugging mode again. Then, I held the throttle plate down about1/4" and started the car. Started right away and it ran much smoother. Question is this. Is there any harm in holding the throttle plate in this position? Or, what is it in the engine that makes the throttle plate depress to obtain a good start and idle?

Any and all help would be greatly appreciated. Thanks.

gtwendt

[Gurunutkins]

as i remember on your 76 450sl with the CIS injection - you have the fuel pump cut off switch on the side of the air plenum where the air restrictor plate sits in its opening. as the air plate moves down as the car starts it either makes or breaks contact (I'm pretty sure it makes contact) to allow the fuel pump to run after the initial prime. If that switch is faulty then you will get the sort of symptoms you describe try shorting the wires together on the plug to the switch but remember it will make the fuel pump run all the time so just do it as a test and let us know what happens

I have included a pic with the air flow interlock switch circled in red.
cheers
barri

ignore the d jet connectors it was my only picture and its halfway through a conversion to megasquirt

[Wodnek]

Some ideas:

Could the timing chain have jumped a tooth?

Could your cats be plugged? (are they under the hood on a 76?)

Electronic ignition issue?

Coil? (my 380 SL had an aftermarket coil, threw a junkyard bosch coil in it and it ran great.)

[gtwendt]

Thanks for your thoughts. Are you referring to the electrical connection that leads to the warm up regulator? If I pull the lead from that connection, with the ignition on I can hear fuel entering the fuel distributor, just as I do when I depress the throttle plate down. Can the position of the throttle plate be adjusted? Timing chain has been inspected and all is fine there. Bosch coil has also been checked and it is good. I appreciate you thoughts and guidance.

[Gurunutkins]

HI
the switch i am talking about is on the air inlet just in front of where the FD would be as per the picture. the switch is a secondary control for the fuel pump and turns the pump on once the air plate moves down as the engine starts. if the ignition is on and the engine stalls (or similar) so the plate is in its resting position then the switch stops the pump from running.

The air plate is adjustable in several ways, it can be re centered in the hole, there is a weight on the arm underneath it that can be moved up or down the arm to change the way the plate moves as air flows over it and there is the fuel adjusting mix screw which acts on the arm to regulate how much fuel is injected as the arm moves.
cheers
barri

[meltedpanda]

the throtle plate can be cleaned and serviced and the relationship to the air flow sensor is key
The Airflow Sensor
The airflow sensor, in most cases, is located on the air filter housing and is responsible for measuring the amount of air
A To fuel injectors
B To warm up regulator
C From warm up regulator
D To cold start injector
E From fuel filter
F Return to fuel tank
Backentering the engine. The sensor housing is conical in shape, into which the airflow sensor plate is fitted. The airflow sensor plate lifts as the throttle is opened by the incoming air.
The amount of lift is proportional to the volume of air entering the engine. The shape and angle of the cone will determine
this ratio.
A neutral plate position is normally level with the bottom of the cone, this is adjustable by bending a small clip / spring that acts as a stop at the bottom of the unit. The purpose of this spring is to allow the flap to move beyond its neutral position to
allow excessive pressure to escape if the engine was to backfire, passing a large volume of air back into the air filter
housing.
If the system did not have this facility the pressure could split or blow off the rubber air trunking. Any splits or ill fitting air
hoses that allow unmonitored air into the engine require rectification.
As the airflow lifts the sensor plate this subsequencially lifts the control plunger - the higher the lift the greater the amount of
fuel delivered to the injectors.
To adjust the fuel mixture a small 3 mm Allen screw is located within the airflow sensor; this alters the relationship between
the sensor arm and the control plunger. Turning the screw clockwise enriches the mixture and vice-versa. It should be
noted that the screw should be turned in very small increments and the Allen key should be removed before the engine
speed is raised.

[gtwendt]

Again, thanks for your help. You have helped me to better understand the workings of the air flow sensor. What do I now need to do to duplicate the starting procedure as when I manually press down on the throttle plate with the ignition on? When I do this, it starts very easily and runs fairly smooth. If I adjust the throttle plate downward and leave it there, does this mean that the fuel pump will always be running when the ignition is on? I don't see any of this covered in either my Chilton, Haynes, or MB CD. Thanks for your patience as I am still learning.

[ctaylor738]

When you press down on the sensor plate you are causing the other end of its arm to push up on a pencil-like piston in the fuel distributor. The piston has 8 slits that are wider toward the bottom, with the result that the further up the piston moves, the more fuel goes to the cylinders. Normally, the plate moves down because of the air being sucked past it through the throttle plate at the front of the engine. This system is very simple, open throttle, more air past plate, plate moves, more gas to injectors.

So when you push down on the plate, you are causing the fuel distributor to push more fuel to the injectors. So obviously you are not getting enough fuel with the plate at rest.

There is one more factor at work: control pressure. This is fuel pressure that pushes back against the piston, basically like a return spring. The lower the control pressure, the further the piston will move when pushed on by the movement of the sensor plate, and vice-versa. Control pressure is regulated by the warm-up regulator at the right front of the intake manifold. It reduces the pressure at cold temps to around 1.5 bars so the engine gets more fuel, and raises it to 3.8 bars as the engine warms up.

But if the WUR is not working correctly, control pressure may be too high and cause the piston to not move at all with airflow. That is most likely your problem. So before you mess with the sensor plate, which is a very unusual adjustment to have to make, buy a gauge, and check your control pressure. There are lots of posts on how to do this.

[gtwendt]

Thanks, again, for the info and guidance. I decided to take the WUR off the car and then blew compressed air into both fuel openings. I re-installed the WUR and the car started better and idled better than it has since I began this project!

However, when I depress the accelerator, I still get back firing through the throttle plate until the engine warms up. Because of all of your help, I just might get this thing on the road. Then maybe my wife will quit referring to it as the "Grease Pot"!! She does, however, support my efforts.

[ctaylor738]

There is a multi-layer mesh filter on the "input" (larger) side of the WUR. It can get clogged, which has the effect of raising control pressure. You really need to fish it out carefully with a pick and clean it really well, especially the fine mesh on the inside layers.

I did a post on how to do this.

The fact that this is occurring during warm up is a pretty clear side that you have a WUR/control pressure problem. BUT, consider that the WUR gets fuel from the fuel distributor and if it has clogging problems, then so does the FD which has even smaller passages. This is common with cars that have sat for a long time and generally requires that the fuel tank be cleaned and coated.

[meltedpanda]

does the air plate "float" up after being pushed down, how much pressure are you using to get it to go down

[gtwendt]

Yes, the plate floats up after being depressed. Not much downward pressure is needed to depress the plate.

[gtwendt]

Thanks again for the good advice. I decided to dig into the WUR on the advice of ctaylor738. I removed it from the car and opened it up to clean all of the inside components, especially the small mesh screen. Put everything back together and walla! It started right up and runs much smoother. I have a feeling that this was my problem all along. Thanks to ctaylor738 for the good advice!

[ctaylor738]

Well, thanks to you for actually following it.