Hard cold start
 

Hello.
I have a 1991 560SEC lately out of the blue, I crank it when cold, it starts for split second then quits. Almost like its a spark issue, but then I go to restart it using the accelerator pedal and I'm able to keep it running and it's fine there after.
I've cycled the key several times to no avail. That was done to eliminate the fuel accumilator. I have not pulled the cold start injector partially out to see if it spraying only because the engine is fine if at cold start I manipulate the accelerator pedal slightly. The engine readily goes into cold idle after this.

Gogs.

Do a fuel pressure internal leak test as described in the factory service manual. This checks the fuel pump system.

And check the cold start injector system.

Cold start injector
 

I'll check the cold start injector first. That reminds me I should check temp. sensor that activates that injector too.
Good advise, thank you.

Gogs.

Right! That is a "system" which includes wiring and gizmos to activate the cold start sensor. (And needs the correct fuel pressure to work as well.)

Fuel pressure.
 

Your probably right about the fuel pressure but, wouldn't there be drivability problems too?

G:
Have a look through the secondary side of the ignition system; condition of the distributor cap & rotor, wire connections, wire condition, and spark plugs clean & gapped. Faults in the ignition can be mistaken for injection faults, particularly during starting.

By opening the throttle slightly, the sensor plate is deflected a bit further, thereby providing a little more fuel.

No. The fuel system should keep pressure when the car is off.

Then there would be fuel pressure for starting.

One problem is an "internal leak". That is a fuel accumulator or damper which has a leak and returns gas to the fuel tank. Thus the fuel pressure does not hold when the car is off. AKA an "internal fuel leak".

This is discussed in the factory service manual for the internal fuel leak test. You place a fuel pressure gauge on the fuel distributor, turn the car on, then off. And see if it maintains pressure for a certain amount of time.

You would also want to check that the fuel pressure is correct when the car is running.

So you may have OK fuel pressure when the car is running, but leakage and lose fuel pressure when the car is turned off. Thus takes longer to start up.

Thank you both for the very good suggestions. I will surely check all of those things and I hope it does not turn out to be the fuel accumilator as they are very expensive for what they are.
One other question I have is regarding the throttle plate. I checked my TP with the engine off, as I depress the plate it feels normal but when I release the TP
It will not return to the rest position, I have to hook my finger under it to bring it back up. I don't think that is normal do you? Shouldn't it return on its own?

Gogs

My TP returns to the up position. With that said, try pressing on it right after you shut off the engine. Then wait a half hour and try again. I don't remember if having fuel pressure would help it to bounce back into position or not?

So I'm thinking that maybe if there is an internal fuel pressure leak, and no fuel pressure (with car off), then that might cause the plate to not return up? Whereas with fuel pressure, it would return up?

And there might be fuel pressure there right after you shut off the engine, but would drain back into the fuel tank and be no fuel pressure after a period of time.

If this is the case, then we may have found a new method of testing for an internal fuel leak!

Your symptoms match what happens if the fuel accumulator diaphragm cracks. Search for a thread started by me "fuel accumlator failure analysis". The is a simple test you can run to test for an internal leak, and you don't need to rig up a pressure gage. If it passes the internal leak test then the problem is likely elsewhere. If there is clear evidence of an internal leak, even slight, it needs to be replaced.

Duke

Will do Duke. Thanks each and everyone responding to my thread, it is well appreciated. I have to wait to go up to Edison to grab my fuel pressure gauge as its at my brothers place, in the interim I'll check out your thread Duke and take it from there.

Gogs.

Update
 

After testing the entire fuel supply system the problem turned out to be loss of residual fuel pressure. Tested fuel pressure which was 92psi normal but was dropping to 45psi a little too low, and then suddenly to almost zero within 20 seconds. Tested the FP accumulator and FP regulator both tested good. Since the pressure drop was sudden I replaced the check valves on the fuel pumps. That did not help at all. So there was only one thing left and that was the fuel injectors. Replacing the fuel injectors solved the problem. Thanks everyone for all the great advise.

Dave.

Does your throttle plate now return to the rest position? (Now that you have constant fuel pressure when off?)