550 some times hard to start when warm -why?

Perhaps a faulty ECU? Could be that the ECU stops working in higher temps.

I would take it in to see my specialist regardless - you know the car is not supposed to do this.


Onno

 

Anders, I would have the static fuel pressure checked at the rails when the engine is hot and before you try to crank the engine. It should be around 3.8 bar (that's what the fuel pressure regulators for the North American 550's are rated for).

I would then watch it closely during engine cranking, and if it drops substantially, I would start looking at the fuel supply system for problems. Perhaps the filters on the pressure side of the pumps need changing, or the fuel pumps are weak or perhaps their suctions are becoming blocked (the rubber isolators on the pumps disintegrating due to ethanol and blocking the pump suction?)

 

On a 348 or Mondial t the 3.4 can experience this same issue when the fuel pressure regulator (there are two) fails internally. The way to check that is you can pull the vacuum hose off of the regulator and check for a strong fuel scent (or actually moisture from the fuel).

If that is present then you will require a regulator.

 

My first thoughts also would be the fuel pressure regulators!

 

When starting cold, the ECU is allowing more fuel than baseline (which, of course, it is supposed to do) ---- but, when starting hot, the engine should / does not need the extra fuel ---- so, you have a fuel delivery problem.

In addition to the pressure regulators, I would also check....

1) Throttle position switch & sensor -- since "opening" the throttle (which also tells the ECU to give more fuel) allows the good hot re-starting, which should not be necessary, if the ECU was getting the correct TP signal during start up.

2) Faulty check valve(s) in the vacuum lines supplying the pressure regulators (if there are any ??).

3) Faulty / leaking gas cap ---- pretty common problem on most any newer car. Plus, it tends to come and go and be affected by engine temperature. This really does not fit your symptoms too much, but it is a very simple thing to check and to rule out. Plus, it is a cheap "fix"....if it happens to be the problem

ALSO, do you have access to a good OBD reader ? I would take a look as the cause of this problem should more than likely be prompting a DTC or two.... even though not triggering a CEL.

 

+1 -- this has to be the first thing to check with your symptom.

IF it is the problem, then two possible culprits -- the previously mentioned pressure regulators, or the output check valves inside the fuel pumps. (And you most likely have a problem on both banks; otherwise, the engine would warm start OKish on one bank -- and each bank may have a different problem).

 

+1 what Michael said.

 

Here's another thought, FWIW....

Doesn't the 550 (might not apply to the Euro version, though?) have a special "fuel cooling" system on-board ? If it were malfunctioning, could it be possible that the fuel could be mildly boiling (i.e., creating too much vaporization) prior to delivery at the injectors ?

This would cause a hot start problem, as well...... I would speculate.

 

Would this cause bucking when running in hot conditions as well?

Fuel pressure regulator as prior - see if fuel scent is present.

Then I'd consider electronic means- bad temp sensor on engine?

 

Best of luck working this out. I have had a very similar problem on my 308 that I'm hoping my mechanic will figure out if I don't come up with a simple fix soon.

Given the age difference between mine and yours I wouldn't expect the problem to be the same, but who knows? They still have the same basic functioning process.

 

Not necessarily..... start up (hot or cold) on any FI engine is a fairly radical deviation, with regard to fuel trim / delivery parameters, from other operational (running) ranges ---- start up is always a comparatively "rich" A/F ratio event, even richer fuel demand than WOT in most (normally aspirated) engines. Therefore, it would be more sensitive to fuel delivery deficiencies, even the more minor ones that may not be critical in the other conditions.

 

Could also be an air intake leak, or leaking injectors allowing fuel to pool in the cylinders and also fouling the plugs. If leaking injectors, it will get progressively worse as the plugs foul more. Pull your spark plugs and check to see if they are black from fouling...

 

Coolant temp sensor. Also check all ecu connectors and grounds.

 

Might be item 1 & 2 in this picture (angle speed sensors):
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