Poor starting hot engine

[PvD]

I have been trying to overcome the poor starting when my 6.9 engine hot and has been standing for more than a few minutes. The possible causes are the non-return valve near the fuel pump, the accumulator, fuel vapourisation, etc which all come to inadequate fuel an pressure when stating the engine hot. I have installed a push button to allow the fuel pump to run when I start the hot engine. The engine running signal to keep the fuel pump running after the engne has been started is operated by the air flap (refered to as dummy lid) switch in the air intake, thus if the engine stops running for any reason the fuel pump stops. I initially wire the normally colsed push button in the wire between the air flap switch and the earth, this did not work because the air flap switches uses an internal connection to the engine as earth (auto elecetics are confussing). Now that I have rewired the normally closed pushbutton in the other wire to the air flap switch I can now run the fuel pump with the ignition switch on by pressing the normally closed push button.
I'am trying various durations to use this push button when the engine is hot, has any body found the optimal time to run the fuel pump before operating the started motor to get an immediste start?
Regards,
Peter

 

[Andrew280SEL]

Doesn't turning the ignition key to the start position with the gear selector in "D" cause the fuel pump to run? I thought that's what I've read before.

 

[Michel]

I have that same problem, but it starts eventually (30-45 seconds)

 

[Helmet]

If you disconnect the switch on the air flow meter the fuel pump will run when the ignition is switched on, no need to install a switch. A simple check for fuel pressure loss is feeling the pressure on air sensor plate, it should have some resistance when pushed down. If this is ok then replace injectors. There, I did not say D%#%y lid

 

[Styria]

Hi Helmet, there is also of course the option of disconnecting the relay inside the fuse box, and this would mean that fuel would be flowing through continuously whilst ever the ignition is switched on.

My reading of Peter's situation is more of an accumulator problem that requires replacement. It is a problem that has been raised many times on websites, including TK, thus I pose the question of whether or not he needs to check out the operation of that unit. General consensus seems to be that replacement of that unit effects a proper cure. Very much interested in your valued expert opinion. Regards. Styria

 

[sean sherry]

I was reminded that ,when all else fails " Read the Instructions " when sorting my Hot Start problem. The 107 Owners Hand Book states that when starting Hot engine hold the throttle open a little and when sitting Hot for a while hold throttle fully open. I haven't got the Book in front of me but this is what I recall. So if the car always had an issue with hot starting and the Factory could not cure it due to the design of the system , i just work round it and after replacing all the usual bits it is as good as it gets.

Not a 6.9 I know but perhaps relevant all the same ?

 

[CraigS]

My guess would also be the accumulator. As Helmut says, check the sensor plate for pressure, or put a temporary pressure gauge inline and monitor it from inside the car.

 

[PvD]

Guy the problem is fuel not getting to the cylinders be it caused by pressure drop, fuel vaporisation, faulty accumulator, faulty non-return valve, pressure drop due leakage, etc. The solution is to get the fuel up to pressure before the starter motor is run by the stater key. The 30 to 45 seconds quoted by Michel does eventually start the engine as the fuel pump runs when the starter motor is run by the ignition key, but it is a bit embarrsing. Using the R or D transmission with the ignition ON does not run the fuel pump. Disconnecting the air flap switch or removing the relay are OK for testing but would not consider this as long term solution since one is overriding the saftey interlocks that stops the fuel pump when the engine is not running, like after an accident. I will fit a fuel pressure tranducer the output of which I can view within the car and do a bit of observation, stay tuned.
Regards,
Peter

 

[PvD]

Previous post should have stater 'Guys' , my electrical circuitry ability is much better than my spelling or proofreading.
Peter

 

[SEL_69L]

Andrew280SEL is right.
Putting the gearbox into 'drive' automatically switches the delivery fuel pump on, and after about four seconds, the delivery pipe from the tank is at full pressure.
Mercedes in their wisdom operate the fuel delivery pump with the key in the 'start' position, and so the fuel delivery line needs about four seconds to come up to full fuel pressure. THEN it starts easily.

I think their 'wisdom' is, that if the fuel delivery line has a leak, and you try to start the engine hot, and you already have full pressure, there is a increased risk of fire. I say 'wisdom' because the car can be hot at any time when the fuel delivery line can develop a leak.

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There are OTHER ways around this problem.
I have a FOUR pin fuel pump relay, not the standard five pin one, and the engine starts with full fuel pressure almost every time on first compression.
I WILL concede however, that the engine still needs some small amount cranking time, before it will start when hot, but the starting time is somewhat lessened with the four pin relay.

With every other modern car I have ever owned, I leave the ignition key in 'acc' "ON" 'start' position for about four seconds, before attempting to crank the engine. Some car owner manuals reccomend this as the normal starting procedure.