Hi everybody,

My CLK320 has a problem, and I'm hoping your experience can help.

2000 clk320
150k miles
Normally starts and runs very well, but it fails to start approximately one time each month.
No dash alarms, no "check engine", no basic code alarms.
During the malfunction, the starter turns, the engine spins over, but I cannot hear the fuel pump running,the manifold fuel pressure drops to low, and the car does not fire on a cylinder. If I allow the car to rest for 15-30 minutes, it will start as nothing is wrong, and have proper fuel pressure at the manifold.

Every time this happens, I dash to the tool box to get the HHT and electric meter, but by the time I get it connected to test the fuel relay etc, its stops malfunctioning!

blue

 

Hi Blue,

You are SURE it is low fuel rail pressure?
My reaction is either

Fuel filter valve defect. Has the filter been changed? Should have at been at150k. Was an OE part fitted? Easy and inexpensive to change

or

as you suggest the fuel pump relay. However this is part of an expensive assembly (module) and one missed start in 100 may not warrant replacement on a "might be the cause" basis..

 

Hi Stuart, and thank you very much for answering.

Fuel rail: Ok,when this was happening, I attached the a gage to the fuel-rail test fitting. I lost some pressure when attaching the gage, and the fuel pumpnever came ON to repressurize the rail while attempting to start.

Fuel filter: Yes, I changed it at approx 120K miles with an MB. I know the pressure regulator is actually inside the filter, but the car has"always" had just barely enough fuel pressure to pass spec. literally on the lower limit. I have of course asked if there was a different filter/regulator that would allow a higher pressure to put the reading in the middle of the tolerance to settle the matter, given that all of the troubles with this car have been fuel related, and have been told NO repeatedly.

I bought a new fuel relay yesterday, When the car has the problem again, I will change relays, and see if there is any change. And once again, the car has over 150k miles so I'm not upset with the problem at all, I'm just trying to stay on top of the repairs.

Blue

 

Hi,
In light of the extra info the relay moves up the suspect list.

HOWEVER I would have done a couple of further checks to confirm this before spending my money. Am I correct in thinking that on the CLK the fuel pump relay is part of a multi relay module? How much did it cost?

1. I would investigate the rail pressure (after fixing your leak problem) a little more. What is the rail pressure with engine off (any change over time?), with ignition on, during start and running vs spec. If the fuel pressure is always low it may well be the pump output is low but I still go for the filter pressure regulator.
When did the problem start. Shortly after the filter change by any chance?

2. I would connect a test lamp to the fuel pump contacts and observe the lamp during a "no start". Lack of power to the pump = relay/fuse. Power during no start suggests pump or return valve (subject to the problem being a fuel related problem).

Of course there are other potential causes. Fuel pick up "fallen off" creating problems on low tank starts etc.

My 112's start "immediately"at mileages in excess of 100k in all weathers, (even with a 6 yr+ battery on the SLK which also includes infrequent use!!)

Keep us posted.

Stuart