robreynolds

2006 C230 M272 100,000 KM

For the last two weeks the car has been really hard to start, it will crank and sputter and eventually fire and run normally. It happens mostly when it has been sitting for a few minutes to an hour. It happens about 75% of the time.

Tonight I hooked up a fuel pressure tester.

• With the key ON (but engine off) I get a slight pump in pressure to 2-3psi.
• Turn the key to start the engine the fuel pressure jerks and stutters up slowly with the engines sputters, finally when the engine starts I get 53psi (normal range)
• Once running the car drives normally, zero issues.

The pump should prime when the key is in ON, but it doesn't.


I swapped out the fuel pump relay (traded with the same relay from the horn) and it made no difference.
I shorted out the fuel pump relay to be always on, and with the key at ON I get 53psi all the time. The car starts in a heartbeat, no stutter no delay.

Could there be something going on preventing the fuel pump relay from switching on to prime?

SOLVED: I replaced the fuel filter which has the fuel pressure regulator built into it. Starts great now and hold pressure.

Russell Ormerod

What happens to the fuel pressure once you turn the car off? Does it have holding pressure? Or does it fall to zero!

robreynolds

when the engine is off, the fuel pressure immediately drops to zero

Russell Ormerod

That's your problem. It must hold pressure. It can either be the fuel filter or the check valve in the pump. Clamp the return line and see if it holds pressure.

robreynolds

Video of the fuel pressure.
http://www.youtube.com/attribution_l...action%3Dshare


Forgive me I'm stupid idiot. Where is the fuel return line to clamp?

robreynolds

Also once the return line is clamped, if pressure holds what does that mean? What needs to be replaced?
If pressure doen't hold what does that mean? What would need to be replaced?

Thanks in advance I'm a noob

robreynolds

I blocked the fuel return line at the sender behind the driver's seat where it goes back into the tank.

Key on engine off I got 20psi
I turned the key back to off then back to on and it jumped to 100psi the meters max.
Started the engine it stayed at 100psi
Turned the engine off and it held 100psi and slowly was loosing pressure 1 or 2psi every 5 seconds.

But what does that all mean?

Russell Ormerod

The fuel pump supplies pressure to the fuel rail. The fuel filter regulates that pressure and returns the excess back to the tank. When u switch the car off there needs to be a holding pressure to prevent the fuel from boiling in the rail which causes the air in the rail which will cause difficult starting when hot. To keep this holding pressure the fuel pressure regulator which sits inside the filter needs to fully close and prevent fuel from bleeding back to the tank. Also the fuel pump has a check valve preventing fuel bleeding back to the supply side. So if the fuel pressure holds when u block the return you have a regulator leak and need to replace your fuel filter. If the fuel pressure holds when u block the supply you have a leaking check valve in the pump. The reason why the pressure went to 100psi is because you prevented the regulator from controlling the pressure. So the gauge went to the maximum the pump can deliver. From your test it sounds like you need to replace your fuel filter.

Russell Ormerod

Watched your video. Definitely a problem. No holding pressure at all.

robreynolds

I've ordered a new fuel filter/fuel pressure regulator

robreynolds

SOLVED: I replaced the fuel filter which has the fuel pressure regulator built into it. Starts great now and hold pressure.

Russell Ormerod

Good luck. Let us know the result

Russell Ormerod

Thought as much.well done