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My 2008 CLK550 has recently started having a long starting issue. It began with it not wanting to start on the first try, and usually having better luck on the second try. Then it was even better starting it with the key (normally use keyless start) after turning it to position 2 a few times, to prime the fuel pressure. I put a fuel pressure gauge on it, and two things jumped out at me. First, as soon as the fuel pump stopped, the fuel pressure dropped. And, it was running at 60psi. This seems a bit high. What is the normal operating pressure on this mercedes? Is it 52psi?
Since the fuel pressure drops immediately , I figure its either the FPR or the Fuel Pump. Other options?
Before I start tearing into things, I tried searching here for answers. I found some limited information, but had some questions. Online, I can find the fuel pump assemble for the right side. Does this also contain the FPR? From searching, it looked like there was the fuel pump on the right side, and the FPR on the left? But when I look for parts, I can only ever find the Fuel Pump Assembly for the right side. Example: RockAuto
So is there a way to test between FPR and fuel pump, or is this all in the assembled unit?
The regulator is actually in the sender, which is on the left side of the tank. But 60 psi is fine. It should be between 3.7 and 4.2 bar ( 53.7 and 60.9 PSI). My guess is there is a fuel line leak somewhere that is allowing pressure to drop.
I'll check around for leaks and see what I can find. When I had the gauge on it, the pressure dropped from 60-0 nearly instantly, which was making me with it was either the check valve, which I believe is part of the fuel pump unit, or, something to do with the regulator. If 60psi is within spec, I'm wondering if its just the check valve.
Originally Posted by Rudeney
The regulator is actually in the sender, which is on the left side of the tank. But 60 psi is fine. It should be between 3.7 and 4.2 bar ( 53.7 and 60.9 PSI). My guess is there is a fuel line leak somewhere that is allowing pressure to drop.
One thing you could do is connect a pressure tester directly to the sender's output hose. You'd need a SAFE way to adapt your pressure gauge there, but that would tell you if the leak is internal within the sender or in the external fuel lines. I'm leaning toward the sender, otherwise, you'd probably be smelling an external fuel leak.
I agree that it probably is not external. I have not smelled anything while driving, sitting at a stop light, or when parked. And my parking spot has no visible fluids left behind. And the pressure bleeds off so fast on the gauge, it seems you'd smell or hear or see signs of a leak that size.
I always prime it three times, and try to start it while i hear the pump running. Sometimes it still needs a few tries to kick over. Sometimes it starts right up. So either my timing is sometimes good. Or, maybe its a sticky part in there, which again leaves me thinking the check valve.
I need to put in a rock auto order anyway. I think ill add a new fuel pump assembly to the list.
Originally Posted by Rudeney
One thing you could do is connect a pressure tester directly to the sender's output hose. You'd need a SAFE way to adapt your pressure gauge there, but that would tell you if the leak is internal within the sender or in the external fuel lines. I'm leaning toward the sender, otherwise, you'd probably be smelling an external fuel leak.
You likely need both the pump and the sender unit. The regulator is in the sender, but when you try to detach its hoses from the pump, the plastic nipple on the pump housing are brittle and break. Get new seals, too. They were revised and made thicker - get genuine MBZ seals.
Update! But first question. On the attached picture, there is a black sensor on top of the cap. Its a three pin connection, and I think it might be the fuel vapor sensor??? Well, the kit I got did not have the white plastic cap to plug it into. I dont think I can steal it from the OEM pump without breaking it. So in the mean time, I just zip tied the sensor in place. I got no CELs and everything seemed fine. Is there any issue with just leaving this sensor out in the air rather than in that special plastic fitting?
Black sensor on the top cap, three pins, how important?
Problem is fixed for now! After taking the seats out, I realized I needed a u-joint to get at the bolt for the hose clamps holding the units to the gas tank. With the seats out, you could hear how loud and variable the noises were coming from the pump. The new one is super quiet in comparison. The misfires at idle and some of the stumbles I was having appear to be gone for now. And, instead of priming the pump before I start the car, I can go back to using the keyless go. Yay!
Admittedly, I did buy an ebay fuel pump. Under brand, its listed as "unbranded". Well, it was $180 brand new for both sides, and since it wasn't the worst job in the world, I'm willing to role the dice on it. If there are any updates, I'll let people know. The seals seemed to be good quality and updated from the old version. The only complaint was the tiny pipe on the drivers side was aimed at a slightly different angle than the oem version. This lead to some interesting fitment issues trying to get it in. I ended up just stressing the plastic bits until it fit thru, but I was a little nervous about snapping that nipple and hose off. Otherwise, the rest of the fitment was good.
07-22-2018, 01:03 AM
