Poor performance MB C200

[DavidR]

Hi all. First post for me here.

Bizzare problem with a '99 C200 which I can't seem to trace.

Car started to splutter a little under full power. Then it got worse. Then it got a lot worse, and anything more than about half power would make the engine splutter and cough. It didn't cut out, just ran very rough. If you back off the accelerator when this happens, then engine then runs just fine.

First thought was a partially blocked fuel filter. Changed, no difference.

Next thought was a slightly duff fuel pump. Rigged a fuel pressure gauge in with the fuel rail, and a rock-solid 100-ish psi of pressure, even when the engine splutters.

So thoughts turn to electronics, and big bills.

Next day, the car fails to start. Checked for fuel delivery (as I've just had the fuel system apart, and that's the most obvious problem) and it's mainly air, with a smidgen of fuel in it. Check the output from the tank, and there's no fuel to be found. Gauge reads over 1/4 tank, but when you pull the inlet pipe off the fuel pump... nothing.

Looking at the fuel pump, and the surrounding area, the pump appears to have TWO inlets, One large diameter, one small. There also appear to be TWO fuel tanks on the car, but I can find no pipes joining them (although I didn't look too hard) Can someone explain that setup??

Engine still won't start. V.Odd. Shove 20 litres of fuel in the tank (in the hope of getting air out of wherever I've managed to get it), gauge shows over half, and the engine now starts. Original problems still remains.

I'm convinced there's air in the fuel, and the next thing I plan to do is change the bit of flexible pipe to the fuel rail for a bit of clear so that I can confirm/disprove that theory. If it is air, where the hell is it coming from? Anyone else had similar problems?

Thanks in advance for any explanations/advice/assistance.

 

[DavidR]

Follow-up:

Just had the engine running with the fuel return from the fuel rail pressure regulator going through a clear piece of hose to an external fuel can (rather than returning to the tank) and there was air... a LOT of air in the fuel. So much so that if you revved the engine, it was just air being returned, and no fuel. Allow the engine to idle again, and the air/fuel mix returned.

I'm now completely convinced that the fuel pump is drawing in a massive amount of air from somewhere.... But where? The short lengths of hose that come off the pump to the tank on the R/H side of the car appear fine. I'm guessing it must have something to do with the fact that the car appears to have two fuel tanks, and the interconnecting pipework that must exist.

Unfortunately, I don't have any diagrams of the tank/pipework, so I don't know what I'm looking for.

 

[jberks]

ok - firstly the fuel senders are a very common replacement item on the C, so are you sure you didn't actually run dry? Especially as putting a few gallons in solved it.

As far as spluttering is concerned, I'd start with the MAS sensor. Ideally just have an electronic diagnostic done but this component is the first port of call when things don't run right.
It could certainly cause the symptoms you describe anyway.

 

[DavidR]

Fuel gauge has been entirely reliable AFAIK, and went up by exactly the amount I would expect after shoving 20 litres in, so I don't think it's failed. Certainly take your point though, and I'll be keeping a beady eye on it.

However, I'm completely convinced that the engine is electronically fine, and splutters/coughs simply because it doesn't have 100psi of fuel to work from... it has 100psi of fuel/air mix. Under light fuel usage, the air is all bled off by the pressure reg, and returned to the tank, but under heavier fuel usage, the injectors start trying to inject a percentage of air rather than 100% fuel, with unsurprising consequences.

After all... there really should be no air bubbles in the fuel lines.

 

[jimsinessex]

As Jb says, the symptoms are VERY similar to a dirty or failed MAF sensor. Try cleaning it first.

Jim

 

[arthurk]

Hi,

The symtoms you describe are very similar to what I had with my SL320 2 weeks ago. Also when the engine was cold it ran notibly worse - very poor acceleration and very rough. Floor the throttle and the engine just seemed to choke. When you eased back on the accelerator the power picked up again.

New Air Mass Sensor cured all this and the car is now running like a dream. It is also much smoother changing gears in the auto box also.

Although I had a fair idea it was the AMS on mine I didn't want to risk buying one, so went to a specialist. Probably cost me about £80 more by doing it this way.

Kenny

 

[DavidR]

To prove/disprove the air-in-fuel theory, today the car was set up with a 5-litre petrol can under the bonnet, with an auxilliary fuel pump supplying fuel pressure. The return was put back to the fuel can too.

No air in the fuel, and the car drove perfectly. Full power available.

So I'm back to square 1. Where am I likely to be drawing in air from? The pipes (2 of them) from the suction side of the fuel pump to the fuel tank are fine.