air in fuel - update no 2. HELP!

[turbopete]

ok, im guessing your all starting to get sick of hearing me banging on about this, but it would seem that i may have been blaming the wrong area of the car for the cause of the fault. i had some time to sare today (between job searches!) and went poking around under the bonnet. when car is running, every now and again you can see air bubbles travelling along in the fuel. if iraise the engine sped to about 800rpm, theyre much bigger an clearer. as the revs rise, obviously theyre harder to see but at approx 1500rpm ive noticed a slight misfire, which i assume is due to the air in the sytem. i aso think that the air bubble i keep seeing when its not running is air trapped inside the filter. after a short motorway run and a 1mile thrash during it, i parked for a while, drove gently for a bit, switched off again, went to re start, nothing. pipe was full of air. anyone any ideas where its coming from? tank almost full so not wanting to take tank strainer out just yet! im wondering about hoses or non return valves etc. or things mercshave that my previous fleet havent had! neve had any problems with previous mercs!

 

[television]

It is unlikely to be the main fuel line as this is just sucking up fuel, if there was an leak then you would have fuel leaking under the car. the input to the filter or the filter bowl not air tight would let air in. the return to tank is not on these lines.

 

[turbopete]

the main fuel lines under the car are metal. i didnt notice any corrosion last time i was under it.you think its the tank strainer then Malcolm? or a length of rubber hose/non return valve perished and sucking air through cracks in pipes but not leaking fuel yet? i know vauxhalls with braided fuel hoses were prone to it, but i think that was just petrol models. not sure though

 

[Number_Cruncher]

You can easily test to see if the problem is at the engine end, or the tank end.

Get a gallon tin full of diesel, and two pipes to connect where the feed and return flexi pipes connect to the engine. IIRC, the feed pipe connects to the fuel heater, and the return goes from the filter housing.

Run the car. If you still see bubbles, the problem is at the engine end. If you don't, the problem is further back. You can keep repeating this trick at various points along the fuel system to track the fault.

You will find the fault, but only if you work with method.

Edit - it's also easy to rule out the strainer by disconnecting the feed pipe flexi on the body side - even with the car level, fuel should come out fairly freely.

 

[television]

No I do not think that the fault is on the main fuel line in or the tank, or strainer.

The fault should be around the top filter bowl and pipes

 

[Number_Cruncher]

No I do not think that the fault is on the main fuel line in or the tank, or strainer.

The fault should be around the top filter bowl and pipes

I agree - I would be very surprised indeed to find that the problem was other than on the engine.

 

[sixpack]

Air Leaks

Hi Pete, have you looked at the plastic pipes between the filter/pump/shutoff valve? The end fittings all have O rings in them which go hard and leak. I,ve had to do these on both my diesels, 1994 250 and 2002 270. Its best to replace all the pipes complete because they go hard and split if you disturb them. Theres also a sneaky O ring between the shut off valve (black box on side of pump) and the pump body. New set of 5 (I think) pipes costs about £70. from merc dealers.There is a mod kit to stop fuel running back to the tank when stopped, but if the pipes dont leak this shoudnt be needed.

Hope this helps
Peter

 

[Daeths]

+1 Sixpack. Had identical symptoms a few years bacl on my E320cdi which was solved by replaced the o-rings. Some of the pipes are a bit fiddly but about 1.5 hours and £80 sorted the lot.

Not a problem since.

 

[Daeths]

nother quick test is fill the tank, park it downhill (nose facing down) and leave o/night. If it starts with no or a lot less air....

 

[turbopete]

sixpack and daeths, ive replaced every pipe and o ring including all filters from the plastic pipe that enters the pre filter forwards, including the cut off switch an the o ring! all thats original since ive had the car at that end is the injectors and the injector pump. the bubbles can b see coming through the clear pipe going into the pre filter from the tank when the engineis running, so it has to be tank side of that pipe id reckon, coz air could only appear engine side of the problem!

 

[turbopete]

nose uphill or down makes little or no difference incidentally

 

[turbopete]

or am i wrong with my assumption that if the bubbles when the engine is running are in existence as soon as the fuel enters the clear pipes that they must be generated before the fuel gets to the clear pipes?

 

[sixpack]

Bubbles

Hi Pete,

When I was having air leak problems with my C250, a service advisor at Mercedes Giffnock (Glasgow), said that they had a problem with the ali casting that holds the fuel filter being slightly porous. Seemed like a load of tosh at the time but the guy was helpful other ways.

Peter

 

[turbopete]

i wondered about that sixpack, but with the engine running, air bubbles are in the clear pipes BEFORE it gets as far as the ally casting. it may be that thecasting is contrbuting to te air bubble when engine is off, but toproduce the bubble as fast as it does after switching off, itd need a hell of a big leak through the casting!

 

[turbopete]

i havent noticed any drips on the floor, or smelled any diesely sorts of smells, but its not impossible that ive got a leak. fuel consumption does seem erratic to say the least! 39.2mpg cruise set at 70mph on an almost continuous 330mile trip, whereas 2 trips of 100miles at same speed using same motorway but with 36hr breaks betweem each 50mile trip (each direction) is nearer 35mpg. never worked it out that accurate though. the guage drops loads faster though relative to the distance and i usually have town trips to do before i can fill up again!

 

[turbopete]

going to chech the fuel hoses for leaks etc at the weekend if time and weather allow. if i need any, is it best to buy them from stealer or just buy lengths of rubber hose to make up my own? anyone got any opinions?