W210 E300 TD air in fuel. update and help please!

[turbopete]

im back with this old problem again! lol! 1999 E300 TD gets large air bubble in clear plastic pipe going into fuel pre filter. ive replaced the injector leak off pipes and the blanking plug at the end of them on no 6 injector,replaced all 4 pipes and their seals which connect to the fuel filter alloy 'housing' as well as the bolt and 2 seals holding the filter to the housing, and the main fuel filter! i have noticed however, where the fuel lines to and from the tank go from metal to rubber next to the bulkhead, if i get to the lower, thinner (presumably the return to tank) quickly after switching off (ie out of car with bonnet ready open, clamp ready etc) and clamp the rubber hose near the metal pipe, the problem air bubble doesnt appear! anyone come across this before? does it mean the problem is near the tank, or just that the fuel has nowhere to run to?

 

[television]

Do you mind a little thinking time on this, before any answer

 

[turbopete]

think all you need! im certainly running out of ideas! short of replacing EVERYTHING which i cant afford to do! and sorry if someones had this problem answered before but i havent found the thread if it has been answered before!

 

[HERBIEMERCMAN]

hi turbopete, on a perfect sealed set up you will see air bubbles in your filter intake fuel line, especially on tick over, at higher rpm the fuel cavitates and you cannot see them. i took my car to my local garage as i had a diesel clatter from tick over up to 1200 rpm, i was told this was not unusuall and live with it. some months later the cold winter came, this is the temp, where the moisture inherent in the fuel waxes and the neg pressures created by the pump finds it easier to pull air instead of a more dense waxed diesel fuel. i had so much air the engine would not start. my local diesel specialist "merlin" replaced every "o" ring and every fibre washer, clips etc. problem solved. obviously the nearer the pump the more potential for sucking air. you could consider discon. fuel feed at tank and pressurising to see if anything leaks. best of luck . herbiemercman.

 

[turbopete]

the bubble only appears when engine isnt running. its a pain to start and misfires occasionally on start up, particularly in the morning. it even did it when i got the car in july. i will give it a go though as mine also has an annoying rattle/clatter up to 1200rpm which till now ive been blaming injector nozzles for!

 

[Xtractorfan]

There is a possibility that your tank isnt venting properly and may be creating a vacuum wherby the fuel is being drawn back to the tank when the pump stops..

 

[5pares]

How about fitting a oneway valve in the return?wont fix it but itl cure your problem.

 

[Number_Cruncher]

Has the O ring behind the shut off valve been renewed?

 

[Parrot of Doom]

Is the bubble travelling up the line, or is it just appearing at the fuel filter?

 

[Adrianh1]

I've similar continuing problems with my 190D 2.5 Smartveg conversion with air in the fuel lines so I sympathise... Terrible starting problems when the temp plummets (though fine when mild).

 

[HERBIEMERCMAN]

hi adriah1, cold weather tests batteries and fuel systems as you may know, i lived with a sucking air problem for some time but as soon as the sub zero stuff came the diesel pump decided it was easier to pull air than "waxed" fuel. there are lots of posts which point out that all the o-rings, fibre washers, etc. have to be replaced, not expensive, to cure the problem. even in warmer temperatures the ingress of air causes diesel rattle and reduces injection pressures in the nozzles, poorer spray patterns and less mpg. poorer starting, in cold weather no starting. herbiemercman.

 

[Adrianh1]

Thanks Herbie. Replacing o-rings etc, is this a job for a garage or can a half-competent numpty tackle it...? Sorry to hi-jack this thread, btw.

Adrian

 

[turbopete]

bubble appears from the fuel pre filter and expands towards the tank. havent heard any hissing when the tank cap is removed to indicate vent probs but i may try starting it, removing the cap then swithing engine off to check that. any seals i have replaced to date are the ones at both ends of each of the pipes ive replaced, and any others mentioned at the start of the thread. all ideas welcome. i need all the help i can get! runs fine, but a bit rattly as mentioned earlier once its started. just misfires occasionally for 30 seconds or so after starting following an overnight or all day at work stand without running

 

[turbopete]

herbiemercman my fuel consumption is awful in town 20 - 25 mpg. however when i used the car in october, (its had the problem since i got it in july) i had to go from carlisle to stafford, i set the cruise control at 70mph and got 39.2mpg! not sure if i should get more mpg on a run or what i should be getting, but i thought that the figures might tell a tale? everyones help is much appreciated!

 

[merc convert]

I have had this problem repaired recently. My local indie located the problem at the tank. he changed a sender and some sort of seal on the return fuel side to the tank. Problem solved. He did all the usual seals first with no change.

 

[HERBIEMERCMAN]

hi everyone, the merc. fuel lift pumps are quite powerfull even at tick over. all fuel systems follow "benulies law" ( Q= CD2 root P OVER S). excuse my bull**** but what it means in simple terms is any fluid or gas follows this law, the volume flowing is proportional to the pressure (+ or -) and inversely proportional to the resistance to the sg ( specific gravity of the substance ). cold waxed diesel or vegioil has an increased sg ( specific gravity ), the total pressure drop through the fuel system, flow and return place high negative pressures at the joints / seals in particular nearest to the fuel pump. locating the induction of air is very difficult, which is why most people just go for evrything on the system. this is what my diesel specialist did on my car, £120. the alternative is to change the negative pressure to possitive and see where the fuel leaks are. diesel engines are great, very robust and reliable, especially in wet , cold conditions, but their reputation was built on the old systems which were piped in copper with nuts and compression olives for connections.this would add £50 to the cost of the car and apart from filter changes the system would last for ever. the "fast food world" we now live in employ "o" rings , plastic, pipes rubber hose and clips which all deteriorate with time. all rubbers have a hystoresies and suffer compression set. having said all this i cannot complain, my machine has been with me for 11yrs, and 190,000 miles, so a full set of seals was not bad. with reference to mpg, mercs are heavy, around town they will mop up the fuel, on long cruising journeys they improve circa 30%, in my experience this is the situation. cold weather takes it's toll especially on the big autoboxes, again it's a trade off as with service every 40k miles they are good for 300k miles. hope you find this interesting, best of luck . herbiemercman.

 

[HERBIEMERCMAN]

hi everyone, the merc. fuel lift pumps are quite powerfull even at tick over. all fuel systems follow "benulies law" ( Q= CD2 root P OVER S). excuse my bull**** but what it means in simple terms is any fluid or gas follows this law, the volume flowing is proportional to the pressure (+ or -) and inversely proportional to the resistance to the sg ( specific gravity of the substance ). cold waxed diesel or vegioil has an increased sg ( specific gravity ), the total pressure drop through the fuel system, flow and return place high negative pressures at the joints / seals nearest to the fuel pump. locating the induction of air is very difficult, which is why most people just go for evrything on the system. the alternative is to change the negative pressure to possitive and see where the fuel leaks are. diesel engines are great very robust and reliable, especially in wet , cold conditions, but their reputation was built on the old systems which were piped in copper with knuts and compression olives for connections. the fast food world employ "o" rings , plastic, pipes rubber hose and clips which all deteriorate with time. herbiemercman.

 

[turbopete]

hmm much thinking needs to be done! im told he lift pump in mine is part of the injector pump. think i will check tank for venting etc and check cheap stuff 1st. i must be running out of things to change though surely?

 

[Number_Cruncher]

>> Has the O ring behind the shut off valve been renewed?

and

has the o ring under the fuel pre-filter been changed?

and

have you changed the strainer in the fuel tank outlet.

I wouldn't worry about cavitation - there's nothing moving fast enough in the low pressure part of the fuel system to cause that.

 

[turbopete]

the o ring UNDER the pre filter? wasnt aware there was 1. thought it was just the ones around the top where it goes into the filter housing and where the pipe goes into the pre filter! havent done anything with the fuel shut off valve or the tank at all. its only coz it doesnt start very well that im bothering!