air in first fuel line really ok? - PeachParts Mercedes-Benz Forum

alaskajones · #1 08-12-2015, 10:18 AM

OM606 W210 turbodiesel

so I have read many times that there can be no air in the fuel system....BUT it is normal for air to show up in line from fuel heater to pre-filter at shutoff...this makes no sense, where is it pulling that air from? these systems forever leak air no matter what?

bsmuwk · #2 08-12-2015, 11:21 AM

Mine has been doing that even with every single fuel system component replaced. Yes, even the strainer and rubber line at the tank. IIRC, there's a small section in one of my 606 factory manuals that states this is normal, I'll have to dig that up and post it.

It's not pulling air from anywhere as far as I know, it's simply draining back. As long as you don't have any bubbles while running, or when revving your engine, you're 100% fine.

m1tch · #3 08-12-2015, 04:03 PM

The far left line will fill up with air - its draining back, the issue comes if you have any air bubbles in any of the ones on the right.

mannys9130 · #4 08-12-2015, 05:04 PM

It's totally normal. Air in the prefilter is fine. The lift pump is strong and the air doesn't cause issues. Air in the high pressure lines or IP is bad because it compresses and prevents fuel delivery.

alaskajones · #5 08-12-2015, 08:50 PM

well , if it is an air tight system, then where is it pulling the air in to even allow it to drain back? that is what is stumping me. the fact that it goes back seems legitimate but in my understanding of air tight fluid filled systems, it should not go back because there is no air in the system in the first place? the fact that this is normal tells me that every single one of us has air intrusion in our fuel systems no matter what. otherwise, it would just try pulling back on a sealed system and not work that way.

nelbur · #6 08-12-2015, 09:29 PM

The pre-filter runs with a bubble of air inside because fuel is sucked in at the top and sucked out at the bottom. When the car is shut down the fuel in the first inch or two from the filter runs down into the filter, or said differently, the bubble in the pre-filter climbs up into the line.

alaskajones · #7 08-12-2015, 11:15 PM

so it is also normal for said air bubble to be seen in the line out of the bottom of pre-filter in the form of many tiny bubbles and a little in the line from SOV to lift pump and a little in the line back up to the main filter, after which no bubbles can be visibly detected. I only ask because I have replaced all the lines with new viton o rings and can see little bubbles moving in such a fashion. everyone here is such a stickler about no air in the lines, but if this is normal then why is air in the fuel lines the first thing everyone jumps on when helping others with no start issues? i understand there are tricky hiding o-rings that many people miss, but there are also electrical problems which seems like no one can diagnose other than swapping out parts and hoping for the best...(k40) does MB sweep these forums (and the rest of the internet) and delete all the truly helpful diagnostic stuff? any other vehicle I have ever owned I can find wiring schematics online for...not this Benz though, frustrating

bsmuwk · #8 08-13-2015, 12:14 AM

Once the engine is running there should be no air bubbles in any of the plastic lines. If you have air in the form of tiny bubbles, you have a leak. I'll usually install O rings with a coating of engine oil to prevent them from ripping with install. Viton isn't a surefire way to prevent air bubbling, it'll resist deterioration from bio fuels and dorks that decide to ruin their cars with veggie.

People get on the bubble bandwagon for no starts because these engines are notorious for having air line leakage. A big enough air leak will prevent your engine from starting. It's the nature of the beast, too late to do anything about it now, just have to stay on top of o ring and fuel line replacement.

The pre - pre filter air is not new air being introduced into the system. It's a void in the filter housing that fuel get's pulled through. There's air in every single one of pre-2000 Mercedes diesel prefilters. It's otherwise an air tight system.

What wiring diagram do you need? I'm sure as hell a google will find what you're looking for.....

mannys9130 · #9 08-13-2015, 01:26 AM

As I said above:

Air in the prefilter is normal. It's air in the high pressure side that causes problems. Bubbles in the plastic line are bad.

I suppose you could hold the input side of the pre filter up and fill it up totally with diesel. On a full tank the input line from tank will spill diesel as soon as it's undone. Get all the air out and reconnect the lines. I'm betting the air pocket will reappear after starting the engine.

alaskajones · #10 08-13-2015, 10:09 AM

well, if it is normal....

google does not come up with much of anything pertaining to w210 om606 specific wiring diagrams. or at least i cannot find them as easily as say, wiring diagrams for gasoline models

alaskajones · #11 08-13-2015, 10:38 AM

specifically, i am looking for all related circuit diagrams for the k40 relay, and SOV and anything else related to the operation of these, resistance values, proper diagnostic flowcharts and the such. I suspect my k40 relay is giving me intermittent no starts, but want to properly diagnose it before just throwing a part at the problem. I cannot seem to find what i need anywhere. I will be purchasing a dvd on ebay and hoping it has what i need.

bsmuwk · #12 08-13-2015, 12:49 PM

Quote:

I have replaced all the lines with new viton o rings and can see little bubbles moving in such a fashion.

Quote:

Originally Posted by alaskajones

well, if it is normal....

google does not come up with much of anything pertaining to w210 om606 specific wiring diagrams. or at least i cannot find them as easily as say, wiring diagrams for gasoline models

No, small bubbles in the lines with engine running is not normal. You still have an air leak. While running, you should not see any bubbles in the plastic fuel lines. I went through this a couple times with some local customers, only to find when they installed their new orings, they tore them on the sharp metal edges of the injection pump/etc. If you're getting bubbles beyond the pre-filter, you may have a leak at that oring. My best advice for anyone with an OM606, is to buy a bunch of viton rings from McMaster Carr and keep them in the trunk...you never know.

I have a couple of the discontinued paper service manuals, I'll see if I can dig up some info for you. Give me a day or two to scan everything in. I'll upload it to my blog so it's easier to view. Forums aren't very PDF friendly.

Also, it may be worthwhile to remove you K40 and check all solder joints. All the cars I've had come in with intermittent no starts seemed to have 2 or three bad solder joints on the board. I've only ever replaced two K40's.

m1tch · #13 08-13-2015, 01:54 PM

Its normal in the left hand line when the engine is off, if there are any bubbles in the line when the engine is running then that is an issue.

I had some bubbles on my engine, found out it was the large O ring around the prefilter - starts like a CDI now

alaskajones · #14 08-13-2015, 08:48 PM

i know i did not tear any o rings, I even took them out again to look, they were fine, i put them back, same tiny air bubbles, as far as the air leak, I am actually suspecting the housing itself I have not yet smeared the o rings with silicone or similar. as far as the K40 needing soldering, I have looked at it twice now and everything looks great, it also does not look like other k40 units I have seen pictures of. my vehicle is a 98E300 TD. did they change the design of it? I have seen videos of what other K40 units look like and even the suspect solder connection which looked obviously bad. anyways, my K40 unit has one relay up on the top left of it, then whole bunches of connections on top, some of them small two wire connectors...one single purple wire into a large empty connector, and what I have found to control the IP is a multi wire connector in the bottom right corner of it (as looking at it installed).

wiring schematics for this whole area and associated controls/devices would be an amazing addition to some stickies!!

thanks for your help guys, looking forward to more!

alaskajones · #15 08-13-2015, 11:17 PM

an air bubble hanging out still doesn't make any sense at all, imagine a straw, you are drinking through it, you pick it up out of the drink while still sucking, and then put it back into the drink while still sucking, you will have a trapped air bubble that you just suck up, then its gone. if the system is sealed, the air bubble should just not be there. another straw fact is if you dip it into a drink, put your finger over the top and pick it up, fluid will stay in the straw until air is let in from your finger releasing the top so the fluid going back or whatever doesn't make any sense at all either, if the system is sealed. isn't this basic hydrodynamics? i don't buy it when anyone says there isn't a single bubble in their fuel systems, there are posts where people have claimed not a single bubble only to state later that there is in fact visible bubbles while revving off idle.