OM606 air in the fuel lines: A theory - PeachParts Mercedes-Benz Forum

treetops · #1 01-24-2015, 08:56 PM

Well, I just finished replacing 5 of the 6 fuel lines to and from the IP on my e300. So far so good, no starting or running issues, but what a messy job. Yes, the SO oring and the Banjo bolt line are next when I get the IM off for EGR cleaning.

I did come across this short thread and thought this 'could be' a reasonable theory concerning the chronic problem on the 606s and other models. Curious to know others thoughts as to its merits or not. I certainly dont have any expertise here, but it does somewhat remind me of the cigar hose purpose on the 617(?).

Apparent Air in fuel system, Mercedes-Benz OM.605/6 W210/W124 and 'G' Wagen

TLDR: Lift pump de-pressurizes the fuel line below the vapor pressure of diesel, ergo bubbles.

tjts1 · #2 01-24-2015, 11:05 PM

A small electric pump at the tank that provides positive pressure to the lift pump would solve all these problems.

jay_bob · #3 01-25-2015, 06:40 PM

Wow I have been pulling my hair out trying to find the air leak on my E300. I have been through the entire system and still get bubbles and clattering just above idle (like when driving at low speed down my street).

Maybe this is the smoking gun. It sure seems like I have ruled everything else out.

RunningTooHot · #4 01-25-2015, 07:52 PM

That's an interesting theory which definitely holds merit. However I would look at that specific issue as a contributory factor, but perhaps not as a primary cause. Otherwise, how would you explain the 606's that don't have air bubbles?

One explanation could be a weaker return spring in some of the lift pumps so there is slightly less depressurization occurring with each pulse - but I seriously doubt that.

If the normal vacuum pulses in a properly operating system is causing the fuel to be juuusssst barely below the out-gassing threshold, then any additional resistance to flow on the inlet side could theoretically push it over the edge.

So... maybe the fuel tank screen being partially plugged and adding restriction at higher flow rates could be the culprit? I don't know... just an idea.

treetops · #5 01-25-2015, 08:49 PM

Finally found the thread the first link cites here

OM606 fuel supply issues

From my reading, I have come across very few 606 owners who have no (0) bubbles in the fuel lines. Most accounts I've read here mention some or very small bubbles but still they are present in the system. I've read less than a handful of accounts that mention a larger diameter supply line with sucess at getting rid of them. Now that seems like a simply elegant solution this forum is known for, right.

Perhaps one of the members with a fluid dynamics background could chime in with thoughts.

Mxfrank · #6 01-25-2015, 11:37 PM

No fluid dynamics background, but not necessary. If the pressure on the line drops below the vapor pressure of the fuel, you will get vapor pockets. At room temperatures, the vapor pressure of Diesel is very low, less than .01 psi. I doubt the pump can pull that hard, so at first glance it's unlikely that this is the cause of your cavitation issues. Diesels are generally considered immune from vapor lock for this reason.

However, a constriction due to a design error can certainly drop the line pressure, due to the Bernoulli effect. This is why pinching the line results in bubbles. The other factor is that the fuel is heated, and the hotter it gets, the higher the vapor pressure, and the greater the tendency to vaporize. I'm not sure how hot the fuel that's in the tank can get. It will warm slowly as hot fuel is returned to the tank, and can theoretically reach engine operating temperature. At 80C, vapor pressure is probably close to atmospheric pressure, and the pump wouldn't have to pull very hard to cause a problem. It probably doesn't get that hot, but maybe. So it's possible that the combination of a constricted feed line and elevated fuel temperature could result in a bubble.

You need to check two things:
- what is the temperature of the fuel in your tank after a period of running?
- what is the pressure in the line?

To fix this, you would need to increase line pressure or decrease fuel temperature, or both:
1) A larger ID fuel line would increase pressure.
2) Some sort of cooler on the fuel line, preferably close to the tank.
3) Install a secondary pump as close to the tank as possible (in the tank would be best).A pump in this location would increase line pressure on the way to the main lift pump
4) Impractical, but raising the tank would help.

One final thought: Diesel as it comes from the pump isn't perfectly pure, nor is it a single chemical. There can be water, alcohol, gasoline or other components in the mix, with varying vapor pressures.

tjts1 · #7 01-26-2015, 01:20 AM

Maybe the small diameter fuel pipe is just clogged after thousands of gallons of infiltered diesel fuel went through it over 20 years of use. After all the fuel tank strainer does little more than keep out the twigs and small rodents. That fuel line could use a good cleaning.

biopete · #8 01-26-2015, 01:52 AM

Quote:

Originally Posted by tjts1

Maybe the small diameter fuel pipe is just clogged after thousands of gallons of infiltered diesel fuel went through it over 20 years of use. After all the fuel tank strainer does little more than keep out the twigs and small rodents. That fuel line could use a good cleaning.

And breadcrumbs and the occasional frie

biopete · #9 01-26-2015, 01:55 AM

I'll check my 99. I plan to put an airtec 7-10 psi pump under hood before the heater soon.

Mxfrank · #10 01-26-2015, 08:08 AM

Quote:

Originally Posted by biopete

I'll check my 99. I plan to put an airtec 7-10 psi pump under hood before the heater soon.

If the problem is vapor lock, then putting a powerful pump on the motor side of the line is exactly the wrong thing to do. Put the pump near the tank, so that it pushes the fuel through the line. It's all about creating positive pressure.

treetops · #11 01-26-2015, 10:27 AM

Frank,
You mention a 5 psi pump you used to find leaks in the fuel system in another thread, would that be about the right size? The fuel pump delivery rate should be 150cm3 minimum on the 95 NA version going by the online FSM, not sure if its different for turbos.

I agree the build up of crud in the supply line could cause the 'pinch like' restriction by my logic. Then again, I'm a total newb at these things.

Mxfrank · #12 01-26-2015, 01:31 PM

Quote:

Originally Posted by treetops

Frank,
You mention a 5 psi pump you used to find leaks in the fuel system in another thread, would that be about the right size? The fuel pump delivery rate should be 150cm3 minimum on the 95 NA version going by the online FSM, not sure if its different for turbos.

I agree the build up of crud in the supply line could cause the 'pinch like' restriction by my logic. Then again, I'm a total newb at these things.

The fuel pressure doesn't need to be more than the current pump produces, so check the FSM. The more important question is where you locate the pump. If the pump is as close to the tank as possible (as it is on all the gas MB's I've ever seen), then the entire fuel line has positive pressure, and you will have a much lower chance of vapor lock. OTH, if you locate the pump near the engine, the entire length of the line is subject to low pressure.

Another thing you can try is disconnecting the fuel heater. I don't recommend running that way, because warm fuel will give a better spray pattern, which means better economy and performance, and lower emissions.

treetops · #13 01-30-2015, 04:32 PM

I've been pondering this since taking the IM off to clean it and the EGR is soon coming. I'm speculating that a larger diameter hose all the way back to the tank would be the best "hose" solution instead of say just going with a bigger hose from the metal line on the firewall to the preheater.

I'm not up on the "fluidity" of these things to know. When I get the IM off Ill take another whack at this idea.

Anyone else tried this larger hose concept?

KarTek · #14 01-30-2015, 04:39 PM

I'll throw an additional curve ball out there: If you pressurize the fuel feed into the lift pump, you can push fuel across the seals in the immobilizer valve and it will literally leak out of the electrical connection.

It did on mine when I tried it.

tjts1 · #15 01-30-2015, 05:55 PM

If that seal can hold under the vacuum created by the lift pump, it can hold 5psi of positive pressure. Plenty of people have added electric fuel pumps to their diesels with positive results.