FuelFlow Questions for OM606 and Starter/Battery Saver Tips - PeachParts Mercedes-Benz Forum

anghrist · #1 02-20-2008, 11:09 PM

Okay folks. I've been thinking about this for a while and it's really bugging me. The OM606 fuel flow pattern is as one person put it "absurd". From a flow dynamics standpoint, a fluid pump will always push better than it pulls. This would indicate that the W210 chassis diesel would have benefited greatly from a fuel pump either in or under the fuel tank; however the geniuses in charge of the design of the fuel system on this car saw fit to install a mechanical "lift" pump on the engine instead... 1960s GM Tech anyone?

Sorry, I'm just getting gripey after chasing more air leaks in the lines. My real point is this. The lift pump is basically siphoning fuel from the tank. Any air leaks in between the tank and the lift pump can show up in the clear plastic lines (we all should know this by now). However there is another effect that I hope to make a recording of this weekend. What can not be observed with the OM606 engine is that, given the right conditions, there will be a bubble that remains in the prefilter. I, tried replacing the stock prefilter setup with a clear 1977 style prefilter to show that this is the case. This bubble does not dissappear, and will remain in the prefilter. This is where the "shutdown" bubble comes from in the prefilter/thermostat/fuel heater line.

This bubble is common in a siphon environment. Anyone who has conducted a diesel purge using clear tubing can see the effect readily. When this bubble exists and throttle is applied the lift pump can generate enough suction to pull this bubble down trough the prefilter and into the rest of the system. Typically this would not be a problem, the air would be purged in short time and there would no longer be a bubble in the prefilter right?

So why would the bubble come back at shutdown. Answer: there's always a bubble in there (leaks or no). The only way to totally eliminate the bubble would be to positively pressurize the line from the fuel tank. I'm looking into a fix for this that will properly work with the Mercedes system.

Sorry folks, I'm just thinking and typing about this to see if anyone else has any ideas. Now on to the tips

**Starter/Battery Saver tip**

After performing any maintenance on the fuel system in which fuel lines had to be removed follow the following steps:

1) Open hood and make sure all lines are connected correctly (obviously)

2) Remove line from top of prefilter.

3) Turn key to preglow position (opens shut-off valve)

4) Using a can of Diesel Purge, and a turkey baster prime system (fill baster and squeeze diesel purge into the prefilter, make sure you shove the baster in there tightly otherwise this will get messy)

5) Prime until Diesel Purge tries to spray out of prefilter (it will)

6) Replace line to top of prefilter and bolt in place.

7) Re-glow and start.

Gosh that's much nicer than grinding on the starter for minutes at a time.

TMAllison · #2 02-20-2008, 11:33 PM

Interesting thoughts concerning priming. May try that sometime when changing lines or the filter...although, I've never had too much trouble priming via the starter personally. Helps immensely if vechicle is not pointed nose up.

So have your tracked down your leak? From what you stated previously it has to be betwen the bottom of the pre-filter and the tank. In my expirience on the forum, i think only one preson has had a confirmed leak aft of the fuel thermostat. Based on that, yours ought to be one of four orings, two of which are at the top of the prefilter.

anghrist · #3 02-20-2008, 11:53 PM

Replaced prefilter twice, orings once, lines with new orings once. At this point it could be a nick in the prefilter housing (not likely), but the vacuum test negative pressure holds for the housing and lines from the heater to the SOV. So it's a test from the prefilter to the tank this weekend as well. Could just be a dirty tank screen. Who knows?

I have a quick question though I could probably easily answer it myself. What kind of "seat" is behind the SOV (other than the oring). The "nipple" on the back of the SOV was kind of hard to get to go back into it's "seat" when I reinstalled it this time 'round.

TMAllison · #4 02-21-2008, 12:02 AM

To the best of my recollection, it is simple a tube with a lip that sealed with an oring. Doesnt *snap* into place or anything.

ALL of the fuel system orings are a very tight fit when new.

They are tight enough that your old one was probably elongated/oval in profile rather than round as it should have been.

Diesel911 · #5 02-21-2008, 12:52 AM

You could put a generic electric fuel like the purolator faucet uiversal electric fuel pump; one that will work with disel fuel. But they do not put out much pressure 3.5-5 psi. The lift pump on the side of the IP has its lowest pressure of 8 psi. Also, the lift pump would have the added burden of sucking through the electric pump. The higher pressure and output electric fuel pumps that I have seen in adds have been made for gassers and may not be suitable for diesel fuel.
Maybe one of the alternate fuel sites would have some good info on a suitable electric fuel pump as some of those fuels have a higher viscosity than diesel.

Parrot of Doom · #6 02-21-2008, 06:44 AM

With diesel, I think its safer to suck air in through a leak, than to spray diesel out through a leak.

Particularly for other road users.

michakaveli · #7 02-21-2008, 09:38 AM

Not to burst anyone's bubble, but instead of disconnecting any additional lines, etc and potentially causing additional wear on the o-rings from the delicate fuel system, I perform as follows...

Back the car out of the garage into my driveway and get my heavy-duty jumper cables out and jump the diesel to my other car, such that when I activate the glow plugs it's taxing the other vehicle's charging system also, not only the diesel's battery. I let her glow for a good twenty seconds and then start cranking. I've found it catches MUCH faster and easier once she gets the air mostly purged out....

I initially top off the full flow filter if it wear empty first, otherwise I don't touch it.