'97 E300D

I replaced the shutoff valve(& its o-ring) to fix a fuel leak. At the same time I replaced the main spin-on filter and pre-filter(& its o-ring) and most of the o-rings on the snap-on fuel fittings.

Now I have an air leak that I can not locate. The bubbles are worse in the morning. The car starts and runs fine.

Three questions: 1) if car performs well, then can I ignore the leak?

2) If I ignore leak will I damage something in the long run?

3) I used the same size o-rings for all snap-on fittings but the drawings of the system I've seen indicate maybe different o-rings are used. Are all o-rings for the snap-on fuel fittings the same size?

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Neil
1997 E300D - naturally aspirated
280,000 miles (as of November 2015)

If air can get in, fuel can get out. I'd fix it.

To my knowledge all fuel line o-rings are the same size.

This was posted by ncof300D:
Snap-on Hose seals
O-ring
13mm OD
8mm ID
2.5mm thick

Shut-off Valve seal
O-ring
17mm OD
12mm ID
2.5mm thick

Is the top of the IP where the delivery valves attach free of fuel? Those valves have o-rings too.

I've had difficulty getting the pre-filter o-ring to seal after repalcing it. You might check it again. What about the o-rings on the bolt that the main filter spins onto?

I suspect that as time goes by, your bubbles will begin to cause harder starts and eventually a leak will develop somewhere. You could try parking nose downhill to see if you could get fuel to leak so you could track it.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

This is an interesting question. For the most part visible bubbles are in the low pressure side of the fuel system as the plastic lines are not high pressure lines, and are not an indication of any potential trouble for the injection pump. In the high pressure side of the fuel system, those atmospheric bubbles will be collapsed and not noticed by either the pump or the injectors. If the engine starts fine, and runs fine, that is an indication that the high pressure side of the system is solid, at pressure. With enough air in the system at pressure to cause a problem for the longevity of the injection pump, I doubt the engine would run at all.

In my experience there is always a series of bubbles in the clear lines, they just don't fill the entire cross section of the insides of the lines at any particular point. When you say you have a leak, are you seeing Diesel leak out, or are you just concerned about these bubbles?

The flow rates of Diesel through the fuel lines is pretty slow in terms of feet per second or meters per second. Which means it is a task for the system to drag bubbles anywhere. If you want to try to eliminate the bubbles it might be good to take it out for an "Italian tune-up" and drive WOT up a hill for a few minutes to try to up the fuel flow rates. Someone more familiar with the routing of the lines may be able to provide a sequence to close them up or specific instructions for venting.

The owner's manual typically instructs you to crank the starter on these later models to clear the air in the lines after running out of fuel or opening the system and letting air inside. That is typically all I do, and once she fires up and runs smoothly, I ignore the little bubbles. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

I already replaced the delivery valve o-rings at 100,000 miles as they were leaking. They are still bone dry. I already double checked pre-filter o-ring and the new o-ring at the bolt on the main filter.

I have no visible evidence of a fuel leak at this time. I was thinking that perhaps I could wait for a fuel leak to help identify the location of the air leak (but I'm concerned I'll damage something running w/air in the fuel). You're idea of parking nose down is a good one; I'll also make sure the tank is full of fuel to add extra pressure (will that help add pressure?)

Will running the car with air in the lines cause pump cavitation / damage or other engine damage? Again, car is currently running perfect.

__________________
Neil
1997 E300D - naturally aspirated
280,000 miles (as of November 2015)

My concern is air in the fuel. There's definitely more air in the lines since I replaced the fuel shut-off valve, fuel filters, and fuel line o-rings. I now see the downside of replacing so many things at once.

I tried the "Italian turn-up" yesterday and all air was purged after that. But this morning, the air was back.

I found the following information from a manufacturer of centrifugal pumps (like a water pump) but I don't know what type of pump the IP or lift pump is...

"Both vaporization and air ingestion have an affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.

Although air ingestion and vaporization both occur they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump"...

Based on your reply, I'm thinking I can safely wait for the fuel leak to appear (if it ever does). If the fuel leak appears, then it should identify the location of the air leak.

__________________
Neil
1997 E300D - naturally aspirated
280,000 miles (as of November 2015)

Given Diesel fuel is nearly an incompressible fluid, like water, the pump type selected for the very high injection pressures is a positive displacement pump. The injection pump is also a specific type of positive displacement pump known as a reciprocating pump. Under each steel line coming out of the pump and going to an injector (which is actually a pressure relief valve with a mechanically set relief pressure) is a piston in a cylinder that has a mechanical means to change the point in the stroke of the piston where the pump becomes a true positive displacment pump. Up to that point the stroke is "wasted" recirculating any displaced fluid at a much lower pressure, back to the filter and ultimately the tank. After the point where the pump stops recirculating fuel, the pressure in the steel line rapidly rises with very little further stroke, lifting the pressure relief valve (injector), and for the rest of the stroke, all the volume displaced in the cylinder of the pump is squirted out of the injector. The transition from recirculation to positive displacement is controlled by mechanical linkages connected to the pedal under your right foot. At idle, most of the stroke is recirculating, while floored, most of the stroke is in the positive displacement mode.

It should be clear to see why air, a compressible fluid, inside this setup goes next to nowhere - the stroke at wide open throttle moves some very small quantity of fuel per cycle. In an air blocked system, such as after you open the system up and let air in, the stroke of the injection pump never generates enough pressure to open the injector, and the air has to be slowly inched along through the recirculation lines, which operate at a lower pressure.

Mercedes used to provide a separate, hand operated purge pump, which took a much larger stroke and still required enough pumping to give you a blister from grabbing the knurled edge of the pump actuator. Today they just tell you to run the starter, so, I don't think the injection pump is at risk. If you did this daily I am sure it would be an unplanned operating evolution that could prove damaging, but your starter would be the first thing to go. Followed by the battery. And likely the glow plugs.

So, if your car starts and idles normally, I don't think you have an injection pump problem brewing. Unlike pumps with impellers, known as centrifugal pumps, there is no similar tendency for "vaporization," also known as cavitation, that typically is damaging, in a positive displacement pump (the output pressure is typically much higher than the vapor pressure of the liquid being pumped).

I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Thanks Jim. That's a great explanation of how the IP operates.

My car is running fine; appears I was worried about nothing.

__________________
Neil
1997 E300D - naturally aspirated
280,000 miles (as of November 2015)

Update for the archive...

I checked for air in the lines this morning on initial startup for the day. No air was seen. I guess 4 miles of test-driving (after initial repair) was not enough to purge air out of lines after changing the fuel filter and the other fuel-line components. Car is running great with no air or fuel leaks.

Thanks again for all the good thoughts / info.

__________________
Neil
1997 E300D - naturally aspirated
280,000 miles (as of November 2015)