I love this site and I'm thankful for all the help I have gotten. I would have to say, my only complaint is not enough pictures.

Anyway, here is the situation. 1987 190D with a OM602.
The car will not start warm or cold without the aid of the glow plugs. Even when the engine is at operating temperature it will not immediately restart.

For the moment, lets assume the compression is good. The engine seems to run fine and there is barely any detectable blow-by.


The fuel tank return line was temporally replaced with some clear vinyl 5/16 hose so I could inspect the lines for air. As seen in the photo above there is zero air bubbles in the return line when the engine is idling and when the engine is revved. Everything seems fine.




On the other hand, the tiny pre-filter has a crap load of air in it. This filter has always had air in it and I figured it was normal. From what I gather, apparently this is not normal and needs to be dealt with... correct?

Anyway, if I shake the pre- filter around a bit while the engine is running I start getting air bubbles in the return line and the engine will start making a high pitch sound.

The fix....??

I'm assuming there is an air leak on the supply side of the fuel line. This car is very solid and relatively rust free so chances are the hard lines are probably ok. Could the air leak be coming from the connection to the fuel tank?

Update...

So I crawled under the car and had a look at the fuel tank supply line....



The hose going into the tank appears dry.




This is where the tank hose connects to the hard line. the hard line looks a bit crusty but is also appears dry.

Although the hard line looks solid, I suspect the corrosion may be more significant than the pictures shows.... but not enough to cause a fuel leak.

Any thoughts?

The prefilter tends to have air in it. Because of how the fuel system works, it's just about impossible to burp the bubble out.

Be aware that the worm-style hose clamps you have on your car have been known to cause air ingress issues on more than one occasion. I fought with them on my own car. Fuel-injection style clamps are much superior and create a better seal.

Leave the clear line on overnight and check it the next morning. It should still have a solid column of fuel in it.

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Some day try replacing the plastic filter with one that has a 90 degree on one end install the 90 degree end closest to the Lift Pump and point the body of the filter down and see if that does not get rid of your plastic filter air bubble.

I saw a picture one time of test equipment. You put the Injector in the machine and then pressureized air was applied to the Nozzle end of the Injector. If the innards of the Nozzle were not still seated good air get inside of the Injector and would pass out of the top of the Injector including the return.

It that happens it would account for Air Bubbles in the return line because air is being compressed in the Cylinder till the Fuel starts to be injected. And during that compression time air could be getting pushed past the nozzles.

I don't know if it possible on a Mercedes but if you pump faster then the fluid can move you can get a vacuum bubble; but that would be on the suction side.
I am not sure if a vacuum bubble looks the same as an air bubble.

You might try runing the engine with out the Fuel Tank Cap on. If the bubbles go away then you may have a tank vent restriction/issue.

One way to find out on the suction side is to bypass the inlet hose coming from the fuel tank and add a section of hose and draw from a seperate container.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the quick feedback!

I have a 90 degree pre-filter on its way. I'll pick up some FI clamps tomorrow.

Unfortunately I had to remove the clear hose and put the fuel system back to normal so I could drive the car tomorrow.

I'll probably do the overnight clear fuel line experiment on thursday or friday.

Stay tuned!

Check the in-tank filter, its under that rubber boot leading into tank.

This sight is not uncommon



If blocked will draw air no matter how good pipes are.

__________________
David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)

Totally agree too on hose clamps, they don't draw up round more of an oval shape

__________________
David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)

You could pull the supply line off the bottom of the tank and see how fast it drains. You could also swap the suppply and return and see if you are still drawing past a restriction or not.

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Interesting... How much fuel should be in the tank if I flip the lines?

This gets too complicated for me. I'd start at the tank by draining it and pulling that tank screen unless it's been done on your car. Diesel is nasty.

You're correct, the primary filter always has a bubble unless you bleed it off by parking the car front end down on a hill and open the fuel exit end of the filter. It's not worth the trouble, imo.

If the car is parked level and the tank has a significant amount of fuel in it the head pressure of that fuel should push it through the filter. That's one of the reasons the left side engine mounts go before the mounts on the right. Fuel runs through clean lines without assistance other than gravity.

I'd replace all the rubber hoses between the tank screen and the secondary filter while I had the lines empty. Hoses and clamps are fairly cheap and cleaning the screen is a matter of soap, water and something to dry the water off of it.

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84 300SD
85 380SE
83 528e
95 318ic

Should I go ahead and order a new in tank filter in case the old one is crap and the o ring is FUBAR? Probably not a bad idea?

Meanwhile, if I eliminate all or most of the air in the pre-filter and the air pocket returns then is this semi normal or a clue.

Right now the car has about 3/4 tank of fuel. It will take a few days to burn most of it off.

I'd try to clean it first. I've only cleaned a couple of the tank screens but neither needed to be replaced. I bought a new one before I removed the first but didn't use it. The rubber o-ring was fine.

I guess it's ok to buy the o-ring just in case but evidence indicates the screen is good. If it had a hole in it or was torn you'd see the results in the primary filter real quick.

Changing the screen will be fine if you select a good quality. I don't prefer putting junk in if the original is still usable. If you can wait to soak that old screen overnight you can probably spray it out with a garden hose and get it clean.

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84 300SD
85 380SE
83 528e
95 318ic

I went ahead and ordered a new screen plus o-ring and the hose just in case. Chances are the screen hasn't been touched in 30 years. Anyway, the parts should be here before the weekend.

- ? -

__________________
David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)

Depending on what your tank filter looks like you may need to consider cleaning the tank It's not uncommon for tank liner coatings to give way over time re-blocking filters, this tends to happen mostly with bio or veg fuels.

__________________
David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)