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  #1  
Old 08-13-2019, 02:17 PM
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OM617 Air bubbles clear plastic Line New Primer Pump, Filters and hoses hard to start

Hello,


This one has me stumped. Om617 with 125k on the clock. New filters, Newer hand pump primer, New upgrade glow plug system and newish filters clean pre filter (note except glow plugs all added and changed after hard start issue and had no effect). The engine started this yesterday, regardless of glow time or throttle to the floor (might help slightly) it is very difficult to start. It usually takes about 30-60 seconds worth of total cranking throttle to the floor sometimes and starter stays engaged oil pressure never rises above 1 bar. Oil level is spot on. Once the car starts however, I have noticed acceleration is sluggish, but not all the time.



Where are these air bubbles coming from??? I cranked down the hand pump as tight as I could, it doesn't leak at all when I prime the filters. I replaced the rubber hoses and there is no fuel leaking anywhere. I hold my phone's light to the clear plastic line going to the lift pump from the fuel filter (presumable the filtered fuel) and at idle there are very fine bubbles when I rev through the RPMs I can see a stream of bubbles and then a gulp of air bubbles until I release the throttle and it goes back to idle and very fine bubbles again.

Is it leaking at the point where the hand pump primer assembly attaches to the the fuel injection pump where it mates up?


Where do I go from here?


Thank you!
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1979-Oct Mercedes-Benz 300SD Turbo Diesel


450SEL 1980 150k parts for sale



Last edited by Assault; 08-13-2019 at 03:44 PM.
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  #2  
Old 08-13-2019, 03:45 PM
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You might want to check the tightness of the hard lines going from the injection pump to the injectors.
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Old 08-13-2019, 03:59 PM
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Since the lift pump is ahead of the injection pump, the hard lines have nothing to do with air in the system.

The hand primer pump is a very common source of air ingress, even if it doesn't appear to be leaking. The forum consensus seems to be that it should be replaced for the new style if you have air ingress that just won't go away.

Do you have air bubbles and/or foam coming into the clear prefilter? If so, you may have a pinhole in one of the steel lines under the car, a rotten rubber hose from the fuel tank, or the wrong size fuel line causing air problems. It's commonly stated on this forum that 5/16" fuel line works fine, I've not been that lucky. Both of my cars had air ingress issues with 5/16 that went away using the correct 7.5mm hose.
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  #4  
Old 08-13-2019, 04:02 PM
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Try loosening your fuel cap.My son just went through this.I haven't addressed the tank vent yet,but symptoms gone.
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  #5  
Old 08-13-2019, 07:16 PM
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Update: Problem has temporarily fixed itself which means it's likely only temporary.


The sluggishness is constant now, its like trying to pull a semi trailer behind me. Everything sounds right, expect when I idling in Drive the power dips low and the oil pressure dips to almost 1 bar. It is on the verge of cutting out.


Hard lines are tight and straight on, everything I can tighten is as tight as I can make it anymore it will break.


I just replaced the hand pump today with the new style Bosch, to no avail.


Pre-filter is as air free as can be, no foam nothing out of place.


Even filled the tank up to see if that would cause any changes, nothing.


Idling in park or neutral its like the air bubbles disappear (i can still seem them in the line and they still appear if I manually push the throttle. Engage the transmission on it wants to die out. There are no leaks or wet spots anywhere.
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450SEL 1980 150k parts for sale



Last edited by Assault; 08-13-2019 at 09:54 PM.
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  #6  
Old 08-14-2019, 03:37 PM
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Unfortunately I cannot tell from what you have said what is a likely source of the problem.

What people don't think about is that atmospheric pressure which is about 14.5 psi can keep fuel from leaking out of tiny cracks in rubber hoses or I guess in tubing. But, that same crack or hole will allow air inside when there is a vacuum from the fuel supply pump.

When I first got the care I did not realize I had an air leak. As it turned out the hose between the fender well and the plastic filter was hard and in-elastic and the clamp was tight on it. When it got hot under the hood the nipple on the plastic filter distorted and air was sucked in. It never leaked fuel.

A place here it could be possible to have an air leak although it usually leaks fuel is where the tubing runs under the Car and there is little clamps that hold the tubing to the chassis. When the rubber insulator on the clamp crumbles away the clamp can rub a hole into the line or it will rust through.

Because I already have them if I was facing you situation I would go under the Fuel tank and clamp off both sides of the fuel inlet hose and cut it and install an electric fuel pump in between. That would pressurize everything from there forward. You then may be able to find a leak and the air bubbles should also disappear.

Start your Engine and remove the Fuel Fill Cap and rev the engine and see if the air bubbles are still there. If the air bubbles went away then you are having a fuel tank vent issue.


What state and city do you live in. It maybe some Forum Member would come down and take a look.
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Old 08-14-2019, 08:19 PM
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Thank you for the replies.


I swapped out the hard plastic clear line from the filter top to the injection pump to watch the bubbles better here is what I observed.


Once the line was full of fuel again (clear line) I could see as I was pushing the fuel primer pump that air was entering the system the more I pushed. This leads me to believe it is the replacement pump that is defective. I have a new Bosch unit arriving Friday. While the car is idling I can see a small but steady stream of bubbles coming from the injector pump into the fuel filter area.


I tried the gas cap, and understand what is meant with the fuel lines expanding under heat, I've had this happen much with the vacuum system.


I will report back once I reinstall another fuel primer pump, time to pray.
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Old 08-14-2019, 10:38 PM
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Problems that people have had with replacing the hand primer are they often don't remove the old copper or aluminum crush washer that usually stays stuck on the Fuel Supply/Lift Pump.

In removing the stuck crush washer it is possible to scratch the surface on the lift pump. Next in line would be if some dirt got between the new crush washer and the sealing areas.
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  #9  
Old 08-19-2019, 04:47 PM
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No cigar.

There is no air entering the plastic pre-filter, and smaller rubber hose is the correct diameter using OEM clamps. The prior hose was 9.5mm and i could pull the pre-filter out when the clamps tight. Now I cannot.
I noticed there are also bubbles going into the fuel lift pump from the filter housing (clear plastic line), in addition to bubbles from the clear plastic line opposite the fuel lift pump.

I'm out of ideas shy of swapping in another injection pump I know to be in solid working order. Both are MW pumps the spare is from a turbo though. Are these interchangeable?

I park the vehicle nose up at a 10-15 degree incline and it has started up always on the 2nd try including today when engine is cold and has sat for 12+ hours. However once stared it is difficult to start regardless of position parked, throttle engaged, short vs extended cranking, or manual priming with hand pump. Regardless of combinations nothing seems to effect starting. I sprayed the ip to try and see any air bubbles forming, nothing. Sometimes I can crank the engine up to 2.5bar and only keep the engine running if I hold the throttle most the way down, when I let off more than 80% the engine dies out abruptly as though one turned off the ignition.

Tank is full, I think we can rule out the tank since no air is entering the pre-filter.
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  #10  
Old 08-19-2019, 06:23 PM
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Quote:
Originally Posted by Assault View Post
No cigar.
........................
Replace the cigar hose with a temporary clear hose, that's the highest point of the fuel system and where all fuel return to the tank. It should be bubble free always, engine running or not. Find and fix the leaks till you see no more bubbles there.
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  #11  
Old 08-19-2019, 06:43 PM
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Have you tried the check valve banjo bolt on the side of the fuel injection pump?

https://**************.com/store/injection-pump-check-valve-troubleshooting-removal-and-repair-demand-video
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  #12  
Old 08-20-2019, 11:58 AM
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You may be fixating on the bubbles too much. I would establish I had decent fuel pressure in the base of the injection pump first. Some simple tests. close off the return line from the injection pump. If a lot better you could have an open relief valve. If no improvement you could have a weak lift pump.

Another test is to make sure there is overflow out of the return line when he engine is ideling. This would be a test after the closing off of the return line with no improvement.

The lift pump should be able to produce an overflow from the relief valve as this is the control regulator for the operational fuel pressure.

To me it sounds like the potential of having two problems is a possibility. If you do see a stream of small bubbles processing through the pre filter. You know an issue exists before it.

I guess I am anal at checking the fuel system periodically even when the engine is running good. Otherwise you can have compound problems develop where you land up kind of chasing your tail. Even Mercedes service literature asks for the fuel pressure to be checked periodically. On the vast majority of these cars it has never been checked for decades. Besides system reliability it can affect power, good clean quick starts, perhaps even fuel milage to a slight degree.

I also suspect damage to the engine itself if fuel pressure is really poor for a very long period of time. Or reduced engine lifespan. It also reduces the probability of a road breakdown. To maintain the fuel system.

So even if your current problem is only air. It is cheap and pretty easy to check out the system. In the process you also get to know it better.
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  #13  
Old 08-20-2019, 06:19 PM
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A clogged tank screen can give the same symptoms as a weak lift pump. The best way to check this is to connect the fuel source and return lines to a remote fuel source, in the engine bay.
At idle, it won't take much. It will run for several minutes on a Quart of fuel.
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Old 08-20-2019, 06:19 PM
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A clogged tank screen can give the same symptoms as a weak lift pump. The best way to check this is to connect the fuel source and return lines to a remote fuel source, in the engine bay.
At idle, it won't take much. It will run for several minutes on a Quart of fuel.
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  #15  
Old 08-23-2019, 12:51 PM
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The culprit has very likely been discovered. To all of you who said it was a fuel hose you were correct! Turns out to be a pinhole small enough to only leak air into the system, since the lines are gravity fed and low pressure no fuel leaked out until we agitated the hose and a wet spot developed. The hose is the one leading directly from the tank screen to the supply line: 1164701475. Guess the injection pump gets to stay in after all. Thank you very much, sometime you need a new perspective. The pre-filter really threw me off. Evidently there was air getting in, it is strange that I couldn't see it through the clear pre-filter.


Since the tank is going to be drained and likely refurbished since its rusted out inside, is 11 ID x17 OD mm cloth braided hoses pn 1124761226 the correct size for the return and vent line? I believe this is what I've used before.


Also, when the tank is pressured from the tank vent using compressed air and fuel cap in place, do not remove that fuel cap until the pressure bleeds off. You will catch a face full of fuel.
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