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Old 08-03-2013, 12:57 PM
barry12345 barry12345 is offline
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Originally Posted by nikkondy View Post
There does not seem to be any air bubbles. At first I also though that air was entering the fuel so I replaced all rubber fuel lines, any o-rings or washers that I could. There is no stream of bubbles going through the prefilter, although I have noticed that my prefilter does not fill up all the way and there is an air bubble at the top. I have attached pictures to show what it looks like. Is this normal? Also I am wondering if my racor filter is causing a fuel flow problem as I don't have any type of electric fuel pump to aid in fuel delivery. When I hooked up the racor filter I used a handheld vacuum pump to suck fuel through it and then connected the line to the primer pump. I also disconnected the return line and put it in a jar filled halfway with diesel and there was no air bubbles.
Retained pretty static air bubble in the pre filter is totally normal. If you have a little time available as what I suggest is a partial tune up on an older diesel. If it is a 616 four cylinder this part especially applies. Make sure you have 15-19 pounds fuel pressure in the base of the injection pump. There are cheap but adequate 0-30 pound liquid filled Chinese pressure gauges on the market for this. Usually about 10.00 at harbor freight.

I have decided that if when using the primer pump it gets substantially harder to push prior to hearing the squeal of the relief valve opening. This indicates to me that at least the relief valve is probably in decent condition. If when priming there is no or very little noticeable resistance increase either the relief valve on the injection pump is soft or fuel is going back by the check valves in the lift pump.

One other test that does not have great meaning or is particularily conclusive. If the return line from the injection pump is closed off. If everything is in good condition you should notice a change of some sort. More power, smoother idle etc. If no change you usually will find some deficiency in the fuel supply system. This closing off the line will not hurt anything as the lift pump does not care if the output from the injection pumps return is closed off for a temporary test.

Personally I like the 616 engines. Keeping the fuel supply system in good condition may avoid a fairly common type engine failure on them. I cannot be totally certain but have a strong suspicion that low fuel pressure may just contribute to the number one rod bearing failures we see on the 616. This requires low fuel pressure over a long time frame though as well.

Running with old partially obstructed fuel filters combined with a weaker output from the lift pump by manufactures design can contribute in my opinion.

The lift pump is more suspect on the 616s simply because it does not operate at as high an output pressure as on the 5 cylinder turbo engines. It is actually another part number but can be upgraded with a tubos lift pump pressure spring installed or substituted with a whole turbo lift pump. Easy junk yard item.

When you have restored proper fuel pressure if not present and problem remains. Study the milli volt thread in the archives if you have a digital meter or can spring five dollars for one.

You could previouisly loosen one injector at a time line nut. Looking for any imbalance from any one given cylinder. Another item is to quickly check that the rebuilt injection pump was installed correctly using the drip method. I really try to keep troubleshooting as cheap as possible but still effective.

The ideal of shot gunning and guessing as a troubleshooting method is both costly and still too easy to miss a problem. I guess this qualifies me as a cheap Mercedes benz owner. Actually I am not really cheap by nature but do not like to waste money particularily.

A working knowledge of the milli volt method can really help locate or clear many types of problems. It is not rocket science but instead gives you a reading of the cylinders individual burn temperature in comparison to the others. The better this is the better the life of the engine and more efficient it may be. You also need or should want all the power that the engine can potentially deliver. The 616 can idle pretty smooth for what it is if everything is in good shape. It also is a possibility that your idle speed is a little low of course but these engines after all these years should undergo the tests mentioned unless running perfectly. You can usually get a little improvement over what is felt to be good. These systems are old and so logically they partially deteriorate with time and milage.

I was also wondering if you had the same symptoms before installing the rancor fuel filter before the lift pump. At low rpm I was just thinking if suction pressure by the lift pump might be marginal. Just a thought by the way.

On the other hand since you had overflow from the return line at idle I suspect it is not an issue. Keep us informed as I suspect there is something to be learn by people like myself as you run this down.
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