Diesel - starved, air?

[concentrik]

Our BMW D35 started behaving oddly..... without warning the revs dropped from 1800 to about 1000, then recovered, then continued doing the same thing...... revs dropping then increasing. It would idle OK and never stalled.

Thinking it was fuel starvation I changed the filters ( three of them inline) when we berthed ( we struggled in ok) but wasn't completely sure how to bleed it after changing all three. There is a push pump on the last filter (Delphi 296 or 629, can't recall) which primed the system enough to get it going but it would run for 30 secs or so then stall. More pumping produced the same thing. I 'cracked' the union after the lift pump and pumped what seemed like a lot of bubbles out but it still drops back the revs under load, wavers a bit at idle and won't rev past 2000.

No history of probs like this.

Am I bleeding it wrong? What's the correct way to bleed this arrangement?

There are banjos on each of the inlets to the high pressure pumps - do these need cracking too?

And if it's not fuel starvation / air in the system, what might it be please?

Pictures are:

first -injectors
second - high pressure pumps
third - lift pump
fourth - filters two identical blue ones, then the Delphi.



View attachment 31864View attachment 31863View attachment 31862View attachment 31861

 

[nigelmercier]

We had a similar problem, turned out to be a minute air leak in the bleed screw on the pre-filter. I'm thinking of fitting some form of transparent inspection tube near the lift-pump inlet, when I can find one.

 

[concentrik]

I'm thinking of fitting some form of transparent inspection tube near the lift-pump inlet, when I can find one.

Is that so you can see if any bubbles are getting in?

I'm guessing that there must be not the slightest tiny amount of air in the fuel feed?

If someone could give me an ABC of bleeding that would be great....

 

[nigelmercier]

Is that so you can see if any bubbles are getting in?
I'm guessing that there must be not the slightest tiny amount of air in the fuel feed?....

Yes.

A few small bubbles are OK, but not a constant stream of them.

 

[cryan]

You still have air. You should have a bleed screw on top of each filter pot. With the fuel turned on from the tank crack open the bleed screws one at a time until the bubbles are replaced by a solid flow of fuel with zero bubbles, tighten the screw and move to the next filter and repeat. The filters in the photo look like primary filters from the tank, you may have final filters actually on the engine. When you have done the filters start by cracking open the bleed screw on the injection pump then the injector pipes then the injector connections. do each one in turn remembering to tighten before moving on to the next. you may need to prime the fuel lines with the hand pump.A bad air contamination can take a long time to clear. did you fill the filters with fuel before replacing them? The only way to clear all the air is to methodically work your way through each bleed screw and pipe connection one at a time until its gone. If you were at sea when it first happened perhaps you were low on fuel and the rolling of the vessel uncovered the fuel suction point? Another thing I have seen on small boats is that a flexible fuel pipe from the tank was crushed by something pushing against it in a heavy sea? Any time the engine revs start "hunting," up and down it is usually fuel starvation caused by blocked filters or air. It could be more serious as in a problem with the governor but make sure all the filters are clean and your absolutely sure there is no air first.

 

[savageseadog]

Make sure all washers are sound, copper washers should be annealed by heating them up to red head and quenching in water or replace them.

 

[nigelmercier]

Make sure all washers are sound, copper washers should be annealed by heating them up to red head and quenching in water or replace them.

Does one have to do this with new washers?

 

[pampas]

Ideally YES as copper age hardens and one does not know how old they are even of bright and shiney.

 

[JomsViking]

Have had the exact same symptoms on two different engines (own boat and a delivery) - Both time it was the tanks airvent that was blocked.

 

[KellysEye]

If you have a bleed screw on the engine and a fuel lift pump wih a manual lever then that's the place to bleed. Use talcum powder on all joints, where the bowl screws on to the filter, the bleed screw on the bottom of the filters and the engine, lift pumps on filters. Run the engine until the revs drop and come back.Turn the engine of and wait for a while, any leak(s) wil show in the talcum powder.

 

[leszek]

Welcome
I had a problem with my D35.
The reason was clogged tank. At low speeds, the flow was OK, but at higher engine speed, fuel flow from the tank was too small, and the engine slowed. At sea, enough to blow the tank (before the filter), and helped. The tank was cleaned in the port and the problem has not returned.
Regards
Leszek