Air in fuel system?? - Volvo 2020

[bluedragon]

Could do with some opinions and advice. We've just had an upgraded primary filter fitted (Racor 500FG) in a nicely accessible position in a locker, just above the fuel tank. The outlet of this goes as usual to the lift pump and then engine filter.

Since fitting the new filter last week, the engine speed has become erratic, especially in the 1500-3000rpm range. After a short while at constant throttle the revs begin to fall away and won't always pick up on increasing the throttle lever setting, though shortly afterwards the speed will increase again of its own accord. The engine starts immediately and runs OK on tickover and low rpm. It's as though the engine is struggling to get fuel perhaps because of a blockage, then clears itself, but the lift pump primes the system very easily and all filter elements are new and the fuel is clean so this seems unlikely. We can only assume that air is getting into the suction side between tank and lift pump. Bleeding the system as far as the injector block a number of times has not solved the problem. We've not bled the injectors themselves so far.

One other thing I've noticed, is that after a short period of running, on removing the engine filter bleed screw, the filter seems empty of fuel at the top, even though when bled a short time ealier it was full. I expected the fuel to be at the bleed hole level all the times in a fully airtight system but I wonder if that is correct and I'm misunderstanding the set-up? The filter outlet pipe at the injection block has a direct connection to the fuel return line it seems (I can see the fuel level in the filter move up and down as I waggle this line). So does air at the top of the filter come through the return line as part of normal functioning and is nothing to do suction leaks?

It's all bit puzzling...

Thoughts and ideas are welcome!

 

[MrCramp]

I have never fitted a filter at a higher level than the fuel tank, unless it is after the fuel pump (as on my Perkins Perama M20). I am not sure how you can bleed the fuel filter if it is higher than the fuel level.

I think you need to reposition the filter so it is lower than the level of the fuel in the tank.

 

[Stu Jackson]

Many boats have their fuel tanks in the keel, and there's no way to get a fuel filter lower than that. That's why they make fuel lift pumps.

Since you just installed the filter, check for any leaks.

 

[bluedragon]

Since you just installed the filter, check for any leaks.

That's what we are doing. I just wondered if we're jumping to conclusions here that it must be air in the system. Could other factors be at play? I don't know what, but somebody might have an idea.

 

[jonathanhsm]

Bypass

I too am having similar symptoms on similar engine (perama M20), with high revs drop off etc and then refusing to come to speed. Start by running the engine from a tankful of fuel going directly to the lift pump - if it runs OK then you know it's an upstream problem - and vice verse.
Mine is a downstream problem - and now I have bought relevant fittings I have been able to briefly bypass the engine filter -and lo the engine runs perfectly so I assume it's a filter seating problem. I've taken it off the boat - renewed junctions etc. and reseated filter on cleaned up housing. I then tested it to 40 psi with no leaks so here's hoping it is that.
It takes quite a blow to get air through the engine filter (which is new) is that normal? How much pressure difference should one expect?
Sorry didn't mean to hijack your thread -start off with a fuel can and see where you go from there.

 

[Ross D]

If you have any air in the fuel system it will not work properly, there must be an air leak to get air on top of the filter, there should be no air in the return line either. I was having similar problems and traced it to a copper washer on the filter bleed screw, if you are having troubles with minor air leaks try changing your copper washers, they eventually harden and do not seal.

 

[ianat182]

Possible crushed breather to the tank causing periodic vacuum?

ianat182

 

[bluedragon]

The only copper bleed washers in the the system right now are downstream of the lift pump (engine filter bleed and injection block bleed). As these are not on the suction side I wouldn't have thought they'd be an issue...but am I wrong?

 

[bluedragon]

Possible crushed breather to the tank causing periodic vacuum?

ianat182

I wondered about that but fuel spurted out of the breather yesterday when filling the tank, so I guess it's OK. Also the problem only started after a new filter unit was fitted.

 

[bluedragon]

I too am having similar symptoms on similar engine (perama M20), with high revs drop off etc and then refusing to come to speed. Start by running the engine from a tankful of fuel going directly to the lift pump - if it runs OK then you know it's an upstream problem - and vice verse.
Mine is a downstream problem - and now I have bought relevant fittings I have been able to briefly bypass the engine filter -and lo the engine runs perfectly so I assume it's a filter seating problem. I've taken it off the boat - renewed junctions etc. and reseated filter on cleaned up housing. I then tested it to 40 psi with no leaks so here's hoping it is that.
It takes quite a blow to get air through the engine filter (which is new) is that normal? How much pressure difference should one expect?
Sorry didn't mean to hijack your thread -start off with a fuel can and see where you go from there.

I assume the engine filter is on the pressure side of the lift pump? Wouldn't a sealing problem cause a fuel leak?

 

[ianat182]

Just did a search for RACOR 500FG and a U.S company MAESCO have diagram identifying the complete assembly and their part numbers relative to the type that may be of help.

ianat182

 

[jonathanhsm]

Reply to Bluedragon

You would have thought so wouldn't you - all I know is that with the engine fuel filter in the circuit great gobs of air come out of the return line (which I replaced temporarily with a see through pipe) and the engine runs badly and won't rev properly. Take the filter out of the line the engine runs perfectly. As I said I have now pressure tested the filter and connections and when I get a moment will put it back in the circuit and see what happens!

 

[jonathanhsm]

As for your fuel filter not being "full" when you undo the bleed screw - this might provide an answer although I can't pretend to understand it.

http://www.aftermarketsuppliers.org/Councils/Filter-Manufacturers-Council/TSBs-2/English/94-6R2.pdf

 

[david_bagshaw]

I would check for air leaks prior to the new filter, including the pickup in the tank as you have raised the suction height.. I would also take lid off the racor and fill to the brim, replace, next time the problem occurs stop the engine & remove the lid, if there is now an airspace, this might be a good indicator of a small air leak.

 

[bluedragon]

As for your fuel filter not being "full" when you undo the bleed screw - this might provide an answer although I can't pretend to understand it.

http://www.aftermarketsuppliers.org/Councils/Filter-Manufacturers-Council/TSBs-2/English/94-6R2.pdf

Not sure I understand the physics here either, though I get the drift. It's confirmed by a comment in the Racor datasheet when discussing a suction gauge accessory on top of the unit to indicate when the filter needs replacing. It refers to "fuel degassing" if the vacuum becomes too high.

Anyway, the fitter who came today (and did find a small leak on pressure testing the system) said they always see an "air" space when taking off the top of these Racor units, and we discussed how to know whether the it was really there al the time, or created by the act of taking the lid off! So it's good to know our hypothesis may be correct from this document...

Anyway, many thanks for finding this!

 

[bluedragon]

I would check for air leaks prior to the new filter, including the pickup in the tank as you have raised the suction height.. I would also take lid off the racor and fill to the brim, replace, next time the problem occurs stop the engine & remove the lid, if there is now an airspace, this might be a good indicator of a small air leak.

The tank pickup is the last thing to check if the problem persists, but fingers crossed we might have found the leak today. The engine filter is now full to the top after running and although there is still (probably) a space at the top of the Racor unit, it may not be an issue. See the reply above. We topped it right up with diesel before putting the lid on, and so far so good on the engine tests. I've love to look inside but it's a bit like quantum theory or was it Schrodingers Cat (?)...too many years ago... the act of observing is going to change the outcome

 

[david_bagshaw]

The tank pickup is the last thing to check if the problem persists, but fingers crossed we might have found the leak today. The engine filter is now full to the top after running and although there is still (probably) a space at the top of the Racor unit, it may not be an issue. See the reply above. We topped it right up with diesel before putting the lid on, and so far so good on the engine tests. I've love to look inside but it's a bit like quantum theory or was it Schrodingers Cat (?)...too many years ago... the act of observing is going to change the outcome

we have the same racor units, 2 of which are on the 2 generators.

The smaller one, a 7.5kw unit, so I presume is similar to your engine, also with the filter above the tank, remains full , other than after a bunkering.

So there is some removing disolved gases taking place. With this engine we had to fit an additional electric pump to lift the fuel from a nearly empty tank, about 1 mtr lift.

 

[KellysEye]

Racors have a pump on top of the filter unit to lift the fuel from a tank in the keel, also a screw-on bowl under the filter and a bleed screw on the bottom. All of of those can cause your symptoms. Firstly if you have not tightened the pump properly after using it to bleed the system then it will let in air. Check where the bowl meets the filter with your finger to see if fuel is leaking, the bowl is not tight or over tightened if it is leaking. Do the same with the bleed screw at the bottom, if it's leaking it's probably under tight. Then check all the fuel hose connections for leaks, if you don't find a leak tighten the hose clips anyway. If you find no leaks it is probably dirt in the fuel so check the bowl for dirt. If you can't see the dirt easily bleed the bowl to relieve the vacuum and take it off to have a good look or unscrew the filter with bowl attached to see if that has dirt in it.

The reason the revs drop and come back is generally dirt rather than leaks. An injector gets a dirt speck in it the revs drop, the dirt is blown through and the revs pick up. If you find dirt in the filter you will have to have the tank cleaned. Air in the system generally cuts the engine out but with a small air leak it the engine can run anywhere from 10 minutes to an hour before cutting out. So if you find an air leak run the engine for an hour at 1,200 rpm to check you have fixd the leak.

 

[Mozambezi]

If you haven't sorted:

1. Check fuel lift pump (might worn)
2. Check throttle mechanism, where cable connects to the engine. Nut might be loose and travelling from vibration

 

[jonathanhsm]

for what it's worth here is an old video of my perkins perama m20 rev counter veering wildly (without touching the throttle) eventually after many hours of anguish replacing lift pump etc traced it to leak in the copper washer connection to primary filter ! Has been fine ever since
http://www.youtube.com/watch?v=lre3iKfwSM4&list=HL1372799114&feature=mh_lolz