Jump to content

Air in diesel lines

Recommended Posts

Over the Christmas period, I've been having a few problems with our Volvo-Penta 2003T (43hp turbo) with air in the fuel lines. It's a bit of a long story and I'd thought I'd got it fixed twice, but no. I think my question here is am I missing something? Is there some other reason for this problem?


Anyway here's the timeline: 


May 2017 November 2017: No issues with engine including during engine test/check during pre-purchase checks

November 2017: Serviced - filters and oil changed.

December 2017: Used for two weekends away no issues (including one long motor)

January 6th 2018: Motored to Kawau (no wind). For 4 hours no issue but in last half an hour slight hunting on revs. Throttled back and all good into Kawau.

January 7th 2018: Motor to Gt Barrier from Kawau (still no wind). Again, all good until last hour or so with same issue, slight hunting on revs. Throttled back and then all good.

January 8th 2018: Motoring around FitzRoy, all good.

January 9th 2018: Motored to Kawau from Gt Barrier (still no bloody wind!) all good, didnt miss a beat.

January 10th 2018: Motored out of Kawau, engine behaving erratically, hunting revs all over the place and stalling at idle. Diverted to Gulf Harbour to get sorted. Engineers diagnosed sucking air in fuel (no surprises there) due to deteriorating fuel line from primary filter to lift pump. Suspect fuel line and copper washers on banjo connectors replaced.

January 13th 2018: Returned boat to Westpark from Gulf Harbour. Sailed at last! Motor out of Gulf Harbour and motor into Westpark up channel no problems yea, shes fixed.

January 19th 2018: PCCs R&R Friday night race. All good until returning to Westpark when same old story, hunting revs etc. Stopped and bled air from fuel line (lots of manual pumps before done). Restarted and motored up channel to marina just fine.

January 21st 2018: Scratching head and wondered maybe the primary filter? So did the following:

·         Bled air from fuel line

·         Ran engine under load (pulling the marina hoping my lines were strong enough) for 10mins

·         Stopped and re-bled lines about 20 pumps.

·         Replaced primary filter with spare and also the 5 o-rings in the assembly

·         Re-ran engine under load for 10 mins

·         Only 2 pumps required to squirt out diesel from bled screw. So maybe fixed?

January 26th 2018: Headed to Mahurangi from Westpark (no bloody wind again!). For 4 hours+ ran sweet then would you believe it the same again. Just about to enter Mahurangi and hunting revs again! Bled lines of air and motored into Mahurangi just sweet.  

January 27th 2018: Motored from Sullivans Bay to Scotts landing. Rebled lines afterwards needing about 20 pumps.

January 28th 2018: Sailed Home, great breeze! Bled air from system just before motoring up channel to marina about 10 pumps.


Ok, now the questions:

1)      In a normal engine, how many manual pumps of the lift pump would you expect to do before diesel squirted out of the breed screw on the secondary filter? One, two, ten, what is normal?

2)      The air must be coming in somewhere between the tank and the lift pump right?

3)      Why would it work just fine for hours at a time then be a problem? Seems odd to me.


I guess my next option is to replace the line from the tank to the primary filter and also the shut-off valve on the tank. Once Ive done that then the whole line from tank to primary filter to lift pump to the secondary filter will have been changed.


Am I missing something? Is there any other reason air could be getting in?


Any ideas? 

Link to post
Share on other sites

If you track from injectors to pump and pump to filters/tank is there any wetness or smell of diesel? When was the pump last serviced? is tank above pump height or below? if below I would suspect pump.


Thanks. No obvious signs of diesel (other than when a squirt it everywhere when bleeding). No idea when the pump was last serviced. Pump around about the same level as the top of the tank so diesel is definitely having to be pumped uphill. Is the lift pump itself a possible source of the air? Is there any simple way I can test the pump itself? 

Link to post
Share on other sites

Nah we don't need no Nigel Calder. Crew is full of very capable people ;-)

Dtwo has a great possible. Definitely check the breather.

Yes the pump can cause the issue too. I assume the lift pump is the engine driven diaphragm type?? Two things to check here. There is a bolt in the middle of the cover. Remove that and then right behind that, there should be very fine gauze filter. This can block. Then check the diaphragm itself. You will need to unbolt the pump body to do that though. They crack/perish around the outer edge, and under the small metal plate in the middle. Very hard to see at first glance. You have to take the diaphragm out and then pull the rubber and any pin holes will open up. If it looks even slightly suspect, then replace it. The diaphragms are cheap.

Moving along.....Even though most all fuel systems work by suction to pull the Fuel through the line, through the filter, from the tank, it actually is not the best way to get Fuel to the engine. Everything tends to work best when under pressure rather than vacuum. Great work on trying the Orings on the filter by the way.
If there is a restriction to the fuel entering the line at the Tank for some reason, then enough suction could then suck in air through something that would normally be sealed. Some Filter Housings are designed for fuel to be pushed through them rather than sucked and some are designed for suction rather than pressure and some are OK either way. So if a blockage in the air breather, or the fuel intake at the tank connection, then enough vacuum could be created to eventually start sucking in air at some joint that would normally be just fine. And most certainly the Fuel line. Is the fuel line rubber? Check all Hose clamps are tight. The rubber hoses can shrink and then the clamp does not hold the hose tight on the fitting anymore. Another possible is the Shut off Valve at the tank. They can wear around the shaft and then under vacuum, let air seep in.
       I imagine that everything runs fine till such time a the Fuel gets to a certain level in the tank after the 4 hrs of running and there is then a little extra work and the fault then causes enough suck to then let the air in somewhere. While frustrating, the fuel system is fairly simple and certainly logical. So just start at the beginning and work along the line.


Link to post
Share on other sites

Thanks for the replies. This group has some great knowledge out there and I've got some more avenues to investigate now :-)


I had an hour to spare this morning so went to the boat to check the breather on the tank. I managed to get the breather pipe off the tank easily enough and gave it a blow and suck. It did seem a little restricted at first but after a couple of puffs it did take air in and out pretty easily. I may have imagined it but it's possible I did dislodge something partially blocking the pipe. It would take more time and effort than I had to spare this morning to investigate further as the breather pipe run to the through-hull is incredibly awkward to get to. Just to access the inside of the hull fitting I'd have to remove the autopilot ram and the large bracket it's mounted on just to be able to get far enough inside the lazarette to reach it - but what did I expect, it's a boat.


***warning - breathing in and out through a breather tube can give you one hell of a diesel headache***


Anyway, over the weekend I'll go down and do some more investigations and tests. If I have cleared the breather I'll see if that makes a difference. I'll report next week. 

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...