Air in Fuel Line???

[glenn.225]

I went out for a sail the other day and when returning to my slip the engine started missing and died. I managed to restart it to get in but the engine would only run at higher RPM. I didn't have time to look into it any further then. Yesterday I had an couple of hours at it, checked tank, all the lines and filters and tried bleeding it. Everything appears good, sight glass in primary filter shows good clear fuel, all connections tight no signs of leaks,lift pump moves lots of fuel, over a litre of clean fuel out of the return line in 20-30 seconds. But symptoms persist.

Hard to start, have to give it full throttle for it to catch, it won't run below 1500 RPM and misses at higher RPMs, will develop full RPM at no load. I tried to move boat, it will do it at higher RPM but misses and will motor at near full speed (5.5 kts for a very short while anyway that's all I tried) but stilling missing occasionally.

The motor is a Volvo 2001 9 hp single, it's 1986 model that was stored new for years I installed it last spring, still had the clean factory oil in it. All new tank, lines and filters, I put 46 hrs on it and it's been working like top till now.

Other than sucking air some where I can't think of anything else it could be. Is there another possible problem? Couldn't be bad valve spring or something else in the head.

I know air leaks aren’t always easy to find but this has me stumped. I will get another couple of hours at it today to see if I can find something, maybe try to find a way to pressurize the tank.

[Charlie]

Hi Glenn:

I hate these kind of problems. To find out if it is an air problem I have plumbed in a small piece of clear hose this will let you see if there are bubble going in to the engine. I had a problem once where the hose was cut by the hose clamp and the cut was underneath the clamp and on the bottom side of the hose (away from the eye). I found it by feel. Cut a short piece of hose off put it back together and it worked. Symptoms were similar to yours. Good Luck.

[zeehag]

check all connections. check banjo fittings. check the fuel pick up--check everything-it took me 3 days to find all the air leaks in my perkins' system, but it was worth it.

[skipmac]

I had a really aggravating air leak and was going nuts trying to track it down. What I finally did was fill a small, clean container with clean diesel, drop a clean hose in and connect it to the intake on the secondary filter.

That bypasses 80-90% of the potential air leak spots and, if the engine then runs well, confirms that the problem is an air leak. Then just work backwards from there step by step, using the small container as the supply until you find the spot where the problem reappears.

[Kettlewell]

If it's an air leak I doubt it would restart after dieing--at least diesels I've had wouldn't restart without rebleeding first. It does sound like a fuel problem of some sort. I like the idea of connecting a small can of fuel directly to the fuel filter to eliminate all the connections prior to that. You'll have to have it above the on engine fuel filter to set up a gravity feed. I wonder if it might be your single injector being partially clogged due to sitting so long?

[glenn.225]

Well it's definitely sucking air somewhere. I took off the secondary filter and it was half empty, refilled it and bled system, ran great for 20 minutes and started to miss again. I shut it down and removed filter, half empty again. Repeated process and same result. I'll have to try the clear hose method tomorrow.
Thanks all.

[Albro359]

Poor man .. I know how you feel !
I had a minor air leak that was coming from a water trap...I found it by installing an electric fuel pump (primarily as I was sick of bending my finger on the diaphragm pump to bleed it). When I turned the pump on without running the engine and checked all the fuel lines, filters, I saw diesel oozing slowly out of the clamshell joint on the water trap.
But as was said previously when I had this issue, the engine wouldn't start after it died without bleeding it.
I had exactly the symptoms you describe when I had water in the fuel.
The engine wouldn't idle...it would run at 1500 rpm, but drop it to idle and it would die.
I had to suck out 100 litres of diesel and clean out the tank. The SS tank had got condensation in it cos it wasn't full over the hot summer here...

[Lagoon4us]

I had same systems on a 4-108 years ago it turned out that in the top of the CAV filter there was a pressure return spring with a small steel ball on a seat, this was sticking and flooding the engine, it would run for a while and die for all the world looking like air in the system. Worth checking...

[bmm32005]

I had a similar problem, after replacing filters, fuel lines etc. I found a air leak in the brazed fitting on the pick up tube in the tank. Boat ran great with a full tank. I hit 3/4 full and she would sputter and die. Runs great now. This was on a Ericson with a Universal diesel that I no longer own.

[glenn.225]

Well finally had a couple of days to check out the fuel problem. I'm certain it's the mechanical fuel pump, eliminated everything else. The diagram is probably gone after sitting for 25 years. Any suggestions for a electric fuel pump? I looked at a couple on line but not sure what is available locally, I'll check if I can find a Walbro in town. I can have a couple of lines made up and will install tomorrow morning to get me back on the water. Season is getting short now 6 weeks till lift out.

[Vasco]

Very easy to check the lift pump, just pump. Probably the crush washers in the copper fuel lines, replace them. It's a Volvo 2001 isn't it?

[glenn.225]

Rick

I did, put on a new fuel line to inlet side of pump with a differernt type of fitting, the old line had a lot of age cracking. Then ran engine with new line and rtn line submerged in a gallon can of fuel and it behaved the same way. I would have eliminated the pump and gone to directly to secondary filter if I could have rigged a gravity feed system. The pump is not leaking fuel out but has to be sucking air in, there is no where else for it to come from.

Unless it is on the pressure side of the pump.