To prime, or not to prime...

Project_Mayhem · Saturday at 6:28 AM

jaysweet said:
Last weekend, I changed lots of filters on our Yanmar 2GM20F, including the primary and secondary fuel filters. I've been struck many times in the past by how many threads I've seen about the difficulty/necessity of having to prime the fuel system. This is totally at odds with my experience with this one engine: I've never had to use the priming lever. I just make sure the filter bowls are mostly full of fuel before screwing them back on. Let me be clear: I don't think this is because I'm particularly good at it, or other people aren't. I'm just confused about why it's totally unnecessary with this 2GM20F, but seems to be a regular chore with some other engines, or maybe some other installations. You read about people spending like an hour trying to get their engine running properly after something is disconnected in the fuel system.

The only thing about our installation (vs. engine) that might be different is that our fuel tank is mostly above the level of the filter and engine. So I can shut off the fuel petcock at the tank, change the primary, fill the bowl to around 90% full and screw it back on. Then when I turn the fuel back on, opening the bleed screw at the top of the filter seems to top off the remaining 10% just via gravity. Maybe this is the opposite in a boat where the fuel tank is lower than the engine?

Tank fittings almost always take a 90 degree turn when transitioning from tank to hose. Would air float up to this fitting since it's the highest point in the system?

I've attached clear filters to help diagnose outboards. I was shocked at how much air a two stroke carb could handle. Maybe your diesel is made to account for a reasonable amount of air in the line?

I'm curious what knowledge others will chime in with

jssailem · Saturday at 7:14 AM

Count your blessings. I can’t speak about all small boat diesel engines, I have personal knowledge and experience with my 53 year old Perkins 4.107 47hp engine.

The reason it needs priming is air in the fuel lines stops the flow of fuel to the injectors. The manual lift pump pushes only a small amount of fuel on each stroke. If the cam shaft lobe is at the top of its cycle the manual pump will not function. Fuel needs to replace all of the air in the lines from the pump through to the injectors. There are several bleed points along the way from the lift pump to the high pressure pump. This second pump is a rotary injection pump. It must be completely full of fuel (no air) for it to pressurize the fuel enough to operate the injectors. Air prevents the pump from building the needed pressure.

One learns to respect the system and control the flow of fuel in the lines to prevent air intrusion. This can reduce or eliminate priming.

Don S/V ILLusion · Saturday at 7:31 AM

Old joke - “ Yanmars run on air”. They have a reputation for being more tolerant of air in the fuel lines. I‘m not sure if that is actually true but mechanics I trust say that. Regardless, bleeding a diesel fuel system isn’t difficult so there is no reason to avoid it whenever a fuel line is open. If nothing else, it saves wear and tear on the starter. Whoever told you it was “difficult” shouldn’t be advising anyone.

dlochner · Saturday at 7:33 AM

The fuel lines can form a siphon which will allow the fuel to flow into the filter if it is lower than the fuel level in the tank. That siphon can continue until the fuel system comes above the level of the fuel in the tank (or the fuel pickup starts sucking air). On my Yanmar, it is only necessary to give the lift pump a stroke or too to make certain the fuel has reached the high pressure pump.

Some Racor filters (not the 500 series) have a pump built into the filter, this makes it easier and faster to fill the filter. The pump can be used to develop the siphon .

east coaster · Sunday at 4:10 AM

When ever I have changed diesel or oil filters I always fill (prime) the fuel filters with Seafoam and the oil filter with the 15w40 rotella oil I use in the engine. Have not had any issues with engine having to be primed by any other means. Using Seafoam is always a debate among boaters, however it serves it's purpose here as it essentially eliminates other priming methods and it doesn't seem to have an adverse effect on the engine or starting it after Diesel fuel filter changes. Again as astute Ralph so often states "your boat your choice".

Barnacle Bill · Tuesday at 12:53 PM

You shouldn't have to prime it unless the engine doesn't start. The 3 usual culprits for a diesel engine not starting are 1. air in the line, 2. water in the line 3. contamination. If it starts, you should be good to go. If you have to bleed, then there is a sequence starting with the primary fuel filter attached to the engine block and then each injector. Closest one first.

dlochner · Tuesday at 2:31 PM

jaysweet said:
I guess my question was really more along the lines of "why doesn't the 2GM20F ever seem to need priming?", or "why do some other engines need it whenever something disrupts the fuel lines?", or "is it mainly having a fuel tank above the engine that makes it easier?"

I think it really depends on how all the fuel lines run between the fuel tank and the injector. If the lines are run in such a way that there is an inverted U to trap air, it needs bleeding. Yanmar engineers are smart. Ever notice that most Yanmars don't have glow plugs? That wasn't by accident.

Ralph Johnstone · Tuesday at 4:40 PM

jaysweet said:
I guess my question was really more along the lines of "why doesn't the 2GM20F ever seem to need priming?"

I think the biggy is having the fuel tank level well above the level of the filter elements. I'm assuming the person changing the filter element does conscientuously use this advantage.

One other unknown (at least to me) is that the fuel flow from #2 fuel filter has a small bypass back to the fuel tank. It actually joins up with the cooling fuel coming off the hot injectors headed back to the fuel tank.

This is a closeup of the device :

Here is the flow control device in theYanmar Book of Words.

The fuel bypass flow is small but enough to discharge any air accumulation which could cause problems.

Put 'em altogether and that's most likely why Yanmars have a minimum of problems with air.

Benny17441 · Wednesday at 8:52 AM

The 2GM20F is notorious for being difficult to bleed all the air out of the fuel system after cracking the lines open. A careful changing of filters and small leaks may only cause minor issues that will clear themselves up as the engine runs. The problem is that in some cases small air bubles that get trapped can allow the engine to run well until they move, disrupt flow and cause the engine to cough or sputter. I always prefer to spend a little time at the dock doing the work, than having to scramble on the water.

Ralph Johnstone · Wednesday at 10:31 AM

Benny17441 said:
The 2GM20F is notorious for being difficult to bleed all the air out of the fuel system after cracking the lines open.

I presume you're referring to YOUR 2GM20F is notorious for its difficulty in bleeding. Certainly not mine, not a cough in 28 years. @jaysweet sings the praises of his little engine.

Tell us, are you able to bleed your filter element and connected lines using the hydrostatic pressure from your fuel tank ?

This is what keeps everything free of air :

I don't use the electric fuel pump when bleeding everything, just the pressure from the slightly elevated tank level to push the fuel to the last bleed screw at the suction to the HP pump :

The air vent bleed screw is notched so just a fraction of a turn starts a fuel flow :

To prime, or not to prime...

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SBO Weather and Forecasting Forum Jim & John

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