10 micron filter or not?

[hans jensen]

I have a 1999 Mercury 250 HP EFI on a 23' wellcraft walkaround. The previous owner had a boat mounted fuel/water separator (23 micron) that leads to the fuel pump and then from the fuel pump to the motor mounted (under cowl) fuel water separator (28 micron).
My question is, could this (2 filters) cause too much strain on the fuel pump?
Should I replace the boat mounted 23 micron filter with a 10 micron? I can't get the motor mounted filter in a 10 micron size so if I need a 10 micron filter it will have to be mounted in the boat.

 

[Texasmark]

Re: 10 micron filter or not?

Hans. Know you from no one but here is a tad of free advice.

The smaller the filter the higher the pressure vs fuel flow. The smaller the filter mesh, the better for the engine.

So I had a big truck (18 wheeler, 800 cu in diesel engine) once upon a time and it had a normal filter which filtered normal oil flow at say 1 gallon per minute (gpm) and the mesh was around 10-15 microns. Well that was ok, but there is a lot of crud below 10 microns left in the oil keeping it black.

So I had a secondary filter installed which was in parallel with the regular filter and it only handled 10% of the regular filter's flow, but was 10x the filtration.
I forget what you call it but it filtered out the little critters in the oil. Amazing how clean diesel engine oil was when you had the secondary filter.

HTH

Mark

 

[hkeiner]

Re: 10 micron filter or not?

To give yourself some peace of mind, you can check the VACUUM level in the fuel line just before the fuel pump to see if there is any excessive restriction to fuel flow caused by the fuel filters (or any other component upstream of the fuel pump for that matter). Just temporarily add a t-connector to the fuel line so that you can attach a vacuum gauge and monitor the vacuum level at idle, while accelerating, and at WOT. Your shop manual should specify the maximum vacuum that should be in the fuel line at idle and at WOT. If the vacuum level is below the maximum spec, then the filter is not causing a problem. Further, you can check the PRESSURE in the fuel line after the fuel pump to confirm that the pressure is alway above the minimum specs at idle and WOT. If the filters subsequently get clogged and cause restricted flow, that is another matter.

I think that placing a 10 micron filter upstream of a 28 micron filter would make the 28 micron filter useless, as the 10 micron filter would filter out everything that the 28 micron filter could possibly filter.

This is based upon what I have learned when installing a new water separating filter on my boat. Others may have additional or better advice.

 

[Texasmark]

Re: 10 micron filter or not?

Like I said, the higher the filtration level, the lower the flow for a given surface area. Agree a 10 in line with a 28 will render it useless and could possibly restrict oil flow to an unacceptable level. That is why OTR trucks use them in parallel like I did.

HTH

Mark

 

[hans jensen]

Re: 10 micron filter or not?

Thanks for the advice guys! I agree that a 10 micron filter ahead of a 28 micron filter is useless, but the second filter has a "water in fuel" sensor and I don't want to eliminate the sensor.

 

[hkeiner]

Re: 10 micron filter or not?

and could possibly restrict oil flow to an unacceptable level

Sorry, I was thinking about a filter for gas. I hope I did not confuse anyone.

 

[hans jensen]

Re: 10 micron filter or not?

hkeiner you are not mistaken. My ? is about the fuel/water separator (filter). I think that Texasmark is just using the oil filter situtation as an example. Thanks for your info.

 

[Texasmark]

Re: 10 micron filter or not?

Yes I was, and the point of my comments was flow restriction as a result of high levels of filtration.

One other point on fuel filtration. When the fuel pump functions it sucks on the fuel line to obtain the fuel. If the flow is adequate it doesn't form a vacuum which will cause the fuel to boil and vaporize under the reduced pressure.

But, if you are running a high filtration filter which restricts fuel, the vacuum will occur and you will wind up with fuel starvation caused by the fuel changing from liquid to gas vapor; especially in hot weather.....call it "vapor lock" and the engine will quit.

BTDT

HTH

Mark

 

[hkeiner]

Re: 10 micron filter or not?

Yes, that is where the vacuum test on the fuel line comes in. If the vacuum level is below spec, you are OK. If it is higher than spec (due to flow restrictions caused by the filter or other reason) then you may have problems, such as a too lean carb or vapor lock (as Mark pointed out).

 

[hkeiner]

Re: 10 micron filter or not?

If you plan to do a vacuum test on the fuel line, the below linked Mercury Service Advisory may be of help to you.

http://www.marinepartsman.com/Mercury-Marine-service-bulletins/Mercury-Mariner/1999/EN_05.PDF