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Primary Fuel Filter


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Just a quick question to put my mind at ease... My Primary fuel filter housing that is mounted on the boat, opposite the engine gets hot. (Input to filter has a rubber hose which is connected to a copper pipe to the fuel tank & output has two rubber hoses, one goes to secondry fuel filter mounted to the engine and the other goes somewhere on the side of the block, i guess to the pump. Both these hoses were also hot) Is this due to the hot diesel flowing back through on the return pipe and back through, or is it just heat transmitted back from the engine. I can't see what else would cause it to be hot, but thought i had better ask.

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The unused fuel should flow back directly to the tank and not the filter and none of it should be actually “hotâ€, though sometimes after a long run it may get warm. What engine do you have and is there any chance of a photo (phone one will do) showing the return pipe where it exits the motor and enters the filter. This is pure speculation based on the brief description you have given, but if the unused diesel is being returned to the primary instead of the tank it may well get hot as it is not being diluted by the contents of the tank and after running through the injector pump is obviously picking up engine heat.

Edited to add that I noticed from your previous posts that it is a 120 Mercruiser, is that the 1.7 Isuzu block?

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I've thought about this a bit more. Interesting to note that if it is the 1.7 Izuzu based Cummins Merc that the designed temperature of the fuel to the tank is approximated at 52°C, so if that is going directly back to the filter instead of being cooled by being returned to the tank it may well be getting a little warm to say the least. Possibly if repowered from an original petrol engine the installer just couldn’t be arsed to fit a proper return to the tank as the petrol tank design would not call for one unless fuel injected.

I recall reading some time ago about big engined boats on long passages at high speed. They noticed degradation in performance when the tanks had a lot less fuel in them and it was strongly suspected that the consequential increase in the fuel temperature was the culprit causing the reduction in power. Modern electronically controlled HP common rail engines suffer less as the fuel air ratio is adjusted to compensate. As in all things marine, they differ from road installations, for instance I doubt this would manifest it'self in a car, van or truck due to the cooling effect of the airflow not present in the enclosed box of an engine room.

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Hi Antares, Thank you for your reply. I did take some photos of the fuel filter & hoses etc and will add them as soon as i have resized them. Yes, the engine is the 1.7 Isuzu with mechanical injection. I am fairley confident that there is only one fuel pipe to/from the tank, so it looks like someody may have set up the return to come back to the filter housing, as i reckon it has the original fuel tank from when she had the OMC Cobra.

I will see if i can upload some piccies for you very shortly.

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I have some pics for you. Don't know if the engine shots are of any help, but one of the primary filter shots, shows the return pipe coming back into the filter.

By the way, thanks for the link to the service manual, but i have a Cd with that already on it.

post-810-136713703455_thumb.jpg

post-810-136713703703_thumb.jpg

post-810-136713703705_thumb.jpg

post-810-136713703708_thumb.jpg

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It looks exactly as you described, I think I would be looking to route the return back all the way to the top of the tank if it was mine, if the fuel does get to a temperature high enough it may start to vapourise and cause problems. At the least it will not give full power when you may want it.

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Thank you for your help regarding that.

Looks like it was worth asking the question with this one, but now i need to think about how to connect this hose to the fuel tank.........

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now i need to think about how to connect this hose to the fuel tank.........

Yes, the fact that it was not done during the repower would point to it not being simple, there may be an inspection cover that you can remove and put a nipple on then replace it. Good luck with it.

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A little while back, when i was sorting out the wiring, i had many access panels removed. One of them was hiding the massive fuel tank which had some sort of grey plastic cover over it and the only part that didn't, was where the fuel sender was. This might be the only area that i could connect it to. Oh well, will add this one to the list........Lol, :lol:

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  • 4 weeks later...

As I have had some involvement with this one, in that I made up the unions and adaptors for allowing the fuel return line to be fed back into the fuel tank, I have been doing some reading on the do's and don’ts, with regards this fuel return line.

I have come across much contradictory advice, and am now left wondering what is best, and what isn't. :?

1. Majority opinion is that the fuel return line should go back to the tank. However, one supposedly "experienced" diesel engineer says that it better to return it to the filter, via a fuel cooler. This he says, is a better method in terms minimising air getting into the fuel line system, and keeping the fuel clean, as it gets filtered on its way from the tank, goes through the pump, and then back though the filter before being returned to the pump.

2. Assuming that the return line goes back to the tank, then tank and line, must be higher than the pump, so that fuel does not drain from the pump to the tank, thus allowing air into the system when the engine is not running.

3. The end of the fuel return line should be well immersed in the fuel at the tank. One proponent says that if the pipe is not immersed, then return fuel will form bubbles on top of the fuel in the tank. Now I have a problem with the 'laws of physics' on this one, as in my experience (especially as a ex scuba-diver) that air bubbles will always go up to the surface, and not downwards. Thus bubbles on the surface of the fuel, will not migrate to the bottom of the tank, where the fuel is drawn off.

4. Another “expert†stats that the fuel return line should be immersed, to prevent air creeping back into the system, if the engine isn’t run for some time. This does seem to have some logic, but then my knowledge of diesel engines is minimal.

5. Another view that contradicts the above is that the return line should have no restriction to the returned fuel. Now, my question here is, does the fact that the return line is immersed in say, a 24†depth of fuel, does this present a restriction, compared an open ended pipe?

6. Finally, given that with diesel engine fuel systems, fuel cleanliness is paramount, does that fact that the exit of fuel return line may be close to the bottom of the fuel tank, pose a fuel cleanliness issue.

I say this because, for obvious reasons, the fuel outlet will not be flush with the bottom of the tank, to minimise the risk of sediment entering the fuel line. If a stream of returned fuel exits the return line, close to the bottom of the tank, this could potentially stir up any sediment lying on the bottom of the tank.

Some engines apparently return fuel at a rate of Litres per minute, whereas with others, the amount is described as a ‘leakage’ quantity.

So, what is right, and what isn’t, as there seems to be contradiction at every turn?

Dave

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Some engines apparently return fuel at a rate of Litres per minute, whereas with others, the amount is described as a ‘leakage’ quantity.

This is the difference between common-rail and older mechanically-injected engines. Common-rails operate like petrol fuel-injection with a pressurised manifold feeding fuel to electronically-controlled atomisers. A relief valve returns excess fuel to the tank. With mechanical injection there is just a small "leakback" from the atomisers. This is designed to provide lubrication to the moving parts of the atomisers.

I know that some common-rail cars are fitted with a fuel cooler in the return line.

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That's interesting info Mike, thanks.

Here, the engine concerned is a Mecruiser 1.7 turbo diesel, which I understand is basically an Isuzu 'lump'.

According to the workshop manual, which we now have, the fuel flow rate is 1.47L per minute, which from memory, I think was at 300rpm. Now if this is the case, then if the return pipe comes close to the bottom of the tank, and is directed downwards (which it is), then it will almost certainly to stir up any sediment laying on the bottom of the tank, which will then get sucked into the fuel system.

I suspect that this is what has happened here, as prior to making the mod to re-route the fuel return pipe, from going back to the filter, and take it to the tank, the engine was running fine.

Since the mod, the engine is now unable to reach anywhere near 3000rpm, and starts hoding back under load.

We are going to bend the bottom of the return pipe, so that the stream of returned fuel is directed across the fuel tank, and not at the bottom. Replace the fuel filter element, as we suspect this might now be restricting the flow, due to sediment, and finallly, install a water seperator in the fuel line, having been advised that this is really a 'must' with diesel engines.

Hopefully, when the work is down, the engine will again perform as it did.

Dave

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