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Old 05-14-2022, 12:05 PM   #1
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Pre fill or not to Pre fill?

2002 vintage ISC 8.3 fuel water separator filter especially. It says not to prefill to just use the ignition on switch for 30 seconds to cycle fuel through and push out any air bubbles, repeat if necessary. Iíve read some old posts dating back many years and it seems that that method has not been very successful in the past? My first fuel filter change out I donít wanna suffer through this, Iím really considering on pre-filling carefully and still prime with the ignition on key. I know the proper prefill method seems to be a little bit of a pain if worried about getting unfiltered fuel down inside the filter. Whatís been some of your experiences with the ISC fuel filters?
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Old 05-14-2022, 12:07 PM   #2
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Yes, with your engine's electric priming pump no problem at all in installing filters dry and using the key/electric priming pump to prime.


Easier to verify with two people. The pump runs about 30 seconds when the ignition key is turned. With two people, have one stand by the back passenger's side of the coach (behind rear wheels). Have your helper turn the key from off to first position (not to where the starter engages). You, in the back with hear the pump start at high RPM (pumping air). After the pump cuts off, have your helper turn the ignition off and back to that first position. Repeat until you hear the pump RPM go from fast to slow (slow= pumping fuel).


Start the engine. That is exactly how I did it with our 2003 Cummins ISL with same fuel system.


For those who FEEL they have to pre-fill, no great harm in pre-filling the primary filter, as that fuel will still be filtered by the secondary fuel filter.


But, if you have only one fuel filter or choose to pre-fill both filters, indeed the fuel will not be filtered.


Yes, I know there are some "work arounds" such as only pouring fuel into the inlets (not center hole), but have never understood with this particular engine why anyone would both to do that.


And, yes, the older mechanical injection diesels ARE different. But following what needed to be done on a 1996 engine is really not relevant to your engine.
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Old 05-14-2022, 12:52 PM   #3
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My engine manual says to install filters dry and cycle the key.


I did this for ~6 years without out a problem and then I was changing filters one time and it would not prime. Tried everything and finally resorted to putting compressed air on the tank and finally got it.

After that I prefilled but I did buy a plastic plug to put in the center hole.


Now I don't have an issue as I have a FASS system installed and it will push the fuel from the front to the back. Have a pressure filter on the secondary filter so I know I've got fuel.
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Old 05-14-2022, 12:57 PM   #4
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My engine manual says to install filters dry and cycle the key.


I did this for ~6 years without out a problem and then I was changing filters one time and it would not prime. Tried everything and finally resorted to putting compressed air on the tank and finally got it.

Absolutely, if the level/elevation of the filter head is above the level of the fuel in the fuel tank, when the filter is removed, fuel will run "downhill" to the tank.


That leaves you with 25'+ of AIR in the fuel line to purge.



Of course if the level of fuel in the tank is higher than the filter head, when you remove the filter, fuel will run out until the levels equalize.


Neither answer is very good!



The answer is to install a BALL VALVE on the inlet side of the primary/only fuel filter. Close it before removing the filter. Also makes a heck of an anti-theft device.
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Old 05-15-2022, 06:25 AM   #5
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If you look at a new Cummins filter there are instructions on the filter explaining how to fill the filter.5
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Old 05-15-2022, 06:29 AM   #6
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I should have added that I usually defer to Wolfe. He is a true Diesel expert. But in this case I disagree
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Old 05-15-2022, 06:38 AM   #7
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On my 2002 ISC I have always used the key to prime...owned it 19 years now. Once I had to repeat the procedure after it died very soon after the initial start.
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Old 05-15-2022, 06:44 AM   #8
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I should have added that I usually defer to Wolfe. He is a true Diesel expert. But in this case I disagree

I understand.


But much of the "I have to pre-fill when changing fuel filters" comes from those with many years of experience with older engines that DID require it and is not applicable to the OP's engine.


Same discussion has surfaced many many times on Caterpillar engines with the manual primer pump. Caterpillar (and my first hand experience) is to install both primary and secondary filters dry and use the manual primer pump to prime. In 170,000 miles with that engine, never an issue-- engine started immediately after using the manual primer pump.


Again, little risk in pre-filling the PRIMARY fuel filter. But pre-filling the only or secondary is what the engine manufacturers are cautioning against.
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Old 05-15-2022, 09:38 PM   #9
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Absolutely, if the level/elevation of the filter head is above the level of the fuel in the fuel tank, when the filter is removed, fuel will run "downhill" to the tank.


That leaves you with 25'+ of AIR in the fuel line to purge.



Of course if the level of fuel in the tank is higher than the filter head, when you remove the filter, fuel will run out until the levels equalize.


Neither answer is very good!



The answer is to install a BALL VALVE on the inlet side of the primary/only fuel filter. Close it before removing the filter. Also makes a heck of an anti-theft device.
Have to agree, my CC came with a valve before the primary filter. Never prefilled any of my fuel filters.

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Old 05-16-2022, 05:27 AM   #10
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I'm a "follow the manual" kind of guy. For our 2005 ISL9 (in a 2006 coach), Cummins says to prefill the first and use the electric lift pump to prime the second. The empty second filter takes 8-9 30-second cycles to prime before the engine starts.
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Old 05-16-2022, 06:03 AM   #11
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It does say not to pre fill the the separator filter but many have had issues it seems. I donít want to pre fill any of them so I can have confidence if ever needing to change them on the rd. I can start the motor from the rear Eng compartment so that makes it easy to do myself. I can turn the key on and go to the back and listen for the pump to fill it. I can do it as many times as needed. I asked on the start of this thread to get opinions and feed back of others experiences especially with the ISC. All the suggestions have been helpful.
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Old 05-16-2022, 07:37 AM   #12
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One thing to keep track of is the lift pump, it is prone to leak and if it does could cause lots of problems with the injector pump. If it leaks while running it will suck air when it shuts off, this is what damages the CAPS pump.

I knew of this problem and kept an eye on mine. In 2020 I did some work on the rig, no leak. Took it for a good drive and parked it and was going to clean the jack stems. Crawled under the rig and found the leak. This it what drove me to install the FASS system. Turn the key and I see ~16psi at the injection pump.
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Old 05-16-2022, 07:44 AM   #13
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Pre fill or not to Pre fill?

So cycling the pump to prime the filters isnít any more difficult on the pump than when it is running is it?
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Old 05-16-2022, 07:53 AM   #14
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So cycling the pump to prime the filters aren’t any more difficult on the pump than when it is running is it?

Nope, that's what it is designed for. It shuts off after ~30-60 seconds. The pump is controlled by the ECM so to get it to cycle you have to turn the key off an on.

One thing you could do is install a pressure gauge on the top of the secondary filter. It would tell you when you are getting pressure while you cycle your key BUT (and this is a big but) it will show negative (0) pressure while the engine is running because the CAPS pump is pulling fuel all the way from the tank. IMHO this is a problem.


With the FASS system it provided (+) pressure the whole time. Even at hard pull my pressure gauge shows ~13psi. As an added feature it continually filters the fuel. My pump is a 100 gph pump, the engine uses 10 gph max, so 90% of the fuel gets continually filtered. A big plus.
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