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Old 10-11-2012, 04:09 PM   #1
kdnelson
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Default Losing Prime 6V92

I have an older coach (1976) with the 6v92 detroit that has lost its prime.
The motor was running well, though may have run dry.
I pulled both fuel filters and they were all but dry. (less than 1/4 full)
I have re-primed by refilling the filters and giving it a shot of ether to get started, which it did.
As long as I kept the RPM high it ran well. When I moved the coach or let it idle the motor died.
When I rechecked the filters they were again less that 1/4 full. First it was just the second filter then also the primary filter.
I cleaned out the strainer and blew out the supply line and filled up the primary filter and manually filled the secondary.
Again, the same scenario played out.
I am able to fill the primary filter by pressurizing the fuel tank and I got a face full of diesel.
I opened the cap on the back of the secondary filter and with the fuel tank pressurized and the secondary filter full of diesel never got fuel to the opening. I was able to get lots of air coming out but no fuel.
With the tank pressurized it was still rough at an idle.

What next?
I understand that this fuel system is a closed system and there should be no air to function.
So the only options that I have left is a bad check valve, fuel injector, fuel pump or a broken line. It would not surprise me too much if theses were the factory lines. I did not see any areas of leaking in the motor compartment
Am I missing something?
Any suggestions on what to do first, or am I in over my head?
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Old 10-11-2012, 05:40 PM   #2
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It sounds like you obviously have air getting into the system somewhere and it could be coming from the check valve on the filters and may be bad. Make sure the check valve is not installed backwards. I'm assuming you have not noticed any leaks around fittings, lines or a crack in the pick up tube?
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Old 10-11-2012, 06:01 PM   #3
kdnelson
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The valve may be broken. I have ran the coach for about 20k miles without a fuel problem.
Though at this time I have not replaced the check valve.
Is this the most likely problem?
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Old 10-11-2012, 06:26 PM   #4
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I have owned two coaches with the 6v92 and on one I had a similar problem. The last guy whom had worked on it replaced or added a check valve on the primary filter, but it was installed in the wrong direction which was causing a siphon effect back to the tank.

How full is the fuel tank? My other guess could be the pick up tube.

These are a couple cheap guesses to try first. Before going on to the fuel pump.
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Old 10-11-2012, 06:33 PM   #5
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Have you put a fuel gauge on it and checked the pressure?
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Old 10-11-2012, 10:07 PM   #6
kdnelson
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I have a fuel pressure gauge at the secondary filter. at an idle it reads 0. When I rev it up the pressure does come up (50-60) but does not stay up.
I ran the motor while I pressurized the fuel tank and still was not able to maintain a prime.
I can and did repeat this process several times.
I may not be patient enough. But I felt that it should not take over 3 min. to prime at a high RPM.
I am able to see the fuel in the tank at about 3/4 full.
When I pressurize the tank, I get a full flow of fuel and the primary filter is able to fill with ease.
I am not able to fill the secondary filter this way. I tried for ten or so min. but only air came out.
Thanks for all the assistance.
I will keep trying.
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Old 10-18-2012, 09:03 PM   #7
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A quick test would be to disconnect at the check valve on the inlet of the primary filter the fuel line and then hook a temporary line into a bucket of fuel, fill the filters and see how the engine runs. If it runs fine the problem is somewhere upstream of the check valve and if it still fails the problem is downstream of the check valve. The DD shop near Washington DC did this and eventually found that the pressure relief valve in the fuel pump was stuck open.

The fuel pump is a gear pump and you will not be able to blow fuel through it, I suspect the air you are getting is coming backwards through the fuel return line to the tank when you pressurized the tank.

Since you didn't mention changing anything between the time before the problem existed and now when it does I do not think the check valve being installed backwards it at all likely. Either air is getting to the system through a small leak or more likely if even when the tank is pressurized the engine fails is that the interior of the fuel hose is collapsed somewhere or the hose has become kinked somewhere prior to the check valve since both filters empty when the engine is running.

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Old 10-18-2012, 10:08 PM   #8
Ernie Ekberg
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I had a 6v92 in my 83 bluebird. Presssurizing the fuel tank is something new to me. Do you have an air compressor you use for that? I guess I need a little education.
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Old 10-20-2012, 12:43 AM   #9
kdnelson
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1) I presssurized my tank to force the fuel up into the filters, to re-prime the filters. I use an old inner tube folded over with a small hole in it and secured the inner tube to the fueling spout with a large hose clamp and used an air compressor with an air gun poked into the hole in the inner tube to pressureize the tank.

2) Jon I will try your suggestion and repost. Is the check valve an interior part of the fuel filter housing or separate?
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Old 10-21-2012, 12:40 PM   #10
Ernie Ekberg
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I just add fuel to my filters and screw them back on, no pressurizing required
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Old 10-22-2012, 01:42 PM   #11
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The check valve on my primary filter is part of the filter housing.

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