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Author Topic: Losing prime - need recommendations (MCI9)  (Read 1794 times)
tnichol
MC-9 1979 - A work in progress
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« on: July 25, 2011, 07:45:42 AM »


Greetings folks -

I've got a 1979 MCI with "rebuilt" 6v92 Detroit Diesel engine.   It has recently started to lose prime, and become impossible to start. What happens is that sometime after I park the fuel system loses prime.  I can tell it happens because if I drop the 2 fuel filters that they are only about a 1/4 full of fuel. 

 I learned that if I take both of the fuel filters off, fill them to the top then replace them that I can squirt some starting fluid in and it will start right back up.  The problem is happening more and more often now - almost every time I park and shut off the bus. 

I had a guy tell me that I need to install a auxiliary fuel pump which will help me to "force" the fuel back and keep the system primed?  I'm theoritically OK with doing that, but I'd like to hear from some of y'all that is the right answer.  I'd also LOVE some recommendations on how to do it - from someone who knows.  Do you put the new pump inline? Meaning that ALL fuel that runs through that line goes through the pump? Or do you somehow put it off to the side and use it only to prime the system, then again somehow shut it off?   Is there some other problem that is causing this now to happen?  I find it difficult to understand how an engine can run perfectly well for 30 years, then suddenly need a new fuel pump to be ADDED to the system?

Any help/recommendations are welcome.

Tim
(Leander Texas)
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thomasinnv
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« Reply #1 on: July 25, 2011, 07:56:25 AM »


Greetings folks -

I've got a 1979 MCI with "rebuilt" 6v92 Detroit Diesel engine.   It has recently started to lose prime, and become impossible to start. What happens is that sometime after I park the fuel system loses prime.  I can tell it happens because if I drop the 2 fuel filters that they are only about a 1/4 full of fuel.

lets have a little more clarification on this one. Are you saying that when you shut it down and pull the filters they are only 1/4 full? or are you saying that they are full when you shut it off but after time they leak down to 1/4 full? or do you try starting it to no avail and then pull the filters to find them at 1/4 full? these could all mean different things.

Quote
I learned that if I take both of the fuel filters off, fill them to the top then replace them that I can squirt some starting fluid in and it will start right back up.  The problem is happening more and more often now - almost every time I park and shut off the bus.

you wanna be really careful with the ether. A little too much one time can be catastrophic, then losing prime will be the least of your worries.

Quote
I had a guy tell me that I need to install a auxiliary fuel pump which will help me to "force" the fuel back and keep the system primed?  I'm theoritically OK with doing that, but I'd like to hear from some of y'all that is the right answer.  I'd also LOVE some recommendations on how to do it - from someone who knows.  Do you put the new pump inline? Meaning that ALL fuel that runs through that line goes through the pump? Or do you somehow put it off to the side and use it only to prime the system, then again somehow shut it off?   Is there some other problem that is causing this now to happen?  I find it difficult to understand how an engine can run perfectly well for 30 years, then suddenly need a new fuel pump to be ADDED to the system?

yes this does seem to be an approved method, and you can do it either way. what ever tickles your fancy. However, the best solution would be to find the problem and fix it. Do you have a check valve installed? how much fuel is in the tank when you park it? have you tried parking it with a full tank and see if you get different results? Somewhere you have a leak and air is infiltrating into the system. you need to find out where.
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1977 MCI Crusader MC-8
8V71N/740
95% converted (they're never really done, are they?)
luvrbus
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« Reply #2 on: July 25, 2011, 08:13:09 AM »

If you losing fuel from the filters the check valve is bad on the filter or installed in the wrong direction

good luck
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tnichol
MC-9 1979 - A work in progress
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« Reply #3 on: July 27, 2011, 04:30:27 PM »

Thanks for the questions/commments - I'll try to be a bit more clear:
 
The way I found this problem the FIRST time it happened was that I pulled into a camp site parked, shut the bus off - the next morning I started the bus, it started up, ran for a few minutes then stalled out.  It never behaved that way before - always started and continued to run without a problem.  I was unable to start it at all, ran the battery down, etc.

I had an older gentleman (diesel mechanic with experience with Detroit Diesels) come out and help me get started.  He showed me how to pull down the filters and check - he predicted that I'd lost prime and that they would be almost empty - they were.  We filled them back up HE shot a bit of ether into the port that seems to be designed for this and it started and ran like a champ for the next few weeks/months.

The very same symptom happened again a few months later - started the bus, ran for a few minutes - stalls out, never to restart again.  I drop the filters - sure enough they are almost empty, etc.

Back then this was during the winter months - I thought it was related to temp outside.  It was so rare that I did not worry about it. 

Now - it seems to happen every time I run the bus.  meaning that every time I start it I need to prime the filters.  The bus is showing about a 1/4 tank - perhaps a bit less.  I will try to fill it up and try again to see if the level of fuel has anything to do with it.  Its been on about 1/4 or less for a few months and so I can't really say if that is part of the problem or not.  I'm in the process now of getting it ready for football season and much more road time - which is what started this line of questioning.   I would like to solve the problem the right way, but also need to be careful these days with my costs - so that I can afford more diesel fuel! 

Thanks for the tip about the ether being dangerous - I'll be sure to keep that in mind.

Please let me know if my clarifications help to narrow my problem down.  Next time I start it up I will add more fuel and report back for sure. I'm QUITE certain though that I've had the tank lower than 1/4 before and NOT seen this issue - but that would have been the previous summer/fall.

Sincerely,

Tim
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eddiepotts
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« Reply #4 on: July 27, 2011, 04:50:15 PM »

you are leaking air someware. It may be a crack in your pick up tube. Try filling it up and see if it still does it. Your going to use the fuel anyway. When you get back from the station look over your fittings and look for fuel. If you don't see any and it does not loose prime head into the tank. That will be your cheapest rout if your not getting fuel all the way to the ground.
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gus
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« Reply #5 on: July 27, 2011, 08:56:13 PM »

I agree with the check valve theories.

If your supply line check valve is leaking the primary fuel filter will empty because fuel is being siphoned back to the tank. I'm not sure that will happen to the secondary because it is after the fuel pump, but maybe it does?

When my check valve failed (At the primary filter inlet) I think it only emptied the primary filter but that was years ago and I'm not certain.

Please let us know what you find out.
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PD4104-1274
Ash Flat, AR
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