Fuel Priming Setup

[Lin]

Of course, the best way to deal with priming is not to need it.  However, with Murphy always nearby enforcing his laws, it could happen.  The easiest prime method I've seen is the bug spray bottle one.  I have a bottle and have attached an old R12 freon hose with fittings to it.  So now, as I understand it, I should replace a plug on one of the filters with an R12-type shreader valve, and I will be all set.  My question is about which filter to attach it to.  Attaching it to the primary would allow me to prime the whole system from there forward, but I was once warned that there is some danger of damaging something that way (I don't remember if it was the fuel pump or something else).  Therefore, I was advised to tape into the secondary filter.  This would get the engine running and allow the fuel pump to complete the prime process.  Which is it?  Thanks

[Ncbob]

Lin, if you're talking about about your average 8V-71 the filters are on the input side of the fuel pump which is not a positive displacement pump...which means that it's not self priming.

Unless you've run your engine out of fuel (which can happen if you replace your primary and secondary filters without filling them first) the the only option I know of is to use the fuel shutoff on the input side of the primary filter and force feed the filters and the fuel gallery with a hand priming pump or a clean proven pressure system as described.

Since the fuel pump cannot prime itself...it just stands to reason that the injectors (without a solid column of available fuel) won't allow the engine to start.

And has been mentioned in these threads for eons do make sure that the Emergency Air Damper located above the Roots blower is OPEN.

Welcome to the wonderful world of troubleshooting a Detroit. Though it has its' heataches it also has tremendous rewards via the school of hard knocks.

Bob

[lostagain]

I would attach it to the primary filter. So when you pump your sprayer, or turn on the auxilary electric pump, it will fill the primary filter, then go through the pump, then fill the secondary filter, then go to the injectors.

JC

[TedsBUSted]

In the typical OE spec'd installation, the primary filter is on the pump's suction side and the secondary filter on the pump's pressure side. From the primary (suction) filter, fuel flows to the pump. From the pump's pressure side fuel then flows to the secondary (pressure) filter. From the secondary filter fuel flows through the injector supply lines and then unused fuel  returns to tank.

So... simply pressurizing the primary filter without turning the engine over (and thus fuel pump) makes for a slow bleed-through-the-pump prime. Charging the secondary filter with fuel directly feeds the injectors and would be the quickest way to get good flow through the injectors. However, when charging through the secondary the primary side should also be flooded with fuel to insure quick pick-up.

For me, usually the decision on how to prime would be based on engine temperature and the amount of engine cranking power available. If the engine  needs to start with the first few precious cranks, then the way to go is to charge through the secondary filter. If some cranking and a  few false starts can be tolerated, then it's okay to charge through the primary.

Ted

[Don4107]

It is so much simpler to plumb in an electric fuel pump and switch.  Need to change the filters?  Change them and flip the switch.  No muss no fuss.  No need to mess with diesel to fill the filters.  Nothing to carry except the extra filters.  I can't take credit for the setup.  PO installed it in our 4107.

Best use I can see for the other priming systems is to assist another busnut that does not have one.

Good luck
Don 4107

[JohnEd]

What Don said.

The most common design of in place priming system is the electric pump mounted at the tank and that pump has a by-pass check valve so the system runs with the restrictive de-energized electric pump turned off.  Clifford has a better idea and he recommended using a pump sold by NAPA that had a built in by-pass and check valve.  Using that pump would greatly simplify the install/modification.

Good luck,

John

[Ncbob]

As Richard Nixon always said Ad Infinitum, "Let me be perfectly clear about this....."

First Lin didn't tell us about which bus this engine was in, and no information about the sequence of the filters. You younger fellas might be more aware of the arrangements of filters, etc. so that may make my 40+ years with Detroit's obsolete. But in the spirit of this group I only offer my meager knowledge in the hope that it might help a fellow BusNut over a minor hump in the road to enjoying  his hobby.

Well wishes to all,

Bob

[Ncbob]

All true, m'friends. But if that miracle priming pump isn't already installed and you're on the road with a problem you're shoveling $hit against the tide aren't you?

Monday morning quarterbacking a solution for a problem without sufficient information has been one of the greatest frustrations has been a problem here for years.

It's great to sit at home in front of the keyboard and design a system to circumvent a future problem but it doesn't do much for a solution to the one at hand.

Not wishing to offend anyone,

Bob

[gus]

Bob,

My 4104 is 57 years old and my 4107 is 45 years old and they both have the  secondary filter after the fuel pump! Makes sense to protect the injectors after the pump.

I presume this is still the same setup with modern buses but I don't know that for sure.

[Lin]

Okay, I should clarify since a reread of my post left room for interpretation.  This is an MC5a with an 8v71.  I do not have a problem now.  I am talking about having a system in place should there ever be a problem.  I thought the filter layout was standard, but mine is primary filter to fuel pump to secondary filter to engine.  The inline electric pump seems like a nice idea, but for now I just wanted a quick fix before our next trip.  One flaw in my plan seems to be that I can't find an R12 service port.  I had thought it would be easily available, but it is not (around here anyway).  So, for the moment if a problem arises, I will have to pull the plug and thread the pump hose directly into the filter port.  The quick disconnect idea was more elegant.  Once the engine was running, one could disconnect the hose without a shut down.  In the later case, I would need to shut the engine down to reinstall the plug.  Oh well! 

[JohnEd]

Bob,

I, and I am sure almost all the others here, sit up and take notice when you share.  You ain't EF Hutton but then EF Hutton ain't you either.  Thanks for the help you provide.

John

[TomC]

To protect the fuel pump from having its' seals blown out (and this is directly from Don Fairchild) you should only pressure prime the engine through the secondary fuel filter.  The usual way the fuel runs is the fuel from the tank is sucked through the primary fuel filter, then to the fuel pump, then to the secondary fuel filter, then onto the engine.  Hence only pressure prime at the secondary fuel pump-after the fuel pump. To get the pump to pick up the prime again, you manually fill the  primary fuel filter and fuel lines with fuel.  This may take three or four tries, but then at least you won't blow out your fuel pump. Good Luck, TomC