Detroit 8V71 in MC-7 Bus - where's the fuel line check valve?

rdiercks · April 11, 2016, 01:15:19 PM

O-K! I give up.
It's a 1971 MCI with a DD 8V71 and 4-speed manual.
It loses prime in the fuel system, all the time now.

Before I install an in-line electric pump (maybe), can someone give me clue as to where in the fuel system I can find that "check valve" I keep reading about?
I want to "permanently" fix this issue.
I'd do the check valve replacement first and see if the "loss of prime" goes away.
If not, I'll also install an electric in-line pump.

Thanks for any clues.

Richard D

luvrbus · April 11, 2016, 01:18:00 PM

It should be on the primary filter and I am not kidding either,check for fuel wet spots on the hoses , filters and around the fuel pump it may not be the check valve causing the problem

rdiercks · April 11, 2016, 06:43:58 PM

There's not a trace of a fuel leak, anywhere.
Also, I'm still not sure about what someone means by "primary filter" and "secondary filter".
The filter on the fuel pump's input port would sound to me like the "primary" side. Is this correct?
If so, then the filter on the pump's output port (pressure side) would be the "secondary".
Have I got that right?

Unless the check valve is buried inside one of the filters, I don't see one right now.
I think I'll go over to the local Detroit service/parts place and see what they can come up with.

Richard D
Milwaukee, WI

gumpy · April 11, 2016, 07:28:23 PM

Quote from: rdiercks on April 11, 2016, 06:43:58 PM
There's not a trace of a fuel leak, anywhere.
Also, I'm still not sure about what someone means by "primary filter" and "secondary filter".
The filter on the fuel pump's input port would sound to me like the "primary" side. Is this correct?
If so, then the filter on the pump's output port (pressure side) would be the "secondary".
Have I got that right?

That's correct. follow the line from the bulkhead to the filter. The first filter is the Primary. Then it goes to the pump. Then to the Secondary. Then to the rack.

The check valve might be located at the bulkhead.

LuckyChow · April 11, 2016, 08:01:45 PM

If my memory serves me right, the MC-8 I owned many moons ago had the check valve at the bulkhead.

thomasinnv · April 12, 2016, 07:36:34 AM

Yes luckychow has it right. Follow the line from the primary filter up to the bulkhead and you should see it.

jackhanow · April 12, 2016, 09:45:51 AM

You can get a test plug from the plumbing supply store and pressurize the fuel system to 20 psi and find the leak that way. Measure the inside diameter of your fill hole. They usually have a tire valve on them. I see people put Teflon tape on the line fittings at the pump and filter ends and that I will cause a leak like that as well. They are line fittings not pipe fittings.

eagle19952 · April 12, 2016, 04:44:35 PM

20 psi....

.... good luck ....

jackhanow · April 12, 2016, 06:22:23 PM

It's a heavy tank and stronger built than the radiator and they test those at 30 psi and it's not like he's gonna use a bicycle pump. May need to plug the fill vent. It whistles when you fill at the truck stop. But it's probably sucking air at one of the line fittings. My mc5 had a dinge on the filter can and I didn't see it right away. Had to glue emery sheets to a piece of plywood and sand it off by swirling it for what seemed like forever. Oh yeah. The only way it can leak back at all is if its sucking air somewhere which means it's probably on the low pressure side, otherwise it would spray while running. It only needs to be a pin hole. If you put pressure on it and spray the fittings with extra soapy water you may see tiny bubbles form. They are usually honeycomb looking. Just like plugging the end of a straw. I use a modified cap on pickups and cars so I can use compressed air. And I pressurize it up to 20 psi or as soon as I or an assistant sees it spray or drip.