tpitt

Helping on a friends 99 Dodge. Sets overnight and is real hard to start. I've pulled the check valve and can blow air through it with my mouth. The air comes out the little hole. Is this normal?

Thanks, Terry

johnh

only in one direction

tpitt

Thank you. I can suck air through it also. I'll order another.

KATOOM

Dont order a new one. This is completely normal as the check valve is not a "check valve" but an overflow valve that regulates a set amount of pressure in the VP while running. Meaning that the valve keeps 14-16 psi in the VP as long as the fuel pump is capable of maintaining that pressure or more. If the VP receives any more than 14-16 psi then the overflow valve allows the excess to escape, all while maintaining the 14-16 psi in the VP.

The reason you can blow though it is because if you look, there's a very small hole in the valve. That hole is not restricted by any pressure and will allow full flow, but because of the small size, only minimal volume will ever pass through it. No one really knows if this hole is to allow fuel to pass all the time in order to allow "some" cooling during weak fuel pressures, or if the hole is to allow the VP to bleed down upon shutdown, kinda like a refrigeration unit, so its not under the same internal pressure during engine starting.

No matter because if the overflow valve is sticking open then it would cause a hard start issue all the time and if it was stuck closed then fuel temps would get to high and trigger the VP to reduce power.

What you're describing about your friends truck is a fuel leak. You need to look at all inlet and return line hose fittings for any leaks. The banjo on the back of the head and the T-fitting just below it are common spots for leaks. And cracked or corroded metal fuel lines will cause a leak. If parking with the trucks nose pointing down hill stops the hard start then he clearly has a leak.

tpitt

It has a leak. I've changed the sealing washers on the back of the head. Didn't change the o rings on the tee. Just tightened them. Still cant find the leak. Could it be in the tank itself?

Thanks, Terry

KATOOM

If you cant find leaks in the locations I a;ready suggested then you can wash all the fittings off with warm/hot water and let them sit to thoroughly dry. Then dust them with baby powder and start the truck. Diesel creeps like oil and can be hard to detect.

NM outlaw

Thanks for such a well presented answer on the overflow valve, from another frind

KATOOM

You're welcome.

NM outlaw

Katoom ,whats the reason if someone wanted more cooling flow and had a lift pump with enough flow to keep up, why don't they open up that pilot hole a little to let more fuel cerculate? Don't get me wrong I would not do such a thing. But wouldn't that do what everybody wants, get more fuel flowing thrught V/P pump? Let me go alittle farther, when this 70% fuel returned to the tank is talked about, dont you think thats represented by the extra fuel from the pop injection system plus the metererd fuel returning through the overflow valve. If you want more fuel flowing then open up the metering orfice dont work the pressure releif valve to death with just more pressure. Im I making any sense, dont you think that is a flow metering orfice?

KATOOM

Well that scenario would cause more than one problem. One being that opening up the hole would create less pressure within the VP while running, unless you have a fuel pump which could increase the fuel volume enough to offset the loss by allowing all that more flow. What would that amount of volume be? And how would you determine what the pressure was on the pressure side of the overflow valve? Remember that increasing flow to increase volume will reduce pressure unless there's more volume to retain the pressure. Example would be changing the nozzle tip on your garden hose. I think messing with this would be something I wouldn't want to do since you may be affecting the VP's ability to retain fuel for optimal fuel timing. Kinda like how a forced induction engine works more efficiently than a naturally aspirated engine since there is always pressure against the intake valve on a forced induction engine.

The overflow valve currently has the ability to flow enough volume over and above the 14-16 psi.....unless we all learn something new about that valve in the future.

And second, I understand that the overflow valve bolt is hardened and would be far harder to drill into than you'd think.

NM outlaw

Sorry man I went and added alittle to my last post. Im'e just asking if anybody can cosign my thinking Im'e not saying Im'e right.

KATOOM

No problem NM outlaw. Just glad I can help at all.

rickf

What Outlaw is saying has merit and is measurable. It is fluid dynamics. I do have to say that it has been way too long ago for me to remember the math involved but it pertains to the viscosity of the fluid, size of the hole and pressure applied. With these known factors and the proper math you can easily figure the flow rate and resulting residual pressure, ie; the pressure entering the pump. I will dig through some of my books and see if I can come up with the formula we need. Meanwhile someone else may already know it and step forward. Drilling a hardened bolt is not a big deal, you just need a carbide drill. Now getting one that small may be pricey! I am thinking take the temper out of it, drill it and retemper it. Here is an idea for you. Drill it oversize, say 1/8" and then braze it closed and then you can drill it to the original size and work your way up bit by bit.

Rick