Fuel check valve

[2fast]

[SRR2]

A nitpicky point for my edification if you don't mind: my experience with vapor lock was on cars built in the 50s and 60s with mechanical fuel pumps mounted on the engine that relied on air pressure in the tank to suck the fuel all the way to the pump. I had one problematic car in particular (I think it was a 62 Lincoln) that refused to run once run and parked on hot summer days. The fuel line was routed over one of the resonators and presumably the fuel just evaporated there and that was that, the car was immobilized. It sometimes took a couple of hours, and once in a while a jump start due to all the cranking, to get it going. I finally got fed up with that BS and put in a primitive electric diaphragm pump to replace the mechanical pump and the problem was essentially solved, though it was pretty funny listening to that diaphragm pump buzzing like pulling vapor out of the line until the evaporating gas cooled the line enough to allow liquid to pass.

OK, so that bit of history said, how does a fuel system that uses a pusher pump in the tank ever get vapor locked? Doesn't seem possible to me. Check valve or not, once the pump runs and pressurizes the lines and rails to operating pressure, any vapor would immediately recondense, and 'locking' in the classic sense just can't happen.

[Bill R.]

into the top of the fuel pump... thats not the in line repair that they came up with.. In order to install this one you have to pull the pump out since its the original check valve.

I think the p/n is no longer up to date, latest one should be 16141179282.

Copy that. It might be and it is not the first time that local parts store will look at you funny when you ask for it.

[tim]

That 62 Lincoln had a one way system. All the pumped fuel was blown into the carbs, and any excess was held in the bowl. If the carbs flooded when warm, and the fuel line wasn't totally airtight, the fuel would back down into the pump and created a bubble in the line in front of the carb.. A check valve would fix that too.

On the BMW we have the recirculating type, where the pump goes to the rail and back again, so the whole line is pressurized. When you shut off the car and it's warm or you have leaky injectors, the fuel travels in both directions back to the tank, leaving a bubble of air on the fuel rail. As long as you can keep the gas up to the rail on the supply side of the system, it's fine. The return line is only a factor if it leaks.

So to answer your question, it's not vapor lock, and unlike the lincoln, if you keep cranking it will eventually start as long as the pump can get the system up to pressure.

[SRR2]

Tim, I understand the concept of vapor lock and know exactly how the fuel system in the Lincoln and the BMWs works. That wasn't the question, and for that matter, your diagnosis is wrong. Vaporization leading to "lock" occurs in areas of low pressure. Assuming that the fuel feed to the Lincoln's mechanical pump was normal, any bubble occuring between the fuel pump and the carb would simply have been pushed into the carb and vented into the intake the moment the float called for fuel. The vapor lock in the Lincoln was caused by 1) heating of the fuel line over the resonator, and 2) use of a suction type of fuel pump. Bad design combination.

The question, if you reread my post, was how come Bill R. calls the problem "vapor lock". The entire fuel system is pressurized, therefore I see no way for vapor to form in any part of the system that would 'lock' anything longer than a second or two that it takes for the pump to build full pressure. And in that case, I would hardly call it a lock. The Lincoln, when this problem occured, was "locked" in the sense that the engine was incapable of running. Now that's a LOCK, in my opinion.

As to "air on the fuel rail" Don't think so. There's no way for air to enter a properly working fuel system short of running out of gas. At most you'd have some fuel vapor, and that would recondense and/or recirculate back to the tank as soon as normal pressure is restored.

[tim]

Sorry, man. didn't mean to piss you off.

[DanDombrowski]

Im not sure if I'm pointing out the obvious here, but you can get vapor in the lines with the BMW system because the entire fuel system ISNT pressurized (at least when the fuel can drain back to the tank when the original check valve fails). Thats the whole point of the check valve, to keep it pressurized. I'm sorry if I'm misreading your question.

Now, would fuel vapor in the line lock it per say? I don't think so. Probably just people using the same terminology now as when they started using it.

[SRR2]

Right. We're in violent agreement. I wouldn't be a bit surprised to find vapor in the engine fuel rail if the check valve had failed. In fact, I'd be surprised if there weren't vapor there! My point, and I think we're on the same page, is that because the system is pressurized from the tank, it can't "lock" in the classic sense of the concept of "vapor lock" which is why I mentioned the Lincoln fiasco. Anyway, at worst, it will take a second or two to purge the vapor as the pump pushes enough fuel into the rail to recondense it and/or push it back into the tank.

I think this has been a productive argument. I bet there are at least one or two members who understand the role of the check valve a little better than they did before.

[Bill R.]

thinking that lock means to lock from starting and it doesn't. A vapor lock is strictly a bubble in a liquid line similar to the terminology air lock. Vapor locks on fuel lines with the old mechanical fuel pumps you referred to could occur while it was running causing the car to stall, they could occur after a hot soak as you mentioned, they frequently did occur at the suction fuel line since that was a low pressure area if it passed over a hot spot such as a resonator or exhaust pipe.. a large number of them vapor locked in the steel line coming from the mechanical pump on a v8 up to the carb after it was shut off and hot soaked. These vapor locks didn't always keep the car from starting and they frequently just required long cranking to get the car to start.... On the fuel injected cars the check valve fails , fuel boils in the lines at the engine and fuel rail, fuel is pushed back into the tank and a vapor lock in the line occurs, anybody that has worked on any fuel injected cars that this has happened too can tell you that it doesn't take just a second or 2 to clear, frequently you crank it for 20 or 30 seconds before it will start , acting very similar to the same vapor lock conditions on an old carburated car.

Right. We're in violent agreement. I wouldn't be a bit surprised to find vapor in the engine fuel rail if the check valve had failed. In fact, I'd be surprised if there weren't vapor there! My point, and I think we're on the same page, is that because the system is pressurized from the tank, it can't "lock" in the classic sense of the concept of "vapor lock" which is why I mentioned the Lincoln fiasco. Anyway, at worst, it will take a second or two to purge the vapor as the pump pushes enough fuel into the rail to recondense it and/or push it back into the tank.

I think this has been a productive argument. I bet there are at least one or two members who understand the role of the check valve a little better than they did before.

[SRR2]

I'm having trouble digesting the 20-30 seconds. Are you saying that it takes that long to purge the vapor? And if so, why is that? The volume delivery of the pump should be able to charge the entire fuel system in a lot less time than that, no? I don't have the spec of the fuel pump on hand, but imagine that it's at least a couple of GPM at reduced (i.e. vapor in the system) head pressure. With that assumption, a full charge into the fuel system should take a matter of a few seconds, certainly under 10. Now, I'll grant you that if there's vapor there, no fuel through the injectors, and the engine doesn't start right away, the ECU could get pretty upset and all sort of unusual things could happen causing that cranking time. I just can't wrap my head around that length of time just to purge vapor from the rails.