Fuel Pressure Questions

[TennTechMan]

Lots of later model fuel systems are "dead head" style systems. I assume the new motor and fuel system Ryan is running is designed to be a dead head. Dead head systems use a speed controlled fuel pump and a pressure sensor to regulate pressure at the rail. A return system uses a constant speed fuel pump and a pressure regulator on the return side of the rail.

All that said, I don't think this is fuel related. It sounds too much like a faulty relay or control module of some kind. With any pressure in the fuel rail, you'd at least get a cough or sputter when trying to start the engine.

[MikeKelly]

I assume the new motor and fuel system Ryan is running is designed to be a dead head.

Let me refer you back to:

I originally thought fuel pressure due to the fact that most of these builds run an aftermarket regulator and I'm sticking with the stock f-body in tank unit.

[MikeKelly]

A little google research shows a lot of this type LS3 swaps are using a C5 Corvette fuel flilter to deal with this problem.

The C5 fuel filter has a built-in bypass regulator.

It still has to have a return line plumbed back to the tank.

[jpvette]

Just talked to my friend and he is currently doing a ls swap on a older truck and is using the same filter/by-pass Mike is talking about. His view was vapor lock also with the pressure bleeding down it's going somewhere: back thru the pump or thru the injectors.

[MikeKelly]

with the pressure bleeding down it's going somewhere: back thru the pump or thru the injectors.

I'm going to suggest the "back through the pump" option.

When a positive displacement pump gets deadheaded it either has to lock up or backflow through itself.

Once it's started backflowing through itself, that situation will only deteriorate.

[MikeKelly]

Let me hit the term "vapor lock" one more time. I've been doing the "liquid / vapor" thing for a lot of years, most people absolutely don't understand it.

The reason water was used in steam engines was because it was very efficient for that. 1 pound of liquid water produces 2500 cubic feet of vapor and harnesses a lot of "latent heat" energy doing it. It takes 180 times as much energy to go from 212F liquid water to 212F steam, than it took to go from 211F liquid to 212F liquid water. That same amount of heat has to be removed for it to condense into a liquid again. At 20psi water boils (or condenses - cooling off) at 258F, at 28" of vacuum it boils (or condenses) at 80F. But the same amount of heat has to transfer for it to change states.

Air conditioning uses the same "latent heat" principle to keep you cool. But 1 pound of liquid refrigerant only produces ~1.5 cubic feet of vapor. It still requires a great deal of energy transfering one way or the other to change states - boiling off from liquid to vapor inside at a low pressure + temperature, condensing back from vapor to liquid outside at a high pressure + temperature. That's how every air conditioner or refrigerator you ever saw works.

I don't know how many cubic feet of vapor a pound of liquid gasoline produces. But I know for a fact that it will boil off in a slight vacuum (getting "sucked" out of the tank) a lot easier than it's going to boil off under 60psi on the pressure side of a EFI system.

That's why you'll not likely ever see a "vapor lock" on the pressure side of EFI system.

Does that make sense to anybody?

[MikeKelly]

Just one more thing and I'll quit on vapor lock.

You have pumps that are designed for moving vapor, usually referred to as compressors. They break if you try to run liquid through them. Liquids can not be compressed, something has to give.

You have pumps that are designed to move liguids, they don't accomplish anything if you're trying to suck vapor through them.

'Vapor lock" is when you're trying to suck vapor through a liquid pump. It doesn't work.

[disneyd]

Good info Mike.

Ryan, do you have anything to read the fuel temperature sensor with? I wonder if the sensor good be bad at the high end of the scale or if something is making the fuel extremely hot to the point the ECU can't deal with it?

This is assuming the car has a fuel temp sensor. Some do, some don't.

[MikeKelly]

Good info Mike.

Thanks, I really struggled trying to explain my point.

The hardest part of explaining the mechanical refrigeration process is getting someone to visualize a substance which boils off cold at a low pressure + temperature, and condenses back into a liquid at a higher pressure + temperature.

When you take the pressure off that liquid, it boils off cold again, get it?

But most every substance will do that.

And I've known brilliant people who have been completely unable to grasp that...

[integra55]

that would be me … no wait .. you said brilliant .. ok so that lets me out

[MikeKelly]

And gasoline is a very "prone to change from liquid to vapor at common pressures + temperatures" substance.

There is where you get "vapor lock".

Evaporation is one thing, but boiling off cold because a particular substance cannot exist in liquid form at that pressure + temp is what I'm talking about.


I hope I'm done now...

[thrdeye]

So have we decided the course of action is to put a 13B in the Camaro?

[MikeKelly]

It won't work with a deadheading efi pump either...

[dfoulk]

Looking back at your original post, I have seen bad crank position sensors in GM cars exibit the exact symptoms you describe. They worked until they got hot then once they cooled down would start working again. I don't know if your car even has a crank postion sensor but if it does it might be worth checking.

Sent from my GT-N5110 using Tapatalk

[01badz28]

Mike,

I've seen the Corvette fuel regulator - that looks like my next stop. I tested out the vehicles electrical system this weekend (the shop manual - as much as it applies at this point - identifies a faulty battery and starter as causing hot start issues) but its cleared out ok.

Its either the fuel line vapor locking, or a tune issue at this point.