Vapor Lock???

[protour73]

So I started to notice a drop in fuel pressure. First I replaced the fuel pressure gauge....nope. Then I replaced the mechanical fuel pump thinking a failed diaphragm....nope. I remove all of the braided fuel lines from where they come off of the hard line and checked for blockage.....nope. Checked the fuel filter...nope. Then there is always the sender in the tank (brand new 6 years ago with only 1500 miles on it). I have a small K&N filter tee'd from a vent tube off of both sides of the tank. No vented gas cap, but then it never was on a 73 and I've had no issues previously.

Fuel pressure starts out at 6psi when the engine is first fired up and stays there for about 6 or 7 minutes and then starts dropping slowing 5....4.....3.....2 1/2psi and stays there after about 25 to 30 minutes of idling. The bowls in the carb stay full even after running the RPMs up several times and cracking the throttle several times.

The fuel line runs past the passenger side headers on the frame rail......vapor lock? I have never noticed this previous to now, so I don't know if this a new issue. I can buy something like http://www.heatshieldproducts.com/pr...-shield/102/43

Thoughts? Theories?

[Sieg]

Scott,

First off a disclaimer that I have little knowledge of 2nd Gens.........

I had a similar experience with drop off in fuel pressure and in my case a new pump solved it.

> If the cap isn't vented how does the fuel tank vent?

Another connection I spotted that concerned me on my car was the connecting hose between the tank hard line and the chassis hard line in this pic:

I replaced it years ago, but it felt soft enough that I was concerned about it collapsing under heavy demand so I replaced it.

First guess is a venting issue as the tank has to vent for the oneway system to work. The car sat for a while right? Bug debris effecting venting?

[protour73]

Originally Posted by Sieg

Scott,

First off a disclaimer that I have little knowledge of 2nd Gens.........

I had a similar experience with drop off in fuel pressure and in my case a new pump solved it.

> If the cap isn't vented how does the fuel tank vent?

Another connection I spotted that concerned me on my car was the connecting hose between the tank hard line and the chassis hard line in this pic:

I replaced it years ago, but it felt soft enough that I was concerned about it collapsing under heavy demand so I replaced it.

First guess is a venting issue as the tank has to vent for the oneway system to work. The car sat for a while right? Bug debris effecting venting?

Back in the day, venting lines ran from three vents off of the tank to the charcoal canister in the engine compartment. Hence, no vented cap. I ditched that whole charcoal canister system back when I started the build.

I have that same connecting hose basically on the 2nd Gen. Mine seems fine, and since I have a brand new pump installed, I've eliminated that possibility.

The car has been out including track time. There is always the possibility that the ethanol fuel around here has compromised the sock filter on the pickup of the sending unit in the tank. So I may have to drop the tank

[Rick D]

Originally Posted by protour73

Back in the day, venting lines ran from three vents off of the tank to the charcoal canister in the engine compartment. Hence, no vented cap. I ditched that whole charcoal canister system back when I started the build.

I have that same connecting hose basically on the 2nd Gen. Mine seems fine, and since I have a brand new pump installed, I've eliminated that possibility.

The car has been out including track time. There is always the possibility that the ethanol fuel around here has compromised the sock filter on the pickup of the sending unit in the tank. So I may have to drop the tank

Um you don't want to look under there

Did you try to run it with the gas cap off like I said yesterday?? If it doesn't drop then you know it's a vent issue? If it still drops then it may be the pickup in the tank.

[protour73]

Originally Posted by Rick D

Um you don't want to look under there

Did you try to run it with the gas cap off like I said yesterday?? If it doesn't drop then you know it's a vent issue? If it still drops then it may be the pickup in the tank.

Before I even talked to you I ran it with the gas cap off, still dropped.

Originally Posted by Vegas69

Liquid filled fuel pressure gauge in the engine compartment?

Yes, it is a liquid filled fuel pressure gauge in the engine compartment.

[Vegas69]

Originally Posted by protour73

Before I even talked to you I ran it with the gas cap off, still dropped.

Yes, it is a liquid filled fuel pressure gauge in the engine compartment.

A liquid filled gauge will become innaccurate with heat. Been there, done that. Order a non liquid filled or a universal gauge and verify before you spend any more time or money.

[Vegas69]

Liquid filled fuel pressure gauge in the engine compartment?

[Ron Sutton]

Hi Scott,

I have some thoughts, so I'll chime in & see if I can help.

You said ...

Originally Posted by protour73

Fuel pressure starts out at 6psi when the engine is first fired up and stays there for about 6 or 7 minutes and then starts dropping slowing 5....4.....3.....2 1/2psi and stays there after about 25 to 30 minutes of idling.

Do you have an adjustable fuel pressure regulator?
(You didn't mention either way)

If yes, what brand & model?
Does it use a diaphragm ?

We run a brand of piston pump & diaphragm regulator from "Stewart Race Pumps". When the car has been just ambient temperature (55-80 degrees) ... it starts ... and after 4-5 minutes of engine warming up ... and the heat rises under hood ... the fuel pressure always drops 3-4 psi.

It did not do this with the common check ball & spring style of regulator ... just when we switched to the diaphragm style from Stewart Race Pump.

Every since we switched to a diaphragm style regulator, my cars always do this now. But since we learned & adjusted it ... no problems.

We just make sure ours is 7 psi with the engine "warm". Once we set it, it stays correct all day, until the next day, when the engine is cold again. It starts with 10-11 psi of fuel pressure ... then as the engine warms up a few minutes it settles in at our 7 psi #.

Maybe you just need to adjust your fuel pressure regulator up to the desired temp when "warm" ... and see what it does the next time you start it?


If this is a new issue ... could you have a bad regulator?
Maybe the spring or diaphragm is weak, or old, in your regulator?

Just throwing out things for you to look at, since there was no mention of the fuel pressure regulator in your forum thread.