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/productdetail/heat-shield-sleeve/fuel-line-heat-shield/102/43

Thoughts? Theories? :bang:

[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:
http://photos.smugmug.com/photos/i-zsxNk9c/0/M/i-zsxNk9c-M.jpg
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]

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:
http://photos.smugmug.com/photos/i-zsxNk9c/0/M/i-zsxNk9c-M.jpg
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 :confused59:

[Rick D]

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 :confused59:

Um you don't want to look under there :_paranoid

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.

[Vegas69]

Liquid filled fuel pressure gauge in the engine compartment?

[protour73]

Um you don't want to look under there :_paranoid

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. :unibrow:

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


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.

[Vegas69]

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

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.

[Ron in SoCal]

Thoughts...

Scott are there any starting/performance issues?

I'd verify you're getting good fuel pressure readings. You can check this may ways.

[intocarss]

Todd is right, liquid filed gauges can be inaccurate due to heat or altitude change. Some gauges have a vent screw on top. Make sure not to let any of the liquid out. Now, turn the screw two or so turns out to equalize inside gauge pressure(NOTE.. some gauges have a little rubber plug that you maybe able to remove insted of a screw).

This will allow the gauge to release un-wanted pressure and self adj to different altitudes or temperature. I used to have to do this at different race tracks with my gauges. Oil filled is good for vibration but can suck for accurate readings.

Ron Sutton is also right, It could be your regulator. I have had the diagram ones lose pressure and you have to adjust accordingly. You said you replaced the fuel pump so I doubt its the bypass (if any) in your pump.Some pumps have an adj pressure screw..is that set right?

Another thing... are your hoses old? Maybe they are coming apart inside and tiny bits of rubber are in your regulater. This is just my 2 cents..Hope you find it

Also..For safety sake... I do not like to run a fuel pressure gauge under my hood they can leak JMOHO

[protour73]

Wow guys, really great responses and ideas. THANKS

The liquid filled fuel pressure gauge is brand new, but I will check it and attempt to equalize the pressure or adjust if there is a screw.

As far as the regulator theory, I don't have one. The fuel lines from where they come off of the hard line are all brand new.

The only performance issue is due to the fact that I have a brand new carb on the car which is not dialed in yet. The car starts fine, idles great.

My theory is that the filter sock on the pickup tube of the sending unit may have been compromised by the ethanol in our fuel around here. I had some serious corrosion issues with a previous Holley Street Avenger (yes Mr. Weld I know!!). I've purchased a new sending unit and will be here early next week.

[intocarss]

Most of the time It's not that the liquid filled gauge goes bad because its new or old,(unless you drop it or beat on it lol) it's that it builds pressure and can give a false reading.

So by backing the screw or removing the plug, you release this pressure. I have used so many different fuel gauges form brand name Co's and they all differ by about 2-3 even 5lbs in pressure. Pick one and tune your engine to it and that is that.

I didn't read that it may be inside your gas tank, I hope it's an easy inexpensive fix for you

[protour73]

Most of the time It's not that the liquid filled gauge goes bad because its new or old,(unless you drop it or beat on it lol) it's that it builds pressure and can give a false reading.

So by backing the screw or removing the plug, you release this pressure. I have used so many different fuel gauges form brand name Co's and they all differ by about 2-3 even 5lbs in pressure. Pick one and tune your engine to it and that is that.

I didn't read that it may be inside your gas tank, I hope it's an easy inexpensive fix for youThe sending unit will require dropping the fuel tank which I don't look forward to. The sending unti was only $70 shipped. It could be worse!! :G-Dub:

[Rick D]

The sending unit will require dropping the fuel tank which I don't look forward to. The sending unti was only $70 shipped. It could be worse!! :G-Dub:

I would try to change the gauge before I would drop that tank? Just sayin!! :peepwall:

[Ron Sutton]

Now I that learn you don't have a regulator in your system, that changes my view considerably.

So, with no regulator, your fuel pressure reads 6-7 ... then drops to 2-1/2. Now, I'm on the same page as everyone else, you either have a bad gauge, or you really do have a fuel system volume/pressure issue.

Test the pressure gauge first.

[Sieg]

The sending unit will require dropping the fuel tank which I don't look forward to. The sending unti was only $70 shipped. It could be worse!! :G-Dub:

Exactly.......how many cars are easier to drop a tank from? :sieg:

[protour73]

Exactly.......how many cars are easier to drop a tank from? :sieg:

The fuel tank is out, replaced the sending unit. There doesn't seem to be anything visibly wrong the old one (had a hunch it would not be the sending unit).

Tank will go back in tomorrow. I'm going to blow out the hard line from the tank up to the fuel pump.

I picked up a cheap (non liquid filled) fuel pressure gauge from the local auto parts store. I'll let the engine idle for as long as it took the Autometer gauge (liquid filled) to fail and compare the two.

I'm pretty sure this $190 exercise in futility will come down to the liquid filled gauge failing, as you guys enlightened me about. :ohsnap:

Everyday I have to troubleshoot in my job (manager of technical support team) so I'm used to this. :hairpullout:

[Vegas69]

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.

Not a bad idea......

Just giving you hell Scott.

[protour73]

Not a bad idea......

Just giving you hell Scott.HA, Todd I have a pretty thick skin after years of hanging out on these cars sites!! LOL

Liquid filled fuel pressure gauge in the engine compartment? But when you posted this I went, OH CRAP, he knows something and that's just the teaser!!

[Rick D]

HA, Todd I have a pretty thick skin after years of hanging out on these cars sites!! LOL

But when you posted this I went, OH CRAP, he knows something and that's just the teaser!!

So did you get it back together Scotty?? :poke:

[protour73]

So did you get it back together Scotty?? :poke:
Pretty much. I have to get back under the car and go over everything to double check for any loose ends.

I also got a small tank of Nitrogen and blew out the hard line from the tank to the fuel pump. I would have always wondered about this too if i didn't do it. It was all clear.

I'll get the car fired up later today and post up the results. :thumbsup:

[intocarss]

Pretty much. I have to get back under the car and go over everything to double check for any loose ends.

I also got a small tank of Nitrogen and blew out the hard line from the tank to the fuel pump. I would have always wondered about this too if i didn't do it. It was all clear.

I'll get the car fired up later today and post up the results. :thumbsup:

angi1vwUkQc

:hello: :hello: :hello:

[Rick D]

angi1vwUkQc

:hello: :hello: :hello:

Man that never gets old!! :hello: :RunninDog:

[intocarss]

you have a pm

[protour73]

Well..............it WAS the frigging gauge.

On the top of the Autometer fuel pressure gauge there is a little rubber disc with a pin in the center of it.

I went over the whole backend of the car, made sure everything was properly reinstalled. Fired up the car and let it idle. Fuel pressure was initially at 6psi, just like it should be. Then after 10-15 minutes of idling it started dropping just like before. I grabbed the head of the pin on the gauge and it immediately snapped back to 6psi. So........there it is. A $190 lesson on how liquid filled fuel pressure gauges work. :G-Dub:

Aside from a valuable lesson, I found some issues that surely would have reared their ugly head down the road anyway. So it made this exercise somewhat worth while.

Earlier in this thread, the spot that Sieg posted a picture of on his car, there is a similar rubber fuel hose transition between the sending unit hard line and the rear section of hardline on a 2nd Gen as well. The 4" section of hose was hard as a rock. So I replaced that with better quality material from Earl's. Then under the passenger side, where the fuel line runs along the frame rail the is an identical transition - same deal there, rubber fuel line was also compromised so I replaced that as well.

Then while swapping out the sending unit I noticed the ground wire had been pinched between the tank and the body and was smashed completely flat with the casing split. So that was taken car of when I installed the new sending unit which came with a new ground wire. :thumbsup:

Now to finish the lesson, I'm going to run down to Advance Auto and pick up a cheap Mr.Gasket non-filled fuel pressure gauge and install it and stand back and be amazed.

Thanks to all that posted to this thread!!

[camcojb]

perfect Scott, exactly how you want things to turn out. :thumbsup:

[Sieg]

-h8pbFY3f54

:thumbsup:

[protour73]

edit*** just like the gauge, my .gif FAILed

[Rick D]

-h8pbFY3f54

:thumbsup:

Nice Sieg!! :D

Scott now take it and beat on it!!! :poke:

[intocarss]

S5-Kyl5Bmx8

[protour73]

S5-Kyl5Bmx8

Nice!!!

Switched from the pink to the pink to the blue accelerator cam in the #1 position, and WOW what a difference. Got rid of my off idle hesitation and this thing pulls like a beast now.

The only thing I noticed is that in 4th or 5th at cruise, it shows on the lean side on the wideband O2 sensor gauge -- 15.3 to 16. just a slight press of the throttle and it goes away but I don't want any collateral damage if you know what I mean.

[Sieg]

Nice!!!

Switched from the pink to the pink to the blue accelerator cam in the #1 position, and WOW what a difference. Got rid of my off idle hesitation and this thing pulls like a beast now.

The only thing I noticed is that in 4th or 5th at cruise, it shows on the lean side on the wideband O2 sensor gauge -- 15.3 to 16. just a slight press of the throttle and it goes away but I don't want any collateral damage if you know what I mean.
You went from one extreme to the other with the blue cam. Might try the orange, green, and white also. They're more in the middle of the fuel curves.

You might try richening the fuel mixture screws an 1/8 turn and see what happens to the cruise fuel ratio, or step the primary jets up one size.

[protour73]

You went from one extreme to the other with the blue cam. Might try the orange, green, and white also. They're more in the middle of the fuel curves.

You might try richening the fuel mixture screws an 1/8 turn and see what happens to the cruise fuel ratio, or step the primary jets up one size.Actually I did have that lean issue at cruise before the blue cam.

I do like the jet increase idea and have a set on the workbench, so that will be the next-next move. First will be the fuel mixture adjustment which will be the easiest to try.

I might actually get this carb thing figured out yet. (with the help of some smarter people than me)

:thankyou: