[freebird69]

I posted about this already but through process of elimination I have narrowed the problem down to fuel delivery. Since my last post I have checked a bunch of things including fuel psi and that's where I'm at. When I start the car my fuel pressure gauge reads 5 psi, then I drive around for a while and the car runs great for 20 minutes or so and then it wants to stall, so I pop the hood and check the gauge and it's at zero. My assumption is that the pump is ok but something is happening between the FP and the gauge, which is connected about 6 inches from the carb, and there's about 15 inches of hard line between the FP and the gauge. The hard line is about an inch from the motor.

Is the fuel vaporizing? If so what can I do?

[steve25]

Is your fuel tank cap vented?

Take a drive and leave your cap loose, does that releave the issue?

[JLMounce]

Sure sounds like the fuel is boiling into vapor and the pump can no longer push pressure.

Modern e10 pump fuel begins to boil as low as around 110* at zero pressure and is mostly vapor by about 175* under zero pressure. Modern pump fuel as a result is designed to be delivered under pressure, typically 43-58psi where it's boiling point is much higher.

In a car like your 69 Firebird, with a factory style fuel system, you are sending fuel from the pump to the carb between 5-7psi, so you're not getting much of a pressure bump there. Add to that the fact that the fuel once bypassed from the pump to the carb sits there in the fuel line and in the carb bowls for a period of time before being consumed. This is why most vapor lock incidents tend to occur at low speed cruise, or while sitting at a light. This is where fuel consumption is the lowest and the fuel has more time to soak heat from the engine bay.

There's a couple things you can do.

1. Source ethanol free gasoline. Your car was made to function on pure gasoline. The problem here is that only a few stations here and there usually have pure gasoline and in many cases it's low octane designed for lawn equipment. If you need premium fuel, it's hard to find pure gas. This means you have to buy and pump your own and that leaves you without the ability to travel beyond a certain point.

2. Attempt to remediate the heat from your engine bay into the fuel system. Get a thicker heat shielding gasket for the fuel pump. Cover the fuel line in a heat shield material. Add a phenolic spacer under the carburetor. If the car has a vapor return line, make sure it's not clogged in any way and is returning vapor to the tank.

3. Fix the problem through modernization. A carb can still be used with modern fuels, it just needs a bit more help. An electric pump, inside the fuel tank with a bypass regulator close to the carburetor allows fuel to continuously circulate. This reduces the amount of time that fuel is sitting in the hot engine bay. Because the fuel is always there for you, it allows you to also run the floats a little lower, so you're not relying on a pool of fuel that is heat soaking in the carburetor.

4. EFI

If you're a stock appearing purist, you can build a system from option 3 that looks really close to what a factory setup would look like.

For what it's worth, I dealt with the exact same issue with my 1969 Firebird. The cramped engine bay doesn't breath well and during the summer I usually got 15-20 minutes of drive time before it started acting up at stop lights. I had every heat shield and phenolic barrier I could put between the engine bay and the fuel system. That bought time, it never cured the issue. I put a Throttle Body Injection system on the car with a proper fuel system and never dealt with it again, even in temperatures above 100 degrees and altitudes in excess of 8000 ft.

[freebird69]

Originally Posted by steve25

Is your fuel tank cap vented?

Take a drive and leave your cap loose, does that releave the issue?

Hey Steve, yeah, after it started acting up I took the fuel cap off but it didn't help.

[freebird69]

Originally Posted by JLMounce

Sure sounds like the fuel is boiling into vapor and the pump can no longer push pressure.

Modern e10 pump fuel begins to boil as low as around 110* at zero pressure and is mostly vapor by about 175* under zero pressure. Modern pump fuel as a result is designed to be delivered under pressure, typically 43-58psi where it's boiling point is much higher.

In a car like your 69 Firebird, with a factory style fuel system, you are sending fuel from the pump to the carb between 5-7psi, so you're not getting much of a pressure bump there. Add to that the fact that the fuel once bypassed from the pump to the carb sits there in the fuel line and in the carb bowls for a period of time before being consumed. This is why most vapor lock incidents tend to occur at low speed cruise, or while sitting at a light. This is where fuel consumption is the lowest and the fuel has more time to soak heat from the engine bay.

There's a couple things you can do.

1. Source ethanol free gasoline. Your car was made to function on pure gasoline. The problem here is that only a few stations here and there usually have pure gasoline and in many cases it's low octane designed for lawn equipment. If you need premium fuel, it's hard to find pure gas. This means you have to buy and pump your own and that leaves you without the ability to travel beyond a certain point.

2. Attempt to remediate the heat from your engine bay into the fuel system. Get a thicker heat shielding gasket for the fuel pump. Cover the fuel line in a heat shield material. Add a phenolic spacer under the carburetor. If the car has a vapor return line, make sure it's not clogged in any way and is returning vapor to the tank.

3. Fix the problem through modernization. A carb can still be used with modern fuels, it just needs a bit more help. An electric pump, inside the fuel tank with a bypass regulator close to the carburetor allows fuel to continuously circulate. This reduces the amount of time that fuel is sitting in the hot engine bay. Because the fuel is always there for you, it allows you to also run the floats a little lower, so you're not relying on a pool of fuel that is heat soaking in the carburetor.

4. EFI

If you're a stock appearing purist, you can build a system from option 3 that looks really close to what a factory setup would look like.

For what it's worth, I dealt with the exact same issue with my 1969 Firebird. The cramped engine bay doesn't breath well and during the summer I usually got 15-20 minutes of drive time before it started acting up at stop lights. I had every heat shield and phenolic barrier I could put between the engine bay and the fuel system. That bought time, it never cured the issue. I put a Throttle Body Injection system on the car with a proper fuel system and never dealt with it again, even in temperatures above 100 degrees and altitudes in excess of 8000 ft.

Jason, thanks for that detailed response. I think that I will start with the easy stuff first and work my way up... Fuel pump spacer and some well placed heat shielding plus a carb spacer.

[mgarblik]

Make sure all the rubber lines in the system are in good shape and are not sucking shut or collapsing, especially from the tank to the fuel pump.

[Goatracer1]

See my post about a "Nagging Problem". I couldn't believe what a difference it made.

[JLMounce]

Originally Posted by freebird69

Jason, thanks for that detailed response. I think that I will start with the easy stuff first and work my way up... Fuel pump spacer and some well placed heat shielding plus a carb spacer.

Easy first steps. If you have one of those glass filter deals between the pump and carb, get rid of it. Those things sink heat very rapidly. If you must have a filter there, run one of the plastic filters.

[chiphead]

Is this a liquid-filled underhood gauge? Those are not accurate at all when they get hot. The air bubble expands with heat and presses on the bourdon tube, which then reads lower than actual pressure.

Only trust an underhood gauge when cold.

[GOAT8U2]

Raise pressure to 6-7psi

Swap out the liquid to a mechanical gauge

Can you see AFR?
We'll assume simple things you took care of, you'll find it.

[freebird69]

Originally Posted by JLMounce

Easy first steps. If you have one of those glass filter deals between the pump and carb, get rid of it. Those things sink heat very rapidly. If you must have a filter there, run one of the plastic filters.

I actually have one of those metal ones which I assume is as bad as glass.

[freebird69]

Originally Posted by chiphead

Is this a liquid-filled underhood gauge? Those are not accurate at all when they get hot. The air bubble expands with heat and presses on the bourdon tube, which then reads lower than actual pressure.

Only trust an underhood gauge when cold.

thanks chip. I have a liquid filled but I did check it cold and it read low so I will replace the pump and see what happens.

[freebird69]

Originally Posted by GOAT8U2

Raise pressure to 6-7psi

Swap out the liquid to a mechanical gauge

Can you see AFR?
We'll assume simple things you took care of, you'll find it.

I ordered a Carter mechanical pump rated from 5 to 6.5 psi. Hopefully that will be better than what i have now which reads 4.5 cold on my liquid filled gauge. If the new pump plus shielding solves the problem I'm hoping to not need the gauge anymore. Also, I have no way to read the AFR.

[JLMounce]

Originally Posted by freebird69

I actually have one of those metal ones which I assume is as bad as glass.

Yeah, metal, glass, stuff like that will really pull the heat from the engine bay and the fuel takes the heat. Same concept as your radiator and engine coolant.

Remember, from the factory, there was no filter between the fuel pump and carburetor. Filtration occurred in the tank at the pickup sock.

I'm not saying it's a bad idea to put an in-line filter in place, but the Pontiac engineers didn't include one there. They typically are included now because people run around with 50+ year old lines that are constantly sluffing off corrosion into the fuel.

If you have new fuel lines and a fuel sock that isn't 50 years old in the tank, you probably don't need the in-line filter.

[RAIIIJudge]

Hope I'm not diluting thread here but I'm going through exactly some of this right now.

JL, wasnt there, at least w QJ outfitted Poncho's, always a filter at carb inlet? Just yesterday I put a Robbmc FP on motor and I thought I'd be able to have the filter down at pump but no way. My builder says QJ inlet filter is too restrictive so I guess I'm going to have to put together some kind of AN run and have filter in space over the water pump? I hadn't thought about heat at that area.

I've got a new tank, new 3/8 sending unit > 3/8 line to 100 micron filter tucked in frame rail and a straight shot (no angled fittings) to fuel pump.
I understood a finer filter was needed between pump and carb to filter the real fine junk that comes from tanks fuel is originally stored in?

Next season I'll go w/ a 1/2" sending unit and line to pump.

appreciate the conversation,
m

[JLMounce]

Originally Posted by RAIIIJudge

Hope I'm not diluting thread here but I'm going through exactly some of this right now.

JL, wasnt there, at least w QJ outfitted Poncho's, always a filter at carb inlet? Just yesterday I put a Robbmc FP on motor and I thought I'd be able to have the filter down at pump but no way. My builder says QJ inlet filter is too restrictive so I guess I'm going to have to put together some kind of AN run and have filter in space over the water pump? I hadn't thought about heat at that area.

I've got a new tank, new 3/8 sending unit > 3/8 line to 100 micron filter tucked in frame rail and a straight shot (no angled fittings) to fuel pump.
I understood a finer filter was needed between pump and carb to filter the real fine junk that comes from tanks fuel is originally stored in?

Next season I'll go w/ a 1/2" sending unit and line to pump.

appreciate the conversation,
m

Today I learned, that they did. Or at least some of them anyway. I've never owned a car new and stock enough to really verify it, other than you can tell from historical archives that there wasn't anything in the line itself.

I'm not sure why the inlet filter on a QJet would be any more restrictive than the same type of paper or sintered element that you'd find in any other type of filter housing assembly. It looks like the replacement filters are all in the 25 micron range, which is fine.

[b-man]

Examples of a factory fuel filter inline before the carburetor.

1964 factory air conditioned Le Mans, this happens to be a 421 HO Tri-Power pictured but the 2 and 4-barrel 326 used the same filter attached to the water pump stud.

Non-A/C cars didn’t have the fuel filter that year. The filter for A/C equipped cars had the vapor return line fitting built into it, later years had the vapor return incorporated into the fuel pump itself.

Last picture is a 1966 Tri-Power non-A/C, it had the filter mounted to a bracket held onto the front carb by the frontmost 3 screws on the float bowl. The filter is obscured by the upper radiator hose in the photo.

[Shiny]

Does your fuel pump have a vapor return port and line?

If not a gauge failure, it's just my opinion that zero fuel pressure is more likely a pump "inlet" issue than between the pump and carb (outlet or pressure side):

- poor venting at the tank (removing the cap eliminated this one)
- collapsed hose between tank and pump (per mgarblik)
- pump getting hot and fuel boiling inside the line or pump without a place to go (vapor lock on the suction side will destroy flow)

If fuel is boiling on the "pressure" side of the pump, I don't see it completely shutting off fuel flow, let alone causing the pressure to drop to zero. The needle and seat are a flow restriction so even if vapor is present and messing up flow, I think you should still see pressure. It might make the engine run bad, but I don't see it completely stopping all fuel flow which would happen if there was truly zero pressure.

I'm aligned with a gauge issue contributing to your challenge.

Is there a chance the carb itself is getting hot enough to boil fuel while the engine is running? This would also make it run badly.

[Schurkey]

Originally Posted by chiphead

Is this a liquid-filled underhood gauge? Those are not accurate at all when they get hot. The air bubble expands with heat and presses on the bourdon tube, which then reads lower than actual pressure.

Only trust an underhood gauge when cold.

ALL the liquid-filled gauges have a lil' rubber plug on them. "Burp" the rubber plug to relieve pressure. Gauge returns to it's original accuracy.

Do this at EVERY temperature change when you read the gauge--cold to hot...hot to cold.

In some applications, there's a small nipple on the rubber plug that can be cut to create a permanent vent. This works fine if the gauge doesn't vibrate enough to shake the fluid out, (as in, NOT "mobile" applications that vibrate and are subject to G-forces) AND if the rubber plug is at the top of the gauge (in the air pocket) in it's mounted position.

[freebird69]

Thanks to all of you for your suggestions... my problem is FIXED!!! Once I narrowed it down to vapor lock I used all of the suggestions for a process of elimination and my issue of the car stalling after running for 20 minutes appears to be gone. By using a fuel pressure gauge I determined that my old pump was a little weak so I replaced it and while I was at it I added a phenolic spacer. I also added a 1/2" phenolic spacer under the carb and then I drove around for an hour in 86 degree sunshine and the car ran great and no stalling.