# Fuel Pressure Drops- 2 DIFFERENT cars



## BlackCherry1966 (Jul 28, 2021)

Long story short (but still trying to give details), same problem with 2 cars, with different set ups- We've had a 1966 GTO with a 4 speed for over 33 years. It had a 400 (.30 over) in it that has always ran good, only took it out to put a 389 in it (to be more period correct, as that is what the car came with). The 389 was rebuilt, put in the car and driven with no problems for 1 whole season. While driving it earlier this spring, it began to shut off when taking off from a stop or while coasting to a stop (every now and then, not every time). While doing basic checks when the car shut off one day, we could hear a small whistle coming from the "weep hole" of the fuel pump-thought that was the problem so replaced it (stock style mechanical about 5.5 psi idling) and the problem still persists. Rebuilt the carb (just because). Didn't change anything for the worse or better. Both cars have new fuel filters. Both cars have a fuel pressure gauge on a fuel log by the carb (one is liquid filled, the other is not).The other car (a Firebird with automatic) has the 400 ( the one that was in the GTO for 30+ years with no problems). Firebird has PTFE lines, GTO has rubber. The Firebird also has a high pressure/ high volume mechanical pump (7psi when idling) Both cars will start fine, have great fuel pressure, and idle all day with no problem. However, when driving around the block and come back to check fuel pressure, both cars will be below 3psi and keep dropping the longer they run after that. I drove the Firebird for over an hour a couple of days ago and didn't have any problems running wise, but I checked the fuel pressure again when I got home and it was at 1.5 PSI. I checked to make sure the GTO had the right gas cap, and it does (same one that has been on it for years). I did drive it around the block with the gas cap off and it did the same thing. (Haven't done that with Firebird yet) Both cars have full tanks of gas (from 2 different gas stations with different brands, in different areas). Tanks have been replaced on both cars. GTO is basic stock style tank. The Firebird has a sump in stock style tank. Neither car has a sock in the tank. Both fuel lines are routed differently, as they are different cars. GTO has never vapor locked before ( even with the 400 in it). Can't say that about the Firebird as we haven't had it running like this before- has always had an electric pump on it with previous motors, and the fuel line was routed differently). GTO has Holley 650 DP that was purchased new a few years ago and we ran on a different car with no problems. Firebird has a Holley 750 DP. Timing is different in both cars. GTO has basic HEI set up. Firebird has MSD HEI setup. Like I said, main problem is just fuel pressure dropping when they get warm- the fact that both cars are doing the exact same thing with totally different set ups and different gas is very mind boggling to multiple people. Figured I would post something on here to see if anyone has some insight. Thank you


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## PontiacJim (Dec 29, 2012)

BlackCherry1966 said:


> Long story short (but still trying to give details), same problem with 2 cars, with different set ups- We've had a 1966 GTO with a 4 speed for over 33 years. It had a 400 (.30 over) in it that has always ran good, only took it out to put a 389 in it (to be more period correct, as that is what the car came with). The 389 was rebuilt, put in the car and driven with no problems for 1 whole season. While driving it earlier this spring, it began to shut off when taking off from a stop or while coasting to a stop (every now and then, not every time). While doing basic checks when the car shut off one day, we could hear a small whistle coming from the "weep hole" of the fuel pump-thought that was the problem so replaced it (stock style mechanical about 5.5 psi idling) and the problem still persists. Rebuilt the carb (just because). Didn't change anything for the worse or better. Both cars have new fuel filters. Both cars have a fuel pressure gauge on a fuel log by the carb (one is liquid filled, the other is not).The other car (a Firebird with automatic) has the 400 ( the one that was in the GTO for 30+ years with no problems). Firebird has PTFE lines, GTO has rubber. The Firebird also has a high pressure/ high volume mechanical pump (7psi when idling) Both cars will start fine, have great fuel pressure, and idle all day with no problem. However, when driving around the block and come back to check fuel pressure, both cars will be below 3psi and keep dropping the longer they run after that. I drove the Firebird for over an hour a couple of days ago and didn't have any problems running wise, but I checked the fuel pressure again when I got home and it was at 1.5 PSI. I checked to make sure the GTO had the right gas cap, and it does (same one that has been on it for years). I did drive it around the block with the gas cap off and it did the same thing. (Haven't done that with Firebird yet) Both cars have full tanks of gas (from 2 different gas stations with different brands, in different areas). Tanks have been replaced on both cars. GTO is basic stock style tank. The Firebird has a sump in stock style tank. Neither car has a sock in the tank. Both fuel lines are routed differently, as they are different cars. GTO has never vapor locked before ( even with the 400 in it). Can't say that about the Firebird as we haven't had it running like this before- has always had an electric pump on it with previous motors, and the fuel line was routed differently). GTO has Holley 650 DP that was purchased new a few years ago and we ran on a different car with no problems. Firebird has a Holley 750 DP. Timing is different in both cars. GTO has basic HEI set up. Firebird has MSD HEI setup. Like I said, main problem is just fuel pressure dropping when they get warm- the fact that both cars are doing the exact same thing with totally different set ups and different gas is very mind boggling to multiple people. Figured I would post something on here to see if anyone has some insight. Thank you


Just a suggestion. The fuel pump may be of the older and uses a rubber diaphragm. It may have been eaten up. softened up, or pin-holes from ethanol fuel. Age can also do this.

Just for fun, when it drops at an idle, and seeing you have a gauge, unscrew the gas cap and see if that does anything. Maybe the tank vent is plugged or you got a wrong gas tank for your application and it has no vent?


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## Lemans guy (Oct 14, 2014)

Interesting issue, first I would posit that different gas is not different at all. I hope Army will chime in about this as we go, but gasoline is refined at only some refineries and even those are not dissimilar formulations. They are made by “cracking” crude oil in a catalytic cracker and brands own their own additives.

One other thing is that the vaporization level of all these fuels is very low. The temperature at when they turn to vapor. Summer blends are higher temps than winter, but they are still low.

when your fuel pressure drops and you see the 1.5 Psi it may be some vaporization occurring in the line, not complete enough to stall it, but enough for you to see the pressure drop. When it gets enough then it stalls.

Do you have return fuel lines to the tank on both cars? Return fuel lines are the fix for vapor lock. If you have no return lines on either car this may be what is occurring.

Gasoline formulations are also held to emissions standards that sometimes have waivers attached for certain states and cities. In general cities will hold to higher standards that keep the vaporization rates lower, while a drive an hour out in the country may yield a gasoline formulation with a higher rate of vaporization, go figure. There are maps that actually show some of this variation. Some states are very strict, others get EPA waivers.

Now this is just a theory, a working theory. You can try wrapping the fuel lines in heat resistant material like DEI sleeping. Some of it has Velcro fasteners, and you can wrap the lines especially from fuel pump to carb in hot engine compartment. You can buy direct from DEI or Summit, JEGS sells it. Get the right size for your lines. That may keep some heat off the lines.

Try to use pure gas as 10% Ethanol has a lower vaporization rate than pure. In the summer blends pure is about 170 degrees and 10% ethanol is about 140 degrees. (I run 10% ethanol but my lines are wrapped) and I run a return line and other things.

Then try a run out in the Country, maybe check some of those emission maps and get some gasoline away from a big city. The gauges are evidence of the issue. Two gauges on two different cars, in July telling you the same thing.

I suspect modern gasoline formulation, July Heat, Maybe no return line? Leading to a slow boiling of the gas and eventual stall. They must start back up after a couple of minutes, as you dis’t mention wreckers. Vapor lock will leave. Once the lines cool.

Could be other things, but you might want to look at this. Good luck and let us know how you do.

By the way where are you located? Hot climate?


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## BlackCherry1966 (Jul 28, 2021)

PontiacJim said:


> Just a suggestion. The fuel pump may be of the older and uses a rubber diaphragm. It may have been eaten up. softened up, or pin-holes from ethanol fuel. Age can also do this.
> 
> Just for fun, when it drops at an idle, and seeing you have a gauge, unscrew the gas cap and see if that does anything. Maybe the tank vent is plugged or you got a wrong gas tank for your application and it has no vent?


Unscrewing the gas cap doesn't change anything. I took it all of the way off and drove around the block too and the problem persisted. Both fuel pumps are new and are from two different sources.


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## BlackCherry1966 (Jul 28, 2021)

Lemans guy said:


> Interesting issue, first I would posit that different gas is not different at all. I hope Army will chime in about this as we go, but gasoline is refined at only some refineries and even those are not dissimilar formulations. They are made by “cracking” crude oil in a catalytic cracker and brands own their own additives.
> 
> One other thing is that the vaporization level of all these fuels is very low. The temperature at when they turn to vapor. Summer blends are higher temps than winter, but they are still low.
> 
> ...


Neither car has a return- that has been a thought. However, the fact that it has never been a problem in over 30 years, but now all of the sudden is... That's puzzling. As far as not needing wreckers, that is true. The Firebird hasn't stalled out... yet. The GTO will usually fire back up pretty quick or pedal it a few times when it starts to stall and can save it. We are located in Indiana. It has done it on a 65 ish degree day and an 80 degree day.


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## Lemans guy (Oct 14, 2014)

Well over 30 years the gas has been reformulated. It does not need high vaporization rates now as fuel injection systems, with ever rising pressure requirements, keep the fuel under high pressure which reduces the boiling point. Similar to your radiator cap, a 16 lb readiator cap raises the boiling point to 240 or so, when water boils at 212.

a 65 degree day and an 80 degree day will make engine compartment temperatures up to the 150 degree area with hood shut. Plenty of heat to effect fuel.

Try wrapping the lines, real fix if this is the problem Re return fuel lines.


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## BlackCherry1966 (Jul 28, 2021)

A couple more things to add.... On the GTO, it is almost like it is flooding out. One of the times that it stalled, we started it back up and pulled into a parking lot, took the air filter off, and had 2 people watching the carb & pressure gauge while one person was cranking / starting the car. It fired back up and while running the fuel pressure was low ( around the 1.5psi mark), but fuel was coming out of the air horns on the carb- at idle. 
2nd thing, as we have been talking to more people about the situation, apparently we are not the only ones with this problem, but have yet to come up with an answer as to why... Another person had a 383 Chevy in a 66 Chevy II, said his car ran good since the 90s (20 something years) and never had a problem. About 5-6 years ago his car would randomly start loosing pressure while driving down the road (it would idle in the garage with no problem) , would sometimes cut in & out and/ or stall. He had a mechanical pressure gauge in the car. He had an electric fuel pump, did have a return style system, and a fuel cell. He ended up changing the fuel pump ( twice, thinking maybe the replacement one was bad), changed the filter ( Said it wasn't dirty, but he was trying anything just to get it to run good again) , changed the filter housing, changed the regulator, and changed the one piece of rubber line that he had ( most of line was aluminum, as it was a street/ strip car and he couldn't have more than 6" of rubber line). Said that he could not ever figure it out and he had asked multiple people and found 1 person that had a similar problem, but all that guy did was change fuel and didn't have a problem after that- that he knows of. In the mean time of dealing with all of that, he ended up going to a carbed LS motor because one of his friends gave him a good deal and hasn't had a problem since. Said he is still running the same fuel cell, but everything else is different, as he is running a lot of nitrous now. 

That just makes us wonder even more what is going on, as that was more of a race motor, electric pump, different style of everything (filter, lines, return, etc) and he still had the same problem....


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## BlackCherry1966 (Jul 28, 2021)

Lemans guy said:


> Well over 30 years the gas has been reformulated. It does not need high vaporization rates now as fuel injection systems, with ever rising pressure requirements, keep the fuel under high pressure which reduces the boiling point. Similar to your radiator cap, a 16 lb readiator cap raises the boiling point to 240 or so, when water boils at 212.
> 
> a 65 degree day and an 80 degree day will make engine compartment temperatures up to the 150 degree area with hood shut. Plenty of heat to effect fuel.
> 
> Try wrapping the lines, real fix if this is the problem Re return fuel lines.


If I get some time this weekend, I may try that. The Firebird has braided nylon all of the way up with a couple of inches of rubber by the pump to get the right size/ fitting coming out of the pump. The GTO is steel hard line from the tank to the pump and then braided nylon up to the fuel log.


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## Lemans guy (Oct 14, 2014)

Yes, I think you are vapor locking on today’s gas. Those braided lines get hot too. Get a temp gun and check carb and lines after driving, if you see those elevated temps it could be the gas. I like to add a little Sta-bil Marine 360 to a full tank of gas. About two ounces. Helps just a tad to keep the volatility up.


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## Lemans guy (Oct 14, 2014)

Actually I said it wrong on the gas emissions. You want the gas from where the fuel volatility is the highest. States get waivers for areas sometimes to not have to meet the standard. The standard actually helps you as it keeps the boiling point higher. So city areas and suburban counties have often higher boiling point gas than out in the country. So try a tankful near a city.

A quick search showed Floyd and Clark counties in Indiana adhere strictly to the 7.8 RVP standard. Every small edge helps!


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## Lemans guy (Oct 14, 2014)

When your fuel was coming out the air horns…it was likely boiling out..and that whistle you are hearing could be vapor pressure from the boiling gas escaping…..

could be, worth further investigation, just a line to work on and not definitive yet.

Good Luck!


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## Lemans guy (Oct 14, 2014)

Heat and Low volatility gasoline……maybe


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## Lemans guy (Oct 14, 2014)

Your race car friend had a carb and carbs operate under low pressure 5 or 6 lbs PSI fuel injection systems operate at near 60 psi.

2 things happen, one is vapor lock. It occurs in the “suction” line from the tank to the pump. Here the pump is pulling a vacumn on the line. Vacumn further reduces pressure in the line and therefore lowers the boiling point even further. Vapor won’t pump,so pressure drops, then you stall.

Now once shut off you get the separate but related phenomenon, Heat soak. This only happens once the car is shut off, but it happens fast. Once the cooling system stops circulating coolant the engine “Heat Soaks” and the temperatures rise fast. Higher than the vaporization rate of that fuel sitting in your carb bowls. So they start to boil, and pour gas into the carb throat and guess what, they flood the engine.

Pumping the pedal will flood it further. The fix for heat soak is phenolic spacers under the carb to reduce those temperatures.wrapping duel lines helps here too. When you broke down you probably got some of both. Both caused by heat and the low temperature volatility of modern fuels.

for now this is something to consider, could be something else. Let us know how you do and good luck!


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## BlackCherry1966 (Jul 28, 2021)

I will try the temp gun first and see what that shows- hopefully this weekend. Ill go from there as far as wrapping the lines. Yes, all 3 cars have a carb. If we get anything figured out, I will post it on here. Thank you for the direction!


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## Lemans guy (Oct 14, 2014)

When you stop hit that carb front and back with temp gun, hit the fuel pump lines and filter. You will get a picture of the temps they are operating at.


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