# Is this vapor lock??



## leeklm (Mar 11, 2012)

Been reading about vapor lock, which I think I experienced today. Drove the car home from work in heavy stop/go traffic in hot weather today, car ran fine during the 50 min trip. Parked in the shade at home for about an hour, went back out to move car, it started, but ran poorly. 

As I looked at my clear fuel filter, it was almost empty, with fuel barely trickling in. Fuel pressure at the carb was a little low at 4lb, normally at 6-8. 

I loosened the fuel line on the carb side of the filter which of course relieved the pressure that was built up. Attached the line, started the car, which filled the filter as normal and car ran fine. Holley 650 carb.

Is this a typical vapor lock scenario? Surprised I never an into this issue In my carb street cars or race cars 10-15 years ago. Looks like I may need to check into the LARS fuel return line...

Any other thoughts, or is this a classic vapor lock scenario?

Thanks.


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## lars (Sep 28, 2004)

As you probably know from reading my posts, true vapor lock actually occurs between the tank and the pump on the suction side of the pump: Hot temperatures and the low pressure on the suction line combine to raise the vapor pressure (especially easily done on the modern fuels, which have a naturally high vapor pressure) of the fuel in that line, causing the fuel to vaporize or boil. The problem is further exaggerated at higher altitudes and when climbing hills. To solve this problem, GM (and most other auto makers) installed a fuel return, or "bleed line", at the fuel pump, so that the pump keeps fuel circulating at a higher rate between the tank and the pump. This higher rate of circulation keeps cooler fuel from the tank in the fuel line, and assures that the pump sees only liquid fuel - not vapor. A stock, engine-driven mechanical diaphragm pump cannot deal with fuel vapor, and its output pressure will go to -zero- when the pump starts sucking vapor instead of liquid. This condition is vapor lock.

There is a second, similar, problem that is particulary a problem on the Holley-style carbs. The Holley carb has a float bowl that hangs off either end of the carb, and in the bottom of that bowl is the accel pump well. After shutdown on a hot day with a hot engine, radiant heating from the engine will hit the accel pump well and float bowl, and our crap high vapor pressure pump gas will start to boil (vaporize). Since the accel pump well has a checkball in it, the expanding, vaporizing fuel in the accel pump well will seat the checkball, and fuel will actually squirt out the accel pump discharge nozzle, just as if someone were sitting in the car pumping the gas pedal. Once it shoots and relieves, the fuel starts vaporizing again, seating the ball, and the "pumping action" will continue for multiple cycles. This results in the engine being flooded upon restart, and it will run like crap until the flooding condition has cleared out, and further run like crap until all the vapor has been cleared out of the carb bowls & pump wells and replaced with liquid fuel again. GM's fix for this was the reflective carb heat shield between the carb and the manifold: The heat shield must extend out far enough to provide reflective shielding for the bowls and pump wells. Keep in mind that this problem is not caused by conductive or convection heating, so installing thick gaskets under the carb won't help: It's radiant heating, and it is solved with the reflective shield. Many people see the results of this as fuel dripping or seeping out of the throttle shafts and puddling on the intake manifold - the incorrect "diagnosis" is that the carb needs to be rebuilt and throttle shaft bushings must be installed...

If you drop me an e-mail request for my "BG Carb Installation Paper" it has a section and some photos of a reflective shield installation along with part numbers. The shield, along with the return line, solves all the heat and vapor-related problems, whether they are true "vapor lock" issues on the suction side, or "fuel boiling" issues on the pressure and carb side of the fuel system.

Lars
[email protected]


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## geeteeohguy (Feb 2, 2008)

Well, there you go. The old timers with vapor locked flathead Fords used to simply install a half of a grapefruit over the top of the fuel pump and continue on their way....The newer fuels seem to be very volatile and prone to vapor lock issues, particularly on hot soak.


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## lars (Sep 28, 2004)

Just some intersting follow-on facts about vapor lock and boiling fuel (at least I think this is interesting, but I'm an engineering geek...):

Gasoline, not being a "pure" substance, but a mix of various compounds (all with differing boiling points), does not have one single boiling point. The lighter, more volatile (higher vapor pressure) substances in gasoline (at sea level pressure) will actually start boiling at 90 - 100 degrees F. The heavier substances can resist boiling up to 300-400 degrees. This means that the suction side of your fuel line, which operates below atmospheric pressure due to the fuel pump suction, will have boiling and outgassing substances on a moderate 80 - 90 degree day. If the fuel in the suction line is not moving along at a reasonable pace, these boiling substances will quickly form gas bubbles in the suction line big enough to "stall" the diaphragm fuel pump.

Many people think that the vapor lock occurs in the hot engine compartment in the metal line between the pump and the carb. This line, being under 4 to 8 psi of pressure, is like a pressure cooker: It dramatically increases the boiling point of even the lightest compounds in the fuel, and the fuel cannot boil under this pressure at temperatures less than about 300 degrees.

However, once the fuel is allowed to enter the non-pressurized carb float bowl, the lighter elements can, once again, boil (vaporize) and cause drivability problems if the fuel has been allowed to heat-soak in the line between the pump and the carb. The drivability problems will persist until enough fuel has flowed through the carb float bowl to cool it down below the boiling point of the lighter substances. For this reason, I advocate installing a return bleed line as close to the carb inlet as possible in order to keep cool fuel from the tank constantly circulating through the fuel line all the way from the tank to the carb.

This quote from a recently published SAE paper reflects the same concepts regarding gasoline/ethanol blended fuels and their Reid vapor pressure (RVP):
"There are two concerns with blends: too-high-RVP vapor lock behavior and water-induced phase separation. For automotive use, vapor lock concerns vanished with modern electronic fuel injection. These feature a fuel rail with a return line. The fuel circulates continuously, so it never gets hot, and thus never vapor locks."

Since modern fuels are designed for modern fuel-injection systems, those of us insisting on using "antique" carburetors must adapt these old systems to accommodate the unique properties of the modern fuels. That involves running injection-style return lines and keeping the heat out of the stagnant fuel in our fuel supply lines.

Lars


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## FlambeauHO (Nov 24, 2011)

Very informative, didn't know that.


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## geeteeohguy (Feb 2, 2008)

Nor did I, not in that kind of detail. Makes sense, though....you raise the boiling point of a cooling system with a pressurized radiator cap, so it stands to reason that pressurized fuel has a raised boiling point, and fuel at atmospheric pressure or in a vacuum has much lower boiling resistance. Makes perfect sense. Thanks, Lars!!


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## leeklm (Mar 11, 2012)

Thanks Lars!


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## cij911 (Oct 25, 2017)

lars said:


> Just some intersting follow-on facts about vapor lock and boiling fuel (at least I think this is interesting, but I'm an engineering geek...):
> 
> For this reason, I advocate installing a return bleed line as close to the carb inlet as possible in order to keep cool fuel from the tank constantly circulating through the fuel line all the way from the tank to the carb.
> 
> Lars


Lars - I believe I am experiencing the vapor lock issue. When the car is hot, I have to either crank the car for a while or hold the gas pedal down ~ 1/3 of the way. The hot engine runs rough after start-up or until I hit the throttle. 

Would you expand on how you would run a return line on a Tri Power setup ? And also post a part number to the reflective carb shield you mentioned in another post ? Thanks

Chris


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## PontiacJim (Dec 29, 2012)

cij911 said:


> Lars - I believe I am experiencing the vapor lock issue. When the car is hot, I have to either crank the car for a while or hold the gas pedal down ~ 1/3 of the way. The hot engine runs rough after start-up or until I hit the throttle.
> 
> Would you expand on how you would run a return line on a Tri Power setup ? And also post a part number to the reflective carb shield you mentioned in another post ? Thanks
> 
> Chris


You are talking to a ghost!! LOL Last post was 2012, look at the top left corner found on the post before yours. Always check this as a post can be pulled up from the past and be many years old - thus no reply.


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## cij911 (Oct 25, 2017)

PontiacJim said:


> You are talking to a ghost!! LOL Last post was 2012, look at the top left corner found on the post before yours. Always check this as a post can be pulled up from the past and be many years old - thus no reply.



I chatted with Lars this morning. He is super knowledgeable, friendly, and helpful. (Similar to you Jim ...)


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## My65goat (Jul 26, 2017)

cij911 said:


> PontiacJim said:
> 
> 
> > You are talking to a ghost!! LOL Last post was 2012, look at the top left corner found on the post before yours. Always check this as a post can be pulled up from the past and be many years old - thus no reply.
> ...


 Hello cij911, I'm interested to hear what Lars said about the 65 tripower hot start issue. I also have the 65 tripower and experience the same thing that you mentioned when engine is hot and parked for a little while. 

I have to crank the engine quite a bit before it starts. 

I think that the gas in my carbs is evaporating as it sits hot ?

Please let me know if you find a solution. It drives me nuts when I get together with a bunch of other cars and I have to crank my engine over more than I should have to. 
Thanks


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## cij911 (Oct 25, 2017)

My65goat said:


> Hello cij911, I'm interested to hear what Lars said about the 65 tripower hot start issue. I also have the 65 tripower and experience the same thing that you mentioned when engine is hot and parked for a little while.
> 
> I have to crank the engine quite a bit before it starts.
> 
> ...


There appear to be three different common hot start problems with the GTO. 

1. Car won't crank or turnover when hot. This is most likely caused by excessive heat on the starter wire or solenoid, rendering the starter useless until it cools. (I don't have this issue likely because I installed a Ford solenoid (i.e. MAD electronics) to only have the starter wire be positive hot while cranking.)

2. Car will crank but not fire up for a while, but runs fine afterwards. This is most likely caused by the gas in the bowls boiling and flooding the motor. You can try to reduce the heat as much as possible - phenolic spacers, heat shields under the carbs, etc. I have the phenolic spacers and may look at fabricating a heat shield for the carbs, but am skeptical if that will do much. I think the bigger issue is that I have the crossover ports open, so exhaust gas flows under the center carb to assist with cold starts.

3. Car seems to die, not start, or run rough when hot. This could be a vapor lock. Best solution for this is to use a return line back to the gas tank. I don't have this setup, nor am I experiencing vapor lock as my car runs great cold or hot.


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## My65goat (Jul 26, 2017)

cij911 said:


> My65goat said:
> 
> 
> > Hello cij911, I'm interested to hear what Lars said about the 65 tripower hot start issue. I also have the 65 tripower and experience the same thing that you mentioned when engine is hot and parked for a little while.
> ...


I only have a problem like 2. I think I will try the phenolic spacer a block off the crossover.
Thanks cij911


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## cij911 (Oct 25, 2017)

My65goat said:


> I only have a problem like 2. I think I will try the phenolic spacer a block off the crossover.
> Thanks cij911


If you plan on blocking off the crossover ports, then you'll need to plan on converting to an electric choke. IMHO I'm not sure it is worth it. My car starts fine when hot if I hold the throttle down ~ 1/3 of the way while cranking. For me, this is an easier / cheaper solution than removing intake manifold (possibly head depending on how you want to fill the ports), etc.

Good luck

Chris


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## My65goat (Jul 26, 2017)

cij911 said:


> My65goat said:
> 
> 
> > I only have a problem like 2. I think I will try the phenolic spacer a block off the crossover.
> ...


Actually I am planning on getting the electric choke conversion that the guys at Pontiac tripower fabricate.I will also get the phenolic spacer,to see if it helps. If not, and when I build the spare engine I will be blocking off the crossover. 
There is no reason I shouldn't be able to just turn the key and have it pop right over. I will get it, I just need to keep fine tuning. 
Thanks


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## dave84 (Nov 3, 2010)

cij911 said:


> I chatted with Lars this morning. He is super knowledgeable, friendly, and helpful. (Similar to you Jim ...)


I too am experiencing the same as what you described, starts fine, after driving a bit, start up is takes a minute. A buddy brought up vapor lock due to the hotter running Pontiac motor. Got a 455 in a 66 lemans. Did you find a good solution?


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## Lemans guy (Oct 14, 2014)

Dave, Hotter running Pontiac motor is basically a myth. If folks set up the water pump wrong for example that would happen.

Set up correctly they run cool. My 461 in 66 Lemans runs 180 degrees.

What you are having is heat soak. Fuel in the lines and bowls heats up after shutdown flooding the intake. Hard start is because it is flooded and bowls are not full.

The heat soak fix is a phenolic spacer for conductive heat and carb heat shields for the radiant heat. Wrapping your fuel lines in DEI heat wrap helps as well.

Vapor lock occurs when you are driving and the car stalls because the pump cannot pump a vapor, reread Lars description of all this above. He lays it out.

A return line fixes vapor lock, phenolic spacers and heat shields and Heap wrap fix heat soak.


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## dave84 (Nov 3, 2010)

Lemans guy said:


> Dave, Hotter running Pontiac motor is basically a myth. If folks set up the water pump wrong for example that would happen.
> 
> Set up correctly they run cool. My 461 in 66 Lemans runs 180 degrees.
> 
> ...


Thank you lemans_guy, not ever having to mess with a spacer before, after looking up what my options are, does height help x amount more the taller you go bearing what will fit? My current setup topend is an edelbrock intake and a Weber carb. I'll first wrap the lines where heat is closest to see if that works. Originally planned to swap to a Holley carb.


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## Lemans guy (Oct 14, 2014)

Dave, yes thicker phenolic spacer is better against conductive heat, but space matters. Since you are thinking Holley Check out Quick Fuel's Technology (QFT) Black Diamond series. They are jet coat treated black to reduce heat,...they say 6% reduction. That is the Carb I use.

also it is just like a Holley, only better looks the same and the bowls hang out back and front so you have to also beat radiant heat. The easiest way is to use Quick Fuel's 
"cool Carb" spacer...it has a composite inside and hangs out to beat the radiant heat as well, and it is thin. That, a black diamond carb and some DEI heat wrap would all contribute to cooling. But the Black diamonds are expensive so you could go with a QFT street carb with chrome or diachrome etc finishes and just use the cool carb spacer.

The story on QFT is 3 Holley Engineers in the 90"s tried to get Holley to change the carb systems to make all the circuits more adjustable. tuning it in with air bleeds, Power Valve channel restrictors, more see through sight glasses, 4 corner idle, etc. But corporate would not go for the changes so they started their own company. It took off racers loved it, they were highly successful. They sold the company t o Holley in 2012 or so, and to Holley's Credit they kept the name and the carb line and they sell and service it well. Many parts are interchangeble.

I can tune in my circuits and if you get idle problems, any running problems and you have a quick fuel they can more readily be fixed.

Go to the Holley web and click on QFT and watch their video on "cool Carb", also Dashman on the web is an expert on Phenolic spacers if you need one.

I can't use cool carb because I keep my exhaust crossover open, and it would melt. so I use a Phenolic spacer sandwiched between two aluminum Radiant heat shiels that I have covered with DEI 2000 degree heat tape, works great,

be like Elvis....Stay Cool!


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## cij911 (Oct 25, 2017)

Does anyone know of any heat shields for a tri power setup?


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## Lemans guy (Oct 14, 2014)

Cj....Would 3 2 bbl Rochester phenolic spacers work?

Have you talked to Dashman?....he is good on the phenolic spacers


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## Lemans guy (Oct 14, 2014)

Cj..........will 3 2 bbl Rochester phenolic spacers work?

Have you talked to Dashman? He is an expert on phenolic spacers,

He has a web page...


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## cij911 (Oct 25, 2017)

Lemans guy said:


> Cj..........will 3 2 bbl Rochester phenolic spacers work?
> 
> Have you talked to Dashman? He is an expert on phenolic spacers,
> 
> He has a web page...


Yes I actually did talk to Ben / Dashman and already have the spacers. I am hoping to add a heat shield as well and nobody seems to sell them for the tri power setup. Maybe I have a new product idea lol....


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## Lemans guy (Oct 14, 2014)

Right!....maybe sone sheet metal or aluminum, painted with high heat engine paint, and I use that DEI heat tape under my shields for extra...

Seems like on a tripowner you might could bolt the sheet to the front bolts of one carb and the rear bolts of the next carb, two of those and a short piece on each end maybe two inches long that bolts to the front and rear end carbs. Maybe some cutouts adjustments for fuel lines etc.

Could be done, of course Rochester’s do not have the bowls hanging out over the intake like Holley and QFT, so it may not be as needed, but if one is having heat soak issues it may help, and could probably be made to look ok.

Anything that reduces heating up carb and fuel lines will help to prevent it


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## dave84 (Nov 3, 2010)

Used the DEI velcro shroud on the line running from pump to carb and take out the lemans for a bit today to test. On the higher dollar end, has anyone change to a Holley EFI setup? From what I see at the tank and mechanical fuel pump, I have two lines in place already, unless pontiacs have two for a different reason, I think there's already a return line.


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## GTO44 (Apr 11, 2016)

dave84 said:


> Used the DEI velcro shroud on the line running from pump to carb and take out the lemans for a bit today to test. On the higher dollar end, has anyone change to a Holley EFI setup? From what I see at the tank and mechanical fuel pump, I have two lines in place already, unless pontiacs have two for a different reason, I think there's already a return line.


The Sniper needs 3/8” fuel lines. You can use the old feed line as a return. The factory 5/16 return can not be re used. You’ll have to run a new 3/8in efi rubber hose or -6an black braided nylon or stainless steel hose as the new feed line.


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