Vapour Lock Definition

[240znz]

I have seen many thread about vapour lock but nothing about what it actually is or how it is caused.

Is it only found on Smog equipt or all 70-78Z's

Can someone who knows about this in detail enlighten us all?

[guy_geo]

I'll give it a try...

If the gasoline was all made up of high volatility compounds, then the gasoline would evaporate too easily during hot weather and vapour lock could occur. Vapour lock refers to the situation when the fuel boils in the fuel lines so it cannot be delivered correctly to the engine. When vapour lock occurs, the engine operates as if it was running out of fuel.

The gasoline we buy at the pumps is not the same year around. During the summer, the refiners supply gasoline with a lower volatility to reduce vapour lock problems on our vehicles. This low volatility gasoline also helps protect our environment by reducing hydrocarbon emissions caused by gasoline evaporating. During the winter months, high volatility gasoline is produced to help our cars start better during cold weather. If we try to use gasoline made for warm weather use during a winter cold snap, then the engine may not start! During fall and spring, the gasoline is blended to an intermediate volatility to enhance vehicle driveability during those climatic conditions.

Am I correct?

[guy_geo]

Or htis:

Vapor lock occurs when the vapor pressure of the fuel is higher than the surrounding environment. In older engines with "sucking" fuel pumps at the engine the pressure in the fuel line to the tank was reduced by the sucking action of the fuel pump drawing fuel form a fuel tank nearly twenty feet away, and when heated, the gasoline actually boiled creating a vapor of gasoline which the fuel pump could not handle, thus the name "vapor lock".

Today's fuel pumps are in the fuel tank where they push the fuel under pressure to the engine systems. Excessive fuel which is not used is sent back to the fuel tank. This causes a constant flow of pressurized, cool fuel in the lines to the engine system, be it fuel injection or carburetor. As a result you would have to heat the fuel line with a propane torch to get the fuel to boil and even then it would be cooled by the flowing fuel circulating from the tank to the engine and back again to the tank. Vapor lock as we once knew it is a thing of the past.

When gasoline overheats and boils inside the carburetor bowl or fuel pump of a hot engine, it ceases to flow. This can cause stalling or hard starting. This is called vapor lock, and it usually happens during hot weather. If a hot engine won't start, all you can do is let it sit and cool off. You should check the cooling system to see if anything is causing the engine to run unusually hot (a bad thermostat or cooling fan, for example). Switching brands of gasoline may also help.

:eek:

[Daniel]

I think guy must have had an encylopedia for lunch!! (just kidding guy).He is correct. The carbs on the Z are on top of the exhaust.On hot days when at idle that heat boils the fuel in the carb bowls.Just like boiling water this causes the air in the fuel to expand.Now instead of liquid fuel for your carbs to atomize it has gas vapor.Kinda like when they pump the bottom of the keg at your favorite pub.Beer vapor. Have fun!! Daniel -Oh yeah, the best description I have heard is your car starts bucking like a bronco as the engine gets fuel then vapor then fuel.

[guy_geo]

Yep Daniel , I spend my whole lunch time finding the correct answer, so do I deserve an A+?????

[240znz]

OMG...information overload.......cheers.

[jcdozier]

Good job, Guy. BTW, the early Z's didn't come with the tank-mounted electric fuel pump, just the mechanical "sucker" mounted on the engine block.

I wouldn't think vapor-lock would be much of a problem in the UK, but in the southern reaches of the US the 100+ degree days can cause problems. It may help to insulate the fuel rail - I've seen small-diameter tubing insulation on some of the auto parts sites.

My '71 is a "pre-electric", with only the mechanical fuel pump. I wrapped the fuel rail with aluminum foil, shiny side out, ("real" insulation is on my to-do list, albeit near the bottom). Doesn't look very professional, but I don't have vapor lock problems. There is no doubt also a way to insulate the carb bowls, but I haven't tried.

JIM DOZIER

[Daniel]

Hey Jim, The foil insulation has a gotcha.It keeps heat out until it gets in.Then it acts like a blanket in july and holds the heat IN.The shield between the exhaust/intake is the answer. Daniel

[jcdozier]

BUMMER, Daniel ! You're right, insulation can work both ways. I understand the heat transfer "stuff" (I did stay awake MOST of the time in ChE Thermo class ) and also understand that the "heat entrapment" problem could arise. (Driving a hundred miles and then parking in direct sun for a few hours in August might do it.) However, I haven't had any problems so far.

I agree about the intake heat shield - no Z south of the Mason-Dixon line should be without it:finger: .

JIM DOZIER

[Zvoiture]

Hey, you think you got problems? Here on the Central Coast of CA we still get 'winter gas' even though the temp never goes below 50 most of the time. It is often 70 or 80 here for good stretches during the 'winter'. This stuff vapourizes at the drop of a hat! Thank God for electric fuel pumps. Just turn the key to 'on' and listen until the bubbles hit the tank (blub-blub-blub-blub) and hit the ignition.

steve77

[v12horse]

Thank you so much guys. Now I know why my car has been broncoing in the heat. I thought it was vapor lock, but you verified my claim. Now I have to insulate my hoses or get a heat shield to cover my exhaust manifold. Any suggestions? The only wierd thing is that my car just started doing this. I had driven it in the heat before with no problem until now. It was 88 degrees F both times when my car got vapor lock. Any more suggestions why it would all of a sudden happen now and not before?

Thanks ahead of time,

Ben

[240ZX]

Maybe try a different brand of gas!!?