Not sure if its a myth or not, but I read somewhere that some of the LG4s had in-tank fuel pumps to assist with the mechanical ones. My car is originally an 86 firebird SE and sported the anemic LG4 and a 700R4 auto. Can anyone tell me if I've got the mystery pump or If I'm just really gullable. I'm trying to figure out a fuel delivery issue and don't want to be surprised when I drop the tank.

 

Its True

I believe some of the 87"s had both...thought to help solve vapor lock issues.....if you have one in the tank, you should be able to hear it run.

 

oh ok. Well I'm a year behind it looks. I removed the computer a while back so I wouldn't be able to hear it anyway.

 

and most in tank pumps cannot be siphoned through so I would say you are probably OK.

 

I have a 91 RS, and my fuel pump is in the gas tank.

 

Yes, but if you have a 91 RS you dont have an LG4, your car is fuel injected.

From what I have read, the L69s pretty much always had the in tank pumps and the lg4 got it in 87.

Ben

 

And we are talking about having Both The in-tank and mechanical...

 

my '87 LT was originally an LG4 and has the in tank pump that i'm not using. i'm using a mechanical one and it sucks through the in tank pump just fine. from what i've heard only '87s had in tank pumps

 

You cant suck through the one in my 89...as is the story with alot of pumps...seems this has been a mystery for awhile now, some will most wont....

 

The in tank pump started in 1984 with just the L69 to help alleviate the vapor lock condition...

I believe that all carbed engines (LG4 and L69) received the electric helper pump starting in 86...

Any EFI applications (TPI or TBI) have the fuel pump in the tank (no mechanical pump stock)

 

Actually, I dont know the year for the l69 but I am almost positive that the electric pump wasnt on the lg4 until 87.

Ben

 

My '86 LG4 donor car did not have the in-tank pump.

There are stories of a retrofit kit the dealer could install if a customer complained of vapor lock. It included the in-tank pump, and a little blower mounted in the passenger fender that would blow air onto the carb when the engine was off and hot. My '86 did have that little blower (which is now in my '82).

 

What exactly is vapor lock?

Ben

 

Its a condition that happens when there is a strong vacuum created by the sucking action of a pump. The gas is leaving the tank but nothing is filling the void (like air), so the vacuum becomes stronger than the sucking force of the pump creating surges in the line and a lack of fuel where needed. I've also heard people refering to fuel boil as vapor lock. Excessive heat expands the fuel in the lines causing it to be forced past the needle and seat in the carb.

I have a fuel boil issue that needs to be addressed. I was asking about the pump because I'm dropping the tank and installing all new fuel line. I wasn't sure if I had the "helper" pump or not, guess this is still a mystery considering all the theories going around.

 

On the helper pump year...I am wrong as to when it applied to the LG4, but it started on the L69 in 84...1 for 2 on that one..

As for vapor lock, the ONLY definition I have ever heard for it is when the fuel boils and it turns to vapors, thus vapor lock. It doesn't push the fuel past anything in the carb, the mechanical pump simply can't pump fuel vapors only liquid fuel.. This can be an issue on carbed cars where there is no fuel return line...

The way to solve this is to insulate the fuel line from the heat and/or install a fuel pressure regulator and return line to the tank thus ensuring a constant supply of fresh (cool) fuel..

The vacuum explanation sounds cool...but it simply isn't vapor lock...what you are describing is when the check valve on the tank vent fails...that can actually partially collapse your tank...

 

I've heard the term used for both and this was coming from an ASE master technician. Not to say this means its correct. BTW- If you don't believe the expanding fuel is forced past the needle and seat then read the edelbrock Performer Series carburetor manual. I guess you could disagree with them....they're not experts .

 

If anybody can remember their chemistry.. a liquid will boil at a lower temperature when the pressure on the liquid is decreased. So when a mechanical pump applies vacuum to suck the fuel from the tank, it lowers the boiling(vapor point) of the fuel in the line. This can cause the fuel to boil and vapor lock the pump. Heat applied to the line anywhere does the same thing.

If you want to test the theory put some water in a syringe. Now block the opening to the syringe and pull the plunger back. The water will start boiling in the syringe at room temperature.

 

This I gotta try...now if I can just find that darn syringe......

 

No4NJunk:

I worked at an auto parts store for 7 years...I have met some ASE certified mechanics whom I wouldn't let touch the model of my GTA (for fear they would somehow start it on fire ) Remember, just because you read something in print doesn't always make it true...

Check out the April 03 issue of Super Chevy and read their explainations for "Horsepower Myths Revealed" to understand what I mean...

After all, 2 correct out of 4 ain't bad for them these days

 

On the Edelbrock website, I only found the following text in the carb troubleshooting section:

"Excessive under hood temperature: Ensure fuel line is not located too close to heat sources such as the exhaust or block, causing expanding fuel to be forced past the needle and seat. Fuel can also boil inside the carburetor due to missing gaskets, spacers, or heat shields. Also check to see if the exhaust heat riser is stuck, allowing excessive heat under the carburetor. Use Edelbrock Divided Heat Insulator Gasket #9266 for dual-plane manifolds, Square-Bore Heat Insulator Gasket #9265 for single-plane manifolds, and #9267 Heat Insulator Gaskets for dual-quad manifolds. "

You are right...it is in print...but every experience of vapor lock I have ever had is a lean condition caused by not enough fuel reaching the carb in the first place...not an overly rich condition due to fuel being "forced" into the carb venturi...

I think Edelbrock is full of it...any one else who has experience fuel being forced into the carb due to boiling?

 

There is nothing wrong with the statements by Edelbrock you quote. It just isn't the classic "vapor lock" scenerio being described.

Fuel boiling in the line between the mechanical pump and carb has the potential to overcome the pressure capability of the needle/seat. Probably happens most when a hot engine is shut off, rather than a running engine.

Another way to see the boiling point reduction effect of reduced pressure: Boil water in an open test tube, remove from heat, cork the tube. As it cools, the internal pressure will reduce, and it will go through several boiling cycles before it reaches room temp. Kinda cool.

 

88TPI thats it! I definitely agree on the ASE techs. They don't know everything (nobody does) but this guy is credible....trust me. Anyway I agree with you about everything siad but I just think there are always "other" conditions. I've got a horrible vapor lock issue right now. My fuel line runs 2" away from the whole exhuast and headers.

 

I would wrap the fuel line with some insulation and then some aluminum foil or aluminum tape...like HVAC duct tape...the silver stuff, not the regular duct tape...

I agree that anything is possible...ie boiling the fuel past the needle/seat...but I haven't actually seen any real cases of it...merely boiling in the lines on the way to the pump...

I guess that with a mechanical pump, if it can reach the inlet side of the pump in liquid form and there is a heat condition in the tube on the way to the carb, wrapping the line from the pump is the way to go...most of the time though, it is getting the liquid fuel to the pump that is the key...

 

Test tube? darn, I just through my last one away... Seriously, i am going to try this, Ive never seen this or even heard of it..(lack of education shows..

5-7, how hard is it to rebuild a 600 edelbrock? Are there directions with the kit?

 

bla bla bla bla bla bla bla are you now done talking carberator chinese.... maby one day i will understand or maby not...