Ethanol Gas and ECM Carb

[Tommy Almond]

Hopefully some here are still running the stock setup with the ECM controlled q-jet, here in NC when summer temps starts to rise into the 90s and even those water temps run 180ish my oil temps runs around 220ish and when I shut the lovely lady down the heat buildup in the engine compartment does have an effect on the gas in the line and carb's bowl and due to at least 10% Ethanol blend after sitting awhile she does crank up but have to spin her some to pump new fuel into the carb and I am told by the carb's experts this is mainly due to the 10% Ethanol turning to vapor easier than non-Ethanol gas. So my question is, do other have this problem and if so what do you do? Run higher octane gas that does not have Ethanol, pop hood when stop to let heat escape or some other means????? Once she fires up she runs fine as can be just harder to crank once sits awhile due to sorta like vapor lock effect. Oh and long live the '81 Corvette, a one year wonder but she is my wonder and still as close to factory as I can keep her.

 

[TimAT]

I don't have an 81, but ethanol does turn to vapor easier than straight gas. It also attacks the rubber parts in the fuel system, including the accelerator pump in the carb. Send an email to Lars Grimsrud at v8fastcars@msn.com and ask him for his recommendations. He has the part number for the pump that is not affected by the ethanol. Since you're trying to maintain the stock appearance I'll leave the suggestion for a carb heat shield out.

 

[Wajulia]

Mine cranks for a long time whenever I leave it for a long time. I dont think that it is vapor lock. I believe that there is a check valve in the fuel pump that if it is not working correctly will allow the gas to siphon out of the float bowl. Many blame it on the wells in the carb leaking, but Lars says that problem was fixed in the early 70's and is often blamed for the check valve failing. I have also leak tested the carb and there is no issue. I have been meaning to fix it (new fuel pump) but it is low on the list. Actually kind of like it, if the car sits for a while it empties the carb of gas -- no old fuel gunking up the carb if don't get around to driving it as much as I like.

 

[jj_long81]

maybe you are talking about the fuel inlet needle and seat. its the valve that hangs from the float with a rubber tip. the brass seats are know to leak even brand new ones. they should be pressure tested before they get installed. a trick it to form a new seat with an old check ball. also depending on the seat you installed a solid or slotted. slotted seats allow the fuel to drain to a lower level when sitting for a long period and will make it more difficult to start. hope this helps. all my info has come straight from cliff ruggles book how to rebuild and modify Rochester quadrajet carburetors.

 

[Jon S]

Hopefully some here are still running the stock setup with the ECM controlled q-jet, here in NC when summer temps starts to rise into the 90s and even those water temps run 180ish my oil temps runs around 220ish and when I shut the lovely lady down the heat buildup in the engine compartment does have an effect on the gas in the line and carb's bowl and due to at least 10% Ethanol blend after sitting awhile she does crank up but have to spin her some to pump new fuel into the carb and I am told by the carb's experts this is mainly due to the 10% Ethanol turning to vapor easier than non-Ethanol gas. So my question is, do other have this problem and if so what do you do? Run higher octane gas that does not have Ethanol, pop hood when stop to let heat escape or some other means????? Once she fires up she runs fine as can be just harder to crank once sits awhile due to sorta like vapor lock effect. Oh and long live the '81 Corvette, a one year wonder but she is my wonder and still as close to factory as I can keep her. ip

Tommy -

You are 100% correct! There is NOTHING wrong with your car, its the E90 Ethanol gasoline! My 1973 Lincoln experienced its first vapor lock in 40 years of ownership 2 weeks ago and my 1958 Cadillac (in the family since new) also experienced vapor lock 2 weeks ago. These cars were both well designed relative to gas line routing by the manufacturer . . . the only variable is the E90 gas. I haven't used my 1981 Corvette yet this year other than starting it up, but I'm convinced it will experience the same problem on warm re-start in this hot weather. I did some research and discovered that adding real tetra ethyl lead (which I have) not only raises 93 octane gas to 101 octane, it also helps to prevent vapor lock. Additionally the addition of Marvel Mystery Oil also reduces vapor lock. I've added these to all of my cars and also bought a case of Lucas Ethanol Conditioner. Just came back from a ride in the Cadillac, and no vapor lock. It's NOT your car!

Jon

 

[jj_long81]

I live in south florida. never happened to me.

 

[Jon S]

Just helped a 1978 Anniversary Edition yesterday - same problem - Vapor Lock!!!

 

[Tommy Almond]

Just helped a 1978 Anniversary Edition yesterday - same problem - Vapor Lock!!!

I'm finding on these hotter days when running for a while and engine oil temp gets up above 200/f and shut the engine off is when my initial harder to crank happens but if I pop the hood open and let the heat escape then no problem so the Ethanol in the gas is playing a part I do believe. I was going to fill up this weekend with non-ethanol 93 octane but at 55 cent more a gallon I passed to save some money. Well at least I know I am not the only one. The q-jet on the engine currently was re-manufactured by All American Carbs in Florida so it is right, and they told me this blend of fuel was causing this issue. Again LONG LIVE THE 1981 Corvette!

 

[beige81]

Greetings all,
Thanks to for the info! Thought I was leaking gas thru the carb but inside and outside the manifold was always dry. Have been having the problem since April, after driving, the next day it would always crank(just like filling the float bowl) and finially start. Not much problem with heat in this area yet. All the fuel locally has "up to 10% ethanol added"? Didn't drive it yesterday but did start and run it with the choke set for around 90 seconds, turned off ignition. Tried it this afternoon and started up like it is supposed to.
Should I be using an additive and what kind?
Thanks
Keith

 

[Jon S]

Greetings all,
Thanks to for the info! Thought I was leaking gas thru the carb but inside and outside the manifold was always dry. Have been having the problem since April, after driving, the next day it would always crank(just like filling the float bowl) and finially start. Not much problem with heat in this area yet. All the fuel locally has "up to 10% ethanol added"? Didn't drive it yesterday but did start and run it with the choke set for around 90 seconds, turned off ignition. Tried it this afternoon and started up like it is supposed to.
Should I be using an additive and what kind?
Thanks
Keith

Adding Marvel Mystery Oil to the gas raises the boiling point of E90 gasoline. Also, Tetra Ethyl Lead also increases the boiling point. I've added both to my gasoline.

Jon

 

[Tommy Almond]

What Ratio?

Adding Marvel Mystery Oil to the gas raises the boiling point of E90 gasoline. Also, Tetra Ethyl Lead also increases the boiling point. I've added both to my gasoline.

Jon

Curious, how much or what ratio do you add to the gas using the Marvel Mystery Oil or Tetra Ethyl Lead?

Of course here in NC we had so much rain in July barely had a chance to get the old girl out and temps have only been in the 80s mostly, maybe 3 days in July in the 90s......funny weather here this season.

 

[Jon S]

Curious, how much or what ratio do you add to the gas using the Marvel Mystery Oil or Tetra Ethyl Lead?

Of course here in NC we had so much rain in July barely had a chance to get the old girl out and temps have only been in the 80s mostly, maybe 3 days in July in the 90s......funny weather here this season.

I follow the directions on the Marvel container 8 oz to 20 gallons of gas and add 32 ox/20 gallons of Real Lead. So far, no more vapor lock.

 

[BigJimZ16]

Adding Marvel Mystery Oil to the gas raises the boiling point of E90 gasoline. Also, Tetra Ethyl Lead also increases the boiling point. I've added both to my gasoline.

Jon

Be REAL careful with that Tetra-Ethyl Lead... a great way to poison the Catalytic Converter and lose performance and function!

 

[motorman]

Mine cranks for a long time whenever I leave it for a long time. I dont think that it is vapor lock. I believe that there is a check valve in the fuel pump that if it is not working correctly will allow the gas to siphon out of the float bowl. Many blame it on the wells in the carb leaking, but Lars says that problem was fixed in the early 70's and is often blamed for the check valve failing. I have also leak tested the carb and there is no issue. I have been meaning to fix it (new fuel pump) but it is low on the list. Actually kind of like it, if the car sits for a while it empties the carb of gas -- no old fuel gunking up the carb if don't get around to driving it as much as I like.

there is a check valve in the fuel filter and if you use a filter without one the float bowl will drain down

 

[motorman]

I don't have an 81, but ethanol does turn to vapor easier than straight gas. It also attacks the rubber parts in the fuel system, including the accelerator pump in the carb. Send an email to Lars Grimsrud at v8fastcars@msn.com and ask him for his recommendations. He has the part number for the pump that is not affected by the ethanol. Since you're trying to maintain the stock appearance I'll leave the suggestion for a carb heat shield out.

here is the GM heat shield and they also have one for the Q jet