missing when quarter tank gas or less
Thread Starter
Instructor
Joined: Aug 2007
Posts: 224
Likes: 1
From: SHERMAN TEXAS
missing when quarter tank gas or less
my car has developed this problem. once the gas level reads under 1/4 tank it runs rough has a hesitation and misses real bad under a load like its running on 6 cylinders. i have replaced every electrical part to figure this out until a friend of mine mentioned water in tank or bad gas. i tried a bit of b12 and put in 5 gall 100 octane tehn it was at 3/4 tank. i drove it all day and the sputtering problem slowly went away to finally no problem at all. today i was out and the problem started again to the point i thought i was going to pull over. i went to nearest station and filled my tank full the problem went away immediately.
anyone ever heard this before. thanks colin75
anyone ever heard this before. thanks colin75
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
my car has developed this problem. once the gas level reads under 1/4 tank it runs rough has a hesitation and misses real bad under a load like its running on 6 cylinders. i have replaced every electrical part to figure this out until a friend of mine mentioned water in tank or bad gas. i tried a bit of b12 and put in 5 gall 100 octane tehn it was at 3/4 tank. i drove it all day and the sputtering problem slowly went away to finally no problem at all. today i was out and the problem started again to the point i thought i was going to pull over. i went to nearest station and filled my tank full the problem went away immediately.
anyone ever heard this before. thanks colin75
anyone ever heard this before. thanks colin75
Racer
Joined: Jul 2007
Posts: 459
Likes: 1
From: Lindenhurst, Long Island, New York
Well if you have water in the gas, I would siphon the tank dry and flush your fuel lines. I'm sure most would also suggest emptying the carb as well. I've never done this, but I believe all you have to do is unbolt the carb, flip it over and let it drain.
If that doesn't solve your problem, I would look into what 81pilot suggested by pulling the tank and inspecting.
You do have a liner in your tank that could have come free and maybe giving you a false reading on the gas level or blocking the pick up when the level gets low.
If that doesn't solve your problem, I would look into what 81pilot suggested by pulling the tank and inspecting.
You do have a liner in your tank that could have come free and maybe giving you a false reading on the gas level or blocking the pick up when the level gets low.
Race Director
Joined: Oct 2000
Posts: 12,679
Likes: 3,129
From: Shamong, NJ
Well if you have water in the gas, I would siphon the tank dry and flush your fuel lines. I'm sure most would also suggest emptying the carb as well. I've never done this, but I believe all you have to do is unbolt the carb, flip it over and let it drain.
If that doesn't solve your problem, I would look into what 81pilot suggested by pulling the tank and inspecting.
You do have a liner in your tank that could have come free and maybe giving you a false reading on the gas level or blocking the pick up when the level gets low.
If that doesn't solve your problem, I would look into what 81pilot suggested by pulling the tank and inspecting.
You do have a liner in your tank that could have come free and maybe giving you a false reading on the gas level or blocking the pick up when the level gets low.
Sounds like you may have water in the tank, or a liner problem.The 75-82s, all had a rubber "safety" bladder in the tank. New repalcement tanks don't have the bladder.
If the bladder starts to leak, you get fuel between the tank and the bladder. With fuel under the bladder, the sender thinks the tank is full, but there may only be a little fuel actually in the bladder, the rest is under it.
Le Mans Master
Joined: Jul 2000
Posts: 6,001
Likes: 113
From: Saginaw Michigan
The most recent issue of Vette Vues (March 2010) mentions a Chevrolet Dealer Service Technical Bulletin 76-T-32 Fuel Restriction - High RPM 1976 Corvette) concerning the pickup tube in the 1976 Corvette fuel tank. The article about the Bulletin indicates that the tank needs to be dropped, the pickup tube removed, shortened, and a new sock strainer and coil spring installed on the end of the tube.
My 1975 had the same problem (missing and bad stumble at full load). My mechanic at Dynamic Corvette here in Saginaw dropped the tank, removed the pickup tube, removed the strainer sock, and cut off about 3/8 inch of the tube. He replaced the sock. He also stated that the sock was fouled after 30 years. (It could have been the combination of the pickup tube reaching the very bottom of the bladder, the sock being fouled, and the bladder itself restricting the flow of fuel.)
I know that the fuel tank and bladder is the same 1975, 76, 77. The article mentions a redesigned pickup tube with an end configuration that was revised the very last week of 1976 production. No mention as to the 1975s. I would guess that the pickup tube would be the same between 1975s and 1976s.
Jim
My 1975 had the same problem (missing and bad stumble at full load). My mechanic at Dynamic Corvette here in Saginaw dropped the tank, removed the pickup tube, removed the strainer sock, and cut off about 3/8 inch of the tube. He replaced the sock. He also stated that the sock was fouled after 30 years. (It could have been the combination of the pickup tube reaching the very bottom of the bladder, the sock being fouled, and the bladder itself restricting the flow of fuel.)
I know that the fuel tank and bladder is the same 1975, 76, 77. The article mentions a redesigned pickup tube with an end configuration that was revised the very last week of 1976 production. No mention as to the 1975s. I would guess that the pickup tube would be the same between 1975s and 1976s.
Jim
Last edited by Jim Shea; Feb 25, 2010 at 05:12 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If the gas you use has ethanol in it (usually 10%), water shouldn't be a problem. Ethanol and water will mix and dissolve in the fuel. So, if there is water, it should be mixed and have that trouble all the time. If the fuel you get does not have ethanol in it, buy a can or two of "Heet" fuel 'de-icer'. It is essentially alcohol and that will do the same thing with water.
Since it doesn't start this until 1/4 tank, I suspect the fuel pump is not 'up to snuff' (poor pressure) and is inadequate when the fluid height in the tank drops to a certain level. You can test this with a fuel pressure gauge...before you go to the trouble of changing the pump. If you do change the fuel pump, remove the right front wheel so you can get to it easier. Much more pleasant a job that way.
Since it doesn't start this until 1/4 tank, I suspect the fuel pump is not 'up to snuff' (poor pressure) and is inadequate when the fluid height in the tank drops to a certain level. You can test this with a fuel pressure gauge...before you go to the trouble of changing the pump. If you do change the fuel pump, remove the right front wheel so you can get to it easier. Much more pleasant a job that way.
Le Mans Master
Joined: Jul 2000
Posts: 6,001
Likes: 113
From: Saginaw Michigan
My 1975 had a new fuel pump. I really think it was more of a "head" of fuel above the pickup that made my car run better with a full tank. As the fuel was used the problem became much worse.
Jim
Jim
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
Jim's "head of fuel" above a too-tall pickup & fouled sock scenario makes the most sense to me ... then couple that with that history of the service bulletin & successful repair via shortened pickup. Moreover, he probably forgot more than the average bear ever knew.
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Le Mans Master
Joined: Jul 2000
Posts: 6,001
Likes: 113
From: Saginaw Michigan
The bladder can be removed. You need to drop the tank. Remove the pickup and pull the bladder. The upper part of the bladder forms a seal between the pickup plate and the tank. You need to cut off the neck of the bladder to reuse it as the seal. Nobody makes a replacement bladder or seal.
Jim
Jim
Safety Car
Joined: Nov 2005
Posts: 3,961
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From: Lompoc, CA. Santa Barbara County
The bladder can be removed. You need to drop the tank. Remove the pickup and pull the bladder. The upper part of the bladder forms a seal between the pickup plate and the tank. You need to cut off the neck of the bladder to reuse it as the seal. Nobody makes a replacement bladder or seal.
Jim
Jim
Bee Jay
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
OP, your symptoms match mine EXACTLY, for water in the gas...I bet you drain that tank and then look at the residue in the bottom, pour it into a clear glass, and you find something looking like watered down cream coffee....and smells like elephant's ***....
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
I removed the bladder in my early 75, but never had problems before or after. The pickup screen was in bad shape though.
The added fuel weight of the full tank will help a poor fuel pump as Jim Shea said.
Water in the tank will also cause the same problems.
You may as well dive in and do it all at once.
Drain the tank and clean (good idea after 35 years) , do the pickup mod and sock (obviously if there was a TSB it was a known problem), maybe remove the bladder and if you still have problems, work on the pump.
Win Win situation
The added fuel weight of the full tank will help a poor fuel pump as Jim Shea said.
Water in the tank will also cause the same problems.
You may as well dive in and do it all at once.
Drain the tank and clean (good idea after 35 years) , do the pickup mod and sock (obviously if there was a TSB it was a known problem), maybe remove the bladder and if you still have problems, work on the pump.
Win Win situation
