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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
My wife's 1987 IROC 5.7 TPI was having issues with high fuel tank pressure and a strong fuel smell on hot days. So much odor that I thought she had a fuel leak. I did a lot of research on the internet and this forum and this seems to be a common problem with a lot of people giving up and just living with it.
We bought the car last summer and I ended up replacing the in-tank fuel pump. Several junk pumps later I ended up with one that seemed okay, but at some point after that I began to notice the fuel tank was building a lot of pressure and when you parked the car you would hear the loud mooing sound of pressure through the charcoal / vapor canister lines and container. Winter came and the issue went away with cooler weather, but returned this summer with the hot weather.
I believe the issue is primarily the aftermarket pump creating too much pressure and heat. Much more than the original pump (the GM pump I removed definitely looked original). The engine had no fuel leaks, so I used these forums to diagnose the vapor canister system. I even rebuilt the vapor canister (different post). The vapor system was original and working exactly as designed, and I realized the mooing noise was coming from the in-line valve (canister purge valve) that allows pressure to vent from the tank to the canister. The way this car's system was designed the valve is activated by a ported intake vacuum line that opens the valve and allows pressure to release into the charcoal canister above idle. The problem was at idle there was no vacuum from the engine and the valve doesn't open, so high pressure from the tank overcomes the diaphragm in the valve and forces pressure out of the tank SLOWLY, causing the valve to make a "mooing" sound. The pressure release was much too slow so tank pressure remained high on hot days and caused the fuel smell.
I figured with the high tank pressures the valve needed to also be open at idle, so I removed the ported intake vacuum line from the valve and hooked it up to the round vacuum reservoir next to the canister under the car. This is vacuum from the intake manifold and has very high vacuum at idle and keeps the valve open almost all the time, allowing fuel tank pressure to vent pretty much constantly (except maybe under hard acceleration when vacuum is low, but fuel demand is high for acceleration so tank pressure should remain low). This seems to have fixed the problem and now the fuel smell is gone, tank pressure remains pretty much neutral, and the annoying "mooing" sound is gone.
Just wanted to offer this to hopefully help others with a similar problem.
Last edited by Aaron R.; 05-10-2023 at 09:09 PM.
Reason: Cclarify ported vs manifold vacuum
Next time i am in Missouri, I am going to look you up and take you and your entire family to a steak dinner! I have been pulling my hair out on this and found your post. Thank you for taking the time to write it!
Can you describe this in a bit more detail? I know there's the ball-canister stuffed down inside the fender. Hooking what to what now?
My '86 TPI reeks of fuel vapor after being run. Engine bay, no vapor at all. Seems to be emanating from the tank itself. Checked the relief valve back there and checks out OK. So either the intake isn't vacuuming the canister enough or I've got a bigger problem with the tank itself blowing off vapor.
I've noticed this on particularly hot days too, along with an occasional pop from the fuel tank expanding. I have been considering doing the same thing, but I wasn't sure if letting it vent constantly without purging would overwhelm the canister and cause it to smell while driving.
Can you describe this in a bit more detail? I know there's the ball-canister stuffed down inside the fender. Hooking what to what now?
My '86 TPI reeks of fuel vapor after being run. Engine bay, no vapor at all. Seems to be emanating from the tank itself. Checked the relief valve back there and checks out OK. So either the intake isn't vacuuming the canister enough or I've got a bigger problem with the tank itself blowing off vapor.
The ball canister inside the fender is a vacuum reservoir that just basically stores air in a vacuum for things like the brake booster and the vacuum lines under the dash. By taking the vacuum line from the valve on top of the vapor canister and hooking it up to the ball canister you have constant vacuum applied to the vapor canister valve, which keeps the valve open constantly (when the engine is running) and allows that pressure to escape.
The relief valve back by the tank is a one-way valve that lets air INTO the tank, but does not allow air to ESCAPE. The only way for air to escape the tank is to loosen the gas cap or apply enough vacuum to the charcoal canister lines or relief valve.
By hooking my lines up this way it permanently fixed the fuel smell and tank pressure problem at idle, while driving, and after shutting the car off. The original design worked fine when the car was original, however I believe the aftermarket in-tank fuel pumps caused the higher pressure. I have found that many aftermarket parts do not actually function like the original parts. After all of this I ended up finding an ACDELCO brand fuel pump. Some day whenever the aftermarket pump wears out I'll throw in the ACDELCO one and maybe test that whole theory.
"The relief valve back by the tank is a one-way valve that lets air INTO the tank, but does not allow air to ESCAPE. The only way for air to escape the tank is to loosen the gas cap or apply enough vacuum to the charcoal canister lines or relief valve."
.Actually Aaron, the valve back at the tank WILL open at 8psi positive pressure to let excessive pressure OUT of the tank. But mainly opens to let air IN.
"The relief valve back by the tank is a one-way valve that lets air INTO the tank, but does not allow air to ESCAPE. The only way for air to escape the tank is to loosen the gas cap or apply enough vacuum to the charcoal canister lines or relief valve."
.Actually Aaron, the valve back at the tank WILL open at 8psi positive pressure to let excessive pressure OUT of the tank. But mainly opens to let air IN.
I don't have the time to search right now for you, but do a search for "UFO valve". You will find it there. Also it is NLA - no longer available.
If yours is in decent shape, you need to carefully take it apart and clean it. Otherwise you have to find a good used one.
[QUOTE=Aaron R.;6249891]The ball canister inside the fender is a vacuum reservoir that just basically stores air in a vacuum for things like the brake booster and the vacuum lines under the dash. By taking the vacuum line from the valve on top of the vapor canister and hooking it up to the ball canister you have constant vacuum applied to the vapor canister valve, which keeps the valve open constantly (when the engine is running) and allows that pressure to escape.
The relief valve back by the tank is a one-way valve that lets air INTO the tank, but does not allow air to ESCAPE. The only way for air to escape the tank is to loosen the gas cap or apply enough vacuum to the charcoal canister lines or relief valve.
By hooking my lines up this way it permanently fixed the fuel smell and tank pressure problem at idle, while driving, and after shutting the car off. The original design worked fine when the car was original, however I believe the aftermarket in-tank fuel pumps caused the higher pressure. I have found that many aftermarket parts do not actually function like the original parts. After all of this I ended up finding an ACDELCO brand fuel pump. Some day whenever the aftermarket pump wears out I'll throw in the ACDELCO one and maybe test that whole theory.[/Q
I have the ball you are referring to, it has one hose attached to the vacuum canister. which line are you talking about hooking to it the one from the intake manifold? do you have any pictures o know it has been a while since your post
You just need to hook it up to a constant vacuum source, usually a large vacuum source at back of engine on most of these cars (like the large line for brake booster or the smaller lines going to interior vent control).
There were different canisters used in these cars but all were similar. In this picture below you will see two lines. You want the smaller one t-ed into a constant engine vacuum source so that any time the engine is running there is vacuum applied to the valve to help vent the fuel pressure and fumes.
To clarify further, there are two types of vacuum sources: ported and manifold. You want manifold (constant). Google search will give you more info.
Edit: I fixed some typos in my original post to clarify the vacuum source.
Just cleaned my charcoal canister bottom out with compressed air, replaced filter, inspected lines and all that, and replaced the purge valve with the 3 outlets just outside the canister with a new AC delco one. For this fuel tank purge valve, amazon has a new aftermarket one, just not sure how high quality it is being on amazon:
08-23-2016, 03:40 PM
nla gm 10033125
