Vette(s): 1981, dark blue w/carmel interior. 350 stock, auto tranny, Q-jet carb, ECM box
Joined: 8/9/2010
Posts: 91
1981 Corvette, what is a fuel tank pressure control valve? I see in the manual where this is listed as a possible problem but no details are in the manual.
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Joined: 11/8/2003
Posts: 19512
I'm not sure...there may be a terminology issue. The only "pressure relief valve" of the fuel tank that I am aware of is built into the top of the tank itself. It's purpose is to blow off if the tank reaches a dangerous level of pressure. The only other thing I can think of is the purge valve on the charcoal canister. The fuel tank is a sealed type, but vapors are supposed to be routed to the canister, and then sucked into the engine while running.
What problem are you having that lists this as a possible cause?
____________________________________ Joel Adams C3VR Lifetime Member #56 My Link
Vette(s): 1981, dark blue w/carmel interior. 350 stock, auto tranny, Q-jet carb, ECM box
Joined: 8/9/2010
Posts: 91
My re-occurring loss of fuel issue surface again yesterday. At times when out for a drive, about 25 miles and temps above 70/f when I stop for a short time, say 5 minutes, every once in awhile when I recrank only the fuel in the carb's bowl is used and then she shuts down and I have no fuel to carb.
Researching I found I did not have fuel some out of the "S" hose to the fuel pump. I could pressurize the tank by blowing air into the filler neck and fuel would flow from the "S" hose. Stop air pressure and fuel also stopped.
Car is jacked up slightly at front but should fuel not flow freely from tank? Is the pump strong enough to suck fuel that far? The entire fuel system has been gone thru, tank drained and cleaned, new pickup tube sock, lines blown out, new "S" hose, new fuel pump, new line from pump to carb and new carb.
I saw the Tank Pressure Control Valve listed in the manual but did not know what this was.
Could my loss of fuel at times not be mechanical but pressure? A problem with the evap recovery system or cansister?
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
Joined: 11/8/2003
Posts: 19512
I thought we had already suggested loosening the cap when this occurs, didn't we? Sometimes, the tank may not vent properly thru the vent line to the canister(I've blown MANY spider/bug webs & even rust out of those vent lines in my time), and while drawing fuel from the tank, the pump could be drawing a vacuum in the tank, keeping fuel from going to the front. It's a rare deal, but entirely possible. As far as raw fuel running out of the "S" hose, the tank would need to be almost completely full for that to happen, since on your car, the fuel has to come out the top side of the tank, unlike the older Sharks, where the fuel came out the bottom of the tank. Were you having this problem before changing the fuel pump? I gotta ask, even tho I think you prolly replaced the pump because of this problem. It's possible the pump is just not working correctly, and drawing enough suction. The only real way to test that would be to install a pressure gauge on the line to the carb. You could also temporarily put a vacuum gauge on the INLET side of the pump, and crank the engine, to see how much vacuum it is capable of pulling. My only suggestion is to drive it with the fuel cap loose for now. One other really off the wall idea is that it is possible the cam lobe on the cam that the fuel pump runs off of is worn, and the pump is just not making a full stoke. This usually makes a noise, but it's not always audible. If yo remove the pump, and the pushrod, take a good look at the end of the pushrod that rides on the cam, up in the engine. There should be NO wear at all on it. I have seen this happen, but not very often. I do know the cams from the '78-mid '80s Chevies had a bad wear problem from not being properly heat treated at the manufacturer....
____________________________________ Joel Adams C3VR Lifetime Member #56 My Link
Vette(s): 1981, dark blue w/carmel interior. 350 stock, auto tranny, Q-jet carb, ECM box
Joined: 8/9/2010
Posts: 91
I spoke with my mechanic, Dub at Corvette Etc in Charlotte this morning, he did the last work, replacing the fuel pump due to this problem. When he had car then it would crank up okay then so was kinda chasing a problem that wasn't there at that moment. Dub feels may be the pump again, although it has an AC Delco pump on now. He did check the lobe when he swapped pumps and felt it was good.
Sometimes this loss of fuel happened when I stopped to gas up, so cap was removed and reinstalled and then she just stopped after fuel in carb bowl was used up. So lossening the cap did not change the problem. Yesterday was a 90/f day so only occurrs when it hotter, but not every time so what can work and then not work just to heat?
As I now going to get a rollback in the morning and get it back to Dub and hopefully still be lacking in getting fuel to carb and maybe he can nail down the problem. Either way I will get a new gas cap just in case.
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