QuestionQUESTION: Brad, I have a 75 vette with a 5 yrs old GM crate motor. The stock Quadra Jet was replaced about 3 yrs ago with a model 4175 Holley which is a "direct bolt" replacement which worked great. About a year ago I had a problem with the rubber fuel bladder so I replace the tank & sending unit. Then I had a problem with the tank pulling a vacuum causing the the car to shut down. After checking fuel ines & hoses I replace gas cap with a vented one this solved that problem. But now the car will run great sometime and then miss after running for few minutes or so. (engine temp @ 200 ) The next day it will run ok for about 1 hour or so before acting up. I've reset the float, checked the filter, and used gas dry. When I've checked the carb after it has stalled the primary chambers are wet. When at idle (missing) gas seems to be oozing from the Phillips head screw that holds the pump shooter tower. The fuel pump is less than a year old.
Is there a seal under the Phillips head screw that can be replaced? Could the fuel pressure be too high? The last time I checked it was @ 7 psi. Any Ideas???
Thank you, Buddy
ANSWER: Based on what you tell me I think that the original problem that appeared to be solved with a vented fuel cap and the problem now are one in the same. I would dive into the charcoal canister evaporative emissions canister and its associated valves and plumbing. This device is supposed to vent the vapors from the fuel tank and store them into the charcoal. When the engine is running the vapors are drawn from the charcoal and burned in the engine. This device also vents the fuel tank to eliminate the vacuum build up. However when the valves and plumbing get a bit confused it can add vacuum to the tank and draw raw fuel into the intake system. There should be a vacuum diagram somewhere under the hood that should show the routing of the vacuum lines. The Chevrolet shop manual for this year car will give the test and repair procedure for the system. There is a filter pad on the bottom of the canister that if plugged will cause this problem as well. This filter is supposed to be changed at set intervals.
Brad
---------- FOLLOW-UP ----------
QUESTION: Brad, 1st of all thanks for the quick response. What I failed to tell you in the back ground info was that when I replaced the tank & fuel hoses I cleaned, checked out, and reconnected the charcoal canister. (Back when I got the car @ 30 years ago it had the line from the charcoal canister was removed from the carb & plugged) After I started having problems following the tank replacement I pulled & plugged the hose between the charcoal canister removed from the carb.
Next the troubleshooting effort has been on hold for more than 7 months (total knee replacement & a new C-6) Because of this I went back to the basics.
I pulled the fuel line at the carb and found the fuel clean and free of water, next I checked the fuel pressure and this time it is very erratic & high (between 8 -10 psi). I guess my next step is to get under the car and pull the fuel pump return line & check for blockage. If that check good pull then replace the fuel pump. I replace it when I was having problems following the tank replacement. It May be defective or got dirt resulting from the tank replacement. Hopefully this will fix it, if not ??????????????
Thanks again, Buddy
Answer8 to 10 PSI can be too much of a good thing and be holding the needle valve open a bit. Especially if the composition float is starting to show signs of being too heavy. Fuel pump pressure is strictly controlled by the spring under the diaphragm. 7 to 8.5 PSI is the recommended range for that car.
Brad
