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Tim's Auto Repair
Tim's Auto Repair, mechanic
Category: Chevy
Satisfied Customers: 16031
Experience:  Have owned a repair shop for 25 yrs.
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GM/Chevy P30 truck chassis: 1986 GM P30 with 454 Quadrajet

Customer Question

1986 GM P30 with 454 Quadrajet carb engine under a motorhome. Engine runs fine when cold, 7lbs fuel pressure per guage. Operating temp fuel pressure drops to near 0 on guage. Driving at thirteen miles hints of fuel starvation, Up to 50 mile acts like lack of fuel overheats. Installed new manual pump twice, added 6 lb electric pump by tank switch. Gas mileage down from 9 to 5.3 MPH in 50 miles.
Submitted: 2 years ago.
Category: Chevy
Customer: replied 2 years ago.
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Expert:  Tim's Auto Repair replied 2 years ago.
Hi, My name is ***** ***** for the delay.I will do my best to assist youplease explain,,you do loose fuel pressure when the engine gets hot?with nthe mechanicaland the electric pump?
Customer: replied 2 years ago.
Yes, wondering if the bypass/turn line off the pump back to the tanks is causing the pressure loss when up to operating temp or beyond. My next move is to bypass the mechanical pump and straight from electric in back (6lb) to carb which eliminates the return line setup complete. That should give me a true pressure reading to the carb. While I'm at its since she did get hot last night and spit I'm going to replace thermostat with new 180 degree and all fresh antifreeze/water. My gut tells me that direct electric pump will solve the starvation problem. I've even seen the fuel pump push rod seize up in the down position when a big block gets hot and cause pump not to operate fully or at all. I cleaned and inspected the rod. No apparent problems with that.
Expert:  Tim's Auto Repair replied 2 years ago.
from the way you explain and sense you are using the q-jet carb,,i think it is vapor locking the fuel line,,this is when the fuel line to the carb gets hot and causes the fuelin the line to vapoize,,this make an air pocket in the line and the fiuel stops flowing....rune the fuelline from the pump to the carb a different way,,get it away from the engine the best you can..i wouldnt think it is the return line or the fuel pump rod..though an electric pump may fix the problem,i have more faith in the mechanical pumpthanksIf you are satisfied with my answer, Please Rate my answer by clicking the rating button so I can get credit for my work. I’m not always going to be giving you good news, so please don't let this stand in the way of you accepting my answer. .it does not cost you more money and we will still be able to communicate, ,.please understand, i can not see, hear or touch the vehicle and only know what you tell me. Bonuses and positive feedback are appreciated.. if you are not satisfied with my answer, please do not leave bad feedback ,i will gladly opt out and let another expert handle the question. .if i have sent any diagrams, please print them. .sometimes you may need to right click the image [,then save picture as]to save it to your computer so you can open it with your picture program...Please ask if you need more help.
Expert:  Tim's Auto Repair replied 2 years ago.
also,consider swapping out that q-jet for a holley or edlbrock..the q-jet has a very small fuel bowl,,and doent take much of a fuel loss to run it out of fuelthanks