CrimsonDragon

85 Z28, 305, Quadrajet, mechanical fuel pump - This afternoon we pulled the carb to have it rebuilt as it was flooding/hard starting. When we disconnected the fuel line, fuel continued to pour out of the line. It seemed we had a vacuum condition, unscrewed the gas cap hearing the sound of pressure being relieved and the fuel line stopped leaking fuel. Couple Questions? Would excessive pressure in the fuel lines put undue pressure on the needle and seat in the carb causing a flooding condition? How/why did fuel flow continually out the disconnected fuel line while the engine was off/fuel pump not working? I am looking at testing the EVAP system - Canister purge valve and solenoid thinking there is a malfunction. Any thoughts would greatly be appreciated. Thanks again.

zed-028

excess pressure will play havoc with these carbs and cause all sorts of wierd and wonderful issues. A while ago when i had to replace my mech pump on the 87 i went to the in tank pump temporarily to get by, i had high enough pressure to cause exactly what you are referring to, then the bowl would overfill and spew out the sides of the carb. good times.

Tank venting can be an issue but im trying to get my head around this. If the vent were blocked, there would be a negative pressure in the tank, this would theoretically suck the fuel back down the line towards the tank, not force it out.

In saying that, if its a stock style mech pump, it should have a return line which is a smaller diameter of tubing, if you introduce a return to that same system with no venting i suppose it is theoretically possible to have a self serving circuit of fuel but i dont think its likely.

At the same time, your high pressure at the carb has to be coming from somewhere. when fuel pumps fail they generally dont start producing more pressure, quite the opposite.

The canister purge valve is an interesting idea. temporarily block the hoses off and see if you get the same vacuum condition.

zed-028

I rebuilt a Quadrajet the other day that had a fully calcified seat under the needle, also had a cracked float which caused a tonne of flooding

CrimsonDragon

The rebuilt carb will be ready Friday afternoon. I am going to test the snot out of everything with gauges and such and see if we can figure this out. Will let you know how it goes. Thanks again

Saxondale

iTrader: (2)

I didn't read the posts above in detail, so maybe mentioned already. But I had a failed diaphragm in canister control valve that caused rich operation and tank pressure. Pull a few lines off the CCV. Should not be fuel in any of those lines.

CrimsonDragon

Yep I forgot to say that an earlier post but I did replace that a canister purge valve. I've got to work all weekend we're doing inventory at work so first part of next week I'm just going to disconnect every line off and plug it in then just through deduction see if there's a line that's causing it.

naf

iTrader: (7)

there is an overpressure relief valve connected to the tank and mounted above the differential. there is a vent line running to the evap canister also but it's orifice is small and enough pressure can still build in the tank to push fuel out through the supply line. this is normal.

if you pressurize the tank through the open supply line with compressed air you should be able to hear the pressure relief valve open and vent. it's a combo valve, actually, will allow air in to prevent negative pressure within the tank and vent overpressure at a pre-determined set point.

the intent is to force fuel vapors to the charcoal canister from the tank but keep pressure within the tank from becoming dangerously high.

sounds like your system is functioning normally. your carb problems are likely related to your carb. testing the evap system is, however, always a good maintenance thing to do.

also, a good practice is to always loosen the gas cap before opening a fuel line.

CrimsonDragon

Thanks, we have noticed that every time we pulled the carb initially to rebuild it then test it on the test engine ( where it runs just fine ) we've had to open the gas cap to relieve the pressure, as fuel would continue to run from the supply line to the carburetor.

naf

iTrader: (7)

anyway, excess fuel pressure flooding your carb won't cause your high idle. it will cause idle problems by flooding the motor. an easy check is to run the motor with the gas cap loose.

CrimsonDragon

Thanks, I've loosened my gas cap and didn't affect the idle. I appreciate the reply

John in RI

iTrader: (167)

If you don't have one already,...... do yourself a favor and install a Fuel Pressure Regulator.

Also be sure that your using a return line ( some aftermarket fuel pumps don't have it. ) If yours doesn't have the return line,... you can buy a 3-way fuel filter (3rd line is return ) to overcome this.

As you and others have already suggested; the tank purge valve , canister purge and EVAP system should be checked too.

Good luck !