Quadrajet and intake flooding

[John P.]

Re: Quadrajet and intake flooding

Hello Patrick, On the way to work and another thought came to mind. I believe your year corvette had a fuel vapor cannister and purge system as part of the emission system. If this system is faulty, it will cause raw fuel to be drawn into the intake manifold through the purge valve. Locate your vacuum source for this system at the intake or carburetor and disconnect. I do not have access to a schematic, however it is possible there is a decal in the engine compartment that illustrates the components. If the purge valve is faulty, it will allow engine vacuum to constantly draw fuel through the purge line directly into the intake. This is probably why the fuel does not appear in the intake when the throttle is held open.

[Patrick H.]

Re: Quadrajet and intake flooding

John,

Thanks for the tip. We may also try this with all exterior vacuum lines disconnected.
One thought - after sitting for days between experiments, wouldn’t any vacuum in the vapor system be gone?

Dave,

The long block is untouched - except I installed a new timing chain 2 years ago. The old one lost so many teeth you wouldn’t believe it.
It made a huge difference in idle stability and overall running. So, I doubt this new chain died. Sorry for not mentioning that earlier.

PH

[John P.]

Re: Quadrajet and intake flooding

Hello Patrick, Trace the vacuum supply lines to the vapor cannister and block off lines. If the cannister is full of fuel, cranking the engine with the throttle closed as you stated would create the vacuum required to draw fuel through the line. This is why the fuel does not appear when the engine is cranked with the throttle held open. I believe the main vacuum source to the cannister is connected to the PCV valve hose. Again, not having a schematic to reference, I am not sure of this connection point, however in the past I remember the cannister purge line was inline with the PCV valve. There should also be a smaller vacuum line from the carburetor to the purge control valve. I also believe that there is a liquid/vapor valve located on the fuel tank that prevented this system from drawing liquid fuel from the tank when the tank was full or fuel sloshing in the tank. If you plug the lines and disable the system, make sure to install a vented fuel cap. When you have the plugs removed, it would be a good idea to run a compression test. This would eliminate any doubt of a failed timing chain.

[Patrick H.]

Re: Quadrajet and intake flooding

When you have the plugs removed, it would be a good idea to run a compression test. This would eliminate any doubt of a failed timing chain.

See first post - compression test was excellent.

[Joe L.]

Re: Quadrajet and intake flooding

Hello Patrick, On the way to work and another thought came to mind. I believe your year corvette had a fuel vapor cannister and purge system as part of the emission system. If this system is faulty, it will cause raw fuel to be drawn into the intake manifold through the purge valve. Locate your vacuum source for this system at the intake or carburetor and disconnect. I do not have access to a schematic, however it is possible there is a decal in the engine compartment that illustrates the components. If the purge valve is faulty, it will allow engine vacuum to constantly draw fuel through the purge line directly into the intake. This is probably why the fuel does not appear in the intake when the throttle is held open.

John------


Patrick has an EXTREMELY vexing problem as described. What you have suggested here is the absolute best theory I have heard as to the root cause of the problem. I'm actually surprised that it had not crossed my mind as I pondered solutions to this.

[Mark E.]

Re: Quadrajet and intake flooding

John------


Patrick has an EXTREMELY vexing problem as described. What you have suggested here is the absolute best theory I have heard as to the root cause of the problem. I'm actually surprised that it had not crossed my mind as I pondered solutions to this.

I too like this theory because it explains all the symptoms... even though I haven't experienced or heard of this happening before (but there's a lot of stuff I haven't experienced :-)).

[Patrick H.]

Re: Quadrajet and intake flooding

So, to understand all, if I block off the vapor canister system but leave the distributor and power brakes attached, I should give it a try. Correct?

Extremely vexing? Try being on my end.

[Joe L.]

Re: Quadrajet and intake flooding

So, to understand all, if I block off the vapor canister system but leave the distributor and power brakes attached, I should give it a try. Correct?

Extremely vexing? Try being on my end.

Patrick------


I would say yes. However, I think I would try to determine if the vapor canister is full of liquid gasoline.

[Patrick H.]

Re: Quadrajet and intake flooding

Yes. I will also see if those lines back to the fuel tank are open or full of gas.

[John P.]

Re: Quadrajet and intake flooding

Hello Patrick, I believe that the vapor cannister is located on the left side inner fender well close to the firewall. There should be (3) hoses connected to the purge valve on the cannister. One smaller vacuum hose is the control (signal) hose to the carburetor vacuum source. This hose controls the opening of the valve predicated on engine vacuum. There are two larger hoses. One is the vapor hose going to the fuel tank, and the other hose connects inline with the PCV system. You can just cap off these hoses at the cannister, or at the connections to the carburetor and PCV hose connection. You will probably find this system filled with liquid fuel. This would be an indication that the fuel liquid/vapor separator at the fuel tank has failed. I would inspect the valve and hoses at the tank due to the fact that they are probably located above the left side muffler. You don't want to chance a fuel leak on the exhaust. The best bet to repair or replace this valve is to obtain a service manual for that year to illustrate the location of all components. Again, capping these hoses at the vapor cannister will correct the problem at hand and can be driven without any issues as long as you install a vented cap. Due to the age of this system, you may want to replace the cannister unit, vapor separator, and vacuum hoses. This will prevent the gasoline smell that the older cars had due to the vented tanks and caps.

[Billy S.]

Re: Quadrajet and intake flooding

To complete the theory -- the tank got filled high (maybe brimmed?) on the fill up between the 30 miles of driving and the first bad start?

I say this because I once brimmed my tank and parked on a heavily uneven surface putting a lot of pressure on the fuel vapor separator (the car was tilted to that side quite heavily) and I had fuel pouring out of the vapor canister. Maybe this car has something similiar but maybe not at such an extreme angle to push the fuel out of the canister so that it can pool in there to be sucked up by the vaccum system to start.

When I had this problem I was too afraid to start the car as gas was pouring everywhere so I drained the gas tank below the separator to break suction, dried up all the gas I could out of the vapor canister then started the car with no issue.

[Patrick H.]

Re: Quadrajet and intake flooding

Currently the tank is about half to 2/3 full, so its hard to imagine it being over full.
I can’t imagine how gas would have ended up in the vapor system, but I’ll test it when I can.

[Patrick H.]

Re: Quadrajet and intake flooding

Today's update:

Used a MityVac on the EEC line and PCV line as they would enter the carb. No gas came with application of the vacuum. Plugged ports at the carb anyway so that carb wouldn't pull in any vacuum through the holes.

Tested fuel supply using a clear hose. Fuel pump makes pressure with rotation of the cam as expected. Left line unconnected for the moment.

Installed spark plugs, closed rear 2 barrels, had choke and front 2 barrels open. No gas from carb turning over engine, and it did not start.

Attached fuel line, repeated test. No gas from carb, engine did not start.

Connected accelerator pump, repeated test. No gas from carb, engine did not start.

Closed front 2 barrels, left choke open, repeated test. No gas from carb, engine did not start.

Partially closed the choke, repeated test. No gas from carb, engine did not start.

Closed the choke, repeated test. Gas gushed from carb, engine did not start.
Observation suggests that the gas may be coming from the idle air openings above the venturi, as even the lip of the choke got gas on it, and the rounded "dome" next to the screw next to the venturi was quite wet with fuel.

Later, removed carb and installed it on another 72 Corvette. Car started and ran, no gushing of gas.
Carb is not the problem.

So why would this engine pull so much vacuum that it literally sucks all the gas out of the carb via the idle air circuits?
At least, that's all I can gather is happening.

See attached photo of gas in the intake, much worse on the left than the right. As noted previously, this only takes a few seconds with a closed choke to accumulate.

[Ken A.]

Re: Quadrajet and intake flooding

This was a common problem with all early Qjets. They all have 2 pressed in welch plugs on the bottom of the carb that are guaranteed to leak.
If driven daily it wasn't too bad, but when they get real bad the carbs leaks down a large puddle overnite and your car is flooded the next morning
& the bowl is empty. The solution is to replace the plugs and use fuel proof epoxy to seal them.

[Joe L.]

Re: Quadrajet and intake flooding

This was a common problem with all early Qjets. They all have 2 pressed in welch plugs on the bottom of the carb that are guaranteed to leak.
If driven daily it wasn't too bad, but when they get real bad the carbs leaks down a large puddle overnite and your car is flooded the next morning
& the bowl is empty. The solution is to replace the plugs and use fuel proof epoxy to seal them.

Ken-------


Yes, that's a very common problem with Q-Jets. However, in this case, when the carb was installed on another engine there was no problem. Also, a different Q-Jet was previously installed on the engine in question and the same problem occurred. So, all this pretty much rules out the carb as the source of the problem.