View Full Version : EVAP Canister Purge Valve sucking fuel out of carb bowl??!

04-03-13, 12:13 AM
Hi all, I'm back with another question. This time relating to the EVAP emissions control system.

So a couple months ago I purchased a newly refurbished carburetor and had my mechanic install it in for me.
When I picked it up, he informed me that he noticed the canister purge valve was sucking fuel out of the carb fuel bowl (keep in mind I last changed this purge valve 2 or 3 years ago, so it's not that old).

I double checked the diagram on the emissions label (california/50 state) and my FSM to make sure all 4 of the respective vacuum lines were connected correctly, and they were...so I purchased a new ACDELCO model a week ago and received it yesterday and installed today. Went for a quick drive around town to get her warmed up to operating temperature, came back, pulled the hose off the car bowl and noticed that it's still sucking fuel out of the carb bowl??!

..It would be very odd that I'd have two defective purge valves in a row...and I cant remember if I was having the same issue with it pulling fuel out on my old Carb prior to replacement never checked..

Here's how I currently have the lines hooked up:
1) I have the small top "manifold vac" line on the purge valve connected to the line that goes from the thermac housing control to the MAP pressure sensor (has a Y connector).
2) The other smaller "purge" valve line goes to the bottom of my carb on the front.
3) The larger "carb bowl" line connects to the upper center of my carb (this is where it's sucking the fuel out of).
4) The last line is the "canister" labeled line which is connected to the line that goes to the top thermal valve switch on the front of the engine next to the engine oil filler neck (I think this thermal vacuum switch is hiding under all the vacuum lines & its situated behind the AC compressor). The bottom connector on this thermal switch has the line going from there to the actual carbon canister.

Has anyone ever experienced a similar issue like this?
..I wonder if this would have the ability to cause cold idle stalls, I've been having stalling issues when cold shifting into drive for about a couple years now and could not figure out why..even with my replacement carb.


04-04-13, 10:18 AM
I have no idea what the vacuum connections should be on your vehicle, but am very familiar with that era GM emission control system.

The first check is to remove the carb bowl vent line from the carb, with the engine running test this for vacuum. Should have none. If it does have vacuum, then your valve is bad or some connection is wrong.

Now some small amount of fuel will be present in this line from condensation, that's its purpose. But actually pulling fuel out of the carb is not right. The other scenario would be a mis-adjusted carb float, if the fuel level is too high in the carb it will just overflow out the vent.

Edit: with your TB engine, there will be an electronic canister purge control valve. You may have to test the above by connecting the purge valve to a source of vacuum (bypass the purge control solenoid). The PCM won't usually let the engine purge the canister when not driving under cruise conditions.

04-14-13, 03:36 PM
Could an overly saturated EVAP/Vapor Canister cause the fuel to be pulled out of the carb bowl?...I took out my canister and it just seems a bit on the heavy side to me..

I checked for vacuum presence with the line from the bowl disconnected, and there is none at idle.
As far as the electronic purge valve, my vehicle only has the 4 ported vacuum style purge...I believe the electronic type was only used with the TBI setups, however my 86' has the computer controlled rochester carburetor.

I had a smog pre-test done couple days ago and it shows the car's pretty clean at the 15mph loaded dyno test (all low readings: HC/CO/NoX), however at the 25mph loaded test is where the CO,and, if I recall correctly, the HC was also over the allowable limit. If my float was misadjusted, wouldn't my readings be high @ all times?

..another thing I noticed was that as soon as the transmission shifted when reaching 25mph, that's when the HC/CO readings jumped..hmm.. :/