Vapor lock issues - maybe??

[Chuck Step-a-side]

I believe I'm having vapor lock issues. The truck idles and runs smoothly until about an 30 to 40 minutes of driving and then when coming to a stop it will idle rough to were it's almost ready to shut off. I will maintain my gas pedal at 700 rpm to keep from the engine shutting off . The 1984 Silverado truck originally had a 305 and now has a pre-80s Chevy 350 (no center bolt valve covers) with an edelbrock 1406 carburetor and mechanical fuel pump with 2 outlets. The truck has a steel vapor return line that runs across the radiator cross-member and ends on the drivers chassis. I think there was a charcoal canister that connect to the vapor line at one time, but now it is open line and there is nothing hooked up to it. I have no idea where the charcoal canister was mounted or how it is connected to the return line.

So - should I install a charcoal canister or a phenolic carburetor spacer to help prevent the vapor lock? I appreciate any help. Chuck

[bd]

Does it blow black smoke when it's acting up?  Try loosening or removing the fuel tank cap to see if the symptoms go away.

[Chuck Step-a-side]

No, I have not notice any black smoke. I have locking gas caps for both tanks and nothing indicated that they are vented, so I have taken each caps apart to drilled a small 2/16" hole in each inner gas cap housing for venting. When it started acting up last weekend, I notice the engine compartment was pretty hot, but temperature gauge read at 165 +- degrees. I'm going to replace the temperature sensor for accuracy purpose and test the thermostat and replace if needed. I'm planning on ordering the Edelbrock 1/2" wood spacer and install that as well. Thank for the reply, Chuck

[75gmck25]

The routing of the vapor line sounds similar to my '75, but I do still have the canister.  Even an open metal line should provide enough venting to let air into the gas tank if its connected to the tank vent.

Did this truck originally have a 3 port fuel pump, and someone swapped in a two port fuel pump? 

A three port pump has a line in from the tank, a hard line running up behind the alternator to the carburetor, and a return line running back to the tank solenoid (it switches between tanks the same as the fuel feed).    When the float shuts off the gas to the carburetor and the fuel pump dead-heads, the return line on a 3 port pump allows excess fuel to circulate back to the tank.  This keeps fuel circulating through the pump and it does not build up as much heat.  A two port pump may work fine most of the time, but it does allow more heat build-up in the fuel line, which may cause vapor lock.

Another simple check is to verify the fuel pressure to the carburetor.  Edelbrock's are very sensitive to high fuel pressure, so maybe its forcing fuel past the needle and float.

Bruce

[Chuck Step-a-side]

Yes, this has a 3 port fuel pump (to clarify my original message - 2 outlet and 1 inlet).

I've worked on the engine cooling today and replace the thermostat and sensor and its reading around 165 degrees after letting it idle for about 40 mins. Then I drove it around for about 6 miles while driving around at 45 to 55 rpm most of the way and then as I turned into my neighborhood and the engine just shuts down. I had to crank its over about  6 or 7 times with the gas pedal all the way down and pumping till it started again. I maintain the pedal at 700 rpm to get it back to my driveway a few blocks away. It cut out again once I got in the driveway and then I cranked over again while pumping the pedal and it starts right up and slowly dies as the engine returns to idle. It seems the gas is restricted or not filling the bowls or something else I can't put my finger on. Chuck

[bd]

Single or dual tanks?  Is the fuel return line running between the pump and tank(s) connected?  Physically inspect all of the rubber fuel hoses running between the tank and the fuel pump for cracks and kinks.  Remove and blow through the fuel filter and replace it if it seems restricted.  Replace/reroute as needed any hoses that are suspect then check the fuel pump draw (>20" Hg), pressure (5 - 7 PSI) and delivery (minimum of 1 quart in 30 seconds of cranking). 

[Chuck Step-a-side]

Dual tanks. Yes, all line are connected except for the end of the vapor return. I have dropped the tanks and replaced all the fuel lines and fuel selector valve about 5 years ago. The driver side gas tank inlet was poorly plugged, so I dropped that tank and cleaned it out a best as possible and did all the lines rubber during that time. I have had on-going fuel issues every since I have purchased the truck.  I will start working on the fuel pump draw and delivery checks tomorrow and get back to you. Thanks.

[bd]

Alcohol enriched (oxygenated fuels) are very hard on rubber components, drying them, which results in brittleness and cracks.  You should recheck the hoses in a new unit of time.

Measurements of the fuel pump's performance is a crucial step in your diagnosis.  Under the circumstances, it may be prudent to drop some oil from the dipstick onto the tip of a clean finger to see whether the oil maintains a tight circular shape or quickly tracks out along the finger ridges.

[Chuck Step-a-side]

I was able to do the fuel pump test and other checks on my truck and here are the results.

Fuel pump draw is (>17" Hg) and delivery is (1 quart in 30 seconds).

The oil beads up on my finger and all fuel injected type hoses seems firm with no visual cracks.

Fuel filter is a chromed threaded capsule type that has a metal beaded formed cup shaped filter where I found a couple of tiny rubber shivers lodged inside that would had no effect on the fuel delivery. The filter was placed before the fuel pump insert.

[bd]

Make sure the truck has adequate fuel in its tanks and verify that the tank selector valve cycles full rack and is properly plumbed with 6 hoses.  Fuel pump draw is slightly low, running ~80% of normal.  What is the outlet pressure?  The fuel filter should be on the pressure side of the pump, between the pump and the carburetor.  The sintered bronze type filter you described typically has the shape of a bundt or angel food cake pan. The center spire of the filter should point upstream, toward the filter inlet.  Either relocate the filter to its proper position and orientation or temporarily remove it to see if there is any improvement.  FYI - pleated paper elements generally filter to a finer micron rating. 

What is the ambient air temperature when the problem occurs?  Have you verified a consistent, healthy blue spark during the failure?

[JohnnyPopper]

So the symptoms you cite, are they constant, repeatable? Good cold, then after running 30-40 mins, bad?

Having suffered vapor lock myself in my '73, I can tell you your symptoms don't sound like it.

Vapor lock is when gasoline is not present in liquid form and the truck won't run at all. Yours still runs so.....

I suspect one of your rubber 'shivers' is stuck in your idle jets, OR float valve seat. Look carefully for black smoke especially when your are in Limp Home Mode.