trouble running when warm

[78longbed]

I have a sb 350 with mechanical fuel pump and edelbrock 1406 600cfm carb that my son drives as a daily driver. It started having a problem that I will try to describe. It started coughing and sputtering one afternoon to the point where it wouldn't start. If it did start, it the RPM's would raise up then drop with the throttle in the same position and would ultimately die. We replaced the fuel pump and fuel lines from the frame to the pump and pump to the carb. We also replaced the filter. It ran great for about 3 days. The same problem started again. We pulled the carb and used the edelbrock rebuild kit and the float kit. Put it all back together and the problem is still there.

Anyone have an idea on what this could be? I am out of ideas.

 

[77 K20]

I'd look over your ignition system real well. I've had almost brand new spark plug wires go bad and suddenly when it was warm it would start sputtering and then die. When they were cold they worked better.
Or ignition module...
Just something to look at.

 

[78longbed]

Thank you I will check that out. A friend recommended checking into the torque converter as well. His theory is that it could be locking up when it gets hot. The tranny has never been out of the truck. It is a turbo 350. Thoughts on that theory? Obviously I will check the ignition system before digging into something else.

 

[77 K20]

I thought the lockup torque converter option was only for later year trucks, and not 1978...
And even if it was a lockup issue it should still be fine in park or neutral. If it locked up in gear while not moving it would then cause the engine to die.

 

[MadOgre]

I thought the lockup torque converter option was only for later year trucks, and not 1978...
And even if it was a lockup issue it should still be fine in park or neutral. If it locked up in gear while not moving it would then cause the engine to die.

Ya its only when you come to a stop in gear that it will die.

 

[78longbed]

I have now replaced plugs, cap, rotor and coil. I inspected the plug wires and they were good. I also drained the tranny fluid and replace filter, gaskets, etc. Probably not related, but needed done anyway as it was leaking. So back to the issue. I am wondering if it still is a carb issue. I have a clear filter on it. When cold and warming up, you can see the filter is full of fuel. After about 20 minutes, it isn't empty, but close and you can see the fuel pumping in to it. I don't have a spare carb laying around and was thinking about biting the bullet and buying a new one. Thoughts?

 

[chengny]

There are three basic tests that are done when the fuel system is suspect:

1. Flow rate available at the carburetor inlet (1/2 pint or more in 15 seconds of cranking)

2. Fuel pump discharge pressure (should be 4 to 6.5 psi while engine is running)

3. FUEL PUMP VACUUM TEST

Disconnect inlet hose at fuel pump and connect a vacuum gauge.

Crank or run engine until maximum vacuum is reached. If the vacuum reading is less than (15" Hg.), replace the fuel pump. If more than (15"Hg.), go to step 3.

Check fuel lines and hoses for splits, leaks or kinks by disconnecting each section of line and connecting a vacuum gauge. Crank or run engine until vacuum reaches it's peak.

Vacuum should be at least (15" Hg.). If less, repair or replace the malfunctioning line or hose.

If the fuel lines and pump check OK, remove tank unit, replace strainer and clean the tank.



The flow test is the best place to start. If you have sufficient volume (and enough pressure to make the gas shoot strongly out of the test hose when doing the procedure) - that would indicate that the pump and discharge lines are good.

Watch the stream of gasoline as it is pumped into the test container. Note whether there appears to be air bubbles entrained in the fuel.

If so, check the fuel line on the suction side of the pump. Look for any evidence of kinks (steel sections), collapsed areas (in the rubber sections) and obvious splits in the line. If air gets sucked into the fuel, it will eventually migrate to the gas pump and cause cavitation. Also if enough air is allowed to enter the fuel supply, it will cause erratic carburetor operation.

 

[78longbed]

Appreciate the ideas and steps to take. The fuel lines from the frame to the carb are all new including the pump. When cold the flow is excellent. I still need to test that with the truck hot. I will reply back with what I find out. Thanks again

 

[78longbed]

There are three basic tests that are done when the fuel system is suspect:

1. Flow rate available at the carburetor inlet (1/2 pint or more in 15 seconds of cranking)

2. Fuel pump discharge pressure (should be 4 to 6.5 psi while engine is running)

3. FUEL PUMP VACUUM TEST

Disconnect inlet hose at fuel pump and connect a vacuum gauge.

Crank or run engine until maximum vacuum is reached. If the vacuum reading is less than (15" Hg.), replace the fuel pump. If more than (15"Hg.), go to step 3.

Check fuel lines and hoses for splits, leaks or kinks by disconnecting each section of line and connecting a vacuum gauge. Crank or run engine until vacuum reaches it's peak.

Vacuum should be at least (15" Hg.). If less, repair or replace the malfunctioning line or hose.

If the fuel lines and pump check OK, remove tank unit, replace strainer and clean the tank.



The flow test is the best place to start. If you have sufficient volume (and enough pressure to make the gas shoot strongly out of the test hose when doing the procedure) - that would indicate that the pump and discharge lines are good.

Watch the stream of gasoline as it is pumped into the test container. Note whether there appears to be air bubbles entrained in the fuel.

If so, check the fuel line on the suction side of the pump. Look for any evidence of kinks (steel sections), collapsed areas (in the rubber sections) and obvious splits in the line. If air gets sucked into the fuel, it will eventually migrate to the gas pump and cause cavitation. Also if enough air is allowed to enter the fuel supply, it will cause erratic carburetor operation.

What are your thoughts on bypassing the tank and running the fuel pump from a gas container? My thought is that if I get the same issue, then I know it isn't the tank or steel lines. If it doesn't happen, then I can go through the tank procedures.

 

[Georgeb]

What are your thoughts on bypassing the tank and running the fuel pump from a gas container? My thought is that if I get the same issue, then I know it isn't the tank or steel lines. If it doesn't happen, then I can go through the tank procedures.

I used to use an old outboard motor tank on junkers I was working on. That eliminated crappy gas issues until I could deal with the tank. Used to set it on the passenger floor and run the hose to the pump. Prime it with the bulb and away I would go!

 

[78longbed]

Thank you everyone for the help with this. I ran a gas can bypassing the tank and lines. The issue did not show itself after almost an hr of running in the driveway and some back and forth driving in the driveway and street in front of the house. Now it is on to the tanks. Anyone know a good way to clean the tanks or would it be easier and better in the long term to just by new ones?

 

[chengny]

A couple of things before dropping and changing tanks:

1. It was only when I read the following that I became aware of the fact that you have dual tanks. It wasn't mentioned in previous posts:

Now it is on to the tanks.

2. It is rare enough for a truck with just one tank (and that sees daily use) to have the tanks become filled with debris to the point of restricting flow. With two tanks it is even less likely.

3. Even if the restriction is in the tanks. Don't assume that it's because the tanks are so full of sediment that has reached the pick-up filter. The sock may just be coated and can be cleared by blowing back with compressed air.


I would take a close look at the tank switching valve. Prior to 1981 the tank switching valve was solenoid operated. Without power from the switch on the dash, the solenoid valve would default to the production (LH) tank and was shuttled by the solenoid's return spring. When power was applied it would shift the valve so that the fuel pump would take suction from the auxiliary (RH) tank.

In 1981 the tank transfer valve was changed to a motorized type. This style requires that the dash switch be held in the position of the tank to be used for about 2 seconds. This design was an improvement over the solenoid type because it was electrically driven in both directions - by the operator. The solenoid type, on the other hand, was only driven in the direction of the auxiliary tank.

I don't know enough about the internals of that earlier type of valve to say for sure, but it might be possible that the valve is not stroking fully and could cause a flow restriction.

And, getting back to the idea of blowing back through the fuel lines to try and clear any sediment film on the pickup sock; if you do this watch the rubber sections of the fuel lines closely. If you have any air leaks they will be seen and heard.

 

[78longbed]

Thank you for the additional ideas to try. Sorry for not mentioning the dual tanks. The only tank in use has been the right (driver) side tank. The left side tank hasn't been used in at least 15 years. I was thinking about putting the left tank back into service that would hopefully eliminate the cross pipe from the driver to the left side.

 

[78longbed]

To recap my ordeal to make it easier for all and avoid reading the entire thread. I have a problem that after 15 to 20 minutes of running/driving, the truck starts to sputter and dies and won't restart. I have:
-replaced the mechanical fuel pump
-replaced the rubber lines from the frame to the pump and pump to carb
-new fuel filter
-rebuilt edelbrock carb using their kit and also replaced floats
-new plugs, cap, rotor and coil (it is an hei dist)

The last thing I have done is run the truck from a jerry can directly to the pump. It started well, runs good, and drives well. No stalls. I couldn't get it to any kind of speed, but it would die shortly after selecting a gear and trying to move.

It is a dual tank truck where the right tank (passenger) hasn't been used in over 15 years. It has been running off of the left (driver) side.

next thing to try is blowing out the lines with air and see what happens.

Does anyone have any other ideas if this doesn't work? I am to the point of running out of ideas and getting ready to drop at least one tank and refurb it or buy a new one.

 

[chengny]

Don't get discouraged - this type of problem is easy to diagnose.

Since the engine runs fine when supplied with an external fuel source (to the pump suction), you can pretty much eliminate everything except:

1. The tank-to-pump fuel line restriction (or possibly an air leak)

2. The sender unit (i.e. the pickup filter being fouled to the point fuel flow is reduced)

3. A heavily contaminated tank bottom (so much that the debris is blocking the pickup)

4. Some kind of flow restriction in the "decomissioned' tank transfer valve


The first thing I would do is, disconnect the fuel line at the fuel pump inlet. Connect a compressed air supply to the rubber hose. Like in this area:

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Do it safely - keep the pressure under 20 psi, be sure the gas cap is off the tank (both tanks actually), don't smoke, etc.

While someone applies air pressure at the fuel pump end of the line, you should be at the open tank fill pipe. Listen for vigorous bubbling inside the tank, put your palm over the fill pipe and feel for air flow out of the tank, etc.

Do anything you can do to confirm that there is (or is not) a steady flow of air all the way from the fuel pump, through the suction line, transfer valve, tank outlet line and finally out of the sender filter and into the gasoline.

While all this is going on someone should be tracing the fuel line - following the same route that the test air is taking. They should be watching for any gas - that may have been still in the line - dripping out. They should be listening too - for any sign of the test air leaking out of the lines. A squirt bottle of soap suds would be a good idea.

If you get plenty of flow out of the fill pipe, it most likely means one of two things:

1. That the problem is not a restriction to flow, but rather an air leak into the gas supply. I have been keeping this statement in the back of my mind:

I have a clear filter on it. When cold and warming up, you can see the filter is full of fuel. After about 20 minutes, it isn't empty, but close and you can see the fuel pumping in to it.

That is not always a sure indication of air ingress but it can be.

The second explanation - the good one - is that there was some kind of obstruction in the fuel supply to the suction side of the pump, but it was blown out when the compressed air was initially applied. If that is the case, button everything up and give the boy his truck back.

What if you get no evidence of air flow from the tank fill pipe when compressed air is applied at the fuel pump end of the line?

Kink in the steel tubing part of the fuel line

Some kind of mechanical failure within the dormant tank transfer valve

A seriously broken hose or piece of tubing in the line (you'd hear it when the air was on)

IDK...that's all I can think of. If it isn't one of those, maybe the truck is haunted.