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1980 chevrolet custom delux 350 5.0L pickup

I recently purchased a lovely 1980 chevy truck to haul firewood, etc. The truck seems to be having a fuel problem. It sputters and almost dies particularly when going uphill but also feels a little sluggish when “putting your foot in it”. The Carburetor appears to be functioning fine(according to the man who looked it over), the fuel filter was allegedly changed during this process to no avail as well. This leads me to believe it could be the fuel pump, but am not certain. Would the fuel pump do this for a long period of time and not go completely out? or do you think this could be something else, like a vacuum leak(I did find a dangling hose from a juncture where the four fuel lines meet and it was open and did not have any obvious attachments to other lines that appeared broken)

A number of things can cause sluggish running. Clogged converter, retarded ignition timing, faulty distributor advance mechanism, weak engine, and possibly a failing fuel pump.

On a positive note, this truck is easy to diagnose and service though. What would I do if the truck were mine? I’d run a compression test on the engine to make sure the engine is not weak because if the engine is half worn out then one could replace parts all day long without making any improvement.
Make sure the basic building block is good and go from there.

If you still have a carb, you might also still have a mechanical fuel pump. Most mechanical pumps fail slowly on the way out. The rubber diaphram cracks and gets holes so you don’t get full flow and pressure. This type of failure would show up under load when more fuel is required. After the load is over the carb float bowls will refill and the float level won’t go down again until you need more gas under load again. Generally, a mechanical pump is a pretty cheap and easy switch out fix.

Follow the fuel line down from the carb. If you have a mechanical pump you’ll see the fuel line enter the pump on the front of the motor low down, under the alternator, or AC compressor. Sometimes they can be hard to access with all the accessories and hoses getting in the way. Once you see it and can get to it you are looking at two bolts and the in and out gas fittings to remove it and replace it with a new one.

Connect a timing light and watch for advance when revving the engine. The mechanical and vacuum advance need to work and also, the module must increase the dwell as RPMs increase.

Check the fuel line from the gasoline tank to the engine. If this truck has a carburetor and a mechanical fuel pump on the engine, the pump could be sucking air as well as gasoline from the fuel tank. Your truck is 32 years old, so deterioration of the fuel line is a possibility.

Is this still a Quadrajet carb? Does this sputter happen at steady throttle position or when you change for the onset of climbing the hill? This could be a weak accelerator pump in the carb. Not a terrible part to change but it does take some care. You can test the accelerator pump. Remove the air cleaner. Start the truck. The throttle has a level arm pushing a rod down into the carb on top of the carb on the driver’s side. If you take a screw driver and gently push the rod into the carb without moving the throttle or level arm, you should get a change in run state, usually a little stumble because it will get rich. You can push it down a couple times at varying rates to see if the stumble always happens the same. Any time you move it without a response from the engine leads me to think it is weak or the check ball below it is sticking.

As for “putting your foot in it” if this is still a Quadrajet they are known for something called a choke pull-off failure. There is a round can on the passenger’s side top of the carb and sometimes one on the passenger’s side lower rear. When they fail, they secondary flap opens too quickly causing a lean condition. This is why most people refer to Quadrajet as Quadrabog carbs. Make sure when the truck is started that the lever arm moves into the diaphram. If they do not they need to be checked for vacuum leaks in hoses leading to them or actually replaced.

The fuel filter is very small, located INSIDE the carburetor at the fuel inlet. To change it, you must remove the fuel line from the carb. Do this carefully with the proper tools as you DO NOT want to strip or cross-thread the two fittings involved with changing the filter…

To Uncle Turbos comment… This is probably a small block Chevy motor. When you go to remove the fuel pump from the block, it is a lot easier to remove a short bolt from the front of the engine and install a longer one before removing the old pump, do not torque this bolt… This will hold the pump rod up while you remove the pump and replace. If you skip this, the pump rod is a bear to get back on the pump actuator. Remember to remove the long bolt and replace the short one and make sure it is tight. If you lose that bolt you will have one heck of an oil leak…

There is no need to replace the fuel pump if it’s working properly. This is easy to check…

Caddyman, only pointing out that if the fuel pump were bad and he decided to replace an easy way to handle the pump rod extension. Personally, I think it is carb or timing related.