My little brother recently bought a 1985 GMC s15 2.8L with about 65k miles that won’t start unless the gas is pumped and once it does start it sputters and will die unless the gas is pumped a couple more times, but once you pump the gas again it’ll idle and drive with no problem. We bought a new battery and air filter amongst other small things it needed so at the very least I can (hopefully) check those things off the list. My guess is the fuel pump but I thought I’d seek out the wisdom of people who know more about trucks than I do. Any help would be welcome. Thank you.
Did you check and see if the choke is operating properly on the carburetor?
Tester
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No. How do I do that?
The engine must be cold.
Remove the air cleaner.
Press the gas pedal to the floor and release.
The choke plate should close fully.
Start the engine.
The choke plate should open slightly. That’s the primary choke pull-off.
As the engine idles, step on the gas and release it. The choke plate should open and close. That’s the secondary choke pull-off.
As the engine idles, the choke plate should slowly open. That’s the thermal choke pull-off.
If the choke doesn’t do any of these, there’s a choke problem.
Tester
Tester I should preface this by saying that I know very little about vehicles so please bare with me while I try and describe what happened.
So I pulled the lid off the air cleaner and under it was two metal plate/flap things. One small one that seemed to be under no pressure at all that was all the way open and one slightly bigger one that was under spring pressure that was all the way closed.
I had my wife push down on the gas and the little plate spewed out some gas but neither flap moved.
Then I had my wife start it and push the gas but neither flap moved.
Lastly, I let the truck idle and still nothing.
On a side note since doing all of this the trucks been starting right up.
Those would be the choke plates on what sounds like a four barrel carb. You could also have a leak in the bottom of the carb which allows the fuel in the bowl to drain out over a period of time. Then there is no gas that gets injected into the engine when you press the gas pedal until the fuel pump fills it up again. If it works now, suggest leaving it alone unless you want to do a carb overhaul. Good luck finding someone that even knows what they look like. Before messing with the fuel pump, have a pressure and volume test done to confirm, but if it runs ok after a warm up and acceleration is good, not likely a fuel pump.
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Not uncommon on carburated vehicles of that vintage, I’ve had a few like that. Pump the gas, and keep on trucking!
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Pretty sure it’s a 2 bbl on a 2.8 L V6. You’ll want to find someone familiar with carbs, have them adjust this to spec. You may need to get it rebuilt if the choke linkage isn’t working. Or you could buy a rebuilt one for $300 - $400.
After 34 years who knows how the carbs been misadjusted.
Here’s a pic
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Just thought of something. Back in the old days of carbs, the procedure for starting was to pump the gas once or twice then hold the pedal down half way-depending on the particular car. Point is pumping the gas was necessary and everyone had a particular method for starting in below zero weather. With FI though, you don’t touch the gas pedal but let the computer do the work. Quite possible our OP was schooled on FI vehicles and doesn’t realize what it’s like to start a vehicle with a carb?
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Good idea - OP, try pumping the gas twice, let off the gas, and turn the key.
But the OP did say that “I had my wife push down on the gas and the little plate spewed out some gas but neither flap moved”. Sounds like the choke linkage isn’t working.
If the choke plate didn’t close when stepping on the gas pedal, it usually means the spring in the thermal choke pull-off is broken.
Tester
If nothing above does the trick, and if it hasn’t been done recently, next step is a basic tune-up, new spark plugs, dist cap, ignition rotor, spark plug wires, fuel filter, and air filter. If that doesn’t do the trick, then you’ll need the carb rebuilt. I had to rebuild the carb on my even-older-than-yours Ford truck a while ago due to similar symptoms. The carb rebuild fixed them all. Plus I received an award here for taking the longest time in history to rebuild a carb … lol …
There are other things that might causes, so as part of the tune-up
- make sure that the battery and alternator are working correctly.
- make sure there are no vacuum leaks
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Was the engine cold? The choke won’t close if the engine is hot.
For this to run fine warm but bad cold makes me want to get the choke mechanism checked out, before replacing any parts.
By now your spark plugs should be really full of deposits. When the choke is fixed, get new plugs.
I suspect Tester is correct. I hated those dang spring choke controls and could rust up and break, and hard to really adjust. I don’t understand the “two flapper” thing though. The picture is not clear but there should only be one choke flapper covering the two barrels, or two covering the four if it is a four barrel which I guess it is not.
Primary and secondary throttle valves like most four barrel carburetors, this arrangement is also used on two barrel carburetors.
The engine was stone cold when we tried the aforementioned remedies.
Then it sounds like you have a choke problem (at least). Any chance you could post some pics? With it stone cold and not running remove the air cleaner top and take pics of 1. As it looks right then 2. With the gas pushed to the floor and 3. With the gas pedal released.
The carburetor has a choke plate on the top of the primary bbl and an air valve on the secondary bbl similar to a Q-Jet. At near wide open throttle under load the secondary throttle opens and if there is enough vacuum the air flow draws that air valve open, pulling the power valve up in its bore to pass fuel.