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71 GMC truck have a Q-Jet problem.. i think?

11K views 58 replies 8 participants last post by  cobalt327 
#1 · (Edited)
alright, i have a 1971 gmc pickup, 350 c.i., original Q-Jet, HEI, Electric choke. The truck has new plugs, wires, cap, rotor, coil, and electronic module.

so, on my way back from the gas station, the truck missed and stalled at operating temperature at a 4 way stop. Once it stalls it will start again, but won't stay running for more then 2 or 3 seconds. It has did this twice on me, this time and while on the highway going 55 mph. If you guys could tell me what is wrong with my truck, it would be greatly appreciated.

Thanks for your time - 71GMC

also, sorry for the un-needed extra thread. did not mean to press the button to send it to the board. :/
 
#3 · (Edited)
Hey 71GMC,

My '77 Chevy truck had problems that were somewhat similar to yours, and it turned out that debris and water in the fuel were to blame. The junk clogged the carburetor and it had to be rebuilt. The truck would start, stay on for a few seconds and then die. Couldn't even drive it at first. Aside from the carburetor rebuild, I had to replace all the fuel filters twice.

How long do you have to wait before the truck starts up and runs normally? How long does it continue to run run normally before it stalls? Or has it not run normally at all since the last incident?
 
#4 ·
71GMC said:
alright, i have a 1971 gmc pickup, 350 c.i., original Q-Jet, HEI, Electric choke. The truck has new plugs, wires, cap, rotor, coil, and electronic module.

so, on my way back from the gas station, the truck missed and stalled at operating temperature at a 4 way stop. Once it stalls it will start again, but won't stay running for more then 2 or 3 seconds. It has did this twice on me, this time and while on the highway going 55 mph. If you guys could tell me what is wrong with my truck, it would be greatly appreciated.

Thanks for your time - 71GMC

also, sorry for the un-needed extra thread. did not mean to press the button to send it to the board. :/
Dang you went back and changed the whole orginal thread you had posted. WHY? :confused: This is not any thing like what you had posted to start with. :eek: :nono: :spank:


Cole
:pimp:
 
#5 ·
lt1silverhawk said:
Hey 71GMC,

My '77 Chevy truck had problems that were somewhat similar to yours, and it turned out that debris and water in the fuel were to blame. The junk clogged the carburetor and it had to be rebuilt. The truck would start, stay on for a few seconds and then die. Couldn't even drive it at first. Aside from the carburetor rebuild, I had to replace all the fuel filters twice.

How long do you have to wait before the truck starts up and runs normally? How long does it continue to run run normally before it stalls? Or has it not run normally at all since the last incident?
it will usually start no problem when it's cold. Now when it stalls, it will take on average of 10-15 minutes. It will run for about 5 minutes before it starts missing, while in park, and it dies in about a minute or two after it starts missing. It runs fine until the truck warms up, then it will miss and stall.
 
#6 ·
Just an idea. Did you change the electronic module yourself? If you did, did you put the dialectic grease under the module? Because it sort of sounds like it is overheating and quit working. It cools some then it works.Just one more thing to think about. Good luck.
 
#7 ·
sirron said:
Just an idea. Did you change the electronic module yourself? If you did, did you put the dialectic grease under the module? Because it sort of sounds like it is overheating and quit working. It cools some then it works.Just one more thing to think about. Good luck.
Yes, i did replace the module myself, and yes i did out the grease on it. the truck also has quite a bit of carbon that comes out of the back port of the carb.. i'm not sure what it goes to, but it blows it right onto the distributor. Also, after it stalls, and you take off the air-cleaner, and push the throttle to ***, white-clearish colored smoke comes out of the carb.
 
#8 ·
71GMC said:
Yes, i did replace the module myself, and yes i did out the grease on it. the truck also has quite a bit of carbon that comes out of the back port of the carb.. i'm not sure what it goes to, but it blows it right onto the distributor. Also, after it stalls, and you take off the air-cleaner, and push the throttle to ***, white-clearish colored smoke comes out of the carb.
First, the module is supposed to have a heat sink compound on it not just any ol' grease. You can get heat sink compound from a computer store or Radio Shack.

There should be no open ports blowing anything, anywhere. If there's a wide open vacuum port on the back side base of the carb, that was originally for the power brake booster vacuum supply line. Plug it and see how it runs then.

BTW, see that there are no other vacuum lines missing from the carb or in bad shape. ALL the air going into the engine should be passing only through the open top of the carb and nowhere else. Any vacuum ports not being used need to be capped off.

Be sure the choke is opening up once the engine has warmed up. The mist you see coming out of the carb is normal. It's vaporized gas coming from the hot intake floor when you shoot gas into the carb from the accelerator pump. This happens every time you open the throttle.
 
#9 ·
cobalt327 said:
First, the module is supposed to have a heat sink compound on it not just any ol' grease. You can get heat sink compound from a computer store or Radio Shack.

There should be no open ports blowing anything, anywhere. If there's a wide open vacuum port on the back side base of the carb, that was originally for the power brake booster vacuum supply line. Plug it and see how it runs then.

BTW, see that there are no other vacuum lines missing from the carb or in bad shape. ALL the air going into the engine should be passing only through the open top of the carb and nowhere else. Any vacuum ports not being used need to be capped off.

Be sure the choke is opening up once the engine has warmed up. The mist you see coming out of the carb is normal. It's vaporized gas coming from the hot intake floor when you shoot gas into the carb from the accelerator pump. This happens every time you open the throttle.
I used the heatsink grease on it, i bought it with it, and put it on when i put in the part. also, the line is connected to a "T" on the Intake manifold, not sure what it goes to, but it throws alot of carbon all over the distributor. another stupid question, but where is the choke located at on the carb?
 
#10 ·
Sounds a lot like a fuel starvation problem, like the sock or filter in the tank is plugging up.
You can test it by Putting a "T" in the suction side hose of the fuel pump hose, There should be little if any vacuum. If you get say more than 1or 2 inch/hg of vacuum, chances are its plugged or the cap vent is plugged.
 
#12 ·
I did see you use the word MODULE in your description of the tune up you have done. Do you have an HEI ? did the installer remove the resistance wire from the harness to the HEI distributor, or did you mean points? Is it factory HEI ??? :confused: :confused:
 
#14 · (Edited)
OK, IIRC a 1971 did not have HEI originally. I mean things do happen I guess it could be , but I believe it wasnt used untill 73 or 74.
The resistance wire is built into the harness and you wont see it unless you kow what you are looking for.
I am thinking when cold, the vehicle has been setting, giving any built up vacuum in the tank, time to equalize. Then when started allows the pump to deliver fuel. As driven for a bit and engine warms up, vacuum builds in the tank and evetually the vacuum gets high enough the fuel pump cant overcome it and stops delivering fuel, literally running the truck out of gas.
Also the distributor power wire ...is it a big heavy RED or Pink wire or is it a skinny little frail looking wire?
Also if the tank filter or sock plugged, the vacuum wouldnt be at the tank, but would be in the line , like a blockage, easy to check with the gauge setup I described.
 
#23 ·
OK, well reason I ask is because the HEI needs a pretty good size wire to run it. And if the truck was originally points and now has an HEI it could still have the resistor wire in the circuit , causing a low voltage delivery to the HEI itself. Modules like plenty of power and can overheat and burn out if the supply voltage is not up to snuff. That could be the cause of the trouble.
Also the fuel thing I mentioned could be the issue as well.
What was the reason you had to change the module? did the truck quit running? And do you still have the same problem with all the new parts on it?
 
#24 · (Edited by Moderator)
the truck did it going down the road at 55 mph, no warning, just lost all power, so we replaced the parts in the HEI and it ran fine. Until about 2 weeks ago, and it's starting again.

Well given that I would have to say its pretty likely your problem is with the power supply to the HEI and not fuel.Probably what killed the first module. The OE GM modules were pretty tough, the aftermarket...well anybodys guess. The module could allready be damaged, but try this first anyhow...

Hot wire the ignition system. Be sure to disconnect the existing wire to the distributor. Use a heavy gauge wire, like 12 or 10.
Hotwiring is a bypass to eliminate the vehicle wiring, and any problem it may have , helping with the diagnosis

Use the right connector and make a jumper, be sure it is making a good solid connection at the battery, route it and tie it down so it doesnt melt on the exhaust or get caught in the fan. Allways disconnect the battery negative when working on wiring projects like this, to keep it safe.

1971 did not have an EGR either.
 
#25 ·
71GMC said:
alright, i have a 1971 gmc pickup, 350 c.i., original Q-Jet, HEI, Electric choke. The truck has new plugs, wires, cap, rotor, coil, and electronic module.

so, on my way back from the gas station, the truck missed and stalled at operating temperature at a 4 way stop. Once it stalls it will start again, but won't stay running for more then 2 or 3 seconds. It has did this twice on me, this time and while on the highway going 55 mph. If you guys could tell me what is wrong with my truck, it would be greatly appreciated.

Thanks for your time - 71GMC

also, sorry for the un-needed extra thread. did not mean to press the button to send it to the board. :/
well, you changed out a ton of spark parts,, when it's hot, I'd test for spark.. first,, just because the module is new doesn't mean much..

if you have spark.. after the 5 seconds of cranking, (you said it'll restart but die in a few seconds) then move to the fuel side
it runs good cold but not hot.. and you asked where the choke is.. so we can go with your somewhat new at this, and thats fine we all started that way..
before we move forward, after you check all the vacuum ports like listed above.. look at the top of the carb, that oval looking flapper over the front two barrels, thats the choke, let it warm up.. with it warm that flapper (the choke) should be open, straight up and down..
let us know if it's opening,, a closed choke on a warm engine will stall..

next up. the q jet, at the fuel line inlet has a fuel filter.. has that ever been changed?

and another tidbit.. the fuel tank, the new ethnol blended fuel.. does a good job of cleaning the fuel tank of years of gunk.. and can clog the sock on the end of the fuel line in the tank.. rust also can get sucked to the sock and slowly starve the engine of fuel..

I'd do this , in this order
1)you didn't tell us, but I'm willing to bet this engine still has it's egr valve..
it's on the intake and looks like a mushroom.. THESE ARE KNOWN TO STICK OPEN, MOST TIMES YOU CAN PUT YOUR FINGERS IN THE UNDERSIDE AND MOVE THE VALVE AND GET IT TO CLOSE.. AND START BACK UP..
I'd when cold, remove the vacuum lin going to this and plug it with a golf tee, or a bolt.. drive around, if the stalling issue goes bye bye the valve is sticking open and needs to be replaced



2) warm it up. when it stalls, check to see if the choke is open,
3)check for spark
4) check the fuel filter inline , at the q jet
5) the old fuel line to the fuel pump, they get old and colapse and the pump can't suck fuel..
6) the intank sock


my bet is on the EGR VALVE sticking open
 
#26 ·
71GMC said:
the truck did it going down the road at 55 mph, no warning, just lost all power, so we replaced the parts in the HEI and it ran fine. Until about 2 weeks ago, and it's starting again.]

Well given that I would have to say its pretty likely your problem is with the power supply to the HEI and not fuel.Probably what killed the first module. The OE GM modules were pretty tough, the aftermarket...well anybodys guess. The module could allready be damaged, but try this first anyhow...

Hot wire the ignition system. Be sure to disconnect the existing wire to the distributor. Use a heavy gauge wire, like 12 or 10.
Hotwiring is a bypass to eliminate the vehicle wiring, and any problem it may have , helping with the diagnosis

Use the right connector and make a jumper, be sure it is making a good solid connection at the battery, route it and tie it down so it doesnt melt on the exhaust or get caught in the fan. Allways disconnect the battery negative when working on wiring projects like this, to keep it safe.

1971 did not have an EGR either.

This was my post with a quote, but somehow it ended up with the OP name on the top of the post ...what the ... :confused: :confused: :confused: :confused:
 
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