Stalling at idle; advice needed

[oneday1]

Truck is a 1978 350/th350 C15 "Heavy Half" so no smog stuff, carburated.
I have searched the forums and found many somewhat similar scenarios but not exactly like what is happening to me.
Truck will start and drive fine, but when coasting for a while or sitting at a light it will start to sputter and die unless gas is pressed. Doesn't take much, just enough to keep the rpms up and it won't die and then when given more gas gets up to speed with plenty of power and will drive for at least an hour (just tried) without ever dying as long as it isn't allowed to throttle all the way down. It seems it could be anything from dirty spark plugs to vacuum leak to distributor to fuel pump to carb etc.

Has anyone experienced this symptom? Have any luck fixing it? Any help is greatly appreciated as winter is here now and light is limited. Thanks in advance!

[geezer#99]

Stone stock or modified?
You got headers?

[Dead Parrot]

Check the suction fuel lines. If they are old enough, could be cracked or worse and letting air into the line. The line from the tank(s) to valve(if dual tank) or metal frame line often don't get replaced due to having to drop the tank to replace.

Fuel filter is another suspect.

Let it die in your driveway then see how much gas squirts out of the accelerator pump jets when you operate the throttle. If just a little, probably a gas flow problem.

[andyh1956]

Your EGR could be stuck open.

[oneday1]

Originally Posted by geezer#99

Stone stock or modified?
You got headers?

All stock, no headers, no upgrades. Still running the factory Qjet which I rebuilt around 3-4 years ago. Which I reckon with the amount of ethanol gas I've put through it may need to be done again.

[oneday1]

Originally Posted by andyh1956

Your EGR could be stuck open.

No EGR on this rig. Would definitely be a culprit though. If this was a newer vehicle I would swear it was the MAF sensor. Almost identical symptoms.

[oneday1]

Originally Posted by Dead Parrot

Check the suction fuel lines. If they are old enough, could be cracked or worse and letting air into the line. The line from the tank(s) to valve(if dual tank) or metal frame line often don't get replaced due to having to drop the tank to replace.

Fuel filter is another suspect.

Let it die in your driveway then see how much gas squirts out of the accelerator pump jets when you operate the throttle. If just a little, probably a gas flow problem.

I have replaced both tanks, sending units, and all hard and soft lines about 1.5 years ago. A similar issue appeared shortly after and first thought was fuel filter as the new tanks have that greasy corrosion prevention stuff on them. Swapped in a new filter and seemed to be fine after. That is definitely on my "we'll see" list. Figure I will start with basic things that are probably due to be done anyways, spark plugs, fuel filter etc. and then go from there. Unfortunately I work 10-14 hour days and don't have a garage so week days are hard to get things done when the sun goes down hours before I get home. I will check out the fuel flow as you suggest though, very helpful, thank you!

[geezer#99]

Heat riser still working?
Thermal working?
Thermal is heat to air leaner from exhaust manifold.

[cadillac_al]

It sounds like the carb needle and seat is leaking. Floods it out at idle, runs fine everywhere else.

[wilkin250r]

A gas flow problem sounds unlikely.

It's not impossible, maybe a bad check valve or bad diaphram in the fuel pump that causes it to deliver less fuel than normal, but still pumps some. It may not pump enough at idle, but once you increase the rpms, it pumps more fuel. I suppose it's possible, it's just not likely.

But my first inclination is that it's NOT a fuel delivery problem, because the truck runs and drives fine. The vast majority of fuel problems get worse at higher rpms, not better.

I'm still leaning towards a vacuum leak. There are lots of problems that occur throughout the entire rpm range, the two that only occur at idle are carb idle settings and vacuum leaks. I would concentrate my effort there, first.

[nonstop]

Start it and let it idle for a while. Look down the throat of the carb and see if fuel is dripping from the boosters.

Just had this problem with an off topic car of mine. Turned out to be a small piece of debris had gotten between the needle and seat. Cleaned it up and problem went away.

[andyh1956]

Is the engine "Loading Up, Running Rich" when idling?
Have you done the fuel well mod to the Q Jet yet? Those plugs can leak badly at idle & load up but you may never notice it while driving.

[oneday1]

Thanks all! Checked the heat riser, which I probably wouldn't of even thought about, and it seems to be working fine. I then redid the idle settings and that "solved" it. It seems odd that it would just go out of wack like that. I guess I'm happy for now though!

[Gregski]

I am not sure about your level of expertise, so I do not want to offend you or anyone, but here is what I would do.

First thing I learned, is there is no silver bullet in other words no one single thing is the cause, I think it's a little bit of this and a little bit of that.

Second thing is you/we need feedback, what is the engine doing what is it telling us, checking your initial timing is a good start and a good baseline, hooking up a vacuum gauge is great too if you have one, I like to drive around with a rubber hose running to a gauge zip tied in the dash somewhere and monitor what happens to the vacuum (ie your engines nervous system) at the time of this problem.

But it all comes down to fuel delivery and or ignition.

I would start with fuel and the carb. Short of pulling it off and doing a nice rebuild / refresh. I mean when is the last time that '78 carb took a bath? ha ha. Check the float levels in both the primary and secondary bowls, I am assuming you have a four barrel carb, I may be wrong. Lots of people skip, or overlook this step and go right for turning the idle speed screw, I am guilty of that myself, but doing so you are building on bad foundation but the order is:

1. Float Level adjustment (if you have sight glasses awesome put the float to the middle, if you have a plug, do it so the fuel just barely drizzles out a tiny bit with the truck running)

2. Idle mixture screws (1 1/2 turns out from bottoming out gently)

3. Idle speed screws

then back to #2 then back to #3 a little till the RPMs land where you want them

4. etc...


Sorry to geek out on you, but this stuff fascinates me, and I like to tune the shiz out of my trucks.

[oneday1]

Originally Posted by Gregski

I am not sure about your level of expertise, so I do not want to offend you or anyone, but here is what I would do.

First thing I learned, is there is no silver bullet in other words no one single thing is the cause, I think it's a little bit of this and a little bit of that.

Second thing is you/we need feedback, what is the engine doing what is it telling us, checking your initial timing is a good start and a good baseline, hooking up a vacuum gauge is great too if you have one, I like to drive around with a rubber hose running to a gauge zip tied in the dash somewhere and monitor what happens to the vacuum (ie your engines nervous system) at the time of this problem.

But it all comes down to fuel delivery and or ignition.

I would start with fuel and the carb. Short of pulling it off and doing a nice rebuild / refresh. I mean when is the last time that '78 carb took a bath? ha ha. Check the float levels in both the primary and secondary bowls, I am assuming you have a four barrel carb, I may be wrong. Lots of people skip, or overlook this step and go right for turning the idle speed screw, I am guilty of that myself, but doing so you are building on bad foundation but the order is:

1. Float Level adjustment (if you have sight glasses awesome put the float to the middle, if you have a plug, do it so the fuel just barely drizzles out a tiny bit with the truck running)

2. Idle mixture screws (1 1/2 turns out from bottoming out gently)

3. Idle speed screws

then back to #2 then back to #3 a little till the RPMs land where you want them

4. etc...


Sorry to geek out on you, but this stuff fascinates me, and I like to tune the shiz out of my trucks.

I would never be offended by someone who took the time out to try and help me. I appreciate it greatly. I would say that I'm somewhat experienced but certainly no expert.
I do plan on doing things the right way. Looking back I see that I didn't make that too clear, but I was trying to say that half-a** works for now, but time needs to be taken to do it right in the near future.
I have rebuilt the carb once before but it has been 4 years and I buy cheap gas so I think it's totally possible that a rebuild would be a good idea at this point.
Thank you for the response though because it does seem like you summed it all up right there.

[andyh1956]

Funny story- In high school I worked at the local NAPA in the machine shop. We had a boy at store #2 named Raymond. Now Raymond wasn't the sharpest chainsaw in the bowling alley if you know what I mean (Bless His Heart!)
Anyway one day we were out back moving things around & Raymond walks by & notices a can of carb cleaner. He picks the whole can up & reads the label. Then he goes over to his 66 Nova SS & pops the Rochester 4-jet off & dunks the whole carb down in the bucket of carb cleaner!
We just watched & Raymond got into the delivery car (A B210 Datsun) & went to work.
He returned about quitting time, removed his carb from the bucket, hosed it off with the water hose, re installed it on his engine, then to our amazement he SHUT THE HOOD, STARTED his car & DROVE AWAY!
True story!
So, if Raymond could do it anybody can!

[Jstock85]

HA! Mississippi boy. I love it!