350 tbi stumble. HELP

[Lewis1129]

I have a 91 scsb with a 350/5 speed. Love the truck. Decent power for what it is. Runs down the road awesome. It idles good, but when you move the truck around. Backing out of a parking spot or pulling up in a drive thru or anything where your just letting the clutch out to move the truck and not getting on the gas to accelerate. When doing this, when you push the clutch in it's like it can't recover the idle. It misses bad like its going to die. I've replaced fuel filter, and swapped distributors with another one I had with brand new cap and rotor. Anybody have any ideas where to start. Would it be the IAC? That makes since to me, but have been told it wouldn't idle good all the time if it was bad.

 

[GT_80]

Could be the IAC, could be timing, could be low fuel pressure... How many miles are on it?

 

[Lewis1129]

The timing is set on 2 degrees advanced. I'm going to set it back. To 0. But changing that did nothing with the stumble. What would cause fuel pressure to be low? Would that be fuel pump. It does seem to be draining back when left not driven a day or two. Takes it a minute to start if I don't cycle the key a time or two. If it is the fuel pump, will a vortec fuel pump support a 5.3 bc it will eventually get a 5.3.

 

[GT_80]

I'm not sure if the stock Vortec fuel pump will support a 5.3L swap - I doubt it, but you'd probably want to change it anyway as your stock one is pretty old and may be junk...

The fuel pressure could be low from a bad pressure regulator, or you could have clogged injectors, or the fuel filter could be clogged, the pump could be worn out, lines could be leaking...

Has the stumble been a gradual thing? Only does it when it's cold? does it high idles fine when it's cold and only stumbles after you start moving? was the truck originally a stick or was the manual trans swapped in? Was the computer ever changed?

it could be a lot of things. First try to determine when you noticed the stumble- did you change anything right before it started doing that?

 

[Lewis1129]

It's been doing it since I got the truck. Before I changed the distributor it would do it if you just revved it up to about 1500. Since the distributor swap that went away but the stumble
Is still there. If fuel pressure was off wouldn't it show itself at wide open throttle as well?

 

[GT_80]

Again, do you knowthe history of the truck? Was it originally a manual trans or was it an auto that was converted to manual? Was the computer changed out? Other than that stumble does it run normally throughout the rpm range?

I converted a manual to a auto one time, and the manual computer tells the vehicle to idle lower, and I had some issues once in a while.

If yours is a Vortec, it has the spider injectors versus the TBI unit? I am not as familiar with those.

 

[Lewis1129]

The truck is totally original, my dad bought it brand new and sold it to my grandfather, who I bought it from. So I know everything that has been done to it. When I got it it didn't run good past half throttle, but a fuel filter fixed that. It runs great everywhere on the rpm range. The stumble is only when it's real low rpm with no throttle. If I rev the motor while its doing it, it will clear up. It seems to be fuel related. Just not sure in what context. But to answer your questions, yes it's factory manual and the computer is original to the truck.

 

[John Deere 4240]

my father's 454 vortec would stall backing up slowly up a hill. When we did intake gasket on it we discovered the fuel regulator vacuum hose had a crack so with no throttle the wasnt enough vacuum to support the load. Not sure if its the same problem but maybe.

 

[Lewis1129]

Well I fixed the problem. Started trouble shooting and took the vacuum line off the edge valve and it totally fixed it. So for right now just going to leave it unplugged. I capped it at the source and have no CEL so I'm just gonna drive it.

 

[GT_80]

You mean the EGR valve? I think you had a typo. The EGR could do that if it is either stuck open, stuck closed, or torn diaphragm.