Really bad stumbling on the highway (400 small block)

[D-Day]

I can't figure for the life of me what is causing this problem. I took a trip to Florence, KY last night to drop my girlfriend off (about a 186 mile drive), and in the last 30 miles the stumbling started, and within the last 5 it was damn hideous going up the hills, whether the 700R4 was locked up or not.

Just before the trip I topped off the fluids, replaced a vacuum fitting T nipple, and made sure the choke cable wasn't sticking. Vacuum throughout everything was at normal.

I'm pretty sure it's fuel delivery related. It's either the carb, or the Holley electric fuel pump. I'm gonna replace both fuel filters for lack of anything better to do, take a look at the air cleaner...even if the air filter is dirty, it shouldn't have caused such chugging and lurching that severe.

It genuinely felt like I was about to run out of gas, or - when you forget to push the choke back in before accelerating hard, how it stumbles before you push it back in. However, it wasn't constant. On the way home, I ran for 40-50 miles like everything was fine.

Another concern is the gas cap I have. It's usually sort of loose, but I noticed when I filled up on the way down and back it was tight as hell, sometimes hearing a "pssht" of pressure being let off when I opened it. Thinking it was creating a vacuum in the tank, I let the gas cap hang off for the last mile home, and it still did it.

Right now my agenda is to pull the plugs, check the timing, replace the fuel filters...as for that, I'm not sure what else to check. Anyone have suggestions?

Its intermittent nature makes this hard to diagnose. Would something really bad like a cracked head letting coolant into the engine cause this? Thing of it is, it starts and runs fine at idle and lower RPM's...the stumbling is primarily noticed when the pedal is pushed down, when the need for more gas is called upon.

[old Rusty C10]

bad gas... clogged filter.... someone put something in your tank...carb binding or sticking...

[1LowToy]

Check the the distributer cap and rotor. Check the coil for proper resistance. A failing coil can cause a crapout like your running out of fuel. I don't know the the resistance figures off hand but I Know I've been caught throwing parts at a bad coil problem before.

[PhilaTruck]

I'm solidly with Bob. Had exact same behavior. $1.69 later, it was better.

[spinem]

float in side the carb. Guarantee it. the vertical "/" of the hill or \ made it go lean and ritch.

[chevyrestoguy]

I'm thinking it's bad gas. This condition is something you see pretty often in the wintertime because the freezing and thawing leaves water deposits in the holding tanks at the gas stations.

I had the exact same issue a few years ago, and the truck ran fine until I got down to about a quarter of a tank and she started drinking the water and I barely made it home. I got home and siphoned out the rest of the gas and refilled the tank and the problem was solved.

[D-Day]

Well, fuel filter replaced, didn't do anything...timing checked, still fine...fuel pump running fine and keeping plenty of gas to the carb. So, it really is either bad gas, or something in the carb. Since this carb has hardly given me any problems (Edelbrock 1405), I suppose I should pick up a siphon of sorts to drain the tank out and see if there is an improvement there.

Never had this problem with gas before, but maybe I do now. I might try messing around with the floats and check those too. I'm slightly hesitant to, though, because I first noticed a hint of this problem a month ago on flat land around here. Then it went away...then it came back with a vengeance. The bad gas thing could be it. Different fluids mixing is very unpredictable, so it would explain the intermittent nature.

Euhh. I love and hate spring. Better weather comes, but at a price. Always a bad luck season...

[D-Day]

Another note, the tank and fuel pick up, as well as Holley electric fuel pump, tank switch valve, fuel lines are a little over a year old (replaced in Dec '08), and the glass fuel filter I have installed behind the fuel pump at the tank is clear as a whistle from what I can see. It doesn't seem to be a blockage. The fuel filter is full of gas at the carb.

The distributor cap and rotor and coil is a newer Mallory performance setup, new plug wires as well. NGK Iridium plugs aren't too old either. I wouldn't think this is an electrical issue. I had a stumbling issue at hard acceleration years ago that was attributed to a cap issue, but it was at a very specific RPM range, and not nearly as severe.

This problem is happening throughout, and even under extremely light accelerating and at low speeds (35mph in 4th gear with hardly any pressure on the throttle). It starts and idles fine, it's just when it needs gas it starts to choke. Even when the engine is cool it will do it; and I've been on 17 hour road trips in the summer time and haven't had a problem like this before.

Sticking float or bad gas I suppose are the only things I have for now that I can check or work on until those are done. Either that or I can throw money at all new plugs again...wouldn't be the first time I've done that.

[spinem]

Wait, where is this fuel filter?

between the pump and carb,
or the tank and pump?

Originally Posted by D-Day

Another note, the tank and fuel pick up, as well as Holley electric fuel pump, tank switch valve, fuel lines are a little over a year old (replaced in Dec '08), and the glass fuel filter I have installed behind the fuel pump at the tank is clear as a whistle from what I can see. It doesn't seem to be a blockage. The fuel filter is full of gas at the carb.

The distributor cap and rotor and coil is a newer Mallory performance setup, new plug wires as well. NGK Iridium plugs aren't too old either. I wouldn't think this is an electrical issue. I had a stumbling issue at hard acceleration years ago that was attributed to a cap issue, but it was at a very specific RPM range, and not nearly as severe.

This problem is happening throughout, and even under extremely light accelerating and at low speeds (35mph in 4th gear with hardly any pressure on the throttle). It starts and idles fine, it's just when it needs gas it starts to choke. Even when the engine is cool it will do it; and I've been on 17 hour road trips in the summer time and haven't had a problem like this before.

Sticking float or bad gas I suppose are the only things I have for now that I can check or work on until those are done. Either that or I can throw money at all new plugs again...wouldn't be the first time I've done that.

[D-Day]

Originally Posted by spinem

Wait, where is this fuel filter?

between the pump and carb,
or the tank and pump?

There's a steel mesh/clear glass filter behind the electric fuel pump (which is right in front of the gas tank), and a standard plastic Purolator one just before the carburetor. So, basically, both.

[spinem]

give me a sec.. alot to type

[manimal]

Try some fresh gas from a gas can. unhook your tank to pump and run a hose into a can of NEW gas and run it that way(not driving) and see if it clears up.

[D-Day]

Originally Posted by manimal

Try some fresh gas from a gas can. unhook your tank to pump and run a hose into a can of NEW gas and run it that way(not driving) and see if it clears up.

I wish I could, but it doesn't appear at idle. It's only accelerating under load. Starts and idles fine, just going down the road it presents itself...which...if it was bad gas, wouldn't that cause a stumble or bad running at lower RPM's too?

[spinem]

this is wrong. search for more detals (sorry i typed this huge thing and hit the back button, lost all of it)...

what is happening is ur fuel pump is having trouble sucking thru it, and causing the gas to go under a vacuum and causing what is to refer to as "vapor lock"

place the fuel filter inbetween the pump...and carb. so the pump is PUSHING is thru the filter.....

[spinem]

this is wrong. search for more detals (sorry i typed this huge thing and hit the back button, lost all of it)...

what is happening is ur fuel pump is having trouble sucking thru it, and causing the gas to go under a vacuum and causing what is to refer to as "vapor lock"

place the fuel filter inbetween the pump...and carb. so the pump is PUSHING is thru the filter.....

[D-Day]

Originally Posted by spinem

this is wrong. search for more detals (sorry i typed this huge thing and hit the back button, lost all of it)...

what is happening is ur fuel pump is having trouble sucking thru it, and causing the gas to go under a vacuum and causing what is to refer to as "vapor lock"

place the fuel filter inbetween the pump...and carb. so the pump is PUSHING is thru the filter.....

It doesn't explain why this would start randomly out of the blue. And electric fuel pumps do not like dirt at all. You gotta have one to protect the pump. Edelbrock carbs require a filter before the carb. The one behind the fuel pump is a mesh one to catch bigger particles and junk from getting to the pump and messing that up, and the carb one is to get the smaller stuff. They coexisted fine under the harshest conditions.

There is no lack of gas to the carb. It gets there, but something is not using it properly. Whether plugs or carb or some electrical gremlin, something is awry there.

[f.monroe]

I would double check the plug wires .Fire that truck up at night and look under the hood for bad plug wires . If they are arcing, you will see it at night. I have had plug wires do exactly what you are describing . They break down under a load and cause a bad loss of power . I had what I thought was a new set that broke down very soon .

Since I am not familiar with the carb you have , does it have an internal filter?If so , check it .
Frank

[spinem]

also, the only... ONLY filter that shoul be run on that truck /w/ ur setup is a FRAM G3 ask your local parts store for one. they are OTC

[spinem]

well you didnt grip what i said... can any one explain better?

[joe1982]

I also am experiencing this, I have a 350/t400 with a 600cfm. Any onr else with this setup have this problem? Sorry, I am not trying to high Jack this thread

[D-Day]

Well, pulled and polished the plugs. All of them had bad buildups, clearly from running too lean. Doesn't surprise me with this problem. Took it for a run, almost seemed better, but could still feel it again under hard acceleration. Will have to do more steady driving and see if there was any improvement overall.

Plugs aren't the root of the problem I don't think, even though horribly plagued with deposits. Cleaning them helped the situation, but didn't solve it. Plug wires seem okay, no arcing from what I can tell.

[rtabish]

any chance you jumped a tooth in timing? this sounds like an ignition problem, not a fuel delivery problem. how does it idle?

[LONGHAIR]

Originally Posted by rtabish

any chance you jumped a tooth in timing? this sounds like an ignition problem, not a fuel delivery problem. how does it idle?

That is impossible....The distributor gear cannot become disengaged from the cam gear w/o extremely bad stuff happening...and you would know it, if that happened.
If the timing chain was loose enough to "jump", you have bigger problems than "ignition" timing...Your valve timing would be retarded too. This is as major as it sounds.

[rtabish]

Originally Posted by D-Day

Well, pulled and polished the plugs. All of them had bad buildups, clearly from running too lean. Doesn't surprise me with this problem. Took it for a run, almost seemed better, but could still feel it again under hard acceleration. Will have to do more steady driving and see if there was any improvement overall.

if you are lean, you are not getting enough gas OR you are getting too much air....OK, simple enough.
what carb is on there? could be a bad diaphragm or plugged passage, time for some guitar string and a kit? have you tried spraying carb cleaner around the carb and intake to check for leaks? are one or two plugs worse than the others? can you try a different fuel pump, just to see if that does anything? are the wires to the pump you have good?
i would think if it were bad gas, you would be fouling rich, not lean, but you can add some kind of fuel stabilizer to what is in your tank. if it were not firing properly [ie;bad plugs, wires, coil, etc] the plugs would be wet with fuel or the exhaust would smell "gassy". does it just stumble or does it backfire [either intake or exhaust] as well?
most areas require the use of oxygenated fuels through the winter months. the O2 additives most often used are alcohol based, and alcohol-based additives are pretty good at absorbing moisture out of their environment. also, in high concentrations, alcohol will cause a lean-burn condition much like you are describing. would have to be pretty extreme, though.
just a thought.....never buy gas from a place that is due for a fresh fuel delivery......

[Pyrotechnic]

For awhile my gas cap would make the "pssshht" sound when popping it off. Being that newer cars do it I didn't think too much of it...till I was on the long trip and the truck kept feeling like it would randomly lose all power for a split second at a time. Towards the end of the trip I tried to pass a motor home and the engine basically died. I pulled over and it idled fine, then I popped the hood and looked at my fuel pressure gauge...zero fuel pressure.

Driving around with the gas cap off helped, but the damage was already done and the fuel pump was toast. It was able to move fuel up to the carb, but it was only able to do so with little to no pressure.

I believe that the tank vent was clogged and the pump was drawing a vacuum on the tank, which was really hard on the pump. There is absolutely no reason for pressure of vacuum to form in the tank, because the tank is vented to the atmosphere, at least on mine being that it's non emissions. The temporary solution was to pop off the smaller hose on the filler neck. After that, I had no more "psshhtt" sound as the tank was definitely at atmospheric pressure.

Now as far as the vacuum canister setups that emissions trucks have, that's a different deal I can't speak on as I don't know how they work, but again, with an electric pump, a vacuum in the tank is bad news.

I would recommend a fuel pressure gauge to help diagnose this problem. That was the thing that helped me most in determining my pump was bad.