Really bad stumbling on the highway (400 small block)

[spinem]

move your stupid ass fuel filter.

[D-Day]

Well, put in a set of plain ol' AC Delco plugs since it's all I could afford right now, and I figured if it wasn't the plugs, why waste nearly $50 on new NGK Iridium's and have them get screwed up.

Took it on a test run, wound it up hard a couple times and kept it in the higher RPM's, never missed a beat. Knock on wood...will have to do some more driving and see if it appears again. Hopefully not.

[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.

[leegrady]

Fuel pump must provide pressure and volume. Do a fuel pump pressure check with a fuel pump pressure gauge, often incorporated with a typical vacuum gauge. There can be gas at the carb, but that's just when it's just sitting there, not when the engine is under load. Should read about seven (7) psi. (Check the book. Always go by manufacture specs'). I've even plumped one in and gone on a test drive to see if the pressure drops off when a press down on the loud pedal. By the way, fuel pressure gauges for long term use like a coolant temp. Alt. tach. should always be fitted outside the passenger compartment. Usually a special housing on the hood viewable through the windshield. way cool. Because of possible fire. You don't want gas flyin' all over you in a crash.
Next is volume. Different from pressure this is the amount the fuel pump pumps within a set period of time. For mechanical pumps disconnect the line after the pump and run the engine with the fuel that's still in the bowl and count the seconds prescribed by manufacture specs' it takes to fill a set amount of fuel. Like for example: three pints in 30 seconds. For electric pumps simply run them and perform the same test as with a mechanical pump.
Or, simply change the fuel pump to another hopefully known good one and see what happens.
Another thing too is that electric fuel pumps must be regulated. If the pumps provides too much fuel it will lift the float bowl needle off it's seat and flood the carb. Mechanical fuel pumps do this automatically. The diaphragm is spring loaded and when a engineered pressure is reach the pump "floats" and doesn't pump fuel. True too, electric fuel pumps should be mounted as close to the tank as possible to push the gas not suck it. In this way the fuel will always flow even if bubbles form, (vapor lock, when gasoline boils from a heat source).
Remember an engine basically needs three things to run:
Compression,
Spark at the right time, and...
Fuel
I'll bet you have a bad fuel pump. In troubleshooting always start with the simple, and work your way toward harder, more involved possible solutions.

[leegrady]

I have an '81 GMC K2500 350 Quadrajet, four speed manual, NP-208, Corp 14 bolt semi-floater long bed with twin tanks, tilt steering and sliding rear window. Non a/c, no cruise. I'm the second owner, and it needs a lot of work, but it's real straight and hardly any rust at all. Payed six hundred bucks for it, and I'll get it on the road by this summer.

I prefer to run mechanical fuel pumps because they work in sync with engine rpm.