Car jerks upon acceleration but only when low on gas

[kaloft]

Hi!

I seem to be having this problem only when I am low on gas. Before I start replacing things, I was wondering if anyone could help me diagnose this.

The problem is, when I start to become low on gas - about 1/3 tank left - my acceleration jerks and lags. What I mean by this, is that it feels like I keep tapping on the gas. It goes away upon higher gears and a constant speed. However, when my gas is even lower, it continues to jerk even during constant high speeds (5th gear 70mph).

I have just recently purchased this car and it has not been used for about two years. It's a 93 manual festiva.

I have also just replaced the spark plugs, spark plug wires, distributor rotor & cap, fuel filter, air filter, oil filter, oil change, new cat, and used some seafoam in the gas tank + engine vacuum.

[batstiva]

You seafoamed the fuel system, did you happen to pull the pump/sending unit, and examine the the inside of the tank for crud/dirt/rust/anything else that might be picked up when you get down near the bottom of the tank?

You are not running on the gas that has been in the car while it was sitting, are you?

[Pu241]

I wouldn't waste good seafoam on 2 yr old gas or slug/crud in the tank.
Before you do anything get 3-5 gal of fresh fuel in a separate container.
when it's low on fuel or you can't stand it anymore, open the drain plug on the tank.
If you got crud coming out time to pull the pump/sender unit from under the back seat and give the tank a good scrubbing and flush out the crud.
Put fresh gas in it and should be good to go.
Then you can use some seafoam if you want.

[kaloft]

Shouldn't be any old fuel in there, this has been my third fill up. It was pretty empty the first time I filled up. I was actually going to drain the tank and check it today for crap. When I changed the fuel filter, it wasn't dirty, just a bit hazy. No crap came out of it though.


Such fast responses! Thanks guys!

[Pu241]

Filter might not have crud in it because the pick filter in the tank is doing it job.
Also if the tank was nearly empty when you got the car I would bet on a fair amount of moisture having condensed in the tank.
Which might well have lead to rust and crud blocking the tank filter when your low on fuel.

[batstiva]

Please humor me, pull the pump/sending unit and see what's in there. Or do as Pu241 says and run it as near empty as you can get away with, then drain and see what comes out. I'd be willing to bet one of my Festivas that you will find crap in there that kind of settles to the bottom. Everything is fine as long as you are pulling the cleaner gasoline from up top. As you get closer to empty, it starts picking that crap up. You are probably not seeing it in the fuel filter because the screen in the pick up tube is catching it. Which is clogging it, which is why it starts bucking and jerking.

[kaloft]

Tank was pretty clean. Turns out that my fuel pump was just going bad.

Installed a new fuel pump + strainer and car started up so quickly like she was brand new. No more jerking or bogging when low on gas. Hooray!

[Pu241]

Great news, but how did you determine it was the fuel pump?