virfighter

Ok guys, made a trip this weekend to visit the girlfriend (long distance relationships suck by the way) and on the way there I noticed some slight sputtering and loss of power while cruising. Here's what would happen:

First on the way up to see her; I would be running along at 70-75mph and I would notice the car would start to sputter and I would lose speed. Pushing the gas pedal just made it worse, no power at all. However, if I let off for a few seconds it would come back. It wasn't really that bad, it happened about halfway through the trip and was a minor annoyance every now and then (happened like 6 times in an hour and a half).

On the way back things got worse. I was running fine the whole way. After about 160-170 miles of driving or so, the car started to sputter and miss again. It got so bad that my speed was dropping down to around 50mph so I pulled off and went to a gas station. I checked all my fluids and poked around some under the hood and everything seemed in order. I got back on the road and for about 5 miles it was fine and then it started again.

At this point the car was really struggling, so I pulled over again checked her out again. Nothing seemed wrong, I let it sit for a good 20 minutes and then went back out. After about 10 miles or so it started happening again. I called my Dad for a second opinion and we threw ideas around until he asked me how much gas I had left. I was at about 3/8ths of a tank and the trouble had started at around 1/2 a tank. So he recommended filling her up and seeing what happened.

So I filled the car up and the car ran perfect the rest of the way in. I never dropped below half a tank before I made it back to Atlanta though.

So here's our conclusion, there is something wrong with the venting in the tank, like the lines are clogged up or something but I'm really unsure. It only happens when I'm down to less than half a tank (I plan to take my car back out on the interstate this weekend and do some testing) and it only seems to happen on long trips.

Also, one last piece of information, I've been adding a little bit of MMO to each tank for about the last 6 or 7 tanks of gas. I not sure if this has anything to with it or not, but figured I would throw it out there.

So, where/how does the gas tank vent itself? Its not through the cap since its a non-venting cap. I did some searching and some stuff about a "charcol canister" came up but I wasn't sure what they were talking about. I browsed through my Haynes' manual but didn't see anything in there either.

Can anyone point me in the right direction or spread some more light on the issue?

Thanks.

680RWHP12A

change your fuel filter

usally if there is a problem with the vent system the car will vapor lock and will stop running completely

virfighter

I just put a new on back in July. Besides... even if it was clogged already, wouldn't it affect my driving around town as well? When I first bought the car, under hard acceleration I would miss/hesitate at around 5000 until I let off the gas, that was solved by replacing the fuel filter, this is a little different.

Still might be something worth looking into though, I'll add it to my list of possible culprits

inittab

When you twist the gas cap off is there a lot of vacuum in the tank? I've heard that the charcoal canister can in-fact clog and cause this to happen. Try putting a piece of wire under the gas cap so that the vacuum can't build up. See if that helps and let us know how it works out.

Rex4Life

Sounds exactly like the problem I had with my 85 12a. My wife had gotten some bad gas that had something like the consistency of mud. Anyway, very likely you have a clogging filter. Remember there are 3 filters. One in the tank, one external before the pump, and little strainer baskets in the carb. BTW I had two of the lines leading to the strainers clogged also.

Could be a vacuum problem by much less likely.

Good Luck finding the problem.

Scott

Rx7carl

Around town your not burning as much fuel. A partially clogged filter will allow low power operation, but when your on the highway it could be struggling to allow enough fuel to pass through. Understand?

Also, the tank vents thru a valve thats just in front of the tank, above the rear axle housing. The manual tells you how to test it. A quick check would be if it starts acting up, remove the gas cap and see if it runs better.

Wankelguy

I had a similar problem with my '80, turned out to be a clogged pickup tube inside the tank. I got an old bass guitar string, bent the end over to form a little hook, chucked the other end in my hand drill and roto-rootered it out. The fix would last for a couple of months, but I finally bit the bullet and replaced the tank.

lnbrown5981

sounds like a fuel pump on it's way out, my 87 did the same thing until it became undrivable. i think the fb and fc have diffent fuel pumps though, but it sounds like the exact symptoms i had.

virfighter

Thanks for all the feedback thus far. Sounds like I have some things to investigate but the fuel filter is looking to be the cause based on the majority of the feedback.

A couple of things I'm still unsure about though. Why does it only seem to happen when I at less than half a tank? Also, why doesn't it happen when I push the car hard around the city (i.e. like up to redline a few times). Previously when my filter was clogged I would experience a stutter only when the car was pushed hard.

RotorMotorDriver

iTrader: (1)

When my fuel filter clogged, it would only hesitate around 60 MPH on the highway, then eventually lower and lower, but NEVER in the city driving. I was baffled until I just replaced the filter.

I dont know why it might only happen with less than half a tank, maybe its something besides the filter.

I think you need to try some of what we have suggested and let us know what happens. Next time it struggles like that, take the gas cap off and see if that helps. If not, its not a vent problem. Simple and free. Let us know .

~T.J.

tunafish

I am really being sincere with this question. For many years at least as many as I can remember while being sober, when fueling up or the car just sitting, engine off and I remove the gas cap there is a substantial vacuum I talking about a long woooooish long. Also when I was replacing vacuum lines I disconnected the line from gas tank that goes to the black round thing part of the rats nest when I pulled the line pssssst and strong smell of gas I really thought this was normal are we saying there should be no vacuum? Maybe this part of my lack of boost from turbo?

virfighter

Ok, here's an update on my problem:

I was able to take it for a long test drive period at the end of last week. Unscrewing the gas cap didn't seem to fix the problem. I even ran w/o the cap for a few miles but the problem was still there. So its not the venting problem.

My next step I guess is to replace the fuel filter and see what happens. However, talking to some other people the fuel pump could be the source of the problem as well (as someone already mentioned in this thread).

As far as it only happening when the tank is half full and lower. Someone explained that this is another sign that the fuel pump is the origin of the fault. He said it will work fine when there is more gas in the tank because the extra weight of the gas pushes it on through the pump. However when the gas gets lower the pump doesn't get any assistance from gravity and thus has more stress on it.

I'm not sure if this is true or not but it sure sounds good. Of course, the same thought process could probably be used when talking about a clogged filter too (more help from the weight of gas forces it through the filter, etc).

I'm heading home this coming weekend for fall break so I'll try to replace the filter before I go. If that doesn't work then I think I'll order a new pump and put it on while I'm at home.

Hopefully I'll get this problem solved soon.

Adventurer

Unless you really want to keep your car pure stock, you can get a perfectly good electric fuel pump for around $40 at most auto parts stores. You'll have to do some shade-tree engineering to mount the thing where the original used to reside. For that price, mine came with a new fuel filter attached.