Funky fuel system problems.. starving after 1/2 tank

[gintaras]

UPDATE, the problem was the fuel pump, and I have replaced it, but I am concerned about not having the correct one..

my original pump has 3 lines.. one from the tank, one to the carb, and one, for what I thought was a return line, but now I don't know..

parts store gave me one with only a supply and a line to the carb.. when I told that mine had a return they told me "no, that goes to the charcoal canister, just plug it up.. so I did just that, runs great, but I think I need the one with the extra fitting on it, I think it is a return... anyone?

I just got back from a 1200 mile trip from TX. I will post more about that in another thread after I get some sleep, but boy are those foam bench seats VERY uncomfortable after a long haul like that!


Let me run down the symptoms which I feel is a fuel issue..



First of all, the truck only has 68k miles on it, and only accumulated 1500 miles in the past 4 years until I added 1200 this weekend..

We were going along just fine, about 65-75mph.. when all of a sudden, 120 miles into our trip, I get some hesitation, and I find myself driving slower and slower.. the engine is bucking, and seems to get a lot worse if the secondaries kick in. and it seems to accelerate when the fuel sloshes around, kind of like a rhythmic accelerate, fall, accelerate, fall, accelerate, etc... I get down to 40 mph and have no choice but to pull over and shut it down for about 15 minutes and then it runs fine for a while. Also experiencing very rough idle, no matter if the tank is full or 1/2, but a little pedal smoothes it out.. this was not happening prior to hitting the highway.

I noticed that this happened after I hit the 1/2 tank mark..

I didnt think it was a fuel problem at first, so I cleaned the points on the road with a dollar bill, replaced the coil and the fuel filter in the carb.. we filled up and thought it was fixed but then again at 1/2 tank we were limping again.

I pulled out the feeder and return assembly and the screen was not restriced, so I put it back in.. oh, important note, the return tube was shorter than the supply tube, not sure if that is a problem or not, and the funny thing is that the return line seems about as long as a 1/2 tank...

another full tank and another 125 miles, again, limping..

So, from Houston to Chicago, stopping every 120 miles kind of sucks, and the 18 hour trip turned into 29.. with all the stops and troubleshooting.

Sorry for the long post, going on 36 hours no sleep, I am off to bed now!

[Jtrux]

My first guess, vapor lock. A few things to do, one is put a phenolic spacer between the carb and intake, kinda expensive but works good (around 40 I think). Also insulate the fuel line somehow or route it where it doesn't get near anything really hot. Change fuel filter and don't let it rest on anything keep it suspended from the intake, valve covers etc. etc.

[68C15]

check the hoses between the tank and the pump. it may be collapsing internally. does it get better if you leave the cap off? (thinking it may have a non-vented cap)

the idle deal sounds like either water in gas or crap stuck in one of the idle circuits.

[hotrodxy]

Originally Posted by Jtrux

My first guess, vapor lock. A few things to do, one is put a phenolic spacer between the carb and intake, kinda expensive but works good (around 40 I think). Also insulate the fuel line somehow or route it where it doesn't get near anything really hot. Change fuel filter and don't let it rest on anything keep it suspended from the intake, valve covers etc. etc.

i agree with jtrux. also and this is a long shot but maybe your supply line and return are switched

[dwcsr]

I'll go with crap gas. I'm having the same issue started last week. We are getting nasty gasoline here after the storm. Some of it smells like varnish, like last years lawn mower gas. Pings at 6* BTC on the timing

[68C15]

I side stepped the vapor lock idea due to it having a return line.
if the lines at the sender were mixed it would flat out die at 1/2 tank.

[cdowns]

if you took the off gas cap would it run then?

[haysonj]

Mine was doing this on the rear tank and I think it had something to do with the pumps ability to pull the gas out of the tank. I added a electric pump in line ahead of the pressure pump and the problem went away. I don't think the FI pumps are meant to pull the fuel only push it under pressure. When you get below a half tank the siphon action is not strong enough to keep up with demand. I think this may be part of the reason new cars have them in the tank.

[gintaras]

Originally Posted by Jtrux

My first guess, vapor lock. A few things to do, one is put a phenolic spacer between the carb and intake, kinda expensive but works good (around 40 I think). Also insulate the fuel line somehow or route it where it doesn't get near anything really hot. Change fuel filter and don't let it rest on anything keep it suspended from the intake, valve covers etc. etc.

Changed the fuel filter, it is in the carb, I didn't see an inline filter outside of the carb.

Originally Posted by 68C15

check the hoses between the tank and the pump. it may be collapsing internally. does it get better if you leave the cap off? (thinking it may have a non-vented cap)

the idle deal sounds like either water in gas or crap stuck in one of the idle circuits.

I thought the hoses were metal, but the ones that were rubber, that i could see, were in good shape. i will get under the truck today and look for more. It has a vaccuum vented gas gap, it is the original cap I believe, it has a nipple inside with a spring behind it.

I think you are correct about debris being inside the carb, but only disassembly will tell, I am hoping to pick up a rebuild kit for this quadrajet later in the week.

Originally Posted by hotrodxy

i agree with jtrux. also and this is a long shot but maybe your supply line and return are switched

The supply and return are correct, when I picked up the truck the tank was empty and it started right up.

Originally Posted by dwcsr

I'll go with crap gas. I'm having the same issue started last week. We are getting nasty gasoline here after the storm. Some of it smells like varnish, like last years lawn mower gas. Pings at 6* BTC on the timing

we made about 10 stops to top off for gas, yeah, maybe it was the first station with cap gas, but I would think that after 10 top-offs, it would be mixed enough and burned off

Originally Posted by cdowns

if you took the off gas cap would it run then?

I did try that wile driving and it made no difference.
Also, there is no pressure or vacuum when I remove the cap to fill the tank.

Originally Posted by haysonj

Mine was doing this on the rear tank and I think it had something to do with the pumps ability to pull the gas out of the tank. I added a electric pump in line ahead of the pressure pump and the problem went away. I don't think the FI pumps are meant to pull the fuel only push it under pressure. When you get below a half tank the siphon action is not strong enough to keep up with demand. I think this may be part of the reason new cars have them in the tank.

I was going to swap out the mechanical pump for an electric pump to see if that fixed it.

[gintaras]

Morning update..
starts up cold no problem, has a nice, healthy idle until it warms up, and then the "shaky" idle starts up, at stoplights, etc... At WOT there is a "spitting" like a rapid misfire.. I am pretty convinced it is junk in the carb, how involved is the Qjet rebuild?

[joedoozer]

haysonj brings up a good point. Try an electric pump in place of the mechanical. If it does not solve the problem, that doesn't mean the pump is still good. You may have more than 1 problem. If electric pump doesnt fix it, try running a supply line from the E-pump into a 5 gallon jug. This would eliminate all the old lines, crud in the tank, rusted pick up tube, plugged filter sock.....and a list of other problems. That way you know you are starting with a good pump, good fuel and good lines. Problem still there, then it points to the carb. If the problem is gone, then it is from the pump back. Try different combinations of solutions. Eliminate just the tank, or just the lines to the tank.


On a side note.

I built a lot of Qjets when I was younger, too poor to buy a new one. Qjets are a complicated carb (compared to a Holley), but in this day and age finding some how to's on the web should be fairly easy. And I am sure their is a wealth of knowledge right here.

I had a similar problem with my truck. But it wasn't fuel level related. Not having a choke on my carb, I would richen and lean out idle mixture screws depending on the season. And when it got cold and I was to lazy to back the screw out a 1/2 or so, it would give me a shake at the stop lights, and a definite spitting almost "missing" at WOT. But it would clear up if I left my foot in it long enough.

Not saying its an idle mixture problem. Just my long winded way of saying I would look into the carb.

But it is wierd that it happens at 1/2 tank mark.

Is the truck at 1/2 tank when you started it, stated in your morning update?

[northerngmc]

I had a very similiar problem with a 78 Ford Courier. Over 1/2 tank and it ran great.

We tried almost everthing and when we dropped the tank down we found that it had a small dent in the bottom of the tank, right at the fuel pickup point. With the weight of a full tank it was fine, but when the tank dropped to below half the fuel pickup with hit the bottom of the tank and starve the engine.

Best of luck.

[gintaras]

Hey guys, thanks for all the replies.. it was....



the fuel pump..

question
my original pump has 3 lines.. one from the tank, one to the carb, and one, for what I thought was a return line, but now I don't know..

parts store gave me one with only a supply and a line to the carb.. when I told that mine had a return they told me "no, that goes to the vac canister, just plug it up.. so I did just that, runs great, except for if I stop for a long time like at a light.. it gets flooded and dies.. I think I need the one with the extra fitting on it, I think it is a return... anyone?

[cparman]

my 2 cents worth: If I were to chase this, I would get a shop rag, and a blow nozzle. Put the shop rag and the nozzel into the gas filler neck. Have a friend looking for raw fuel at all of the hoses on the feed side. NOW, that was 1 cent worth. The other penny is thinking that the fuel system is sucking air when the tank gets low. I would think ( bunch on here are lots smarter than me ) that the tube mounted on the sending unit is a solid steel tube, not capable of sucking air, but needs to be checked. Back in the 80's we saw a bunch of our trucks with diesel/gas engines doing this and it was a split in the top side of a hose. would not show a leak, would would run like fido's A **

[gintaras]

thanks for the reply, but it was the fuel pump, please see my question about the fuel pump return line.

[gintaras]

bump for a fuel pump answer, thanks

[68C15]

it is a return to the tank. not having a return can cause too much pressure. so much so that the needle valve may not be able to control it all.
the evap canister is fed from the bowl vent on the carb.
find a different parts guy.

[dwcsr]

I had the same issue and a pump didn't fix it. I used a 3 line pump so I have a idea of why these to trucks had the same issue.

The common factor was gas in Houston just after the huricane. Some of this gas smelled like last years lawn mover gas. I think we both got crud in our tanks from this gas and the filter bag on the pickup got to the point that it was easier to suck gas from the smaller unrestricted line and return it to the larger line. After a half tank mark was hit the smaller line was no longer in the gas and sucked air. Mine was between 1/2 and 3/8th tank when it would die and stumble until it sat for a while. If it was full or below 3/8th it would run fine.

I remembered years ago buying 3 line pumps and the directions saying to plug the small line with a red plug in the box. I plugged the smaller return line and it now runs and any tank measure. This wasn't anything to do with ecc on the tanks or vacuum hoses in the wrong place. It was just a bad design from GM to fix a vapor lock problem in the pump by circulating fuel back to the tank. Once the screen was dirty enough it cycled backwards and sucked air at half tank.

[mnz28]

Just experienced the same issues. The pump was junk. Inconsistent issues the truck would run great then would starve for fuel.