Fuel pressure issue? Comes and Go's

[BARRACUDA1968]

Hello, I have a Y88 78 Trans Am 4 speed car.  Original engine and carb.

My fuel pressure comes and go's?

I've replaced the tank, sending unit, all rubber lines; had the carb rebuilt with Cliffs kit and installed 2 new fuel pumps.

I have a fuel pressure gauge hooked up to the car and taped to the windshield.  Basically the car runs out of gas after running it wide open for maybe 3 or 4 seconds. 

I'm on my second mechanical fuel pump, just an over the counter piece.  That's all I can find that matched the original fuel pump.  I'm trying to stay away from an aftermarket pump because the car is original.  I think the pump is made by airtex?  I got it at CARQUEST.

It will idle between 4 and 7 psi just fine for a while then drop to 2 psi and just lay there.  Reving the engine doesn't change it.  Then if I drive it down the road it will sometime go to normal pressure then after a while it will drop down under 3 psi.

I double checked the lines again today and all seems fine.  I unhooked the feed line and blew air through it and installed a new rubber line back by the tank.  When pressure is down I have loosened the gas cap and no air.

It just seems like something is restricting fuel flow or vapor locking??  I am at a loss at this point.  Maybe the fuel pumps I'm getting are junk but 2 in a row seems unlikely.  My motor is basically stock so a mechanical pump should run it fine.

I do NOT want to through on an electric pump.  I want to figure this out.

I am running a return line and a vapor line to the canister.  I unhooked all the line from the canister just for kicked and plugged them, still the same.

Throw some ideas at me,  I've tried everything I know to do.

Thank you,

Brett

[Malibu72]

          I have read on other sites of problems, usually with too high a pressure output, with that particular brand of fuel pump. I am assuming that when you replaced the sender that you also replaced the filter sock. I would purchase a AC Delco pump. Also make sure you try a new fuel filter.  Hope it helped.

[429bbf]

welcome aboard . im not familiar with your fuel system but this is what i would do. i would unhook the fuel inlet at the pump and put a gallon or whatever i could get under the hood. directly to the fuel pump.this should tell you if its the pump or the supply coming to the pump.one key thing to remember (pumps dont make pressure) they produce volume and it takes some sort of restriction. ie needle and seat ,fuel line size etc.to make pressure. fwiw

[BARRACUDA1968]

Thanks for the information!

Yes I installed a new sock on the sendor.

The only place I've been able to find an AC Delco pump is Butler Performance.  I talked to Tech and they couldn't tell me who made the pump and how good is really is.  It's about $85 and the pump I'm using is $36.  Again the only over the counter pump I can get from 3 different auto parts stores.  They all carry the same pump.

Tell me more about this needle and seat.  I know what it is and I understand it but is there a way to adjust it on the carb?  IE can I set it to withstand more pressure.

When my fuel pressure go's down the car seems to idle a little high and run rich which would seem opposite to me with less fuel pressure but if the pump is blowing through the needle and seat I guess I would have less pressure and shoving more fuel in the motor?

The guy who built the carb is a competent builder.  He has a special tool he made so he can take this plug out of the top of the carb and then he has a short looking screwdriver with a slot in the end of it to adjust something?  I'm thinking float level possibly?  It's toward the front of the carb on top.

I was under the assumption that these stock type pumps pressure between 4 and 6 psi so I really shouldn't need a regulator.  When I do have fuel pressure I am in that range.  Then the pressure go's away and sometimes comes back and sometimes not.

I have responsive idle air screws as well.

I can try the fuel pump without the main feed line to the carb and see how it pumps.  Not sure if this tells you anything but I can take the main fuel line off the inlet on the fuel pump with the car level on the hoist and some fuel will run out and stop.  I do not have to plug the line.  I figured fuel would just run out but it doesn't.  Maybe thats normal.  Just trying to give as much info as possible.  It's probably something very minor I'm missing.

One other thing, I am not running a fuel filter right now.  My carb builder said not to run the filter on the front of the carb so I plan to install an inline filter.  Just need to figure out where I am going to put it at this point.  Right now I'm just trying to get things lined out.  I doubt this would make a difference but I'm throwing it out there.

Thanks for the help so far.

[429bbf]

not sure where to start. your rebuilder is adjusting the APT (adjustable part throttle ) this sets the height of the metering rods in the jets.i think if you hook an electric pump to the carb with the engine off you may find your problem.if the needle and seat are bad you should be able to see the gas run into the engine or out the vent.its sounds like your pump is not putting out a constant volume.if you use the electric to feed the carb you will be able to narrow this thing down.hope this makes sense

[429bbf]

just reread the post. i don't know why a rebuilder would tell you not to run the filter in the carb. this is the fastest way to have to rebuild the carb again. it take very little garbage to mess a good carb.some of the holes in the carb are as small as .025 thats not a very big piece of dirt to cause problems. fwiw

[BARRACUDA1968]

My builder wants me to run a fuel filter just not the one on the front of the carb.  I'm going to install an inline filter. 

Since the tank and all parts are new I wasn't real worried about starting the car without a filter just to get it lined out.  I will install a good filter before I do any real driving.

I still have the electric pump I took off the car so I may try that.

Now that you mention the adjustable part throttle I remember him saying that.

Can the vapor line from the tank cause an issue?  And just so I know my lines are hooked up correctly, (I just hooked them up like they were when I installed the new tank)  Is the return line on the sender or the metal line welded on the tank?

[429bbf]

i can't answer that I've never owned one I'm sure somebody on here will know . sorry i can't help.

[blazer74]

I feel your frustration, sorry I cant help you due to I'm dealing with the same issue with the fuel pressure and mine also falls on its face  @WOT most of the time when it warm outside but not always.

I Think its solved then comes back.

Went thru all with new rubber everywhere, rerouted away from heat.

Blew out lines feed and evap vent to tank. Left gas cap off.

Replaced fuel pump with edelbrock high volume 6 psi, no help.

I get the steady 6psi then drops like yours and can watch the fuel go down in the glass filter just before the carb, then it will recover after some time and repeat. I'm lucky to hold 3 psi when this happens at idle. I dont know what my fuel pressure is  as it falls on its face but am going to install an electric fuel pressure gauge for that.

Mine will also surge a little after shutting off hot for a short time and then restarting and going under light or hard throttle momentarily then its ok.

 Im also going to upgrade  the tank and install a new sender with the extra return line port, currently I dont have the 1/4 inch return line to the tank, just the vapor line to the canister.

Im going to install  an early GM fuel filter with the extra 1/4 vapor port  and connect it to the return on the  new sender and see maybe its vapor lock because the hotter out the worse it gets.

I've also tried different Q-Jet and distributor to rule that out but I may throw on a Holly 600 I have just to try.

This has been going on for quite some time but didnt have this problem years ago, Ive had the turck since 1988. Modern fuel is probably not helping.

Now thats is no longer winter I want (need) to get to the bottom of this already.  Good Luck

1974 K5 Blazer 355

[von]

May be a long shot but it could be possible that the pump pushrod or cam lobe is worn and not giving the pump a long enough stroke.