ive searched and searched...and ive had a stumbling problem on accel for a while.

heres whats happening:

i had the fuel pressure guage hooked up and while runing its showing ~32 psi. when i open the throttle the fuel pressure drops. i assume this is because the fuel pump cant supply the volume as the injector pulse width is increased on throttle opening and the pressure drops. ive already replaced the fuel filter and the injectors aren't clogged.

my questions: is it my fuel pump? and what is the stock pressure.

this is an 86 tpi 305

thanks,
bryan

 

Welcome Aboard!

That sounds like a typical low output volume problem. Since you have determined that pressure drops as a reult of increased flow, either the pump cannot maintain pressure at the increased volume, or there is a restriction in the line. Make sure the fuel filter is clean, and no lines are damaged.

 

lines are good, and filter. its had this problem for a long time...i think its the pump but i dread having to drop the rear end. which side is it that has to come down? or is it both sides?

 

The fuel fill neck is soldered to the tank (it actually reaches close the mid piont of the bottom of the tank.

It comes out from driver to passenger side at an angle, just don't put too much pressure on the fill neck while wiggling it around. Do yourself a favor, and inspect the fill neck neck solder joint for cracks.
I not sure if the 86 has the pump test terminal on the ALDL, if it does you can use that to empty the tank before attempting to drop it.
Terminal "G" to 12volts with the key off, will power the pump, can also be used to test pump pressure.

 

Is it better to just drop the whole rear end out, or do you think I can get by with dropping the driver's side then?

 

The rear end isn't in the way, don't pull it out just down on the ground without the tires, that would give the room to pull the tank out.
The tank would drop straight down if the exhaust, and panhard bar weren't in the way, assuming you have all obstructions out of the way, you would have to drop the passenger side of the tank first because the filler neck goes over the frame on the driver's side in either case.

This is what I did when I pulled my tank.

 

so, the exhaust and panhard bar? how about the shocks? it seems like you would have to undo them to have enough room.

 

Yes, in hind sight putting the jack stands under the body (with the body so high I was afraid the jacks would slip,) removing the shocks, and tires would have given me plenty of room.
The Exhaust, bar, and one other lateral piece that's attached to the frame is all I removed.

I cut the exhaust, as close as possible passed the last bend any longer and it won't drop down far enough (if you drop the rear end I don't think you'll need to cut it.) I think I could have take it out if I had cut it shorter than I did.

I first jacked the body up until the tires were off the ground, then I used 2x4s to block it in that position.
The I jack it up from the axle to the 2nd highest notch of my jack stands, but I could have used another 6 inches. Hind sight is 20-20.

I do not plan to do this again since I put in a new tank, filler neck, a Holly 255lph pump, and (much to the admonition of some 3rd geners) cut an access door above the tank.

 

did you check the fuel pressure regulator? the vacuum line could have been plugged up or maybe come off, sounds like it may just be the FPR went out, i would check that first before replacing the pump

 

i just put in a new fpr diaphram yesterday to see if it would solve my problem without having to drop the tank. the old one looked a little worn out, but not busted. it didnt change anything. the fuel pressure doesnt go over 36 psi whether the vacuum line is on or off, it does drop down some with it on though. shouldnt it be at 43-44 psi at 0 vacuum?

what the engine is doing:

when i floor it the fuel pressure actually drops, and the needle is flying around like crazy but it never gets over 36 psi. it hesitates like its bogging down, then it backfires out the intake loudly a few times if i dont let off of the throttle right away. it bleeds fuel pressure off after i shut the engine off i can watch the guage slowly go down. it takes ~5-10 seconds for it to go down to 20 psi.


it's had this problem since i got the car and ive replaced nearly everything else it could be.

 

are you getting enough vacuum? check that, when you diconnect the vacuum it should make a difference

 

it does make some difference, but the pressure never goes above 36 psi. it does drop when vacuum is applied to ~32 psi. when i open the throttle quickly the pressure actually drops. its not a vacuum problem i dont think, its getting good vacuum.

 

Correct at zero vacuum fuel pressure should be 40.5-to-47.0psi
A weak FPR spring can also cause problems.

 

I replaced the FPR diaphragm, I'm going to try a used fuel pump and see if it helps.

 

If you remove the vacuum, fuel pressure should increase as posted by rgarcia63 to the mid 40's range. The drop in pressure sounds like your pump is DONE!!

If you are going to go to all the trouble to drop the tank, put in a new pump. If you're going to mod the car at all, put in an aftermarket one, because it doesn't cost much more but will be able to support a lot more power than a stock replacement. Racetronix sells a nice kit with new wiring that will support more than you need. Don't go cheap on this one, do it once and do it right.

 

I'm doing this so I can sell the car. I'm building an LS1 Rx7 and need the funds.