pos86vett

I have a 1986 vett with cast heads and tuneport all stock, also auto the idle is set, the timing is set, the tps is set at .50 there are new plugs wires cap rotor and coil, new fuel filter. There is about 32 psi at idle of fuel pressure all the injectors measure out at 17.3 -17.6 on all of them. also taped on the maf with no change. The car ran great sat for about a week now when you give it gas it bogs out and pops, and the idel is a little rought as well but it is manly driving is carrys on to the point that it doesnt pull out. There are no codes it only flashes a 12 nothingelse i will try to get a sound clip
thanks for your help tim

corvette1989bham

Did it do that before the plugs wires, etc.? You probably have a spark plug wire reversed , its easy to do, even AFTER TRIPLE CHECKING for correct spark plug firing order. After you remove and re-install all the spark plug wires again - one at a time by the book, if you still have a problem - are you dis-connecting the computer timing advance when you set the timing, and also are you plugging it back up? Could be a ignition module, pick up, timing off, vacuum leak, iac problem, dirt throttle body, leaking intake, plugs gapped wrong, injector working but dumpingn fuel instead of spraying....

AGENT 86

Check your plugs, good place to start.
Welcome aboard

_twisted_

put the fuel pressure gauge on it and see if it is bouncing all over the place in rhythm with the problem - if so look at your fuel pump

John A. Marker

with above posts. Also TPS should be .54, .50 is real close but should not contribute to the problem.

Check fuel pressure with line to regulator disconnected and PLUGED.

black panther

I also have an 86 all stock same problem

JrRifleCoach

Did you miss this?
Lots of good ideas here.

Might also look (listen) for vacuum leaks. Since the bog appears when accelerating, I'd start with checking the vacuum line to the FPR. You should see a jump in FP when bumping the throttle.

coupeguy2001

my 86 fuel pressure is 39 psi, when I remove the vacuum line, it jumps up to 45 PSI.
1. The knock sensor may not be working, causing it to ping, and detonate, giving it a bog.
2. also, If you changed the oxygen sensor, put the old one back in temporarily and see if it runs any different. I bought a new one that apparently might have fallen out of the box onto the floor, and somebody returned it to the store.
3. also, change the pulsator in the tank, the pulsator is basically two rubber bushings in a metal housing with a tiny bladder in the housing.
That could account for the low fuel pressure.
4. The fuel pressure regulator out of mine was beginning to crack, and pass gasoline through the woven part of the fpr diaphragm, and bleed off some of the fuel pressure when I shut it off.

rons85

It actually sounds like a small vacuum leak - one that's not destroying the idle, as it's small enough to be compensated for, but which is making the thing lean because all of the intake air isn't coming through the MAF. One of the small hoses or hard plastic lines perhaps?

AGENT 86

After reading your post again, I see that you missed fuel pressure numbers with vac line disconnected and fuel pressure under load (driving).
Do you have those numbers ??

pos86vett

i will get those numbers i didnt dfo anything to the car it ran great than started running bad thats when i changed everything (tune up)

CSS996

First thing is to change your screen name.

I used to bug guys on a Ford truck forum, because I had a '73 F250 that hated me. My screen name was UglyF250. I think she knew....

I bought the truck for $800, put about $2,500 into it, and sold it for $500.

Good luck sorting her out. Maybe someday pos will stand for Perfect Old School.

pos86vett

i checked the fuel pressure its about 32-34 psi if u take the vac line off there is no change but if you WOT it will go down to 25-27 and stay there untill you leave off. This is at idle and at WOT, while running

-js-

Thats reverse of the way it should work.
No vacuum = max psi
idle vacuum = lowest psi

pos86vett

yes i know its like its stuck in one position if you take the vac line off no change and if your WOT it goes down not up and there is gas in the tank i added 5 gal to be sure

AGENT 86

Not good
Your diaphragm or pump is going bye bye. You may find some insight from my manual.
http://members.shaw.ca/corvette86/Fu...mDiagnosis.pdf

coupeguy2001

I think you either have a bad pulsator leaking in the tank, or a bad fuel pump.
1.A fuel pump knows no pressure, but it will react to a restriction.
The fuel pump pumps the fuel up to the manifold through the filter, and to the fuel rail, and if the fuel pressure regulator is doing it's job, and restricting fuel, (assuming it isn't running for a sec.) the fuel pressure builds up and the regulator relieves at 40 psi. It relieves the extra fuel back to the tank.
Pull the vacuum line, and the spring in the regulator limits the fuel to approx. 45 psi before it relieves the extra pressure back to the tank.
2. now start the car, the fuel pump pumps the fuel into the rail, and the regulator relieves the excess pressure, and the injectors are opening and closing at idle. The pulsator has some big fluctuations to dampen, and it absorbs the fuel line shock. Some of the fuel is going to the engine to keep it running, and some of the excess fuel is going back to the tank.
3. now accelerate the engine. The vacuum falls off in the manifold, and it also leaves the regulator. The regulator closes, and the pressure jumps up to 45, and the engine continues to accelerate, and the spring in the regulator begins to close the orifice as the accelerating engine requires more fuel.
4. Now there is only a very little amount of fuel flowing back to the tank. Engine is at 5,000 RPM, and almost as much fuel as the pump can pump is being consumed, and the injectors are opening and closing rapidly. The pulsator isn't required to absorb quite as much fluctuation as the screaming engine is using most of what the pump has to offer.
5. The restrictions in the regulator, the fuel rail, and the line losses all add up to a minimum fuel pressure loss that equals enough resistance to flow, but the fuel injectors are letting out the pressure, and the fuel is dribbling back to the tank.
Recap:
low speed, high restriction, high speed, low restriction.
at higher speed, the pump should have a substantial effort to maintain fuel pressure at 45 psi.
Now your car.
1.The pump has a hard time maintaining fuel pressure at idle.
2. as you accelerate, the pressure goes down
3. your idle is lousy
4. your acceleration is lousy
A. fuel pump is going bad
B. pulsator leaks
C. regulator diaphragm is cracked
E. dirt or debris not allowing regulator to close
F. The filter sock on the end of the fuel pump is clogged
G. there is a bad ground at the tank for the fuel pump negative wire
and the pump isn't running full speed
there's all it could be unless your wiring harness is pinched and the pump only gets about 8 volts.

pos86vett

where would i crimp the return line going back to the tank i checked but all i saw was a metal line, also i noticed that the are 2 fuse for fuel pumps Are there 2 fuel pumps. sorry its been awhile i have been busy

_twisted_

take the fuel filler door and boot off/out and the rubber hoses are there

AGENT 86

The are 2 injector fuses, but only one fuel pump fuse. On my 86, the fuel pump fuse is marked "FR".