Just replaced the dying stock fuel pump with a Walbro 255. I had to cut my exhaust to drop the pipes so its running half the the system AFTER the cat converter. (Cant get the car running good enough to get it to a shop to be welded back together)

The car runs great at idle or when at 1/2 throttle plus. But if its opened up to 1200 rpms or so the engine will cough and sputter and try to die.

Things I have done trying to correct the problem:

New diaphragm on fuel regulator
Checked and cleaned IAC valve
Replaced spark plugs and wires
Ensured EGR system was working properly
Checked and adjusted timing at 4*
Checked for vacuum leaks

Things that were replaced recently (3-6 months ago):

O2 sensor (AC delco)
CTS
Cap and rotor

Things I suspect:

TPS giving bad signals
Faulty ignition coil
Faulty wiring in the harness that's giving weird signals

I can make a video of the problem on request. Unfortunatly I dont have the equipment
to data log.

 

All you need to datalog is a computer. The ALDL software is free and the cable can be easily made.

As far as the problem, its unlikely that it would be the tps. The engine would probably still run ok for the most part. The ignition coil is probably ok as well since a weak spark will be most noticable under high throttle.

Have you checked the fuel pressure with the walbro in there now? The walbro may be more then the stock regulator can handle. It could also be a problem with false knock counts if you have a buch of exhaust piping dangling down.

 

You know I never thought about the exhaust doing that. That may well be a possiblility since it is dangling by about 4 feet of piping with alot of bounce. I will try securing the pipes and see if it lessens the problem.

 

As Dim274 mentions- the Walbro is causing your fuel pressure to go through the roof and flooding the car.

Check your Fuel Pressure- it will likely be much higher than the required 12psi. Then you will need an Adjustable Fuel Pressure Regulator.

 

What regulators would you all reccomend?

I did a quick look at summit and all I could find was one made by JET

Part # JET-61500

10-20 psi

66 bucks

 

The stock one could work if you ported out the return side. What kinds of pressure do you have now?

 

Just went out and bought a kit that goes right in front of the fuel filter. It's reading 15 psi at idle.

 

What does it read if you open the throttle up?

 

It will increase slightly to 15.5 then drop back down as throttle is closed.

BTW just to rule out the exhaust theory I shook the pipe and banged it against the floor panels but it had no effect on anything.

 

If all you see is a +/- .5 psi increase with rising speed the pump may not be the issue. Although, that can be a 3-5 psi increase over stock, depending on how the regulator was set from the factory. This will be around 10-20% more fuel for a given PW. While its enough to make it run rich, I dont feel its quite enough to cause it to totally bog down and want to die, assuming the above info accuratly represents what the actual pressures are.

 

Assuming the stock setting would be 12 psi would it be possible for me to set the stock regulator by bending the lil tab and rotating it up the screw 1-2 turns?

I really dont want to go about this is a "cheap" way but my knowledge of the matter limits my options. I really do appriciate all the input I have recieved so far.

Any other ideas that we may have over looked?

 

Theres a plug in the bottom of the regulator housing. If you drill this out and slot the screw there, your regulator becomes adjustable.

If you notice that no matter how far you back the spring off, the fuel pressure remains at 15 psi, then your regulator is maxed out flow wise. This can cause issues in of itself because itll cause the pressure to fluctuate as the flow demand from the injectors varies.

 

So I would have to decrease tension on the spring to decrease pressure? Or is that just to test if its maxed? I'm guessing you would rotate the screw counter-clockwise to put more tension on the spring.

 

15 psi is much lower than I thought it would be especially with a 255 pump. Mine jumped up to 19 and that was with the AFPR at the lowest pressure setting.

15 doesn't sound like enough to cause the stumble you describe, but if the car wasn't doing it before and replacing the pump was the only thing done to the car prior to the problem surfacing, it would be my top suspect.

When I get home, I can check which way the AFPR needs to be turned in order to lower or increase fuel pressure- unless someone knows off hand-
S-D

 

Little update:

During my lunch break i drilled into the plug on the bottom of the regulator. I rotated it counter-clockwise until the tab was about 3mm from the top of the groove. Pressure still read at 15.5 psi to 16 psi BUT when i took it for a test drive the bogging was greatly reduced.

When I come home I will try some more adjustments as I still have some room to play with... well at least I'm keeping my fingers crossed.

 

Problem Solved! Putting more tension on the spring did the trick. Right now I have it about 1mm from the top and I now have no stalling or bogging. Car is very responsive. The exhaust does not smell rich at all.

Thank you all again for your suggestions that led me to the solution!

Now to get some datalogging equipment so I can start tuning it just right....