marc1130

Here are the facts:

Fuel pressure readings:

Car on engine off - goes to 41 then 39psi
car on engine on - idle 34 psi constant
removal of the vacuum line on the regulator at idle - pressure rises 7-10psi
car will hold 40psi of pressure after everything is off so no leaks

WOT 0 - 4500 rpm 41 psi
WOT 4500- 5750RPM - Fuel pressure drops off from 41 goes to 31 psi

I will see an occasional misfire at high rpm. I just changed the fuel filter to remedy the problem, no luck. Checked the vacuum line on the fuel pressure regulator for any leaks with a mityvac - holds vacuum at 20inHG no problem.
Car has 160k miles. Plugs and wires are good. Is this a fuel pump, fuel pressure regulator, injector problem or something else?

Kevin Blown 95 TA

Your injectors could be gunked up a little but that wouldn't cause low fuel pressure at high rpm. Regulator seems to be OK. Seems like the pump from everything you said.

mrmint69

It sounds like your pump might not be able to keep up with the volume of fuel. I have heard of tests where you run the pump and measure the amount of fuel. Otherwise I don't know how to test the fuel pump. If it were my car, I would go ahead and do the trap door method of replacing the fuel pump and see what that does. JMO

mrmint69

You could check the fuel pressure one way. You would need to tape a fuel pressure gauge to the windshield to be able to see if the pressure drops at higher rpm's.

marc1130

Thanks for the advice, the reason I knew it dropped and all the readings above is because i just tested it earlier today .

Its easier to tie the gage with a rope sideways to the plastic windshield wiper arm. Still thinking the fuel pump route. I separated all the spark plug wires with wire separators and that may have cured the high rpm misfire. I think I had a loose plug wire or they were arcing even though I have taylor 8mm plug wires.

Any suggestions on the fuel pump? The delphi one from advance for $99 looks like a winner. Any good writeups on the trap door method?

Injuneer

Go to the "stickies" on the "Fuel and Ignition" forum. There is a huge thread on the "trap door".

marc1130

Thanks, I found a bunch of writeups.

Before I go ahead and do this is there any other explaination for the pressure drop of 10psi at WOT at high rpm? Can it be an electrical connection that needs to be cleaned up or any other cause?

If I have to replace it what is the best stock for stock replacement, and do I need a wiring harness. I see these racetronix kits but my fuel system all stock so I think for me its overkill. I'd like to buy a Walbro I'm wondering if its a direct swap any part numbers would be helpful.

mrmint69

I am guessing its the pump not being able to keep up with the demand on the amount of fuel the car needs. Its using enough that the pressure is lowering.

Injuneer

Low voltage at the pump will reduce the pump's flow capacity, causing the pressure to drop during high demand. Check the voltage at the harness connector above the rear axle.

Guest47904

Why don't you try jumping 12 to the fuel pump prime connector before testing it one more time?

So we take it the plug wires weren't the problem?

marc1130

Two separate problems - plug wires were causing the misfire. But I do have a pressure drop so I'm leaning out a bit at high rpm - not enough to cause a misfire, not sure but I think its the pump. What will jumping the test plug to the pump do for me? I know it will prime fine and operate to a degree as I already tested it, i wanted to clean up the electrical connections at the relay and the ground.

Guest47904

Jumping the fuel pump prime connector may help answer the issue of dropping voltage since it bypasses the relay.

marc1130

I will be checking the voltage to verify if indeed it is an electrical issue or a pump issue.

Injuneer

When the fuel pressure drops to 31psi, you lose 15% of the fuel flow. That would lean a well-tuned engine out to 15+:1 at WOT (PE mode). Could cause detonation, could cause misfires. Definitely will not make good power.

marc1130

Not the most reassuring thought when your going WOT. This guy had a similar problem:

"I went from dropping to 20psi at WOT to staying at 42psi (even looked like it was going UP in fuel pressure at WOT). "


http://www.mifbody.com/vbulletin/sho...res-and-How-To

I checked the pump voltage, steady 12V so without a doubt this is the fuel pump. I ordered a Walbro F20000169 with the harness adapter / sock and clamp kit. I also went and purchased the plastic flex hose and some foam to secure the pump in the bucket from APE. I plan on using a dremel/heavy duty cutoff wheel to create a trap door. I'll use an aluminum plate and RTV to seal it when I'm done. I'll post my results after performing the installation and retesting with a fuel pressure gage. Thanks everyone for your help.


I'm glad we have such an awesome online community. At first I was going to do an optispark replacement or coil/ icm replacement. After checking the fuel pressure I was able to isolate the problem instead of blindly throwing parts until the problem goes away - wasting time and money. I'll end up spending less than $130 total can't argue with that.