Fuel Pressure Drops At Wot, Pump?

[Chuckman]

so i just got this car together again, and am chasing down a miss/stumble in the upper rpm's at full (or even mostly full) throttle. ignition checks ok, but my fuel pressure is acting weird.

the gauge on the shcrader valve bounces 1-2 psi up and down constantly. has been doing this since i first fired it up this year with a stock-replacement regulator, but back then it was bouncing 5-6psi. swapped it to my old kirban (no egr now, so it actually fits) and it seemed to clear it for about a day before the 1-2psi bounce showed up.

i've set the kirban fpr to 40-42psi with vac off, with vac at idle it drops to 37-39 iirc.

with full throttle, the gauge will jump back up to 40-42 for a second, then drop down to 25-30.

i'm thinking the pump may be on the way out, it's a 190lph walbro that i put in back in 2008. are there any checks to verify this?

i also had a crushed section of supply line (car slipped off jack) that i fixed using a couple double-flare fittings and unions. is it possible for this (or some other restriction somewhere, clogged filter etc.) to cause this kind of fuel pressure drop?

 

[Chuckman]

ttt

 

[90sickfox]

Could be a bad injector... Or something as simple as that pesky rubber hose at the top of the pump in the tank.

 

[Noobz347]

This sounds to me like you have an in-tank pump that is internally bypassing.

My guesses so far:

Walbro 190L pump
Anything that says "Airtex" on it.

Your troubleshooting so far has pretty much ruled out regulator (for me at least).

190lph walbro

I just went back and saw this. So yeah, my guess is that it's your pump. These are pretty well known for this. I recommend you replace with a 255 High pressure particularly, if you ever plan to boost.

 

[Mustang5L5]

Ever solve this? Same symptoms here

 

[Chuckman]

Fuel pressure issue ended up being a faulty gauge. I had changed pumps to a 255 just before changing the gauge and had the same wacky. can't remember what solved the miss (probably a spark/timing problem) but it never fell on it's face like yours does.

 

[Mustang5L5]

Thanks. I still figure it's the pump but still just searching around to find more evidence to confirm it while I burn off some gas and wait for a few parts to arrive in mail.

 

[Blown88GT]

...

i've set the kirban fpr to 40-42psi with vac off, with vac at idle it drops to 37-39 iirc.

with full throttle, the gauge will jump back up to 40-42 for a second, then drop down to 25-30.

i'm thinking the pump may be on the way out, it's a 190lph walbro that i put in back in 2008. are there any checks to verify this?
...

Drop down to 25-30 at WOT, it's the pump,
Keep the Kirban, you'll need it with the replacement pump, since Walbro removed the anti drain back valve which will cause hot start problems. The Kirban has this valve,