Fuel pressure dropping during acceleration


Jack's Smirking Revenge
May 24, 2001
Intermittent issue here. Back ground….new tank, non venting so canister was deleted, line is capped and zip tied at the TB. Casper’s Hotwire and 340 pump, volts read fine on scan master G during acceleration. Took it to the track and made three 8th mile runs with no issues. Driving 30 miles home, decided to to make a WOT blast near home to confirm boost setting. Nope car almost dies. Try again slower and as boost begins to come in FP drops rapidly. Found my make shift canister capped line was not capped and thought ok that’s the problem…..nope it’s now capped as I mentioned and this is still occurring. I’m not saying I may not have new vacuum line issues after the track, I’ve done a once over and don’t see any blaring hose issues, so where should I start looking? And is FP drop with boost going above 1 psi a clear vacuum line leak issue? Or should I be looking elsewhere? I’m not opposed to replacing the 30+ year old billet fp regulator if needed. Thanks to any replies.
New fuel filter installed after this issue first started, I thought it fixed it….like I said drove to a track, made three passes with no issues but on the way home it reared its ugly head again….baffled
When you deleted your charcoal canister, did you plug the line to the gas tank? I did that once by accident and the tank had no way to take in air to displace the gas it was using. Ended up with a very low pressure gas tank and a stalling car.

When I opened the gas cap a bunch of air WOOSHED into the fill neck. Quick check.
Exactly what Chuck said. I did the same with a small in-line filter.
Thanks for the replies, tank is vented via the cap. The tank itself is a aeromotive stealth. It does not have a third line coming out the top.
You can , #1 check the billet regulator by removing it and disassembling it and check for a torn / leaking diaphragm . depending on who makes it , parts are usually available . #2 drop the tank and check ALL the electrical connections from front to back . The connection at the top of the tank ( intermediate harness to the bulkhead connector ) are known to melt . #3 pump going bad and can't keep up with demand .
tank is vented via the cap. The tank itself is a aeromotive stealth. It does not have a third line coming out the top.
AIRC, the cap on the tank is not vented. if it was, fuel would leak out on acceleration, and the smog system would not work.
The vent is on the pump hanger.

Stealth hanger.jpg
You can check the regulator without having to tear it apart. Take the boost line off the turbo and run compressed air through it. Turn it down to 20 psi or so.

Put your gauge on the fuel line with ignition on, engine off. Check to see if your fuel pressure rises with boost correctly. You may find your vent problem in the process.
This is a pic of the top of the tank, there is no vent line. Venting occurs with a “venting” cap, non original type of a cap.


  • image.jpg
    2 MB · Views: 73
So that cap looks like it is closed all the time. How does pressure get released out of it?
Tested the regulator, adding air to the system caused pressure to rise accordingly. I’ve ordered a bunch of vacuum line and fittings, going to try to replace all of this 34 year old rubber in the next couple weeks.
Might want to double check that your gas cap is letting in air. I thought mine was vented but it wasn't - had to go with the inline filter to fix my situation.
What's the pt number on that tank assy?
Doesn't look to me that you have the correct assembly for a GN.
I don’t know the PN, and I would agree it’s not the correct version, it was recommended to me by a long time reputable Buick shop. I won’t name drop that just yet as I have to get them on the phone to discuss this issue. I am still working replacing all cracked vacuum lines, but I’m leaning toward thinking it’s a pickup issue, easy enough to test by adding another 5 gallons and testing acceleration. Doing that tomorrow and will post results.
Does the pressure drop look like this? My FP was dropping. Pump was bad
Lol, mine is way worse, by the time I’m doing 45 mph at 5 psi boost, fp is dropping thru 10psi. Keep in mind, this is a new tank, and new 340, and I had run three passes at the track with no issues, it was on the ride home this pressure drop started happening again.