Engine load.

My vote is still to get a new Delphi pump assembly and retune it for 43psi. Don't use anything from the old assembly(pump, tubes, etc, etc.).

 

The fuel level thing makes sense but this happens whether my tank is full, 1/4 or anywhere in between. The tank vent is working as far as I can tell. I tried putting vacuum on it and it didn't hold any vacuum so the line was clear and the diaphragm was working. Same with the evap lines. I put vacuum on the line from the canister to the tank and it didn't hold. Then I put pressure on the canister vent line and the purge line and they were totally clear.

That thought crossed my mind too but it happens at any speed. Doesn't matter if it's 80 mph or at a much lower load going 25 mph. The car does act like high load drains the lines faster and the pump can't refill them again because of how pressure drops and then doesn't recover very fast if at all. To be fair I never watched MAP while driving uphill during this trip so maybe the engine was working harder than I thought while doing 25. I agree that load makes it worse but the root cause is unrelated to load. It's definitely in the tank somewhere and I'm suspecting the flex tube.

As for going back to stock, I'm trying to avoid spending the money on a new stock assembly. Hawks has one for $240 and I still have 2 more things I want to rule out before throwing in the towel. The clue I can't stop thinking about is how steering the car side to side increases pressure to 50 psi and then it falls right back down after I stop. I already reinstalled the assembly using new crimp clips and a new flex tube. I think if this doesn't work, the only thing left to rule out is that black and green plastic clip connecting the flex tube to the hard line.

 

I don't think I've ran across any posts over the years where someone has had issues with liquid motion in the gas tank cause fuel pressure issues. I've experienced it and seen it on the engine oil side....but not the gas tank side. Just to recount the oil pressure issues I had, the stock oil pump pickup is positioned very close to the front bottom of the oil pan. On hard acceleration, oil was getting pushed to the back of the pan and that was causing oil pressure drops. Going to a different pickup that moved it to the center of the pan solved that problem. I changed the pan too, but I think the pickup position is what really solved it.

Is it possible that the fuel pump is too close to the gas tank wall/floor? is It mounted securely to where it can't move on the bracket? The Delphi unit on rockauto($143) does have a plastic spacer on the bottom of the spring bracket that help keep the fuel filter spaced off the tank wall. Does your assembly have that plastic spacer?

 

I had a problem with my jeep where the pump assembly would hit the bottom of the tank when the fuel level got under a half tank (Plastic tanks evidentially shrink when they are not designed for E10) , when it hit it would block the pickup and starve the motor,, And typically only happened going up-hill, I pulled the unit and shortened it by about 1/2 inch and the problem went away. Also the filter sock was scuffed up due to rubbing on the tank bottom so it was letting grunge through to the fuel rail. Drove me nuts for about 3 months.. The weight of a full tank would pull the bottom of the tank down about 3/8 of an inch.. (20 gal tank in a factory bucket style skid plate )

 

The strainer making contact with the tank and suctioning to it like a vacuum cleaner crossed my mind too. My assembly doesn't have a plastic spacer under the bottom of the strainer but it does have a rubber button thing on the back of the metal spine so metal isn't resting on metal. The mounting is all stock and as far as I can tell there's no play in the bucket.

To piggy back on my reply to ACE, I think the pump strainer hitting the wall is unlikely. Although the tank is metal, the little cubby space in the tank that the pump sits in looks plastic. I thought maybe that thing became unmounted from the tank somehow and was free floating and would get sucked up to the pump but I tried wiggling it and it wouldn't budge. Lastly when I installed the pump with no bucket, I mounted the pump a few inches higher on the sending unit so it was even farther away from the tank wall and the problem was still happening.

 

The fuel pressure will be fine for the next 7-8 months as the temps stay cool enough here in Oregon but I wanted to post my final idea. I still have the original flex tube to hard line fitting from the factory sending unit/ flex tube. I used a heat gun to get a new flex tube on it and if I'm still having problems next year, I'll throw this tube in.

My idea is that the current black and green fitting might not have any o-rings in it. I remember looking inside for one but don't remember seeing one. It does have an external o-ring on the barbed end but that's it *if I remember correctly*. This original fitting has 2 internal o-rings although they are a bit flattened. Worth a shot.

 

It honestly sounds like you are boiling your fuel. To specify, it sounds like your fuel is heating up in general, but boiling at the pump because its the hottest part of the system.

At least that is what I think I experienced with my Camaro. However when I pulled my gas cap off when I heard boiling, hot vapor came out of the filler neck. The symptoms I experienced are identical to yours.

Mine is slightly modified and I suspect the headers were heating the fuel because the lines ran pretty close to them. Re-routing my fuel lines to the back of the head from the fender well and adding insulation in all the areas it was remotely close to the exhaust fixed my issue in the short term. It broke for other reasons shortly after.

I would see if you can add insulation and/or shielding to the fuel lines and rails in the engine bay to see if that makes any difference.

The other thing reading your thread was the fact you stated you had fuel spilling out of the center of your vehicle when filling it up. From what I understand the fuel tank should be completely closed off except for the vent that goes to the charcoal canister, an over pressure vent, the fuel lines, and gas cap. Fuel should not be able to exit the tank any other way, and if its able to freely pour out there is an issue. There should be a pressure relief valve that will only bleed pressure off to the charcoal canister after a certain pressure is reached.

Inspect your fuel vent valve to make sure it is working as intended.

 

It's heating up where I live so I'm back at it. The vent valve works fine. There's never any pressure or vacuum built up in the tank and I smell vapor whenever it gets hot enough.

Went for an hour long drive yesterday while it was around 90 outside and the pressure was slowly dropping right on cue. I planned on driving more but once it hit 55 psi I had seen enough.

I swapped in the quick connector and flex tube that I posted above and went for another hour+ drive today. Although not quite as hot, my elevation was pretty high at 5-6000 feet and it's dropped in cooler temps before. It was anywhere from 75-90 degrees during the drive and pressure held steady except for the very end it had fallen to 56 psi. I'm gonna need to drive it more before declaring victory because even though the pressure held nicely, I want to see how it does on a truly piping hot day.

Here's what the old quick connector looked like. I guess it did have an o-ring internally. It didn't feel like it was loose at all...

 

Okay, now I feel way less crazy about my boiling fuel theory because ol' Cleetus himself is going through the same crap on Rocky Mountain Race Week! How about that!

Video starts where he's having fuel pressure issues while cruising down the highway. He even has some video footage of the boiling in his fuel cell.

EDIT: for some reason the video isn't starting at the right time. Should be 11:29.

 

Looking at an experiment on youtube, it appears around 165ish that bubbles will start forming in agitated fuel. This post here got me to thinking.....
https://www.camaroz28.com/forums/lt1...-lines-888925/

Is it possible your exhaust is too close to this above section of fuel lines? You might not be using the stock setup, but I thought I'd ask.

 

Cool to see CZ28 still has activity. I jumped ship a while ago, just wasn't getting any discussion.

I can check the fuel lines where they get close to the exhaust but I've inspected them the whole length and don't remember anything striking me as needing attention.

 

Well I took the car to the most abusive environment I could think of today. There's a mountain near me that's 8000ft with a road to the top. Conditions were perfect for my fuel pressure to bottom out. Ambient was anywhere from 95-100F, elevation averaged probably 5000ft, I only had half a tank of gas, coolant temps hit 208F driving up the mountain, total drive time was 2 hours and included highway, stop and go city, passing on 2 lane roads. Fuel pressure held! It never dropped below 55psi. It was lowest when driving up the mountain and after I got back to town where ambient temps hit 100. Summer is just starting but for now I'm pretty stoked. Here's a pic of what I changed. That's the old Racetronix nylon to metal adapter next to a 5/16" Dorman nylon to metal stock style fuel line adapter and a 300mm x 8mm x 9mm flex hose with crimp clamps. The 9mm end fit snugly over the pump outlet and the 8mm end was slightly too small to fit over the Dorman adapter... just like I wanted. I used my heat gun for a few minutes to soften the flex hose then popped the adapter in and crimped it down for the tightest seal possible.

 

I think they had trouble with those having pinholes several years ago. Hopefully this is a rock solid fix. I know this has been a pain in the back side for a long time.

 

Just put the car through some more punishment today. The airport in Sisters Oregon hosts drag racing every 4th of July and today it was near 100F. I had the car running for about 2 hours since I didn't turn it off in the pits and did 4 races. Pressure never fell below 55 psi except for WOT it fell to the low 50's and on the last pass it fell to high 40's but would immediately recover once the race was over. Then on the way home it was up near 60 psi again. I'm feeling really good about this.