Hey everyone. I have gone through this forum looking for suggestions to this common problem but I still have some questions.

Vehicle: 1989 IROCZ 5.7 TPI
Notes to keep in mind: Fuel tank, sending unit, and fuel pump were replaced 15 years ago. I say this because the tank is not the stock unit.

1) Four weeks ago I went to a gun show that was 45 minutes from home. T-tops off and enjoying the 80 degree weather. On the way back home, close to home, my wife and I heard screaming. Got home to investigate. Being hearing impaired affects my judgement because I thought it was coming from the charcoal canister. Key on, engine off and the normal fuel priming sound turned into the screaming sound. Started the car and the scream was constant. Took the gas cap off and WHOOOOSH! My immediate diagnosis was EVAP venting which brought me back to the charcoal canister area.

2) The charcoal canister had no blockage and the solenoid was working on the bench just fine. I replaced the canister purge valve AND the fuel tank breather valve from Hawks third gen.

3) Everything seemed fine, till yesterday. It was 78 degrees out but I didn't drive 45 minutes like I did 4 weeks ago. I was in traffic. Got home and the scream was back. Remembering the fuel cap "whoosh" from before, I kept the car running and took the cap off. Again, WHOOOSH! This time I am sure that the scream is the fuel pump. Not only did I hear it at the filler spout, I felt air blowing at me. Not a lot but definitely coming out. I checked the fuel pressure but got 43 psi with vacuum to regulator off. There was no fuel coming out of the regulator either.

4) I've scoured this forum and even Facebook groups for similar stories and came up with many theories. One person mentioned "roll over valve" and that when it gets stuck, the tank pressurizes and the fuel cavitates. This reminded me of the many times that I saw my fuel gage go from FULL to HALF randomly. Sometimes down to the 1/4 mark. I immediately thought, fuel floats cannot float with bubbles in fuel. So cavitation seems to be legit because fuel pumps don't like pushing air in fuel either. This explains the screaming noise. Also explains why people keep changing pumps since 1985 (I'm exaggerating this) and the problem persisted.

5) I was confused to how a roll over valve would cause pressure build up and cavitation when there are two methods of venting. The new Hawks breather vent and the EVAP system itself. So to rule it out, I drove to work with fuel cap completely off to negate any pressure build up at all. Before leaving for work, the fuel gage was FULL and just 22 minutes later, the fuel gauge was at HALF. I did not smell fuel and the car was running fine. This leads me to think the fuel is cavitating somehow.

6) Could the sending unit be leaking fuel back into the tank after the pump picks it up? If so, how is the engine still getting 43 psi?

7) I'm also thinking that cavitated fuel would boil in the fuel rail if it's hot enough. Causing the engine to stall (which hasn't happened to me yet), but also makes the pump scream. Only to come back a half hour later and everything seems fine.

8) I really don't think the problem is the fuel pump. There are tons of people here saying that they've gone through 2, 3, and even 5 fuel pumps but the problem persisted.

9) One person used something called a Tanks roll over valve from Summit and apparently it fixed the issue with 3 of his 3rd gens. My question is, if the fuel cap is completely off, shouldn't the problem be gone?

10) Where exactly is this roll over valve? If this is in the fuel neck and it completely blocks the neck, I can see how removing the cap wouldn't work. Feeling air blowing at me from the filler spout tells me that the roll over valve isn't blocking the fill neck.

11) Finally, is there something called a percolator on the sending unit? One person mentioned that it will leak. I cannot find information on this except that some fuel pump manufactures would provide a small hose to bypass it. What is this?

I hope a bunch of you can chime in. If I have to change the tank, I minds well change the sending unit and pump too since I'll be far into it. I just want to get this problem resolved. I am sure many of you do too.
Thanks for reading this far.

 

Rollover valve is on the sending unit in the tank. Right on the top. Check the nylon lines under the plenum for burn holes or breaks. Your solenoid will bench test fine and vacuum test fine but those black nylon 1/8 inch lines are what's putting the vacuum on that solenoid. Check them all the way back to the distributor area.

 

Edge, thank you for your reply.
I removed the EGR and EGR solenoid completely. The charcoal canister is getting vacuum source from the throttle body (ported). You brought up an interesting point. I disabled the EGR solenoid in TunerPro (IROC runs the EBL Flash). Does disabling the EGR solenoid also disable the Charcoal Canister solenoid?

 

It does not. Mine is disabled as well