After driving for an hour the car shuts down like it ran out of gas. It has a full tank. After waiting 15 minutes it will fire right back up and off I go. The next time it shuts off is sooner, a round 30 minutes or less. After waiting another 15 minutes it will fire back up. This continues to happen until I make it to my destination.
There is tremendous pressure on the gas cap. I realize some pressure is normal, but this will blow the cap off.
I have replaced the fuel pump relay and the ICM. I haven't checked the fuel pressure yet, only because it doesn't seem to be a loss of pressure. If it were a loss of pressure, it couldn't build pressure by sitting.
Someone mentioned a possible evap system failure.

I'm at a loss. Please help!

 

Do you have access to a good scanner? That would make solving this issue much easier. How about the Factory Service Manuals for your 1988? They are critical to have when you own a Corvette as this way you can solve many of your own problems, the manuals are full of troubleshooting charts.

Are you getting any indicators or codes? You need to double check them. Use the paperclip method if nothing else is available.

The gas tank pressurizing is new to me. How is the tank supposed to vent? I have never seen a car with the gas cap blow off. I have seen cars with vented gas caps get plugged up and make a vacuum inside the fuel tank. The pressure must be related to your emissions system malfunctioning somehow. I would say that you clearly have an issue with the "Evap System". It is likely that a relay is not functioning in the emissions system and that would be the cause of your problems. Take a real close look at your entire vacuum system under the hood. Be sure that your EGR is working and that it has vacuum at the EGR Vacuum solenoid. My EGR Vacuum Solenoid failed and caused the EGR to fail and get all carbon covered inside.

Try venting for your fuel tank and see what happens with the pressure issue. Just be sure to have a Fire Extinguisher handy whenever working on the fuel system!

Did you put the thermal paste under the new Ignition Control Module when you installed it? You should clean the pad under the ICM and then the heat sink on the bottom of the ICM with alcohol prep pads or something like that. Then you apply the thermal paste and attach the ICM to the distributor. Do you still have the old ICM from before? If you do try and swap it and see if the problem is still there. Was the problem there before you swapped out the ICM?

I see that you are new to the Corvette Forum. there is a Profile page that REALLY helps us if you put the basic information about your Corvette. 1988 is easy to remember for me as I have a 1988 C4 Coupe myself. The profile page also shows where you live so that another member with the right tools might be able to help you out. It is very useful for us here at the Forum but it will benefit you more in the long run.

Having a Fuel pressure gauge on your fuel rail would be very interesting. We need to know what the pressure is while idling, revving it a bit and how long does the system hold pressure after shutdown of the engine? Having pressure in the fuel tank has little to do with your engines operating fuel pressure so we need to know the numbers if you can get them. It sounds electrical and most indicators point to your ICM, but with a new ICM the problem should not be there.

Best regards,
Chris