I'd read the link below to an old thread and remove the IAC valve ( also called the Idle Speed Control Bypass Air Solenoid ) and see if it idles with it out.

Don't just unplug it, remove it. The solenoid may be dead and not opening

It provides a path for air flow around the throttle butterfly valve when the engine is idling and if it doesn't open when it should, the engine won't idle.

It may not be the problem, but it's free and easy troubleshooting without throwing parts and money at it just yet. .

If you haven't already, it's also worth backprobing the TPS and checking voltage to see if it is a nice smooth voltage curve from about .9V to 4.5V as you increase throttle. Quick and easy way to confirm your ECU isn't getting bad data on throttle position.

The vent hole on the cap needs a glob of silicone so that it's not a vent hole anymore. 

Or if your recent new cap has the glob already, use that one and you'll have a spare

Another choice would be Spectra FD-14

You'll find a wide range of prices.

Here is a link to the injectors I installed. Anyone see a fault with those?

I can't help but suspect them since I didn't have an idle problem before...

I may put the old ones back on to rule out the new ones.

Somebody might suspect that there was human error involved and maybe you messed up an O ring or 2 when installing the new injectors. Somebody has to stick up for the injectors 

What gasket(s) did you change?

Back in post #1 you mention having a constant 40 psi, while the engine is running presumably.

It should be down around 31psi plus or minus 3 when it's idling with normal vacuum and 39 psi plus or minus 3  at full throttle or with the Key on Engine off. and somewhere in between the 39 and 31 numbers at part throttle depending on engine load which directly affects vacuum The vacuum is sensed by the regulator and it controls fuel pressure based on that.

Have you checked the vacuum while it's idling ? It should be around 15 in Hg. and pretty steady or if it won't stay running unless you give it some throttle what's the vacuum at that time?

I changed the intake gasket. I took the intake back off to check and the gasket looks good. 

When I hook the pressure gauge to the fuel rail, it takes about half a dozen key turns to get the pressure reading to 40psi. Then it does not fluctuate afterwards. This had me curious as well. I thought it might should drop as it's trying to idle but it does not.

I have not checked vacuum when it's trying to idle because I didn't know what is normal. Now that I know 15 is what I need, I can T into one of the tubes coming off the underside of the manifold and see what I have. 

If the vacuum is low, that would explain the higher fuel pressure and no idle. I'm guessing there aren't many places for a vacuum leak if I've ruled out the intake gasket. On a car, I would spray carb cleaner around the manifold to find a vacuum leak but not sure about this particular engine. 

Look for Ford 24# injectors like these-  https://www.cjponyparts.com/ford-racing-fuel-injector-set-24lb-ev6-style-uscar-connector-2005-2013/p/M9593LU24A/?gclid=Cj0KCQjwm9D0BRCMARIsAIfvfIYp3eLxawAtSheYi7ohpEqElfJYsr6D378cEuXpDlYeTFg4AxND_2waAo5eEALw_wcB" rel="nofollow - link .  Additionally, you will need adapters:  https://lmr.com/item/LRS-9593-EV1K/fuel-injector-adapter-kit-uscar-to-jetronic?gclid=Cj0KCQjwm9D0BRCMARIsAIfvfIaVqVgAzl5YlEMIfZabBeY9t1bIA4IDxzeg7_oMMzLV-ly8lr0JuyAaAtW0EALw_wcB" rel="nofollow - Here  are EV1 to EV6 adapters -- just what you need. You can price shop on Google as the the injectors and adapters seem to have prices all over the place.

If you were to decide to get one as a spare, try the Spectra FD-14 from Rock Auto, you'll probably like the price, even after shipping charges.

How's the fuel pressure when it idling now?

And.............good job on the diagnosis

Yes, the new injectors were purchased to address a hot start issue. 

Those first versions a year or so before PCM started using the engine were the fuel injected OMC/Volvo King Cobra engines in 5.0 and 5.8 varieties used in stern drives.

There are numerous places that the manual gives info that's dead nuts accurate for the OMC/Volvo engines but not too good at all on the PCM gt40

I wouldn't claim to  be able to list them all, but some of the equipment locations are different, some wiring colors and diagrams are different  and the fuel system description and diagrams are different like where they talk about the vapor separator reservoir.

It's enough to drive a poor guy crazy sometimes 

I haven't had a chance to get the boat in the course and see how it does this year with repeated hot starts. 

I never put a fuel pressure gauge on it with the old injectors but now that I own one, I can. This should help me identify if I have a leaking injector. 

Hey........it's a gt40 (but you can pick any fuel injected engine these days) and if you don't have a whole arsenal of diagnostic equipment you're gonna throw some parts and money at it, unless you get just plain ole' lucky. 

That's been shown with numerous gt40 issues over the years, here on CCF.

I think the distributor qualifies as parts and money.

Still wondering what your fuel pressure is at idle after the old injectors went back in  Skeet 

Is pressure still 39 or does it go up by 7 or 8 psi or so?