[Sirrotica]

A thermostat serves another purpose beside controlling flow through the system to stabilize temperature. It provides a restriction to raise the system pressure in the block and heads.

There are areas in the block and heads that are somewhat stagnant like dead end hallways that don't get as much flow due to the design of water jackets, most times towards the upper parts of the block and heads.

Steam can form in these dead end portions, if water pressure isn't sufficient the area that is hot has a steam pocket in it that will grow without enough pressure to keep the coolant in touch with the metal. Pontiac engineers addressed this in certain 421 blocks by adding an extra hole in the block deck to alleviate steam in those pockets. Most Pontiac engines do not have this hole drilled. It's cut out in the head gaskets, but in 99% of the blocks was never machined. Proof though that the engineers knew that there were areas that could be helped by more coolant circulation.

Thinking about your situation I would at the very least try a gutted thermostat, or an aftermarket restrictor washer. I would think a thermostat would also make your aftermarket FI system work more efficiently. I know from working on the later model GM stuff how a malfunctioning thermostat will mess the system up due to the water temp driving the system richer. Plus if your system adjusts timing by the ECM it will also have an effect on that.

I've actually never worked with an aftermarket system, but I would guess that it monitors coolant temp as part of the parameters to decide fuel air mixture as well as timing if that's part of your system.

At idle your water pump is by passing a portion of the flow through the intake crossover, but due to the lowered pressure at reduced RPM I would guess the block and head coolant pressures have also dropped due to no thermostat. This exposes those steam pockets to grow, and steam does nothing to cool an engine, as they grow efficiency nosedives. This could explain temperature creep at idle not just air speed through the radiator, but coolant pressure in the block and heads.

Anyway it's not a huge job to install a thermostat, or a restrictor to see if raising the pressure helps your condition.

With the nylon RV fan you can't get much more air speed at idle without going electric. My own opinion is I would never use an electric fan on an older Pontiac muscle car, most won't come close to an engine driven fan, especially the aggressive pitch one you currently have.

The other possibility is the Champion radiator isn't efficient enough, or it's scaled internally insulating it's ability to reject heat quickly. I used one of the nylon fans on my 73 T/A with a 455 and an A/C car on the street without any O/H problems. Granted it wasn't in AZ but it never even hinted at getting hot. Of course apples to oranges when you're comparing radiator surface area from a first gen F body to a second gen F body. I've owned both so I'm aware of the differences.

You might also try a heat sensing gun to check heat of different components in the system to see where you're losing efficiency. Likely no silver bullet and possibly a combination of small things instead of one single change will fix everything.

I can tell you that my parents lived in Phoenix since 1980, and their 75 Catalina with a 400 2bbl and air, with all stock components was fine for many years, until they traded off for a 89 GP.

Anyway, my next move if I was diagnosing the problem would be a thermostat, or restrictor to raise coolant pressure in the block and heads. You're not helping your problem with no thermostat. Most people will tell you it can help a cooling problem by slowing down how long the water is in the radiator, when in fact pressurizing the block is what actually brought down their coolant temperatures. Electrons move heat, so slowing the water down so it can reject more heat is a wives tale, pressure change though is a real change that can have an effect. Covering more surface in the water jackets with coolant will have a real effect.

Keep plugging away, you'll get it figured out.