Hi,,

I guess I’m still stomping around in the same tub of grapes, like in an old rerun of I Love Lucy.
On theses cars I can make all kinds of funny faces.
I still don’t know where or what you are calling an accumulator.
I thought on these cars LH cars the oversized fuel filters acted like an accumulator?

You said, “ Not having the required fuel pressure at the rail resulted in backfiring and poor accelerator response” is absolutely correct as the engine will suffer various ailments.

A faster than normal idle is unusual since it computer controlled through the IAC.
In order to get faster excessive air has to get around the throttle plate to where the IAC vane cannot close down far enough.

On newer than 1988 cars the IAC is spring loaded to shut down if not energized.
It has to get a signal to open more or it can stick but the spring is supposed to always close it.
The throttle plate can get adjusted wrong and can cause a high idle.
It basically has to be fully closed but not be allowed to get jammed in the bore too tightly.

Make sure you have no other vacuum leaks and check the throttle plate for proper closure.

The coolant temperature sensor affects mixtures but the computer doesn’t add RPM for a different idle speed on purpose.
Unless, Under startup conditions the instant racing of the engine upwards is normal but will immediately drop down to 750 RPM. It seeks loads only right after the startup command.
The mixture is preprogrammed to the engines temperature, yes!
It stays from in the “cold”start point to “warmup” period area very temporarily.
The transition is preset to the coolant temperature and the O2 sensor in the exhaust coming on line.
The RPM is constantly monitored or built to stay on that 750 RPM.
A throttle switch is a snitcher to make idling transition happen.

IF the engine stumbles the ECU plays catch-up or shut down maneuvers. In abrupt cases, the timing gets adjusted and so you might hear a clack and the knock sensor is there for that.

With all this explanation you are looking for too much AIR or the control of it.
A smooth but too fast of an idle is only one perimeter off its baseline.

The ECU is very fast as it operates in steps but maybe even two or more at the same time.
Some “Adaptive” technology was sneaking into scene in the nineties.
IMO, it seems to never stay dialed in, to where when cold, the idle will be lumpy but RPM stays put @750.
Excessive mixture under certain circumstances is a wider chase but a coolant sensor circuit problems are alway possible. It’s important, yes. Otherwise, the AMM and that fuel Rail is in there and causes a wider Trail of thought for troubleshooting.

What’s been nice with the red block’s drivetrain is that it has all been repairable.
IMO reliable in a rather inexpensively way for over the many years of owning my 240s anyways.

Hope this helps check for the air input while investigating or changing the coolant sensor under that manifold.
Hey you got a whole extra year to do the same thing that you do on New Year’s Day.
Be careful of traditions. (:-)

Oh! the BRICKBOARD might have us for another one too. (:)

Phil