1989 740 non-Turbo Auto Trans, 110k miles

My '89 740 was running perfectly until it started getting hard to start when hot (had to use 1/2 throttle while cranking), then it began to stumble when shifted into or out of gear, or the A/C was turned on. It still ran fine on the highway, but would show low RPM at a traffic light. While the idle and hot start were affected, the engine never actually stalled.

Turns out that the Fuel Pressure Regulator had failed, with the regulator's diaphragm passing fuel into the intake manifold (fuel poured out of the regulator when the regulator's vacuum hose was removed), obviously causing VERY rich running. I have no idea what the actual pressure was at the fuel rail, however, but suspect it was high?

Total mileage driven w/ the failing/failed regulator was probably 50mi max.

My wife's son in AZ changed the regulator, and she says it's LOTS better, but it STILL idles roughly, and the idle RPM is low and not super smooth as it was before the failure (when you could honestly balance a nickel on the valve cover). It now shakes a bit. They report good highway performance (even w/ the BAD regulator it ran fine on the highway, however).

I am not where the car is (wife is in AZ with the car; I am in TX), but looking for ideas to pass along.

I suspect that the ONLY real "failure" was the regulator, so I am not thinking something else (like the ECU temp sensor, idle control valve, etc.) concurrently failed.

I am thinking the rich mixture may have fouled one or more of the plugs, and if the pressure went to 100psi (max FP pressure?), the injectors could have been affected. Not sure what else to look for?

Any other suggestions?

Thanks,
Bob