My sister gave me her 98 Buick LeSabre with the upper plenum leaking coolant. I ordered a Dorman kit and pulled the upper and lower intakes and cleaned out all the coolant/oil gunk. I followed the instructions to the letter replacing the lower intake gaskets and putting in a new metal coolant elbow and replacing the EGR stovepipe. I also cleaned the gunk out of the throttle body from the intake side where it bolts to the plenum (I did not do a through throttle body cleaning, just what I could see looking through the butterfly. I follow torque procedure to the letter. The only issue I had after getting the car back on the road was the small evap leak which I fixed by replacing the vacuum lines. When I did the intake gaskets, I also replaced the o-rings on the fuel injectors and put in a new pcv valve that came with the Dorman kit.

I drove the car for about six months after fixing the coolant leak and the car ran just fine without any issues. Then last month I had a code to appear for the bank 1 lean. Then the car would "jerk" randomly while driving and the Mass Air Flow code would set. I replaced the Mass Air Flow sensor and cleared the codes and the jerking went away and no code for the MAF reappeared.. However, the idle would not be constant, it would go up and down and I got code 507, idle speed higher than expected (or something close to that). When starting the car, the idle would go up to around 1500 but then drop back down and fluctuate but not go up that high. I checked for vacuum leaks but could not find any. My friend'* buddy is a mechanic with access to the online database and he stated that most often it was a bad ICV. I ordered a new ICV and put it in today. When I started the car the idle speed went up and back down (the pintle on the ICV extracted out as it should have) but the the idle climbed to around 1800 rpm. I thought maybe I had gotten a bad ICV so I put the old ICV back in, and it did the same as the new, idle speed climbs up to around 1800 rpm. I decided the PCM might have needed to recalibrate or something so I put the new ICV back in and drove the car letting the temp come up to normal operating temp. When I got back home and put the car in park, the idle speed was back up at 1800 rpm. So I remove the new and put the old back in and same thing, 1800 rpm in park. Just a few days ago when I was looking for vacuum leaks the idle would just go from around 700 up to around 1200 and then back down.

Looking for some info on what could cause this and troubleshooting procedure.

I used a computer programme to read some data.

Fuel System 1 closed loop
Calculated Load Value 5%
Engine Coolant Temp 88 C
Short Term Fuel Trim - Bank 1 -0.8 %
Long Term Fuel Trim - Bank 1 16.4%
Intake Manifold Absolute Pressure 25kPa
Engine RPM 1736 rpm
Vehicle Speed 0 km/h
Ignition Timing Advance Cyl 1 25 degrees
Intake Air Temperature 30 C
Air Flow Rate 8.15 g/*
Absolute Throttle Position 0.4%
Bank 1 Sensor 1
Oxygen Sensor Output Voltage 0.465 v
Short term Fuel Trim -0.8%
Bank 1 Sensor 2
Oxygen Sensor Output Voltage 0.475 v
Short term Fuel Trim 99.2%

I hope this might help to indicate where to look.

And the trouble code PO507 has returned of course.

OK so I will tell you how to fix it.
I AM 99% sure it will I have had same problems 8 times on GM and other cars and this fixes it.
Replace the coolant temp sending unit.
IT works the gauge/light and also tells the ECM the engine is cold so idle the engine up
They can go out the gauge/light still works but send the ECM a reading the engine is cold.
So it will idle it up high.
This is from experienced in my own cars and in the shop
I try to give advice from experienced that has happen in real time
I hope it helps you.

Yes, just read on another forum that the Coolant Sensor could cause this so will replace and let you know. Thanks for the reply.

After looking at the coolant sensors offered by the local shops, I decided to get an ACDelco from Rock Auto; will take a few days for it to arrive. Will update when I get the part and get it installed.

Since you can read data, on a cold start, does the coolant sensor data match the IAT sensor data? And does it linearly increase with time?

I will check this tomorrow and reply back with the data; too late at the moment.

I did notice this, when you start the car the idle immediately goes up to about 1800 rpm and then drops down to around 1100 or so then over a period of time it creeps back up to around 1800 rpm.

I asked a tech about the car and he had this to say about the data readings but did not suggest what to do.

"What I see is the vehicle was warmed up, running on the high side at 1736 RPM in neutral. The problem I notice is the Oxygen Sensor 1 and 2 readings are nearly identical. That means the Catalytic Converter is not working at all. Your long term fuel trim is way high, indicating the computer is adding fuel to a very lean condition."