1998 Cadillac STS check engine light comes on intermitantly

Hi pats37, Welcome to 2carpros and TY for the donation.

P0101 there's a problem with the Mass Air Flow (MAF) sensor or circuit. The PCM detects that the actual MAF sensor frequency signal is not within a predetermined range of the calculated MAF value for more than 4.0 seconds

Inspect for the following conditions:
An incorrectly routed harness--Inspect the harness of the MAF sensor in order to verify that it is not routed too close to the following components:
- The secondary ignition wires or coils
- Any solenoids
- Any relays
- Any motors
A low minimum air rate through the sensor bore may cause this DTC to set at idle or during deceleration. Inspect for any vacuum leaks downstream of the MAF sensor.
A wide open throttle (WOT) acceleration from a stop should cause the MAF sensor g/s display on the scan tool to increase rapidly. This increase should be from 6-12 g/s at idle to 230 g/s or more at the time of the 1-2 shift. If the increase is not observed, inspect for a restriction in the induction system or the exhaust system.
The barometric pressure (BARO) that is used in order to calculate the predicted MAF value is initially based on the MAP sensor at key ON.
When the engine is running the MAP sensor value is continually updated near WOT. A skewed MAP sensor will cause the calculated MAF value to be inaccurate. The value shown for the MAP sensor display varies with the altitude. With the ignition ON and the engine OFF, 103 kPa is the approximate value near sea level. This value will decrease by approximately 3 kPa for every 305 meters (1,000 feet) of altitude.
A high resistance on the ground circuit of the MAP sensor can cause this DTC to set.
Any loss of vacuum to the MAP sensor can cause this DTC to set.

https://www.2carpros.com/topics/electrical/map-sensor--2

https://www.2carpros.com/topics/electrical/mass-air-flow-sensor
P0171/P0174 Lean fuel condition

Lean fuel condition could be caused by one of the following below:

1.Vacuum leaks
2.EGR and PCV valve.
3.Low fuel pressure.
4.Dirty fuel injectors.
5.Mass Air Flow Sensor.
6.Oxygen Sensor.

Just to add to rasmataz's post, have them check for a split intake boot between the intake and the throttle housing These usually split along the lower portion, and will cause all three codes to set The 171/174 due to the vacuum leak causing both banks to be lean, and the 101 for unmetered air entering engine. You can sometimes experience higher than normal idle, as well.

Somehow you got into another post, instead of replying to this one until now, and that is how we lost communication. I assume the three codes set at the same time. P0171 and 174 set when the fuel trim values reach a maximum of 25%. The fact that both of the banks are showing lean, is more than likely due to a central induction type fault. Did the plenum boot get replaced, from what you say it appears as though it did. There is a flat plastic piece clamped(or supposed to be) to your air induction pipe. The reason I say supposed to be, is sometimes this clamp is removed for one reason or another, and the piece can be dislodged and causing a central unmetered air leak. Check that this piece is fully connected to your induction pipe. A simple backfire through the intake can unseat it if not clamped. You can use a cable tie to replace the clamp, it should function. There is also a pressure blowoff on the intake below that plenum, it would have been visable with the intake removed. Sometimes, a backfire can deposit a chunk of carbon deposit onto the seat of this valve, and it can cause a vacuum leak. As always, someones work quality can also introduce a concern, I am not doubting the credibility of your mechanic, it is just food for thought. The reason that it is intermittent, is that it could be present at one time, say at idle when manifold vacuum is high, and at cruise it(fuel trim) can be brought back to acceptable limits, or vice versa. Of course fuel pressure is a possibility, lo pressure may cause lean conditions, but I am not sure of the P0101.
As stated, a scan tool that can view data can be helpful. If the fuel trim can be observed going lean, spraying(nothing flammable if at all possible) around the induction area and monitoring for a change, can sometimes be more conclusive than say, a vacuum leak that can't be seen or heard. I am not sure, but there may be a programming calibration upgrade on your vehicle that will address P0101, if I remember it was for 101 only though. It is just something to consider, and maybe coincidence. If you post the full VIN, I can validate that. The maf sensors seldom fail, and it probably didn't need replaced. The quote given that it needs replaced when presented to an oiled air filter is false, it can be cleaned with contact cleaner. I don't suspect you have injectors clogging, that would be more cylinder or bank specific, as opposed to both banks. Have you possibly filled with a high ethanol fuel(higher than 10%), I have not had experience with that scenario, but have heard that it can cause problems. Unfortunately, there are not many vacuum sources out of that manifold. If you can view fuel tim, you can also disconnect and cap the sources, and see if any make a difference. You have a PCV source, possibly a line behind the steering pump, the fuel pressure regulator line, and the power brake booster. The booster hose can be pinched closed for test purposes, no need to disconnect. Hope this helps some, and keep replying to this post.

Pat, did you ever have the lower intake checked as Gstacey suggested? He is the Caddy guy, and pretty knowledgeable. Aslo fuel pressure been tested yet? Results? I will look for TSB's(technical service bullitens). No TSB's But this is what to check:
Circuit Description
This test determines if MAF sensor is providing proper readings to the PCM. MAF sensor is a high frequency device that measures the rate of airflow through the throttle body. MAF sensor outputs a series of pulses that varies in frequency with airflow. PCM uses this signal for fuel control.
MAF sensor readings during acceleration will be much higher than those during deceleration or idle. PCM calculates what MAF sensor reading should be from the sensor under certain conditions using engine speed (RPM), throttle position, and altitude parameters. When these test conditions are met, PCM will compare its calculated MAF value to the actual value received from the sensor. The calculated MAF reading is calculated by comparing MAP, RPM and IAT values. If difference between calculated MAF and actual MAF is too great, DTC will set.

Diagnostic Procedures
1. Perform POWERTRAIN ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK, then go to next step. 2. Using scan tool, check for stored DTC(s). If DTC P0106, P0108, P0112, P0113, P0300, P0401, P0441, P1108, P1404, P1406, P1441, P1508, P1509 or P1645 is also present, diagnose affected DTC(s) first. If DTCs are not present or after diagnosing affected DTC(s), go to next step. 3. Check vacuum leaks at MAF sensor seal, throttle body spacer, intake manifold, PCV, EGR valve, plate or tubes and brake booster and hoses. Repair as necessary and retest. If air leak is not found, go to next step. 4. Check for stuck or incorrect PCV valve. Check for air (unmetered) leaks at oil dipstick tube or oil filler cap. Repair as necessary and retest. If vacuum leak is not found, go to next step. 5. Using FREEZE FRAME and FAILURE RECORDS data function on scan tool, duplicate conditions under which DTC P0101 was set. If DTC resets, go to next step. If DTC does not reset, see DIAGNOSTIC AIDS. 6. Go to DTC P0106 and perform diagnostic test, then return to this step. If repairs were necessary, verify repairs and retest. If repairs were not necessary, go to next step. 7. Replace MAF sensor and retest.

Diagnostic Aids
For intermittent setting of DTC P0101, use FREEZE FRAME data under which the DTC set, looking for faulty data from other sensors which may set this DTC. An improperly installed air cleaner or a distorted snorkel may cause DTC to set. Check for improper air cleaner or snorkel installation.

With the check engine light coming on prior to the stall, Are there any new codes? I don't see in the thread we have created, that I(or anyone) has instructed you on how to get the codes from your vehicle. Do you know how to do that, using the info center? One other possibility that comes to mind that I have seen before, is a faulty ignition switch. It was a probability in the early(98-) Sevilles. It would cause stalling at any time, and I am not sure of any codes, or just a p1599 stall detected. Does this stall occur at any time(cruising, accelerating, coasting to stop)?