2002 Buick rendezvous poor idle

[raenloe]

My 2002 Buick rendezvous runs as if it's going to run out of gas UNLESS the air conditioner is on. As soon as the compressor kicks on it runs fine. Otherwise it idles funny and sputters badly. Any ideas how I might fix this?

[carriedi]

you should have the engine scanned to see what your fuel trims and Idle Air Control solenoid are looking like.

have you ever had your fuel system, top end cleaned? Motorvac is a branded top end cleaner that runs a cleaning solution through your fuel injector system that cleans your fuel rail, injectors, intake, back of your intake valves and cylinder. Deposits build up over time from your fuel. those deposits will eventually start to block the normal operation of the fuel getting into the engine smoothly and atomized fully.

[wastan]

My 2002 Buick rendezvous runs as if it's going to run out of gas UNLESS the air conditioner is on. As soon as the compressor kicks on it runs fine. Otherwise it idles funny and sputters badly. Any ideas how I might fix this?


__________
Moon

[Digger]

1996 Le Sabre has intermittent stalling problems, usually when coming to a stop. It also seems that when stopped and put it into park, idle goes low, then up to about 1500 RPM, then to about 600, the back to 1000 RPM. Changed MAF, cleaned throttle body, changed plugs and fuel filter, but no significant change. Any ideas?

[carriedi]

Most of the OBD2 vehicles really need to be driven with a scan tool; that can view the data stream. Viewing the data stream lets you see what the computer is responding to. O2 sensor, fuel trims, coolant temperature, air flow or manifold vacuum, Idle air control, and if there are any codes stored in memory. and then there is mode 6. pending codes. many vehicles have misfire counters.

I would start looking for a vacuum leak, fuel pressure regulator leaking fuel, fuel pressure, egr...

[Digger]

So, had time to run vehicle with scan tool monitoring. Showed a pending code of P0172 system too rich on bank 1. Pre CAT O2 sensor volts were rapidly fluctuating between 0.? and .850 (hard to read as it changes to rapidly), POST cat O2 sensor value was pretty steady between .450 and .550 volts. MAP sensor showed 12" hg, short term fuel 1 changed between 5% to -20%, long term fuel was between 0% and 5%. All this was at idle.
Car seems to run fine when moving and there is not any loss of power, misfires, or poor economy but, sometimes at stop lights, it will stall. It also occurs occasionally when stopped and then placed into park. What happens then is that the RPM will surge to about 1200 - 1300 then drop to 400 - 500 then back to 1200 - 1300 then to about 1000, OR drop so low that it stalls. Hard to read any/all of these monitor values when this occurs (scanner is a single value at a time display).

Other thing is that I check the test reports I am assuming the readiness tests) and get the following (which I have no clue what they mean):
T=04 C=20 FAIL HIGH = 8002! <=7FFE
T=06 C=35 PASS HIGH = 0026 <=0035
T=06 C=41 PASS HIGH = 0042 <=013C
T=07 C=0C FAIL HIGH = FF97! <= 0000
T=07 C=0D FAIL HUGH = FF97 <=0000

Car has new throttle position sensor, mass air flow sensor, and crankshaft position sensor. So, what values on the monitor should I be looking for that might lead me to the cause of the stalling at idle (mostly when in gear)? Any thoughts or suggestions on a direction? I am about ready to put a for sale sign on it for $750.

[carriedi]

the idle is controlled by the computer. The computer looks at the map/maf, coolant, rpm and O2 to figure where it should be at. Usually isle hunting (surging) is because the computer can't find a steady idle that meets all it's criteria. Vacuum leak, fuel leak, O2 sensor reading... something is off so the computer sets the idle at something and then it sets it somewhere else because the information changed. Your short term fuel trim going to -20 is out of the norm. Computer like it to stay within 0 to +/-10% or less all the time. More than 10% means there could be a problem. It's taking away fuel to compensate for something.

A cylinder contribution test would be a good start. You may not be able to get it with your scanner or for that year obd2 vehicle. You may have to do it on a scope. But you should look to see if there is a particular cylinder that isn't acting like the other cylinders and concentrate on that cylinder for a problem.

[Digger]

Thank you for your reply. I will see about seeing how much to put it on a scope.

Last question though, the voltage value coming from the pre CAT O2 sensor was going from minimum to maximum voltage reading so fast that I could not see any stable reading. The post CAT O2 sensor was pretty stable. How does the PCM/ECM read and analyze a signal that is this unstable? I work with variable frequency drives and PID loops and these processors are pretty quick but, can't process a signal that fluctuates the dramatically and this fast. Is this normal for an O2 sensor?

[carriedi]

the pre-cat O2 sensor should go below .2 and then above .8 back and forth pretty quickly. You would need a graphing scanner to see how fast the O2 sensor changes. (should be fast) The computer uses this reading to see how the engine is running. The after-cat O2 sensor will be fast until the car warms up and start to burn off the extra hydrocarbons (HC). Once the cat is hot most of the HC is burned off the Rear O2 sensor will level out quite a bit (at idle) If the computer see the rear O2 sensor switching too fast after it's hot you will get a code for poor cat efficiency