I have a 95 with 60,000 miles it runs great except at idle either in gear or park. After acceleration light or heavy it smoothes out and runs perfect. Iwould appreciate any help on this before I head to the shop.
Try replacing your O2 sensors. If they are the originals, the readings will be off at start up, causing your engine to idle roughly. Once the computer recalulates your A/F ratio the engine will idle fine, hence only a problem in the begining. You shoud also see an improvement in your MPG, 2-4 increase. You might want to also replace the fuel filter if it has not been replaced yet.
Thanks for the input. Can you clear this up? It is my impression that at start-up the system is in open-loop, that is the 02 sensors are inputting (they do constantly) but the computer system does not use the signals. GM used to call that the "limp home" mode, as I recall. No efficiency-no nothin'. Just enough fuel burn and spark to either 1. get you home, or 2. to get the motor running and nice and warm, at which predetermined time the computer kicked-in to "closed- loop" mode.
Does the idle surge up and down and sometimes it tries to stall??
If the answer is yes, I would check for an intake/vacuum hose leak OR you may need to do a throttle body cleaning. Vacuum leake are the most common and affect the idle the most.
Remove the hose between the MAS and throttle body. Have someone open the throttle blades up all of the way (engine off PLEASE) and take a light and examine the backs of the throttle blades and the throttle bores.
They will be very dirty. You may be able to clean them up with carb cleaner and a rag but sometimes they are so dirty that you have to take the throttle body off and do a more complete cleaning. Give that a shot. If it doesnt help,, check for vaccum/air leaks.
Your idle is controlled by the IAC valve on the throttle body. That could be dirty too.
O2 sensors do not work properly until they are at the proper temp. (HOT HOT!!!) (700-800 deg) The engine operates in OPEN LOOP until it meets a bunch of operating prameters. Coolant temp is one of them.
The shop manual explains it very well.
When it goes into CLOSED LOOP it uses O2 sensor voltages to fine tune the air/fuel ratio.
You can check to see if your O2 sensor is working. Use a digital volt meter. I stick a fine wire into the connector and read it to the negative battery terminal. At idle it will read 300-500 mili volts and swing over to 600-800 mili volts. It will swing back and forth as the computer adjust the air fuel ratio. If you blip the throttle it will peg high or low (cant remember which voltage = rich and lean) and then settle back out. Its a quick check but it will show you a bad one real quick. A bad one will just sit at one voltage and not swing very far from it.
Sometimes an O2 sensor gets sluggish and will still work but it will not give you the readings that you need to properly control your air fuel ratios. Everything works but you get poor efficiency, ie bad gas mileage, iffey idle. A bad idle is probably vacuum leaks or dirt!