Corvette Forum : DigitalCorvettes.com Corvette Forums banner

1 - 5 of 5 Posts

·
Registered
Joined
·
89 Posts
Discussion Starter #1
I just bought an 85 coupe w/auto trans. When the car is cold the idle is good but it stumbles and stalls easily. As it warms up the idle gets faster up to 1500rpm. The engine runs rough and the trans shifts to 3rd early causing the engine to lug. No check engine light. Anyone have any suggestions on where to start.
 

·
Registered
Joined
·
914 Posts
I think I would check the TPS switch if it's bad it could cause this condition. I think I would look at the IAC for proper adjustment and operation carbon deposits can make the IAC stick and that's what controls your idle speed. Check out the sticky in the C4 forum on CODES AND OTHER INFO by willard it has information on how to check and calibrate these sensors. GOOD LUCK:thumbsup:
 

·
Registered
Joined
·
724 Posts
First let me say, you should probably invest in an inexpensive scan tool if you want any hope of really diagnosing problems on these cars. One for your car can be obtained rather cheaply because it is an OBD I system. Don't buy just a code reader because those can sometimes get you in more trouble than you had. Sometimes one thing will cause the motor to react a certain way that will set a code for something entirally different. Get one that will show you the codes, voltages,pulse width and everything so that way you can watch what all the sensors, and injectors are actually doing. Also it makes it easier to adjust the TPS and IAC. If the car runs better cold than hot, it will probably be a sensor problem. Your car runs on the MEMCAL chip when it is cold which has set data in it for a particular air/fuel ratio timing etc. That is called open loop mode. In order for it to go into closed loop mode(normal driving mode), three things must happen. 1.)The coolant sensor must reach operating temp. 2.)The oxygen sensor must reach operating temp. 3.)an internal timer inside the ECM has to expire.(usually 2-5 minutes) Only after all these conditions are met will the ECM go into closed loop and start taking readings from the sensors to make adjustments to the air/fuel mixture and timing. I would probably look at the coolant sensor or O2 but that is just a guess without having some readings to go by. Possably the TPS, and there is a way to check the voltage on that without a scan tool by using a volt meter. Let me know if I can help.
 

·
Registered
Joined
·
724 Posts
1 - 5 of 5 Posts
Top