My 89 L-98 will not idle properly, the service engine light is on, and I tried an OBD reader but it didn't display any codes.
I've cleaned the Mass Air Flow sensor, replaced the Idle Air Control Valve, and cleaned and checked the throttle body.
I can get a fair Idle with the set screw - but when I turn on A/C it all goes down hill and same thing happens when I put it in gear. Any suggestions.

 

The service engine light comes on to indicate that a code is stored. Try clearing the light and see if it resets.

 

How do I clear it??

 

The acronym OBD referres to On Board Diagnostics, the early version (OBDI) uses a different communication protocol than the current OBDII which became standard in 1996, many of the scanners currently in use can only access OBDII information so if you scanned your 89 with such a tool it would not be able to read the OBDI data your ECM stores.

See this link to learn how to read and clear codes with only a paper clip.


http://www.corvettebuyers.com/c4vettes/ecm.htm

Once you retrieve the code or codes stored, for best results you should troubleshoot using the factory service manual. Codes do NOT indicate a part is bad, only that voltage in the circuit was outside of normal parameters, that can be caused by poor connections, open or shorted wiring or mechanical problems not just the sensor or device mentioned in the code description. The FSM uses the fault tree method to eliminate each possibility in turn until the root cause is isolated, following that procedure is almost always more cost effective than throwing parts at the problem, good luck.

 

Thanks. I'll read the link and give it a shot. I have an OBD II, but did use a low end OBD I reader and no codes flashed. Thanks again.

 

Read and understand procedures, had previously diconnected neg term to clean connectors and terminals. Performed paper clip test, Service Engine Soon - still on constantly and no code display, not even 12. Dropped ALDL and verified wires in tact. Car idling better after various cleanings and slight adjust of the TPS. Would still like to see any codes and clear light.

 

And don´t attempt to set idle with the set screw, you only make it harder for the computer to maintain idle according to it´s settings.

There is a procedure for setting the base idle, and the set screw "might" be used but only during the procedure.

http://web.archive.org/web/200402030...D=55&TopicID=2

 

I agree, the idle set screw is to allow the throttle position sensor and the idle air control to be centered in relation to the feedback from the oxygen sensor by using reference voltages and set parameters to allow the computer full authority.
In other words, if you set the idle set screw at the open end of the screw setting, the idle air control valve will be closed to take the information from the computer to regulate idle. So when you turn on something, the idle will be regulated, but when you turn it off, the engine may die.
If you close the throttle blades, the idle air control valve will be wide open, and when you turn something on, the engine will die, or have probems maintaining an idle.
In a fuel injection engine, increasing or decreasing a specific amount of calibrated filtered vacuum leakage is how the idle is controlled.

forget all you knew about a carb in reference to fuel injection except lean is not enough gas and rich is too much gas.
How you arrive at those conditions has little to do with carb technology because carbs rely on float position, fixed orifices, short bursts of fuel from intermittant pump squirts and convergent ducting to create vacuum inside the venturis to draw fuel from a reservoir inside the carb.

 

Thanks. I am a fair mechanic, especially more old school, I am also a measurement tech so I have DVMs - Multimeters - laptops - etc. What I am lacking is accurate information, so the link and advice is much appreciated . I never had these problems with my 1970, but it didn't run and handle like my C4.

 

This is where the FSM becomes critical, there is a procedure to determine why the ECM will not display a code 12. I do not have an 89 book, perhaps someone will help you out if you do not have one. Another option, though not as convenient is an Alldata online subscription. They should provide the FSM in electronic form for less than $20 annual fee.
Since you are familiar with test equipment you should have no problem following typical diagnostic procedures.

 

Thanks I have learned much today.

#1 Never depend on the stock hood support - jamb a screwdriver in it or clamp it.
#2 Never put any tools or multimeters in the Fuel injection cover or area near wiper blades, when the hood falls because of uneven idle your windshield will break.
#3 Followed all steps to properly set Idle and will now take it to a shop.
#4 Probably cheaper to get the work done, than replace a windshield.

#5 This is not my first Vette, but will probably be the last.

 

Can't blame the game for user error..

 

True - I should know better, but to have such a POS hood support on an otherwise nice vehicle, is just poor engineering. A spring release button would save the day and cost less than $2 at production.

 

My 92's hood strut has a spring loaded lock type thing...so if the strut were to fail, the hood wall fall all of about half an inch. You have to pull the lockout tab away to close the hood.

With time comes error correction...The supports for the earlier C4's were the worst of all of them.

 

I would like to thank all that too the time to lend suggestions and help. The information was dead on especially the Idle adjustment procedure. After all the test and adjustments I had decided to take it to a shop, but just happened to take the time to start priging parts upfront. Found that a remanufactured warranted ECM was about the same price as min/labor - diagnostic so I picked 1 up, changed it out, repeated the idle adjust procedure.
Life is good. Now all I need to do is replace the windshield that broke when the P.O.S. hood support allowed the hood to drop.

Thanks again all. Your help was priceless.