89’ is idling rough/surges sometimes in Park (800 RPM) and Neutral (800 RPM). Though it idles smoothly in Drive (600 RPM) and Reverse (600 RPM). I’ve replaced the TPS, IAC (already did the process), Cap and Rotor, new spark plugs and wires, rebuilt the throttle body, new FPR, new Bosch III Injectors, cleaned the MAF, new fuel pump, new fuel filter, new EGR, new O2 Sensor, fuel pressure is perfect at start up, Idle, and under load) The only things that raise my attention are the vacuum lines are old and will probably fall apart if touched, the air intake hose going to the throttle body was wrapped up in tape by the P.O so quite possibly cracked under there, and all of my spark plugs are pretty clean which make me think it may be running lean. The #6 cylinder though has oil on its threads. Any ideas as to what this could be???

 

Do a leakdown to see what is going on.

You said "I’ve replaced the TPS, IAC (already did the process). What process? Did you check timing? Did you set the min air using the FSM method which ASSUMEs many things? What are the IAC counts with everything but the motor off as done on a scanner? That should tell you if there are any air leaks. What size of injectors? How did you observe the RPM? Tach, which can be inaccurate or scanner, which is what the ECM sees? What does the ECM want for an idle at that time? Coolant Temp sensor reading as observed by the ECM vs IR Thermometer?

 

[QUOTE=aklim;1599806622]Do a leakdown to see what is going on.

You said "I’ve replaced the TPS, IAC (already did the process). What process? Did you check timing? Did you set the min air using the FSM method which ASSUMEs many things? What are the IAC counts with everything but the motor off as done on a scanner? That should tell you if there are any air leaks. What size of injectors? How did you observe the RPM? Tach, which can be inaccurate or scanner, which is what the ECM sees? What does the ECM want for an idle at that time? Coolant Temp sensor reading as observed by the ECM vs IR Thermometer? [/QUO

I’ve done a compression test that came out okay but I will do the leak down test too and post the results. The process I’m referring to with the new IAC is turning the key to on for 30 seconds, unplugging the IAC, turning the car off, plugging it back in, then turning on the car. When reading up on replacing the IAC I saw a lot of forum members just calling it “the process” I set timing to 8* above TDC. I’ve consulted the FSM to no avail. I do not know the exact IAC count. Injectors are 22 LBS. I was using the tach. I don’t have a full on scanner for the OBD I. I just use a paper clip. It’s not throwing any codes. And my timing light only flashes, it doesn’t show RPM.

 

I'm not sure that is even the FSM method of setting minimum air. Timing is 6,no 8 degrees. IAC counts should be about 20 to 30. Tach isn't accurate as it should be.

I would get a scanner so I know what the ECM sees. They are pretty cheap, under $100 and save tons of time and money

 

Is the air intake hose that connects the MAF to the Throttle Body connected and sealed so no outside air can leak in under vacuum conditions while the engine is running operating the engine? I think you have a vacuum leak that is causing a lot of this.

I have a smooth tube that I bought someplace but it seals the MAF and the Throttle body very securely. If you have a leak in this section of tubing the car's MAF sensor will not be able to work properly. If the old one is cracked or modified I would order a new one.

Do you have the 9th injector on the end of the intake on the drivers side? If you do you could try disconnecting it to be sure it is not leaking excess fuel while you are operating the Corvette. When those "freeze" open or just stick they can leak fuel into the intake all the time.

Your engine has a Coolant Temperature Sensor and if that goes"wonky" it will send information that is out of range like telling a freshly started engine that it is hot, or having the equivalent of a Choke sticking when your CTS is telling the engine that it is cold after hours of running and adding fuel to start things up because the ECM uses the input of the CTS to decide on mixture adjustments.

Before you go far buy yourself a roll of the right size hose and replace everything in sight including the parts connecting the cruise under the dashboard to the brake switch of the cruise control (If equipped). Good sharp cutting tool is a must here.

Take a few moments and check out the EGR system on the engine in question, are the vacuum lines working and running to and from the EGR vacuum Solenoid working properly? Are you able to move the EGR valve by applying vacuum pump? Are the connections clear?

Good Luck,

Best regards,
Chris

P.S. Aklim knows what he is talking about regarding about the scanner! I recently bought a old Snap On MT2500 and love what it can do for my 1988 Coupe. I have a reader for OBD 1 and another for OBD 2 but this scanner does even more with that old out of date OBD1 system in my 1988 C4. A great addition to my tool collection!

 

1320 electronics and an android phone works for a scanner.