I have my son's 89 tpi vette back together and did initial timing it is running now pretty good but need to set it at 6 degrees with a timing light. I read there was a brown wire with a black stripe that comes from the main loom along the firewall by the heater core follow it and unplug it before timing it and in the service manual it said to unplug some wires from the HEI distributor,
On the dampener the line was lined up with 0 on initial timing do I set at the 6 below the 0 or the 6 above the 0? I have tried it both ways and when I plugged it back in it is supposed to show about 17 degrees and is either way high or I cannot see it at all. not close to 17 degrees. I used the brown wire with black stripe method. I have not tried to unplug the distributor wires.

 

I have 89 also

After warm, unplug the wire you are talking about. Timing is 6 degrees before TDC. Plug wire back in and that should clear your code 42 you will get
After that your Ecu controls timing. This would be above timing mark
Only unplug the one wire.

 

That's odd. On my 89, the timing wire is between the brake booster and the valve cover on the firewall; not near the heater core. Are you certain you've got the correct wire?

Timing is 6 degrees Before TDC.

 

Mine is also between brake booster and valve cover

 

My brown wire comes from the main loom on the pass side but the wire does have the connector on the drivers side between the valve cover and the brake booster. I did try timing it with that unplugged and first I did it with the 6 below the 0 top dead center and then when it did not work right I went to 6 above the center mark and when I plugged it back in and ran the car I started it again and it pinged like it was too far advanced and the idle was and is still too high above 1200. I set the timing the last time set on 4 above center and it starts better but idle is still too high. Also the timing mark with the wire plugged back in is still way above the 12.

 

Sounds like you need to set minimum idle speed with the IAC

 

Sorry i do not know what an IAC is or how to set it. I would like to know. It ran good today I reset the timing again but going down the road it started dropping RPM and acted like it was dropping out of gear Kinda weard. I brought it back home and put a vacuum gage on it and it only came u[p to 12# I know it is supposed to be close as I can to 21. This may be most of my problems now. Too many vacuum lines to suit me. I plugged the vacuum gage into the port below. both in front of and after it.

 

Tools and Equipment
You will need the following tools and equipment:
1. A set of Torx wrenches. ( Mine are phillips head for some reason)
it looks like this
http://www.partsgeek.com/gbproducts/...FdKIfgodzNoMqw
It is on opposite side of throttle to see where the throttle is positioned to tell the computer.

You can buy a complete set in a nice, genuine plastic pouch at Sears
2. A good digital voltmeter that will read voltages less than 1 volt
3. A paper clip
4. A small screwdriver

Procedure
There are two electrical components on the TB that you will be working with: The TPS and the Idle Air Control Valve (IAC). Make sure that the connectors for these two components are easily accessible and that you can easily disconnect the IAC. You will also be playing with the diagnostic connector under the dash. Remove the cover (if it’s still in place). Bend your paper clip into a “u” shape. You will be playing with the two top right hand terminals (“A” to “B”) in the connector.

1. First step is to set the minimum idle speed. If nobody has messed with this on your car before, the set screw will be covered by a pressed-in plug. It’s located on the driver’s side of the TB. Remove this plug if it’s there.
2. With the IAC connected (looks like this

http://www.summitracing.com/parts/df...FQZbfgod_dUP4g

Notice the 4 pin connector just below Tps sensor I use needle nose pliers to remove the connector,
It can be removed and cleaned if needed.)

and the ignition “OFF,” stick the paper clip into the diagnostic connector from “A” to “B.” This grounds the diagnostic lead.
3. Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds.
4. Now, with the ignition still in the “ON” position, disconnect the IAC connector at the IAC.
5. Remove the paper clip from the diagnostic connector.
6. Start the engine and allow it to reach normal operation temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while(I just stick a screw driver between the throttle to keep it going)
7. If your car is an automatic, set the parking brake and put the transmission in “DRIVE.” If your car is a manual, leave it in neutral.
8. Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.
9. Shut off the engine and reconnect the IAC.

That’s it for idle speed. Now on to the TPS.
There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires.
I stick pins thru the wires and hook my meter up to the pins. The wires will be fine.


You can either buy a special harness connector that breaks these wires out (from Min America), or gently pierce the insulation of the wires with the pointy prongs on your volt meter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. Whatever is easiest for you.

1. Turn the ignition to the “ON” position without starting the engine.
2. Loosen the TPS Torx adjustment screws, can be phillips head like mine
3. Set your volt meter to the low scale DC volt setting that will accurately read less than 1 volt.
4. Measure the voltage between the two top TPS wires.
5. Adjust the TPS by rotating its position until you get a reading of .54 volts.
6. Tighten the Torx screws and recheck the voltage. Readjust if necessary to make sure voltage is right at .54
7. Turn the ignition “OFF.”
You will need to do this, the IAC is a little controller valve that you can brush off and oil to refresh it.
I keep this on my phone thank god I haven't had to do it in a year now

 

IAC is Intake Air Control. You cannot set it. That is done by the ECM.

The broken off line goes inside the cabin and operates the doors in the HVAC ducts. With that vacuum missing you will have no control over where the air comes out.

The EST connector is between the brake booster and the windshield wiper motor:



You set the timing so the mark is like this (6° BTDC):

 

I am amazed how similar this corvette is to a 94 F-350 diesel I have been working on of my own. It also has a throttle position sensor but it is called a FIPL and mounts on the engine. It also has a speed sensor that mounts on the rearend housing. It also has a obd1 system. I understand the unit under the dash that has to be jumped a to b top right next to each other but I do not understand what the abbreviation TB stands for. I think the Throttle position sensor is mounted on one bolt of the thermostat housing if I am not mistaken and has 3 wires and two vacuum connections one going to the slobber tank and the other going the round thing (ECM) under the plenum. am I right so far? I think I will better understand where the IAC is when I understand what the TB is and now after reading the post about the throttle position sensor having torque screws I am not sure where it is either. This is all new to me last time I had to work on cars was in the 70's. I am feeling pretty good just getting the head gaskets on and back running this good. LOL I knew I was in trouble when the distributor did not have points or a vacuum diaphram. I am also getting the check engine light and think it may be because I do not know how to reset after changing the oil. I do not know that process.

Cliff do you know where the vacuum line from the cabin comes thru the firewall? I know I have low vacuum it was only 12 and I know it has to be closer to 21. Either the transmission is dropping gears or the rpms are dropping immediately during a shift and the transmission was working fine before the head gasket problem but I realize the transmission will not work properly if the engine is not running right.

 

I edited my post to help explain the parts involved, check it out

 

Thanks I think I finally got it. should have been obvious TB(Throttle Body) I understand the procedure I have seen the diagnostic point under the dash on the drivers side. My son will be here today to help and I will give it a try. Thanks for editing I think even I can understand it now.

Cliff the blend door does not move except with a long screw driver in the inspection plate next to the heater core. Could this vacuum line control the blend door operation as well?

 

We completed testing and resetting the low idle adjustment and it was already set for 400 in neutral and drive but when we plugged the connection back together it idles about 800 rpm so we did not mess with the mechanical screw that adjusts the idle down further. When we started to test the throttle position sensor there was already tape on the two top wires where someone already checked before so we removed the tape and checked voltage with a good volt meter and got no reading at all. Clearly touching both wires with the probes. Does that mean there is no power to the throttle position sensor or is it bad?

 

unplug the connector and see if its corroded and plug it back in turn the car to "on" position only and you should have a reading make sure your tester is set properly, mine is 20 dcv for that test.
But yes this could be a problem also after you get it set, rotate the throttle and you should see a steady raise in the reading if it is erratic you need a new module.

 

The blend door is moved by an electric motor. The motor is controlled by the HVAC programmer, mounted on the firewall above the gas pedal.

The vacuum line goes into the engine wiring harness and then through the bulkhead feedthrough into the cabin. It's shown in this picture from Agent 86 (behind the pink distributor power wire):