1987 c4 where to start sometimes it will idle when started and some not sometimes will shut down at a stop sign give it some gas and it will run fine? whats the first clue to look for I've already changed the throttle position sensor so whats next?

 

After changing the tps sensor you must do a idle adjust it is mandatory.

Tools & Equipment
You will need the following tools
and equipment:
1. A set of Torx wrenches. 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” and “B”) in
the connector.
� 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.
� With the
IAC connected and the ignition “OFF,” stick the paper clip
into the diagnostic connector from “A” to “B.”
This grounds the diagnostic lead.
� Turn the ignition to the
“ON” position without starting the engine. Wait 30
seconds.
� Now, with the ignition still in the “ON”
position, disconnect the IAC connector at the IAC.
� Remove the paper
clip from the diagnostic connector.
� Start the engine and allow
it to reach normal operating 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.
� 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.
� Adjust the idle speed
screw to obtain 400 rpm in drive or 450 in neutral.
� Shut off
the engine and re-connect 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 use pins. You can either buy a special harness connector that breaks these wires out
(from Mid 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.
�
Turn the ignition to the “ON” position without starting the
engine.
� Loosen the TPS Torx adjustment screws.
� Set your
volt meter to a low scale DC volt setting that will accurately read less
than 1 volt.
� Measure the voltage between the two top TPS
wires.
� Adjust the TPS by rotating its position until you get a
reading of .54 volts.
� Tighten the Torx screws and recheck the
voltage. Re-adjust if necessary to make sure voltage is right at
.54.
� Turn the ignition “OFF.”
You are now in
perfect adjustment on idle speed and TPS output. Start the engine. It
may take a few seconds for the car to “catch on” to its new
settings.

 

Same procedure for a 1989 coupe?

 

Before we toss parts at it, perhaps we can try something else like cleaning the entire TB. That means take the IAC off, take the IAC housing off an clean the TB and IAC housing with brake cleaner. I would wipe the IAC pintle with brake cleaner on a paper towel.

Since you took off the parts, change gaskets. If you don't reuse condoms, why reuse paper gaskets?

Setting the minimum idle that way is only good for stock cars. Once modification is done, all bets are off.

After cleaning the TB, set the timing to stock 6 degrees and take the car for a drive. Check for leaks. Go get a scanner and read IAC counts. If it is higher than say 20 counts, screw in the adjustment screw. If it is less, screw out the adjustment screw. With each adjustment, rev the engine and watch the counts settle and adjust as necessary. When that is done, set TPS. All of this done with only engine running and no AC or anything else.