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Michael, Auto Service Technician
Category: Car
Satisfied Customers: 919
Experience:  21yrs.Foreign and domestic driveability diagnosis and repair/ ASE Cert./Fuel Injection Specialist.
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1989 chevy s10: temp sensor,oxygen sensor..throttle body..tps

Customer Question

I have a 1989 chevy s10 that has two problems, when started it idels at high idel for about five seconds and then goes to the proper idel this happens every time you start it even when at running temp. The other problem is that after it is warm when you revv the engine it stays at high idel for several seconds after you let off the throtel. I have replaced the following things: IAC,EGR solinoid,temp sensor,oxygen sensor,map sensor,cleaned egr and ports, completly rebult throttle body,tps,and cleaned everything and it still has the same problem. If you could tell me what the problem is if it need a new computer chip (prom)or what, that would be great, thanks

Optional Information: 1989 Chevrolet S 10 4.3 v6
Submitted: 11 years ago.
Category: Car
Customer: replied 11 years ago.
I wanted to let you know that when you rev the throttle and it stays at high idle for several seconds, it does not do it when it is cold
Customer: replied 11 years ago.
Relist: No answer yet.
I am very anxious to find out what you have to say about my problem
Customer: replied 11 years ago.
Relist: I still need help.
Expert:  Michael replied 11 years ago.

S-10 2wd/4wd?Blazer/PU?Is the actual throttle cable returning to it's stop on the throttle cam.You can check this by having a helper snap the throttle pedal and watch the linkage to see if it returns quickly to it's stop or creeps to it.The cable may be gummed up inside or the pedal pivot or return spring on the pedal.If you have addressed the other items,thats all that's left.Unless someone fooled with base idle or timing on the vehicle.What is the TPS voltage at idle position and at WOT?You rebuilt TPS?Not likely.If you replaced it,did you set it properly or just bolt it on?Is the check engine light illuminating and if so what codes were retrieved?So far with the info you have provided the most common cause is a vacuum leak or bad PVC valve which in effect would be considered a vac leak as well.To find vac leaks use a can of carb clean with extension nozzle attached and with engine running spray in small spurts all around the intake and vac hoses while listening for idle fluctuations to occur.You should have an underhood vac system decal to guide you here. The other thing is a faulty Coolant temp sensor that will not always set the MIL on.

For vac leaks refer to this.

Testing the CTS.

Figure 1

The Engine Coolant Temperature (ECT) sensor, sometimes referred to as the Coolant Temperature Sensor (CTS), is a thermistor (resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant temperatures produce a high resistance - 100,000 ohms at -40°F (-40°C), while high temperatures cause low resistance - 70 ohms at 266°F (110°C).

Click image to see an enlarged view
Fig. 1: Coolant temperature sensor and connector

The control module provides a 5 volt reference signal to the sensor through a resistor in the module and measures the voltage. The voltage will be high when the engine is cold and low when the engine is hot. By measuring the voltage, the control module knows the engine coolant temperature. The engine coolant temperature affects most other systems controlled by the module.


TP sensor is connected to the throttle shaft of the throttle body or carburetor. It is a potentiometer with one end connected to 5 volts from the control module and the other to ground. A third wire is connected to the control module in order to measure voltage from sensor. As the throttle angle is changed (the accelerator is pressed down), the output of the sensor changes.

At a closed throttle the output of the sensor is fairly low (0.5 V). As the throttle opens, the output voltage should rise towards 5 V. By monitoring the sensor output voltage the control module can determine fuel delivery based on throttle angle.


  1. Disconnect the negative battery cable.

  2. Remove the air cleaner assembly or air duct, if necessary for access.

  3. Disengage the TP sensor electrical wiring connector.

  4. Loosen and remove the sensor attaching screws.

  5. Remove the sensor from the carburetor or throttle body.

To install:

  1. If equipped, position the seal over the throttle shaft.

  2. Make sure the throttle valve is closed, then position the sensor over the shaft.

On most applications, it will be necessary to rotate the sensor counterclockwise to align the bolt holes.

  1. Make sure the retainers are covered with a suitable thread-locking compound (Loctite®262 or equivalent) then install the bolts and tighten to 18 inch lbs. (2 Nm).

Some earlier model vehicles, such as some applications of the 2.8L engine, utilize an adjustable TP sensor. If so equipped, the sensor should be adjusted after the retainers are threaded, but BEFORE they are tightened. Please refer to Fuel System in this repair guide for TP sensor adjustment procedures.

  1. Engage the sensor electrical connector.

  2. If removed, install the air cleaner assembly or duct, as applicable.

  3. Connect the negative terminal.

Battery Low Mode

If the computer module detects a low battery, it will increase injector pulse width to compensate for the low voltage and provide proper fuel delivery. It will also increase idle speed to increase alternator output and, in some cases, ignition dwell time to allow for proper engine operation.

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Customer: replied 11 years ago.
Reply to Michael's Post: I have replaced pvc and temp sensor and tps,forget to mention it, I have checked for vaccum leaks and there are not any. My tps does not have an adjustment that I know of or could find in the manuel. I have replaced throttle cable and checked it the you said to but it still has the same problems. The check engine light does not come on.
Customer: replied 11 years ago.
What is the base idle?

Expert:  Michael replied 11 years ago.

It's the factory setting as adjusted by a torx screw then plugged against tampering.If you had to ask you havn't touched it which is good.Did you even read the last post completely where I refered to Low voltage or CTS(ECT).For the low voltage simply check the voltage at the battery before start up.Is it at least 12v?If not the engine is overspeeding to charge the battery.I will also post the correct timing procedures here for you to check for accuracy.I'm really leaning toward a bad ECT sensor on this one though.You sure you replaced the sensor and not the switch which has only a single wire?

Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.

Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.

The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5° BTDC, the spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.

As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC. To do this, distributors have various means of advancing the spark timing as the engine speed increases. On some earlier model vehicles, this is accomplished by centrifugal weights within the distributor along with a vacuum diaphragm mounted on the side of the distributor. Later model vehicles are equipped with Electronic Spark Timing (EST) in which no vacuum or mechanical advance is used. Instead, the EST system makes all timing changes electronically based on signals from various sensors.

If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and have started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.

Timing marks consist of a notch on the rim of the crankshaft pulley and a scale of degrees attached to the front of the engine (often on the engine front cover). The notch corresponds to the position of the piston in the number 1 cylinder. A stroboscopic (dynamic) timing light is used, which is hooked into the circuit of the No. 1 cylinder spark plug. Every time the spark plug fires, the timing light flashes. By aiming the timing light at the timing marks while the engine is running, the exact position of the piston within the cylinder can be easily read since the stroboscopic flash makes the mark on the pulley appear to be standing still. Proper timing is indicated when the notch is aligned with the correct number on the scale.

There are three basic types of timing lights available. The first is a simple neon bulb with two wire connections (one for the spark plug and one for the plug wire, connecting the light in series). This type of light is quite dim, and must be held closely to the marks to be seen, but it is quite inexpensive. The second type of light is powered by the car's battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter. This type of light is more expensive, but the xenon bulb provides a nice bright flash which can even be seen in sunlight. The third type replaces the battery source with 110 volt house current, but still attaches to the No. 1 spark plug wire in order to determine when the plug is fired. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.

Never pierce a spark plug wire in order to attach a timing light or perform tests. The pierced insulation will eventually lead to an electrical arc and related ignition troubles.

Since your truck has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps onto the No. 1 spark plug wire, eliminating the adapter. It is not susceptible to cross-firing or false triggering, which may occur with a conventional light, due to the greater voltages produced by electronic ignition.

following procedure requires the use of a distributor wrench and a timing light. When using a timing light, be sure to consult the manufacturer's recommendations for installation and usage.

The vehicle emission control information label, which is found underhood, will often contain specifications or procedures for checking and adjusting timing that have been updated during production. The information contained on the label should always be used if it differs from these instructions.

On most HEI systems, the tachometer connects to the TACH terminal on the coil and to a ground. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Also, note that some tachometers won't work at all with an HEI system. Consult the tachometer manufacturer if the instructions supplied with the unit do not give the proper connection. WARNING Never ground the HEI TACH terminal; serious system damage will result, including ignition module burnout. Set the parking brake and block the drive wheels, then warm the engine to normal operating temperature.

  1. Shut the engine OFF and connect the a timing light to the No. 1 spark plug (right front 2.8L engine, left front 4.3L engine or front plug on in-line engines):

    1. If using a non-inductive type, connect an adapter between the No. 1 spark plug and the spark plug wire; DO NOT puncture the spark plug wire, for this will allow a voltage arc.

    2. If using an inductive type, clamp it around the No. 1 spark plug wire.

    3. If using a magnetic type, place the probe in the connector located near the damper pulley; this type must be used with special electronic timing equipment.

      Do not under any circumstances pierce the insulation of a spark plug wire in order to connect the timing light.

      Click image to see an enlarged view
      Fig. 1: A common timing mark scale

  2. Clean off the timing marks, then label the pulley or damper notch and the timing scale with white chalk or paint for better visibility. If the timing notch on the damper or pulley is not visible from the top, the crankshaft should be bumped around using the starter or turned using a wrench on the front pulley bolt, in order to bring the mark to an accessible position.

  3. On early model vehicles not equipped with EST, disconnect and plug the vacuum advance hose at the distributor to prevent any distributor vacuum advance. The vacuum line is the rubber hose connected to the metal cone-shaped canister on the side of the distributor. A short screw, pencil, or a golf tee can be used to plug the hose.

Click image to see an enlarged view
Fig. 2: Aim the timing light at the mark and scale, but be careful not to contact the fan or drive belts

Click image to see an enlarged view
Fig. 3: Timing marks are often difficult to see; this engine is equipped with a magnetic probe holder which may be used with special equipment

  1. On later model vehicles equipped with EST, the electronic spark timing must be disabled or bypassed to prevent the control module from advancing timing while you are attempting to set it. This would obviously lead to an incorrect base timing setting. There are 2 possible methods of disabling the EST system, depending on the type of engine:

  2. On 2.5L engines, ground the "A" and "B" terminals on the ALDL connector under the dash before adjusting the timing.

  3. On all other engines using the EST distributor, disengage the timing connector wire. On a few of the earlier model vehicles, the 4-terminal EST connector must be disengaged from the distributor. Most later model vehicles are equipped with a single wire timing bypass connector. On these later model vehicles the bypass wire is usually a tan wire with a black stripe. This wire usually breaks out of the wiring harness conduit adjacent to the distributor, but on some vehicles it may break out of a taped section just below the heater case in the passenger compartment.

  4. Start the engine, then check and adjust the idle speed, as necessary. The idle speed must be properly set to prevent any centrifugal advance of timing in the distributor.

  5. Aim the timing light at the timing marks. Be careful not to touch the fan, which may appear to be standing still. Keep your clothes and hair along with the timing light's wires clear of the fan, belts and pulleys. If the pulley or damper notch isn't aligned with the proper timing mark, the timing will have to be adjusted.

TDC or Top Dead Center corresponds to 0° mark on the scale. Either B, BTDC, or Before Top Dead Center, may be shown as BEFORE on the scale, while A, ATDC or After Top Dead Center, may be shown as AFTER.

  1. Loosen the distributor base clamp locknut. You can buy special wrenches which make this task a lot easier on certain models. Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation (found in the firing order illustrations earlier in this Section) to retard, and against the direction to advance.

  2. Once the timing is properly set, hold the distributor to keep it from the turning and tighten the locknut. Check the timing again after finishing with the nut in case the distributor moved as you tightened it.

  3. If applicable, remove the plug and connect the distributor vacuum hose.

  4. If necessary check and/or adjust the idle speed.

  5. Shut off the engine and reconnect the EST wire (if equipped), then disconnect the timing light and tachometer.

Customer: replied 11 years ago.
I have not checked the timing yet but I am working on it. i was wondering about the temo sensor, the one I replaced was on the intake manifold on the front of the engine and has a plug on it with two wires coming from it. which one are you talking about
Expert:  Michael replied 11 years ago.
Thats the one.I guess that's out.Unfortunately there are only so many things that you can do to isolate this in the driveway.Don't forget about checking the low voltage issue.The way to isolate this properly is to have diagnostic equipment in place while the failure occurs,e.g.,Scan tool hooked up,dual trace scope,Fuel press.gauge atttached,vacuum gauge attached etc.The possibilities for this are almost infinate.Even a bad ECM ground would cause that.This is going to take some serious seat time by a well equipped technician.The only other thing you might try is pinching off vac lines,including brake booster hose one at a time to see if the idle comes down.But check the Timing,The Low voltage issue first.