Racing enthusiasts in the East Coast of South Africa are usually restricted from any advanced engine modifications by either a lack of funds (as a result of the mediocre median wage earned by citizens of a third world country) or a shortage of skilled mechanics eager enough to take on the modification challenge. The severity of such shortages are only multiplied when faced with a problem concerning a previously modified car by an unknown mechanic. One can imagine the intensity of the task which arose when confronted with a 156 Alfa Romeo housing a newly installed V6 engine.
The owner speaks: According to the owner, the car idled fine but was unresponsive once the throttle was pressed. The throttle butterfly weren’t opening at all.
It was decided, after doing a host of checks to verify that there was indeed sufficient current going to the potentiometer, that the throttle potentiometer needed replacement. Whether or not this was the correct thing to do at the time remains to be challenged as we have not heard from the car owner since. However, given the diagnosis of the problem coupled with the experience of Gaven (the owner and chief mechanic), we decided to go ahead with a replacement of the throttle potentiometer.
Step 1: It can be somewhat cumbersome accessing the bottom of the throttle body so you may want to begin by removing the entire battery to provide a clear access pathway. You can then remove the screw holding the throttle body to the intake.
Step 2: Locate the idle stabilizer valve (ISV). It should be located on the intake hose. If, however, you’re dealing with an older throttle body (i.e. pre 1990 production), there could be a separate hose that connects the intake hose to the ISV. If there is a separate hose, it needs to be taken out before removal of the intake hose. If not (i.e. if the ISV is mounted directly on the intake hose), proceed to step three.
Step 3: You will now have to remove the intake hose.
There are usually a number of clamps holding the intake hose to the crankcase ventilation valve. Once the clamps are removed, you can simply pull up gently to remove the intake hose from the crankcase ventilation valve.
Step 4: At the top of the throttle body you will notice a throttle switch. This needs to be unplugged. The two screws on the mounting bracket of the throttle body needs to be loosened and taken out.
Step 5: Next you will have to remove the vacuum tube and vacuum hose. There should be a metal vacuum tube connected to the right hand side of the throttle body. This tube should be connected by a rubber section. Removing the vacuum tube simply involves disconnecting the vacuum tube from the rubber section and the rubber section from the throttle body.
The vacuum hose is quite easy to remove. It can be found at the rear of the throttle body below the idle switch mounting point.
Step 6: Next, the throttle cable needs to be kept out of the way. This can be achieved by firstly detaching the cable from the top of throttle body by removing the holding clips and then pushing the cables out of the way.
Step 7: Using a hex-head socket, some penetrating lubricant and a hammer perhaps, remove the 7mm screws mounting the throttle body to the intake. A few taps of the hammer may come in handy to break the seal.
Step 8: Remove the throttle body from the intake manifold. Be careful! If your throttle body potentiometer is loose, it could get damaged.
Step 10: This step may take a bit of trial and error is you’re not too familiar with the process or with throttle bodies as a whole. This is also arguably the most crucial step. Look at the potentiometer. You should notice three pins. Adjust the position of the potentiometer until the first two pins of the potentiometer and the idle switch are about 1.5 degrees closed before the throttle plate closes. If you’re a perfectionist (like Gaven) and know that the cost of redoing something is perhaps not half worth the “time” you spend into doing it again, then you may want to consider using a multi-meter to check the readings of the potentiometer. If everything is not lined up correctly, you may have to go through steps 1-9 all over again just to re-adjust the potentiometer position.
Step 11: Following on from Step 10 above, if you’re using your multi-meter, you’re looking for a 5V reading from Connector 1 to Connector 2 and Connector 1 to Connector 3 respectively with the ignition on and the throttle closed.
With the throttle closed, you’re looking for different readings on each respective throttle pot. Between throttle pot 1 and 2, you should get a reading of between 1 500 and 2 600 ohms. Throttle pot 2 and 3, you should get a reading of between 750 to 1 300 ohms.
Finally, using your multi-meter with your ignition on, measuring your reading on pin wire 3 and 2 of your potentiometer, you’re looking for a reading of 5V moving down to 0V as the throttle goes from closed to open.
Step 12: Before re-assembly, you might want to clean the throttle body with a bit of carburetor cleaner.
As can be seen, this is not an easy process at all. The DIY of this is clearly only for the avid car mechanic or enthusiast. If, however, you feel up to the challenge, it can be an ‘extremely’ rewarding experience at the end of it all. I may not be able to bear personal testimony to a hands on experience for each step, but I have definitely witnessed the procedure first hand and have gained a wealth of knowledge in its practical implementation.