Intake leak common locations

[RobertGary1]

My 96 SC2 will often idle at around 1500RPM after its been running a bit. I've done a cursory soapy water check around the TB, EGR, and the top of the intake manifold but not found the leak. Any ideas of areas that commonly cause this issue (since some are hard to get to ).
Fortunately this won't affect my SMOG inspection because if I shut the car off and restart it; it returns to normal idle everytime.

-Robert

[adventureoflink]

Originally Posted by RobertGary1

My 96 SC2 will often idle at around 1500RPM after its been running a bit. I've done a cursory soapy water check around the TB, EGR, and the top of the intake manifold but not found the leak. Any ideas of areas that commonly cause this issue (since some are hard to get to ).
Fortunately this won't affect my SMOG inspection because if I shut the car off and restart it; it returns to normal idle everytime.

-Robert

Longshot here I know (due to TPS failures pegging at ~2000 RPM usually), but perhaps the TPS needs to be replaced?

[fdryer]

Replace the throttle position sensor. OEM seems to be the chosen one for reliability.

[RobertGary1]

Originally Posted by fdryer

Replace the throttle position sensor. OEM seems to be the chosen one for reliability.

Interesting. Hadn't thought of that. I have software to read the TP from the PCM so I may just compare the two values (during the problem and during no problem).

-Robert

[RobertGary1]

Originally Posted by fdryer

Replace the throttle position sensor. OEM seems to be the chosen one for reliability.

Just to verify. You're suggesting the TPS not the IAC?

-Robert

[BV22]

My TPS failure led to an idle of 1500-1700 RPM consistently. Granted, mine is an 02 SOHC. Nonetheless, the fact that on restart your car idles correctly points to the TPS.

[fdryer]

Originally Posted by RobertGary1

Just to verify. You're suggesting the TPS not the IAC?-Robert

Yes, tps since its sole job is to provide an electronic feedback of throttle position. A worn out one from years of use creates the non-linear resistance that's misinterpreted by the pcm. A worn out one will read other than reference voltage so the pcm is misinformed. Similar to the faulty coolant sensor; wrong signals resulting in higher idling.

The iacv is a slave unit with no feedback that's commanded to move a valve to adjust idle speed air flow. With two motors, any one that fails keeps the idle speed semi-permanent, stuck. Very few fail.

[td1238]

Two ways to test the TPS: with an analyzer tool that allows for data viewing, watch the TPS reading as you drive. Every time you release the throttle, the reading should fall back to 0. If it lingers above 0 by any amount, then the TPS is worn or otherwise damaged.

Another test is to connect a mechanical ohmmeter to one end terminal and the center terminal. Slowly operate the throttle through its range of motion. The resistance should change evenly, and the ohmmeter's needle should start and stop in exactly the same place every time (provided that the ohmmeter's movement is in good condition). Any jumping around or not stopping in the same place is a sign of wear and improper operation.

Move the meter wire from one outside terminal to the other and repeat the operation. Works best with insulated alligator clips.

A TPS is a potentiometer, or variable resistor. It works just like the mechanical volume control in a radio. Resistance material is applied in a semi-circle (and may contain varying resistance change curves (mathematical) depending on the use), and each end is connected to one of the outside terminals of the control. A sprung wiper connects to the center terminal, and wipes over this resistance material. If the resistance material is dirty or worn, rough changes in resistance will occur. If it is worn away completely, there will be an open circuit at that point. This condition as heard on a radio shows up as a 'scratchy' volume control. Sometimes a volume control or TPS can be salvaged by spraying WD-40 inside. However, concerning radio and television, this can cause trouble with some high impedance circuits, where impedances are over 1Megohm.

[RobertGary1]

Originally Posted by td1238

Two ways to test the TPS: with an analyzer tool that allows for data viewing, watch the TPS reading as you drive. Every time you release the throttle, the reading should fall back to 0. If it lingers above 0 by any amount, then the TPS is worn or otherwise damaged.

I have the full GM extensions on my laptop's interface software so I can read the TPS voltage as well as % of throttle from the PCM. However, the problem doesn't seem to happen whenever I get all the cables and laptop stuff together.

I'm still not following the logic though in how the TPS affects the actual idle. The only way the PCM can modify the idle is by moving the IAC, correct? (at least in our older mechanical TB's). So why would the PCM move the IAC in response to a signal from the TPS? Does it believe the TPS is below a "stall" position???

-Robert