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

[BV22]

I'm no expert, but I believe the TPS input to the PCM/ECU also controls fuel injection rate. A failing TPS tends to give an incorrect resistance when the throttle is closed, telling the PCM to dump a little more fuel than is actually needed to sustain idle. Basically, the car thinks you have your toe on the gas pedal.

IAC controls only the amount of air allowed to leak past the closed throttle.

[RobertGary1]

Originally Posted by BV22

I'm no expert, but I believe the TPS input to the PCM/ECU also controls fuel injection rate. A failing TPS tends to give an incorrect resistance when the throttle is closed, telling the PCM to dump a little more fuel than is actually needed to sustain idle. Basically, the car thinks you have your toe on the gas pedal.

IAC controls only the amount of air allowed to leak past the closed throttle.

I'm not familiar with the term "injection rate" as you used it. Are you referring to the pulse width? I would expect that increasing pulse width without increasing airflow would stall the engine. This is why the car uses the IAC when cold vs just increasing mixture.

[BV22]

As I said, I am no expert, and I do agree that dumping a lot of extra fuel without extra air would stall the engine. Maybe when it is just a little extra fuel, it bumps the idle up. Kind of like the idle surge when you spray brake cleaner at a leaky intake manifold gasket.

For nearly two years, my SOHC would idle fine sometimes, and around 1700 RPM other times. Occasionally, a blip of the throttle would drop it back down, but not all the time. Shutting it off and restarting always brought the idle down to normal. I replaced the TPS about a year ago and never had the fast idle since. So I am convinced that the faulty TPS and associated incorrect resistance at the idle position did something to cause the faster idle. The TPS is a feedback device, while the IAC is a slave, so the IAC can't "cause" anything unless it fails to respond to commands.

[RobertGary1]

Originally Posted by BV22

As I said, I am no expert, and I do agree that dumping a lot of extra fuel without extra air would stall the engine. Maybe when it is just a little extra fuel, it bumps the idle up. Kind of like the idle surge when you spray brake cleaner at a leaky intake manifold gasket.

For nearly two years, my SOHC would idle fine sometimes, and around 1700 RPM other times. Occasionally, a blip of the throttle would drop it back down, but not all the time. Shutting it off and restarting always brought the idle down to normal. I replaced the TPS about a year ago and never had the fast idle since. So I am convinced that the faulty TPS and associated incorrect resistance at the idle position did something to cause the faster idle. The TPS is a feedback device, while the IAC is a slave, so the IAC can't "cause" anything unless it fails to respond to commands.

That sounds like exactly what I'm seeing. I'm not dismissing the TPS idea; but trying to rationalize how it would change the idle. Idle usually surges when you spray break cleaner on an intake leak because it causes a very temporary seal of the leak (i.e. soapy water makes the same affect).

-Robert

[OldNuc]

The usual test for a failed TPS is from the high idle RPM condition shut off the engine and then restart. Do not touch teh gas. If the engine restarts at the normal low idle RPM the TPS is defective. Replace with OEM from a JY or new. Th ePCM holds the idle at 1200-1400 RPM while the car is rolling with throttle closed and then drops to the normal low RPM when stopped. If the TPS is defective the PCM does not know that the throttle is closed. On a engine start the PCM defaults to throttle closed.

[BV22]

Thanks, as always, OldNuc - makes perfect sense.

[RobertGary1]

Originally Posted by OldNuc

The usual test for a failed TPS is from the high idle RPM condition shut off the engine and then restart. Do not touch teh gas. If the engine restarts at the normal low idle RPM the TPS is defective. Replace with OEM from a JY or new. Th ePCM holds the idle at 1200-1400 RPM while the car is rolling with throttle closed and then drops to the normal low RPM when stopped. If the TPS is defective the PCM does not know that the throttle is closed. On a engine start the PCM defaults to throttle closed.

Thanks! That's what I'm seeing. However, can you confirm that the PCM uses the IAC to affect the higher rolling idle? (I understand the rolling idle is a SMOG requirement to "blow out" excess HC's so cars don't arrive at a stop light and blow out everything built up in the cat). I'm just asking for academic reasons.

-Robert

[td1238]

Originally Posted by RobertGary1

Thanks! That's what I'm seeing. However, can you confirm that the PCM uses the IAC to affect the higher rolling idle? (I understand the rolling idle is a SMOG requirement to "blow out" excess HC's so cars don't arrive at a stop light and blow out everything built up in the cat). I'm just asking for academic reasons.

-Robert

There are only two ways for the computer to hold the idle up high, either with the IAC or with the ignition timing, or both, so it would have to hold the IAC open a bit.

I do know that when I had an issue with my TPS, and idle would hold high, if I put my finger over the IAC intake port, the idle would come down, so it definitely holds the IAC open for that fault, and likely does the same while the car is rolling, for the rolling high idle.

I actually find that now, with a new TPS, and with the car running at normal operating temperature, that the idle will actually drop down close to 1K right away, instead of hovering at a higher rate. When the engine is colder (above 1/4, but not fully warmed up), the idle will hold a bit higher while rolling.

[OldNuc]

Originally Posted by RobertGary1

Thanks! That's what I'm seeing. However, can you confirm that the PCM uses the IAC to affect the higher rolling idle? (I understand the rolling idle is a SMOG requirement to "blow out" excess HC's so cars don't arrive at a stop light and blow out everything built up in the cat). I'm just asking for academic reasons.

-Robert

It is done with the IAC. That is why it takes a couple of seconds to come back to 800 when you stop rapidly. If you stop slow enough the PCM has dialed back the IAC for the 800 RPM idle. The ignition timing is a dependent parameter and is not used to modify engine RPM

[RobertGary1]

Originally Posted by OldNuc

It is done with the IAC. That is why it takes a couple of seconds to come back to 800 when you stop rapidly. If you stop slow enough the PCM has dialed back the IAC for the 800 RPM idle. The ignition timing is a dependent parameter and is not used to modify engine RPM

Excellent. Thanks!

-Robert