Odd Rough Idle Continues...

[QueensLacMan]

Hello gentlemen -- hope everyone is well.

My 1995 SLS has this problem where it occasionally rough idles at red lights. I have to press down firm on the brake so that the car ("Leroy") doesn't surge forward. It doesn't do this all of the time but it is quite disturbing when it happens. It has never stalled on me, nor have I ever had a problem starting the car. But when it gets really rough, it seems as if it's going to stall. The car also shakes a little bit and then it stops. Aside from this, the car drives great.

I'm getting two codes: PO22 (Open Throttle Position Sensor) and PO24 (Speed Sensor Circuit Problem).

I know there have been many posts on "rough idle" but I have yet to read anything specific regarding these two codes. Any ideas?

PS - For those of you who care to remember, I also have a 1996 STS that was recently diagnosed with a blown head gasket. I have it up on eBay and it's doing quite well. One potential buyer even phoned me and told me he'd buy it for $3750 as soon as he completes the sale of his car. So... I'll be able to get back almost all of the money I put into it (previous owner returned $1,000). Thanks for your help.

[Krashed989]

Hmmmm.... I'd start with the throttle position sensor. Unplug it, spray some WD-40 in the connectors and plug it back in. Then clear the codes and go for a drive to see if the code comes back. If it does, you might need to replace the sensor. The TPS is located on the throttle body pointing towards the front of the car.

On a side note, do you hear a "ticking" noise from the ICM (Idle Control Motor) when you turn the engine off?

[QueensLacMan]

Ok, I'll try that. I do hear a ticking noise when I turn the car off.

Just got a tune-up and the car is surging like mad. It even goes up to 3000 rpm when in park. I'm also noticing that my car is running a bit hotter than usual. IN traffic, it gets up to 223-225 tops. And then around 192-201 at 60+ mph. That's great. But now it's starting to get up to 234 in traffic. I'm starting to worry. I recently had a leak in one of the hoses. They replaced the hose, etc. Got the tune-up done, coolant flushed and fuel injectors cleaned. Now the car is surging worse and running a bit hotter.

I'm starting to worry.

[Krashed989]

The hotter temps are most likey from the higher rpms that the engine keeps surging up to. Keep an eye on the coolant level though just in case.

The ticking you hear is the ICM trying to adjust itself so the car idles good on the next startup. Each time it ticks, it's a failed attempt. After a number of failed attempts it will give up. Usually it would indicate a bad ICM. But right now I'm going to look up the operation of it to make sure that the TPS is not what the ICM is looking at when it's trying to adjust itself. If it is looking at the Throttle Position Sensor, then the ICM could still be good.

LOL Whoops I've been using ICM for the abbreviation all this time when the correct abbreviation is ISC (Idle Speed Control) motor.
And yes it does recieve direct input from the TPS, so I think you should just replace the TPS or fix the connection to it and you'll be all set.

The TPS is an easy easy job. You can replace it yourself if you have the star driver for the three screws. Just unplug it, and unscrew it and it should come right off. Then do it in reverse for the new one and it's good. The manual says to do the TP learn procedure after you install it, but you don't have to because it will learn on its own after a few drives. But if you want to do the procedure here it is:

TP SENSOR/IDLE LEARN PROCEDURE

1. Outside air temperature is at least 10°C (50°F) so that the A/C compressor will turn "ON".
2. Start the engine.
3. Allow the vehicle to idle continuously until the coolant temperature is 80°C, then idle an additional 5 minutes.
4. At end of 5 minutes, enter diagnostics (engine still running).
5. Turn ignition to "OFF".
6. Wait a minimum of 20 seconds.
7. Turn ignition to "ON" (engine not running).
8. Enter diagnostics.
9. Turn ignition to "OFF".
10. Wait a minimum of 20 seconds.
11. Turn ignition to "ON" (engine not running).
12. Enter diagnostics.
13. Turn ignition to "OFF",
14. Wait a minimum of 20 seconds.
15. Start the engine.
16. Apply the brakes and place the transaxle in "DRIVE" range.
17. Turn the Climate Control Center (CCC) to "OFF" and allow the engine to idle for 30 seconds.
18. Turn the CCC to "AUTO", verify A/C compressor engageent occurs. Allow the engine to idle for another 30 seconds.
19. Place the transaxle in "PARK" range and turn ignition to "OFF".

DIAGNOSTIC NOTES

NOTE: Make sure nothing is touching or obstructing the accelerator or brake pedals during the TP SENSOR/IDLE LEARN procedure because this will prevent the PCM from performing the learn routine.

NOTE: High electrical loads may disable TP SENSOR/IDLE LEARN. Components that may produce a high electrical load are the headlamps, the rear defogger, high HVAC blower and the cooling fans. If difficulty is encountered learning TP SENSOR/IDLE LEARN values (i.e. Code P080 remains current), turn "OFF" these high electrical loads. Also, it is important to accelerate the engine above 1000 RPM after turning "OFF" the above components because the PCM will think it is still under a high electrical load until it checks the system again (when engine rpm exceeds 1000 RPM). Furthermore, since the cooling fans can not be manually controlled, engine coolant temperature must be below 105°C and A/C refrigerant temperature must be below 510°C.

[QueensLacMan]

Great stuff Krashed... thanks a bunch.

I just dropped the car off at my mechanic (i'm not mechanically-inclined nor do I have any tools other than the basic stuff), and passed on your post. I also printed out troubleshooting info regarding the TPS and Speed Sensor (two codes I'm getting). He didn't look too happy to receive the handouts from alldata. Still, I wanted him to know I knew what he was supposed to do in case he tries funny stuff.

I'm going back in a few. I'll let you know how it goes. Thanks again.