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Intermittent High Idle Speeds


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Hello,

My 99 Seville STS has an intermittent high idle concern. The rpm’s hang between 2000 to 2800 RPM. The throttle is not sticking; the IAC is being commanded opened at 161 counts by the PCM. Most of the time, when the key is cycled, the idle speed returns to normal, but sometimes it does not and more than one key cycle is required. Performing a key cycle allows the PCM to do a sweep or reset of the IAC and 9 times out 10 idle speed returns to normal and IAC counts will be 61, still too high. For some reason I feel the PCM is seeing an erroneous input and commanding a high idle speed. Trouble is I do not see the erroneous input on the scan tool, all parameters look normal (except for IAC counts)and no DTCs set current or history. The drivers in the PCM are functioning correctly because I can command the IAC up to 1.400 and down to 500 RPM. Of course, while the engine is in the process of acting up the output controls are disallowed due to an out of range idle speed.

If anyone has battled a similar concern on a 96-99 N*?? Any help/advice would be much appreciated.

Have a Happy Easter, Frank T

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Anytime I had a high idle on my 96, the throttle plate and throat were dirty with gunk and carbon.

Have you checked for DTC codes?, even history or intermittant codes. Look at the bottom of my post here to see how to check for codes from your on board diagnostic computer, you do not need a reader. You can check for codes at the link to OBD2 codes.

An intermittant vacuum leak, bad IAC, may set a code

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Anytime I had a high idle on my 96, the throttle plate and throat were dirty with gunk and carbon.

Have you checked for DTC codes?, even history or intermittant codes. Look at the bottom of my post here to see how to check for codes from your on board diagnostic computer, you do not need a reader. You can check for codes at the link to OBD2 codes.

An intermittant vacuum leak, bad IAC, may set a code

Thank You BodybyFisher, No vacuum leaks, No DTCs current or history.

I cleaned the throttle body the back side of the throttle plate was pretty gummy, clean the mass air flow and replace the IAC valve. The air flter is nasty, I'm going to replace that too. The car is acting normally, but we'll know in a week or two if its fixed.

Re,

FrankT

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  • 2 weeks later...

Well the caddy went a week idling perfectly, then Friday it acted up again, no DTCs. I replaced the throttle position sensor today. I'll keep you posted.

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Glad to hear it, it is necessary to clean your throttle body at least annually, it should take at least 20 minutes to get in there with rags, throttle body cleaner, and brushes. DO NOT spray carb cleaner into the intake. Without DTCs parts replacing is a waste, if it was bad, it would have set a code

Good Luck

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Glad to hear it, it is necessary to clean your throttle body at least annually, it should take at least 20 minutes to get in there with rags, throttle body cleaner, and brushes. DO NOT spray carb cleaner into the intake. Without DTCs parts replacing is a waste, if it was bad, it would have set a code

Good Luck

I think it depends on how you drive it. I cleaned mine a few years ago and have checked it and is still shiny and clean. On my Deville.

the problem sounds like a TPS going bad and not enough to throw a code. coolant temp sensor can cause idle problem also but usually throws a code

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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If you are driving an average of 12000 miles a year, you should clean your throttle body, like you change your air filter. The back of the throttle body is subject to EGR gases condensing on the back of the throttle plate, creating a thick goey ooze, that gets on the throat and edges of the plate. I have seen a long trip where the throttle is open for long stretches and the engine gulping air cause a plate that sticks open resulting in a high idle because the air turbulance pulls the gunk to the edges of the plate creating an obstruction. Its the garbage behind the plate that causes the problem. Cleaning the throttle body grreatly improves throttle response so I cleaned mine all the time.

Cleaning the throttle body is a dirty time consuming task, its the throat and rear of the plate that are hard to clean, especially if it has not been done in a while.

I am going to say it again, PARTS REPLACING IS FOR STUPID MECHANICS AND THERE ARE MANY, INTEMITTANT TPS SIGNALS WILL SET A CODE, so will a dirty throttle body (high idle) kinked linkage of poorly adjusted linkage.

This is probably more like an intermittant vacuum leak or IAC issue if you ask me, we have seen quirky IACs cause this

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Thank You for your help guys. This problem is so intermittent I cannot put my hands on it, it may take up to two weeks to act up, by the time I get a scan tool and dive into the problem its gone. Lucky for me I have access to N* I can rob parts from.

The following has been done:

1. Throttle body has been cleaned, front side and back side

2. Map sensor replaced

3. Mass airflow sensor has been cleaned

4. IAC has been replaced

5. Looked for a vacuum leak, none, fuel trims are between 0 and 10%. If a vacuum leak was present, fuel trims would be over 10%.

6. PCM connections have been checked and all the above components connections have been checked.

Now, I will say this, A DTC will NOT set until a calibrated threshold has been reached. A sensor can fail within a calibrated threshold, cause a drivabilty symptom and not set a DTC until the sensor fails outside a calibrated threshold. So far, I have $20.00 in parts for an air filter and a can of electrical cleaner. The IAC and MAP sensor came off a 99 N* with blown headgaskets. The car ran fine until then, otherwise I would not have replaced them.

In this case, the PCM is commanding a high idle for a reason that I cannot see on the scan tool. A possible cause is the TPS. If the TPS is failing, spiking rapidly, the PCM may command a high idle, the spike will never be seen on a scan tool because the refresh rate of the scan tool is too slow, nor will it set a DTC because it didn't fail time wise, so the controller sees it and responds accordingly thinking it's normal.

I replaced the TPS, yesterday, I want to pinpoint this problem, If the TPS does not repair the my high idle concern, the only component left is the PCM.

The PCM determines the idle speed/IAC counts based off the following inputs:

1. Battery voltage

2. Engine load - based off of MAP/MAF

3. Power steering switch

4. Vehicle speed

5. ECT

4. A/C on/off

All the above parameters check ok on the scan tool, the only thing left that I can see is the TPS. If the TPS does not fix, then it could be the PCM, maybe? I'll keep you posted.

Thanks again, Frank

Banging my head on this one, thanks for the help, Frank

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good luck. I have more than once had a TPS fail enough to not throw a code, but it doesnt happen ofter. Alot of times it happens with using aftermarket sensors that may not have as tight of a threshold for the paramiters as an oem part. Chrysler was big for that one and it can be the case for any of the parts today as the vehicles becomee so highly complicated. Me working with things as difficult as multiplex wiring, sensors and switches have to be exact, any variance can have unwaned operation or consequences

Another thing to mention is you have to be careful of the chemicles you use around the IAC and related sensors as they can get into the sensor and cause faults or complete failure of the sensor.

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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rockfangd,BodybyFisher,

You both make excellect points, thanks for your input, I'll keep you posted.

Regards, Frank

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  • 2 weeks later...

Hello, Just wanted to let you know the TPS has repaired the intermittent high idle concern. No DTCs, this was tough one, thanks guys.

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  • 2 weeks later...

I cleaned the tb and the IAC, Map sensor, etc, My wife and I went for a ride in the mountains (CO) all of a sudden in 1st gear the car was going 50 mph in 1st gear. I had to put my foot on the brake(on and off) and ride it about 9 miles home. I pulled it into the garage with brakes smoking and stinking. Put it in neutral and the engine was running full bore. I immediately shut it down and let it set for 1/2 hr. Restarted it and it idled perfect drove it around town and took it to the garage who sold it to me. The mechanic rode it all over town and it ran perfect. The boss man said the IAC couldnt be the problem because it has only to do with idle and if the car runs wild like it did it couldnet be the IAC. Went to pick it up today and couldnt get it to act up. He told me something he read about the NS engine and found something about a valve with a orange rubber gasket around the lip. Its located right side of engine at the front of the intake manifold. He said if that sticks open the engine wil "run away" He doesnt know if GM did anything about this valve but the NS has a history of this happening My wife wont ride in it anymore because this is a very dangerous situation. I mean can you imagine this happening while going down 6 & 7 % grades down Vail pass and Eisenhower tunnel for miles. The brakes would burn out and we might end up going over the side..Iam darn scared of this thing And dont want to trust it. Its a beautiful car looks like new. But Iam thinking on parking it in the garage covering it up and maybe use it for a museum piece ! Oh yes no codes relating to this are showing. They checked the throttle cables and could find no cause for it acting like this. I'm still going to replace the IAC and TPS I hope I'm on the right track because like I said this is downright scary. I had no trouble before I cleaned the tb with tb cleaner. Aybody got any ideas ??.

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Mechanic couldn't find anything wrong so far it hasn't repeated itself. Hope it stays that way. I may have hijacked this thread if so I do apologize. But my symptoms were similar to Frank T but much worse.

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  • 2 weeks later...

Mechanic couldn't find anything wrong so far it hasn't repeated itself. Hope it stays that way. I may have hijacked this thread if so I do apologize. But my symptoms were similar to Frank T but much worse.

Donald, Replace the throttle position sensor (TPS). It will not set a code. I'm 99 % sure it will fix your high idle. If you can catch it, you will find that the PCM is commanding the high idle because of an errornous, or spiking tps input.

I have not had a problem since, it been almost 2 months.

Regards, Frank

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Very odd that a balky TPS that was bad enough to cause a problem did not set a code, even the TPS on my 91 set a code and that was OBD1

Here are the possible TPS codes on OBD2

  • P0122 - Throttle Position (TP) Sensor Circuit Low Voltage
  • P0123 - Throttle Position (TP) Sensor Circuit High Voltage
  • P0124 - Throttle Position Sensor 1 Circuit Intermittent

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Mechanic couldn't find anything wrong so far it hasn't repeated itself. Hope it stays that way. I may have hijacked this thread if so I do apologize. But my symptoms were similar to Frank T but much worse.

Donald, Replace the throttle position sensor (TPS). It will not set a code. I'm 99 % sure it will fix your high idle. If you can catch it, you will find that the PCM is commanding the high idle because of an errornous, or spiking tps input.

I have not had a problem since, it been almost 2 months.

Regards, Frank

I have a new tp sw. but I cant seem to get to the bottom screw that holds it in the dog bone seems in the way (or at least the bracket that holds it in) cant seem to figure out how to get that out of the way to get to that screw.

Thanks for the tip

Don

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Mechanic couldn't find anything wrong so far it hasn't repeated itself. Hope it stays that way. I may have hijacked this thread if so I do apologize. But my symptoms were similar to Frank T but much worse.

Donald, Replace the throttle position sensor (TPS). It will not set a code. I'm 99 % sure it will fix your high idle. If you can catch it, you will find that the PCM is commanding the high idle because of an errornous, or spiking tps input.

I have not had a problem since, it been almost 2 months.

Regards, Frank

I have a new tp sw. but I cant seem to get to the bottom screw that holds it in the dog bone seems in the way (or at least the bracket that holds it in) cant seem to figure out how to get that out of the way to get to that screw.

Thanks for the tip

Don

Donald, remove the small elbow bracket and you can get to the bottom screw without removing the throttle body. I was able to go in at a slight angle with a magnetic torx bit. Be carful, don't cross thread the bottom screw and make sure you get the small alinement pin in the small hole at the thottlebody.

Regards, Frank

Edited by Frank T
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Very odd that a balky TPS that was bad enough to cause a problem did not set a code, even the TPS on my 91 set a code and that was OBD1

Here are the possible TPS codes on OBD2

  • P0122 - Throttle Position (TP) Sensor Circuit Low Voltage
  • P0123 - Throttle Position (TP) Sensor Circuit High Voltage
  • P0124 - Throttle Position Sensor 1 Circuit Intermittent

BodybyFisher,

I agree with you 100%, thats why this concern drove me crazy. It took me 7 months to pinpoint this problem because I am not a parts replacer. I like to verify, diagnose, and repair the concern once. However, this started out to be very intermittent last November, after a key cycle the high idle would disappear for weeks.

So I had to shoot from the hip. I finally caught it with a scan tool while it was process of high idling and found a parameter in the data list that stated TPS at Idle with a yes/no answer, the paramerter read NO and the throttle plate was fully closed. After a key cycle, the high idles were back to normal and the parameter displayed Yes. Thats when I replaced the TPS, In the end of this headace, I won.

Now, I have to fix a rattle in rear suspension, I found it, the right rear arm from the center rear to the rear knuckle cost for the part $130.00. Labor time 4hrs GM SI states the entire rear suspension and exhaust must come down. Anybody know a short cut? I have almost 200K on this ride, not ready to give up on it yet.

To all here, have happy hoilday!

Thank You for your help, Frank

Edited by Frank T
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Glad you got it nailed down, I learned an important lesson here, Thanks

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Mechanic couldn't find anything wrong so far it hasn't repeated itself. Hope it stays that way. I may have hijacked this thread if so I do apologize. But my symptoms were similar to Frank T but much worse.

Donald, Replace the throttle position sensor (TPS). It will not set a code. I'm 99 % sure it will fix your high idle. If you can catch it, you will find that the PCM is commanding the high idle because of an errornous, or spiking tps input.

I have not had a problem since, it been almost 2 months.

Regards, Frank

I have a new tp sw. but I cant seem to get to the bottom screw that holds it in the dog bone seems in the way (or at least the bracket that holds it in) cant seem to figure out how to get that out of the way to get to that screw.

Thanks for the tip

Don

Donald, remove the small elbow bracket and you can get to the bottom screw without removing the throttle body. I was able to go in at a slight angle with a magnetic torx bit. Be carful, don't cross thread the bottom screw and make sure you get the small alinement pin in the small hole at the thottlebody.

Regards, Frank

OK removed that bracket and installed the new TPS hope that cures my problem. Not too difficult a job lost some coolant when I unscrewed that small hose fitting but all in all a easy job.

Thanks for your help.

Don

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Good Job Don, post back in couple of weeks for the stauts report, fixed or not, I betting its fixed.

Have a great weekend.

Frank

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  • 1 month later...

Good Job Don, post back in couple of weeks for the stauts report, fixed or not, I betting its fixed.

Have a great weekend.

Frank

Your right its been running fine for almost a month and a half.. So replacing the tps cured the problem..

Thanks to all

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excellent. that is wonderful news. Somehow I have seen it many times where the TPS messes up randomly and conditions can be hard to pinpoint problem. It messes up enough to cause havoc in idle but either never throws a code, or throws a code that immediately goes to history.

Very happy to see that this thread has been updated with results. Thank you very much as it can help alot of people who may someday run into the same problem

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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Very odd problem indeed, that no codes were set, this is a good thread, I never would have thought this could happen

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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