View Full Version : 96 STS High Idle After Highway Run

03-17-10, 12:56 PM
Our 96 STS runs normally around town but after 1/2 hr highway idles at 1200 to 1500. Seems like it will come down after shutting off and starting again. Is the computer confused? Not sure at all what is wrong.

Replaced the plugs 6 months ago due to misfire, helped but not completely. Seems the mileage dropped before I changed the plugs. Used to get 20+ average, now getting 17. Driving it hard with WOT will bring on Service Engine light which then goes out after some normal driving. There were a few codes one being misfire. Car has 150K miles and does not overheat.

Thanks for any tips!

03-17-10, 01:36 PM
Did you replace the plugs with AC-Delco? What about the wires; how many miles are on them? I'm not positive about your high idle, but could have something to do with the throttle position sensor.

03-17-10, 01:49 PM
I used #41-950 a few had the center electrode visibly off center. I was in a hurry and used them anyway.

03-17-10, 02:05 PM
You've got the right plugs in there then..and the wires?

03-17-10, 02:08 PM
Didn't do the wires and probably should.

03-17-10, 03:22 PM
Seems the mileage dropped before I changed the plugs. Used to get 20+ average, now getting 17.
Have you checked the FPR for a leak?

03-17-10, 03:58 PM
I'll bet the TB is in need of a cleaning also.

High idle after road trips and a SES after WOT - any vacuum leaks anywhere?

03-17-10, 04:32 PM
carbon on the intake is my bet.. take off you air intake tube and check it out. i bet there is carbon in there causing it.

03-17-10, 04:37 PM
Thanks everyone, I'm going to have to do some reading here since I don't have a lot of experience with the motor. Is there a thread on the best procedure for checking for vacuum leaks?

Edit: I'll check FPR according to this thread: http://www.cadillacforums.com/forums/cadillac-tech-tips/61600-how-check-faulty-fpr-fuel-pressure.html
It does not have hot start issues.

Edit: I'll try the TB cleaner to check for vacuum leaks: http://www.cadillacforums.com/forums/cadillac-seville-cadillac-eldorado-forum/187706-smoke-test-vacuum-leak.html

Edit: Cleaning the TB, 1998 close enough? http://www.cadillacforums.com/forums/cadillac-tech-tips/66742-how-clean-throttle-body.html

03-17-10, 06:29 PM
Pulled the codes:
PC0139-HISTORY P0139 - Heated Oxygen Sensor (HO2S) Slow Response Bank 1 Sensor 2
PC1406-HISTORY P1406 - EGR Valve Pintle Position Circuit
IR1771-HISTORY Could not find specific code Integrated Radio????

On clearning codes it will go for 2 or 3 days without new codes, then the service
engine light comes on, with new codes. Then days later it often goes off again.
I believe that the electrical system is flaky, but am certainly open to suggestions.

Some time after I changed the plugs I put Techron in it and about an hour later
the service engine light came on. After the next fill up it went off.

03-18-10, 10:04 AM
EGR Cleaning: http://www.cadillacforums.com/forums/cadillac-seville-cadillac-eldorado-forum/189886-egr-valve.html
HO2S: http://www.cadillacforums.com/forums/cadillac-cts-first-generation-forum-2003/191549-heated-oxygen-sensor.html

03-18-10, 12:26 PM
I'd start with the EGR valve.

03-18-10, 04:11 PM
I will do that, thanks again!

It seems like it runs better after clearing the codes, is it in some sort of fall back mode when a history code is set but the service engine light is off?

03-18-10, 05:40 PM
Not sure, but I don't think so.

03-19-10, 12:10 AM
Strange, it was doing the high idle thing consistently, now that I've cleared the codes it is not doing it since I've made two highway runs today. And as I said it seems to run better.

I hope to get to cleaning the EGR ASAP.

04-15-10, 12:20 PM
The SES has not come on since I last posted, this is rare and I'm not sure why. I did clean and spray (WD40) the fuse box, reseating many of the fuses and relays some time ago (before the high idle), since the electricals seem intermittent and worse on rainy days. The fuse box was hit by a lot of road dirt since pieces of the under chassis mud guard seems to have fallen off. Any suggestions for a good replacement material here, or where to get a factory replacement?

Took a 100 mile highway trip recently and got 26 mpg, using cruise control keeping it at about 60-65. This seems closer to what I remember, and perhaps we are just doing less highway driving.

I've waited to do any cleaning since I wanted to see how long the normal behavior would last. I'll do it as soon as I have a free day with good weather.

Thanks again for the help!

04-18-10, 09:25 PM
Cleaning it doesn't always work.
I needed a replacement.

09-07-10, 01:10 PM
Checked FPR and it tests OK.

johnny kannapo
09-10-10, 12:16 PM
This TB off my car, Never cleaned, 105,000 & running & idling very well looked like this:


The upper left chamber houses the Idle controller, pic below is the spacer between the intake & TB


I had a high idle issue that turned out being a faulty TPS It behaved differently than the symptoms you describe. The Idle controller is easily remove w/2 screws. Its right on the top front.


09-23-10, 01:41 PM
Thanks Johnny, I will look into that if it continues. It has not done it again since I cleared the codes - strange.

09-23-10, 01:45 PM
Not sure where else to post this so here for now.
I've never flushed the coolant but did replace an upper hose
about a year ago and added new tabs the ones recommended.
The recovery tank has always been dirty and smelly, when I
scape a finger along the inside I get something like vaseline
jelly out. Most of the fluid is of normal consistency, this is
just a film in there. Best way to flush this mess?
Please just point me to the right thread if it has been discussed.
TIA - Pete

09-23-10, 02:37 PM
Not easy to clean that tank. What you are seeing is most likely sealant tab residue. About all you can do is pull the lowest hose and turn a garden hose on it.

09-24-10, 09:58 AM
Thanks Ranger! I was hoping I could add some sort of cleaner, run it for a while then drain and flush.
I know this won't clean the tank but I'm also concerned about what is in the block, etc.

09-24-10, 10:49 AM
It really isn't going to do any harm. Just flush it as best as you can.