: Weird problem... or maybe not.

12-06-04, 06:27 PM
Hi all,

I have a weird problem on my 1996 DeVille. Weird to me anyway. Here's what happens. I am driving. With or without cruise control, doesn't matter, the problem happens in both cases. When I let go of the gas pedal (or when the cruise control doesn't need to accelerate at all), the RPM starts acting oddly. Say I'm going 65. If I let go of the gas, the RPM is usually about 1900 and slowly going down (unless the cruise control maintains it). However, here, it goes down to 1500 then comes back up to 1900, then down, then up... etc. Until I press the gas pedal (or the cruise control decides it needs a little extra gas). Instead of staying stable.

In other words, it only happens when you don't touch the gas.

Another thing. With the car hot (Coolant temp: 200 degrees F), the idle RPM in Drive is at about 1000 RPM. It usually is at about 650. Put the car in Park and it jumps to 1500 RPM (?!?!).

Now this doesn't happen all the time. It's about the fourth time since I bought the car 2 months ago. So it's not that bad, in a way. I turned the car off, then started it again and the idle RPM was back to normal, at about 650 (in Park). And it usually solves the problem for the driving. Until next time. Like a RESET on a computer I guess ?

Now I'd like to know what causes this. I have a code set and I get the message that I should service the transmission. The code is P0717: Input Speed Sensor. I know it's in the tranny and that it doesn't prevent the car from driving. However, could it cause this ? And only intermittently, as is the case ? If not, what else is the problem ?

I appreciate any help. Thanks in advance.

12-08-04, 06:52 PM
Nobody knows ? :/

12-08-04, 07:43 PM
It sounds like a problem with the idle air control system. I would search the forums for IAC or idle air control and you'll probably find plenty out there. As for the input speed sensor, same thing. Search the forum for that and I'm sure you'll find it out there somewhere. Sorry I can't help you directly but wanted to at least tell you how to find help.

Good luck...

M C :cheers:

12-08-04, 09:01 PM
Hey it's better than no replies lol. ;)

Thanks. I know about the sensor already and I am not sure this has to do with the IAC since I've had weird behaviour while driving too. I was curious to know if the problems were related.

96 STS
12-08-04, 10:03 PM
I'm 1/2 speculating here, but I tend to think that the 0717 code could be related to your varying RPM.

I'm not exactly sure if the input speed sensor has anything to do with it, but maybe if the sensor is occasionally giving false reading then this is affecting the engagement of TCC, which would affect your RPM slightly.

Can anyone else see this possibility, or am I just "loco"?

BTW...where's our resident wizard, bbob? He hasn't posted an about a week. I bet he could shed some light on the matter.

96 STS

12-09-04, 02:12 AM
The IAC or main throttle butterfly.

I would clean both and see if it helps.

Carb cleaner and toothbrush and rags.

Don't get carb cleaner into electrical part of IAC.

12-10-04, 01:38 PM
Yep....first step is to clean the throttle body bore and the back side of the throttle plate to see if there are any deposits built up on it that might be restricting the blade travel. Lots in the archives on throttle body cleaning if you search for such.

Clean out the IAC passage also as described.

You might also check the floormat interaction with the accelerator pedal.....aftermarket floormats, particularily, are notorious for interfering with the throttle return operation. Sounds too simple but it does happen frequently. A bunched up floor mat could cause exactly what you describe.

12-11-04, 05:18 PM
Thanks for all the replies. :)