High Idle, Delayed Shifting - 03 Buick Lesabre 3.8L

gah_not_again

Hello, thanks in advance for any input.

After I start my Lesabre when cold, after about 15 minutes or so it starts idling high, usually about 1500 , but sometimes it's as high as 2300 while stopped at a light. And a few times it has been 3-4000 rpm after putting it into PARK. After it starts idling high it starts shifting late. If you shut the engine off and immediately restart it, it usually starts idling fine again temporarily. Engine has normal power and runs smooth and idles smooth. It has also randomly died a couple times while driving at 30-40mph but then immediately restarted as if there was no problem.

It has a P0121 code which is "throttle position sensor switch A circuit range/performance problem". It also has a P0420, "Catalyst System Efficiency Below Threshold (Bank 1) ", which I'm not sure is related to the issue because the check engine light had been on a long time before the idling/shifting issue started happening. Like in the neighborhood of a year and a half before the current issue, but it ran fine the entire time and got 30+mpg on the highway.

First thing I did to troubleshoot was to test the TPS and it seemed ok. Voltmeter readings were .4v with throttle closed and seemed to advance smoothly up to 4.4v at WOT. But, I tested it before the engine was hot and I don't have a Oscope.

I then used some carb cleaner and shot it around the injectors and a few vacuum inlets to check for vacuum leaks, to no effect. Engine runs quiet and I don't really hear any vac leaks.

Next, I cleaned the idle air control valve which was pretty dang dirty, but that didn't seem to resolve the issue, although it may have had a slight effect because the issue seems to come and go some while driving now, but I'm not totally sure. I also tested the connector at the IAC and had voltage on all 4 pins between .4v - 11.4v approximately.

Lastly, I got the engine to reproduce the problem and then pulled and then reconnected these sensors one at a time while the engine was idling high:

MAF Sensor
TPS
IAC
Intake Air Temp Sensor

This didn't have the slightest effect on the engine...

I'm thinking it might actually be the TPS or IAC but I don't want to just start replacing stuff without knowing damn near certain it's going to resolve the issue.

Could a vehicle speed sensor cause this?

I'm stumped, does anyone have any idea how I should proceed from here? :?

atoom

Amazing I have found your post. I have a 2003 impala 3.8 doing everything you describe. I have no answers as I am just beginning this journey. Did you ever find out what your problem was? Any help would be appreciated.

imidazol97

atoom said:

Amazing I have found your post. I have a 2003 impala 3.8 doing everything you describe. I have no answers as I am just beginning this journey. Did you ever find out what your problem was? Any help would be appreciated.

I'd suggest trying carb spray around the edges of the intake manifold as well as all
tubes and connectors that might be leaking. The ABS intake plastic can change as it
gets hot and a leak open due to warping that's not there when cooler. So test when it's
hot.

gah_not_again

For some reason I did not get email notifications that there were more responses. I did spray around the intake with carb cleaner while engine was hot and could not find any leaks. This issues was never solved and we eventually sold the car for salvage and bought something else . We were so disappointed as this was a great car with GREAT gas mileage before this happened.