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? :?

[badbrakes]

I would first troubleshoot the idling problem. I'm guessing the shifting problem is a result of the idling being all over the map.

I would replace the IAC valve.

I would dismantle the throttle body, take it out and clean the heck out of it. I dont know your specific engine but I would try to clean beyond the throttle body. Take the plenum cover off and clean inside.... If your IAC was gummed up, I'm guessing that should also be bad. You may have to consider an EGR valve. ... However what your describing isnt typically caused by the EGR. Get new gaskets for everything you open up, i.e. plenum gasket, tb gadket, iac gasket.

The above may not solve the problem but you wont waste your time. If your IAC was gummed up, the rest ought be gummed up to.

If problem persists, i would look at the sensors.

Lastly, make sure no air in your cooling system... That can also play tricks on you.

Arm yourself with patience and decent hand soap !

[Tech II]

Another possibility is a gummed up throttle bore and plate....when released, it could stick in a partially open position, creating a higher idle.....which may cause a tps code....

Could also be a bad IAC, if no vac leaks.....or it could be the connection in the harness to the IAC, if they have too much play, causing an intermittent connection.......or it could be a bad wire in the harness to the IAC that when flexed, could cause a problem.....may have to check the 3 circuits all the way back to the PCM(mice have been known for building nests in the air box where the PCM is, and eating the insulation off the wires)......

As for the P0420, that is a separate problem....this is usually a catalytic converter problem.....

[gah_not_again]

Originally Posted by badbrakes

I would first troubleshoot the idling problem. I'm guessing the shifting problem is a result of the idling being all over the map.

I would replace the IAC valve.

I would dismantle the throttle body, take it out and clean the heck out of it. I dont know your specific engine but I would try to clean beyond the throttle body. Take the plenum cover off and clean inside.... If your IAC was gummed up, I'm guessing that should also be bad. You may have to consider an EGR valve. ... However what your describing isnt typically caused by the EGR. Get new gaskets for everything you open up, i.e. plenum gasket, tb gadket, iac gasket.

The above may not solve the problem but you wont waste your time. If your IAC was gummed up, the rest ought be gummed up to.

If problem persists, i would look at the sensors.

Lastly, make sure no air in your cooling system... That can also play tricks on you.

Arm yourself with patience and decent hand soap !

Thanks to both of you for your response.

Yes the shifting is definitely related to the idle. It shifts fine up until the exact moment when the idle changes. I will pull the TB off today and clean the hell out of it. Due to the fact that the problem recurs basically as soon as the engine reaches normal operating temperature I don't think it is a wiring issue. Connectors on IAC, TPS, and MAF all look good. It's just weird that when you turn the engine off it will start back up and idle fine for a bit.

However, it does have a slow coolant leak and did get pretty low on coolant at one point, and I sometimes do hear the sound of running water under hard acceleration when the engine is acting up. So, the problem occurs when the engine reaches operating temperature, could this be due to the thermostat opening then and have something to do with air in the coolant system? How would the air in the system cause this issue? I'll have to research how to get air out of the coolant system. Seems like I remember something about jacking the vehicle up at an angle and running it that way to get air out.

Originally Posted by Tech II

Another possibility is a gummed up throttle bore and plate....when released, it could stick in a partially open position, creating a higher idle.....which may cause a tps code....

Could also be a bad IAC, if no vac leaks.....or it could be the connection in the harness to the IAC, if they have too much play, causing an intermittent connection.......or it could be a bad wire in the harness to the IAC that when flexed, could cause a problem.....may have to check the 3 circuits all the way back to the PCM(mice have been known for building nests in the air box where the PCM is, and eating the insulation off the wires)......

As for the P0420, that is a separate problem....this is usually a catalytic converter problem.....

Yeah the IAC was gummed up pretty bad so I'm sure it's pretty dirty in there and could be causing a sticky throttle plate. The plate is behind a honeycomb like screen on the front of the throttle body on this model. Due to how consistent it is about repeating itself I don't really think it's a wiring issue, but I could be wrong.

I'll start today with cleaning the TB and replacing the IAC and then work on getting the air out of the coolant system.

Unfortunately, yesterday when removing the IAC one of the screws sheared off inside the TB housing so I'm going to have to drill it out and use a helicoil I guess. I've never done that before, any input on that?

Thanks again!

[badbrakes]

Depending on how long you're planning on keeping the vehicle, I would definitely get the coolant leak problem fixed. Hopefully the leak is external and easy to diagnose. I do know that air in the system can easily cause weird idle problems... dont know the exact reasons but my guess is on the fact that sensors get confused when air hits them and they send all sorts of misinformation to the computer.

I suggested replacing the IAC because you said you cleaned it.... in my experience, they're hard to clean without damaging them... kinda fragile. Just recently, I cleaned mine and as soon as I reinstalled, the car misbehaved when idling. So I just bought a new one and that immediately did the trick.

As for that broken screw in the TB housing. Been there. If it's broken flush with the housing, you have options (check out youtube). Me when I'm stuck with the problem, I use a drill bit much smaller than the screw to get staeted and helicoil. Needless to say they're just a pain.

Good luck with the project.

[gah_not_again]

Ok thanks, looks like I have plenty of work ahead of me today. Hopefully between cleaning the TB and replacing the IAC and burping the coolant system the issue will be resolved.

The screw is broken fairly flush with the housing, youtube it is! I had that happen on an exhaust manifold bolt once and had to get someone to burn the remainder of the screw out with a torch!

[gah_not_again]

First thing I did was to just replace the TPS. It was strange because after replacing it before test driving the engine would die after revving the throttle a bit. I reset the PCM and took it for a drive and everything was great for half an hour or so, the the intermittent high idle started happening again. So I pulled the new TPS and took it back to the parts store. I haven't had any shifting issues since resetting the computer.

After driving for 30-45 minutes I started to get pre-ignition pinging under hard acceleration. This hasn't happened again yet to my knowledge. I bled the coolant system and replaced the thermostat and radiator cap which were both old. The coolant system definitely needs a flush because there's brown crud around the radiator filler neck.

Next I pulled the throttle body. It was ridiculously built up with carbon and the throttle plate was sticky. Cleaned the TB and IAC thoroughly. It seems as though it's actually acting a bit better at this point, but the issue is still there. Sometimes after revving in neutral the idle will stick at 1500 or so and then slowly return to normal. I can usually turn the engine off and restart and the idle will be fine, but sometimes I have to wait a few minutes. Yesterday it was idling at 3000 rpm when I got home. Turned of the engine, immediately restarted and it was idling normally.

Idle is smooth even when high and engine has plenty of power but I am still getting the P0121 TPS out of range code. I wish I had access to a diagnostic scanner with live data but I don't unfortunately. At this point I'm going to take it to a shop to diagnose and then probably make the repairs myself as long as it isn't replacing the intake manifold. I am at my wit's end on this one.

[Blue Bowtie]

The TPS can be checked with a voltmeter directly at the sensor instead of using a scanner. The connector is not as accessible as one might prefer, but it is possible to back-probe the TPS connector and measure the wiper voltage across the throttle range.

[badbrakes]

I'd probably do the same... Take it in to a decent shop and get an estimate. Depending on mileage and how much u want to keep the car, it may be a good thing to get a precise diagnostic. Quite convinced though that you didnt waste your time so far..... If the TB and surrounding parts were that gummed up, they had to be cleaned.

[gah_not_again]

Originally Posted by badbrakes

I'd probably do the same... Take it in to a decent shop and get an estimate. Depending on mileage and how much u want to keep the car, it may be a good thing to get a precise diagnostic. Quite convinced though that you didnt waste your time so far..... If the TB and surrounding parts were that gummed up, they had to be cleaned.

I completely agree, the TB/ IAC cleaning were definitely needed. The shop down the street charges $98 to diagnose it so it looks like I'm going to go with them. After they figure out the problem I should be able to fix it myself, hopefully. Thanks for the feedback!

[gah_not_again]

Well the issue has been solved. The shop did a diagnostic for me and found that the MAP sensor was intermittently shorting the 5v reference that it provides to the TPS, causing the TPS to behave very strange and making the computer think the TPS had problems. They also said the PCV valve was sticky. I got the PCV for 3 bucks, the MAP sensor for about $55, replaced them and the problem has been solved!

Hopefully this will help someone because I couldn't find anything online that pointed to the MAP sensor potentially causing this problem.

[maxwedge]

Thanks for resolution, unusual failure.

[Blue Bowtie]

Agreed - That's not a common occurrence. It's also something for which a scanner is all but useless.

[DrRadar]

I wonder how the shop came to that conclusion? How did they isolate to an intermittent short in the map sensor?

[gah_not_again]

I was wondering that myself. Well, after driving 25 minutes home from work today the car started idling high again as soon as it was parked. Turned it off and restarted, still idled at 2500. Turned it off again and restarted, idled normally. Turned it off again and restarted and it was over 3k rpm! It drove fine for several days, now this again. I'm very disappointed to say the least. Guess I'll have to call the shop and find out how they came to the conclusion it was the MAP and tell them the issue isn't fixed. I'm sure they'll want to charge $98 for a diagnosis again. I'll pass and take it to a different shop if I have to. So bummed.