2003 STS Idles high when starting

[drcmrn]

my 2003 STS idles for 10-15 seconds at about 11-1200 rpms before dropping. i never noticed this before. could it be something with the IAC? it does this even when its 90 degrees outside

[Ranger]

Actually that's rather quick. Mine does that for 40 seconds while the AIR pump is on. It's normal.

[JimD]

It might be worth your time to remove and clean the IAC valve and holes in the throttlebody. But I would expect a dirty IAC system to reduce idle speed rather than increase idle speed.

Your engine also uses a secondary air injection (AIR) system that is designed to force pressurized fresh air into the exhaust system to accelerate/promote the operation of the catalytic converter. AIR operates on cold engine starts when the coolant temperature is between 41 and 176 F and the ambient temperature is more than 41 F. AIR is shut off when the engine reaches closed loop operation.

How does all that affect idle speed you say? I can't quote chapter and verse from a Cadillac Service Manual, but I can tell you my '04 Deville acts similar to your description and I have searched all the sources at my disposal without finding a hint that the PCM is calling for a higher idle speed for a short time after a cold engine start (sort of like the fast idle cam on the choke linkage of a carburetor). But I believe that is what is happening.

FWIW; if you do cold engine start after dark, you might notice your headlamps will flicker when the engine reaches closed loop operation and the AIR pump is turned off. Normal.

[drcmrn]

i have a catalytic low efficincy error which i think is P0420, how does this all tie together?

[JimD]

I have no idea. But if your vehicle has less than 80,000 miles, you remain covered under the EPA emissions warranty terms and the dealer will have to address the P0420 problem.

[drcmrn]

I have no idea. But if your vehicle has less than 80,000 miles, you remain covered under the EPA emissions warranty terms and the dealer will have to address the P0420 problem.

92,000 miles

[duns]

I have no idea. But if your vehicle has less than 80,000 miles, you remain covered under the EPA emissions warranty terms and the dealer will have to address the P0420 problem.

Vacuum leaks? Usually fast idle is the cause of more air getting in than should be.

[CarlaValentine]

Ok, first off I want to say that I don't know much about newer cars (2000+) so I am taking a "shot in the dark" here.

Is it possible that the problem is being caused by a clogged/partly clogged catalytic converter? According to what I'm seeing in this thread, your car has an AIR system (as Jim mentioned) which forces air into the exhaust when you start the car. If you have a clogged cat then it seems to me that the AIR system can't function efficiently or properly, and could be causing the idle to be high. A clogged cat would also cause the cat not to work efficiently, which I'm guessing could result in your P0420 code.

If I'm wrong or off base with my idea, I'm sure someone will correct me. As I said, it's a shot in the dark, but an idea I felt was worth mentioning.

[KHE]

FWIW; if you do cold engine start after dark, you might notice your headlamps will flicker when the engine reaches closed loop operation and the AIR pump is turned off. Normal.

My '05 Deville headlights flicker slightly after a few miles of driving. It only dims once and returns to proper brightness. I wondered what was causing that!

[JimD]

FWIW; if you do cold engine start after dark, you might notice your headlamps will flicker when the engine reaches closed loop operation and the AIR pump is turned off. Normal.

My '05 Deville headlights flicker slightly after a few miles of driving. It only dims once and returns to proper brightness. I wondered what was causing that!

I can try to explain the "why" of it if you are curious or interested.

[KHE]

FWIW; if you do cold engine start after dark, you might notice your headlamps will flicker when the engine reaches closed loop operation and the AIR pump is turned off. Normal.

My '05 Deville headlights flicker slightly after a few miles of driving. It only dims once and returns to proper brightness. I wondered what was causing that!

I can try to explain the "why" of it if you are curious or interested.

I'm interested. I just figured there was an increased electrical load on the system. Funny how it takes about 3 miles or so for the headlights to briefly dim and then return to full brightness. It always happens in the same spot on my way into work each morning.

[Ranger]

Carla, A clogged CAT would cause problems at high throttle settings. Pinch your nose a little and try to breath hard. No different on an engine.

Kevin, Mine does the exact same thing. So did my previous SLS. Both still do. Nature of the beast I guess.

[JimD]

I'm interested. I just figured there was an increased electrical load on the system. Funny how it takes about 3 miles or so for the headlights to briefly dim and then return to full brightness. It always happens in the same spot on my way into work each morning.

It's actually a decreased load on the alternator.

The AIR pump motor is fused at 50 Amps. I don't know how much current the AIR pump motor draws in operation, but for the purpose of an example, let's use a figure of 35.7 Amps. Thirty five point seven Amps at 14 Volts equals 500 Watts. That is a huge electrical load for an automobile system other than the starter motor.

Your alternator is (easily) supplying this 35.7 Amps and the voltage regulator has responded by adjusting the alternator field current to maintain the alternator output voltage at or near 14 Volts.

When your engine toggles to closed loop fuel control, the AIR pump motor is shut off. That removes the AIR pump motor 35.7 Amp load from the alternator. The instant that load is removed, the alternator is still providing the field current needed to support the AIR pump motor. As a result, the alternator output voltage will rise. It takes a heartbeat for the voltage regulator to detect that the alternator output voltage has risen and make the necessary field current adjustment to bring the alternator output voltage back to the desired 14 Volts.

The voltage regulator cannot anticipate that a large load is being added or removed. It can only respond to detected voltage variations.

Bottom line is you might see your headlamps flicker. And I'll bet the flicker occurs at a different point/distance from the house depending on the season.

All of this occurs at the speed of light.

[KHE]

I'm interested. I just figured there was an increased electrical load on the system. Funny how it takes about 3 miles or so for the headlights to briefly dim and then return to full brightness. It always happens in the same spot on my way into work each morning.

It's actually a decreased load on the alternator.

The AIR pump motor is fused at 50 Amps. I don't know how much current the AIR pump motor draws in operation, but for the purpose of an example, let's use a figure of 35.7 Amps. Thirty five point seven Amps at 14 Volts equals 500 Watts. That is a huge electrical load for an automobile system other than the starter motor.

Your alternator is (easily) supplying this 35.7 Amps and the voltage regulator has responded by adjusting the alternator field current to maintain the alternator output voltage at or near 14 Volts.

When your engine toggles to closed loop fuel control, the AIR pump motor is shut off. That removes the AIR pump motor 35.7 Amp load from the alternator. The instant that load is removed, the alternator is still providing the field current needed to support the AIR pump motor. As a result, the alternator output voltage will rise. It takes a heartbeat for the voltage regulator to detect that the alternator output voltage has risen and make the necessary field current adjustment to bring the alternator output voltage back to the desired 14 Volts.

The voltage regulator cannot anticipate that a large load is being added or removed. It can only respond to detected voltage variations.

Bottom line is you might see your headlamps flicker. And I'll bet the flicker occurs at a different point/distance from the house depending on the season.

All of this occurs at the speed of light.

Jim, that makes sense - thanks for the explanation. I usually don't even notice the flicker anymore.

[CarlaValentine]

Carla, A clogged CAT would cause problems at high throttle settings. Pinch your nose a little and try to breath hard. No different on an engine.

Kevin, Mine does the exact same thing. So did my previous SLS. Both still do. Nature of the beast I guess.

I didn't even think about that. Point taken. Thanks.

As far as the headlight flicker, mine does that too. I actually had my battery and alternator tested shortly after I bought the car because of it, thinking one of them was taking a dive. I'm unsure if mine has the AIR system though. It's a 1994. Does anyone know?

[Ranger]

Jim, AIR pump shuts down about 40 seconds after start up. The headlight flicker occures about 5 - 10 minutes later.

[Texas Jim]

Do the newer cars...like my 2006...have the AIR pump?

[JimD]

Jim, AIR pump shuts down about 40 seconds after start up. The headlight flicker occures about 5 - 10 minutes later.

Larry, I doubt if you could count on the AIR pump being shut off at any given elapsed time after a start. The PCM shuts the AIR pump off when the PCM determines the engine meets all the tests for closed loop operation.

I found the following partial explanation of the switch to closed loop in one of my Service Manuals;

Closed loop operation should begin when the oxygen sensor becomes active, engine coolant temperature exceeds 5 C (41 F) for more than 30 seconds and the PCM has seen a RPM of 1,200 or greater for 10 seconds. At extremely high ambient temperatures or when towing a trailer, it is possible for the system to remain in open loop operation to control catalytic converter temperatures.

[Ranger]

Everything I see in that explanation points to 40 seconds to me Jim. That's when my RPM drops from 1200 to 900. It certainly would not take 5-10 minutes to reach those parameters. When I notice the flicker I am almost at normal operating temperature on the gauge. 10:00 - 12:00 position.

[KHE]

My headlight flicker also occurs between 5 and 10 minutes after startup and the bar-graph temperature display is in the fully warmed-up position. I really don't notice it anymore as it is lighter in the mornings now. It is probably a high electrical load that shuts off but it may not me the AIR pump.