Fluctuating idle speed question

[virafp]

Hi All...the idle speed on my 100 is acting wierd. Last night, driving on the freeway everything was normal. As soon as I decelerated at the off ramp to come to a stop, the idle speed jumps up to around 1000 RPM for about 5 seconds, and then settles back to around 550 - 600. This was also happening at red lights. This was happening with the AC on and the AC turned off.

Is there some sort of idle speed sensor I should check? Should I disconnect the battery and do a ECU reset?

The only thing that was done last week was a timing belt replacement service at the dealer.

Thanks for the help.

 

[OregonLC]

Clean your maf and throttle body and reset the ecu by pulling the battery cable for 30 min.

 

[virafp]

Worked like a charm!! Thanks

 

[virafp]

Hi All...the idle speed on my 100 is acting wierd. Last night, driving on the freeway everything was normal. As soon as I decelerated at the off ramp to come to a stop, the idle speed jumps up to around 1000 RPM for about 5 seconds, and then settles back to around 550 - 600. This was also happening at red lights. This was happening with the AC on and the AC turned off.

Is there some sort of idle speed sensor I should check? Should I disconnect the battery and do a ECU reset?

The only thing that was done last week was a timing belt replacement service at the dealer.

Thanks for the help.

Ok, so I did the following:

Disconnected the battery for 30 minutes
Cleaned the MAF sensor that looked pretty dirty
Throttle body was cleaned about 7K ago, so did not mess with that.

Issue has resurfaced. On a warmed up engine, here is what is happening:

AC is on

Idle Speed with car in D is around 650-700 RPM. Engine at full operating temperature.

Idle speed increases to around 800-900 RPM with car in Park.

Coming to a stop, car in D, idle speed jumps to 1000 -1100 RPM for 5 seconds and then drops back to around 650 RPM.

1. What is the correct idle speed for the car?
2. This is a recent issue, but not related to any work done on the car.
3. Does the battery disconnect not reset the ECU? Do I need to have it done at the dealer?
4. Seems to happen a bit less with the AC turned off.....but that could be my imagination!

It is definitely different to how it used to be a month ago. The main reason I notice it, is that to keep the car stopped, when the idle jumps, I need to apply more brake pedal pressure.

Thanks for any help and advice.

 

[OregonLC]

I would wager it's gunk on the throttle body, even though it was cleaned 7k ago. Pop the inlet hose, and wipe with TB cleaner.

The surge is because the gunk is choking it off and the TBW is reacting by opening it slightly.

 

[PADDO]

Hi All...the idle speed on my 100 is acting wierd. Last night, driving on the freeway everything was normal. As soon as I decelerated at the off ramp to come to a stop, the idle speed jumps up to around 1000 RPM for about 5 seconds, and then settles back to around 550 - 600. This was also happening at red lights. This was happening with the AC on and the AC turned off.

Is there some sort of idle speed sensor I should check? Should I disconnect the battery and do a ECU reset?

The only thing that was done last week was a timing belt replacement service at the dealer.

Thanks for the help.

I'd have the dealer check it out, on their dime. Someone was climbing in your engine bay, lots of stuff disconnected etc and you presumably paid for the work with and associated warranty, so get em to have a look. Hot idle in gear should be around 625 to 650 rpm, sounds like the idle up is randomly licking in, could be a loose vac hose, maybe loose engine temp feedback?

Sent from my iPhone using IH8MUD

 

[virafp]

Worked normally without AC

I would wager it's gunk on the throttle body, even though it was cleaned 7k ago. Pop the inlet hose, and wipe with TB cleaner.

The surge is because the gunk is choking it off and the TBW is reacting by opening it slightly.

Ok, will try that. Drove it about 30 miles today, and it behaved perfectly, BUT with the AC turned off. The AC has never been recharged in 12 years, so it may be low, and causing the pressure switch to cycle it on and off? What do you think? Possible? In anycase, I am going to Phoenix later this month, so I am going to have the AC checked to make sure its charged up.

 

[virafp]

Sure, I can do that, but this started happening a month before the timing belt service. I just responded to OregonLC, and the problem does not appear with the AC turned off. The idle sat at right around 600 RPM.



Thaks for the help.

 

[OregonLC]

Ok, will try that. Drove it about 30 miles today, and it behaved perfectly, BUT with the AC turned off. The AC has never been recharged in 12 years, so it may be low, and causing the pressure switch to cycle it on and off? What do you think? Possible? In anycase, I am going to Phoenix later this month, so I am going to have the AC checked to make sure its charged up.

Does it blow cold? If so it's not the AC compressor. The ECU picks up the command signal from HVAC telling the compressor to engage and then adjusts rpm up slightly to accommodate the load. The bump in rpm is an ECU programmed amount. Your delta from that sounds to me like throttle body.

I suppose you're correct in that no-load on the engine would cause the RPMs to increase higher than expected, but that comes back to my "is it cold" question. Cold=working=load.

 

[virafp]

Ok, I'm going to do the throttle body today and see if it helps with the AC on! Thanks for the advice.

 

[virafp]

Ok, so here is an update. Everything that has been mentioned above was done, with mediocre success. Then, yesterday I put in a can of Seafoam in the gas tank. Drove to San Luis Obispo today, and the idle has returned to normal ranges. I always had doubts on Seafoam, but it really seems to have helped.

Idle in P with AC on 600 rpm
Idle in P with AC off 750-800 rpm
Idle when coming to a stop in D, around 550-600 rpm.

LC feels a lot better!