Cold Start Fast Idle

artbuc · March 31, 2006

SK Performance asked me to re-post this question so here goes.

Re 99 RX, initial RPM on fast idle is too high. When you start engine, RPM goes to 2000 for a few seconds then settles at 1750. As engine warms up, idle speed comes down. At normal operating temp (176 degrees F), idle speed is correct at 600. This started happening this winter when engine temp was 35-40 degrees F. Did the exact same thing this morning with engine temp at 54 degs F. As I understand it, the cold fast idle program runs whenever the engine coolant temp (ECT) is below normal so I would expect to see the same problem at 54 as I see at 35.

The ECT sensor checks out perfectly and so does the IACV. I had the IACV out. It was relatively clean and checked-out electrically according to the shop manual. Coil resistances were fine and IACV fully opened/fully closed when voltage was applied as specified.

I see two possible explanations: (1) Even though the IACV checks-out, it is still opening too much or (2) the ECM is providing the wrong target rpm to the IACV, ie the ECM look-up tables have been corrupted.

I assume the RPM sensor is fine because all other idle functions work ok.

SKperformance · April 1, 2006

2000 for a few seconds on inital start up is not a strange thing at all.

Did it never idle this high before?

artbuc · April 1, 2006

2000 for a few seconds on inital start up is not a strange thing at all.

Did it never idle this high before?

No. The initial cold fast idle used to be 1500 tops. I could live with 2000 for a few seconds but the real problem is that it settles at 1750 and very slowly drops from there as the engine coolant warms-up. Braking when the engine is trying to idle at 1750 is like braking with one foot on the gas.

Thank-you very much for responding.

Kaizen-san · April 1, 2006

Have you checked the intake air temp sensor as well as the air flow sensor? How about the TPS? I believe your car has twin-throttle body... just a suggestion. :-)

artbuc · April 2, 2006

Have you checked the intake air temp sensor as well as the air flow sensor? How about the TPS? I believe your car has twin-throttle body... just a suggestion. :-)

Thanks for the suggestions. I am not a trained auto tech; but, based on the Toyota tech bulletins I have read, the air temp and MAF sensors do not have anything to do with the fast idle program. The TPS sensor is involved insofar as the TPS must give a fully closed signal for the fast idle program to work. Since my fast idle program is working, ie the idle speed does come down as engine coolant comes up to normal temp, I rule out the TPS as a cause to the problem of high initial rpm on a cold start fast idle.

SKperformance · April 2, 2006

Then of the fast idle is much higher than normal but is steps down normally once warm i might suspect a bad temp sender saying it is colder than it actually is ,as well as checking your coolant.

Also TB's can get pluged closed as well as being stuck open which causes a higher idle.

artbuc · April 3, 2006

Then of the fast idle is much higher than normal but is steps down normally once warm i might suspect a bad temp sender saying it is colder than it actually is ,as well as checking your coolant.

Also TB's can get pluged closed as well as being stuck open which causes a higher idle.

Thanks SK. How should I check the coolant? I change coolant every two years (about 20k miles). Are you concerned about low coolant or contaminated coolant?

Does TB mean throttle body?

SKperformance · April 3, 2006

Both with the coolant ,air bubbles or dirt can cause a bad sensor reading.

Yes TB does mean throttle body ,good guess.

monarch · April 3, 2006

Do you use 50% factory red coolant + 50% distilled water at replacement time? Have you ever cleaned the throttle plate ?

artbuc · April 3, 2006

Do you use 50% factory red coolant + 50% distilled water at replacement time? Have you ever cleaned the throttle plate ?

Yes, I only use Toyota red with distilled water. I either buy the water or use water collected from my dehumidifier. As I posted earlier, the ECT checks our perfectly with resistances at various temperatures matching exactly the resistance vs. temp curve in the shop manual.

No, I have never cleaned the throttle plate but I will do so because I can't think of anything else to do.

At this point, I think the problem lies in the ECM. I believe the "look-up tables" used to determine the initial IACV opening are corrupt. Since I am not going to shell out big bucks for a new ECM, I plan to try one more experiment, if cleaning the throttle body/plate doesn't work. I am going to install a valve in the air line between the IACV and engine intake. I will adjust the valve so that initial cold start rpm is 1500. I'll let you know how/if it works.