Engine rpm dropping

[OldNuc]

The tach is not real accurate at low values so before anything else I would verify the RPM with a scanner. Idle RPM is determined by the PCM demand and it will hold it within 100 RPM of desired. Changing the IAC just spends money.

[fdryer]

Originally Posted by Supra83mkii

I cleaned the iacv when I removed it from the tb. Do you think it needs to be replaced? The iacv was black with build up. I put a pic of it though fuzzy but it's clean.

I didn't clean the spring part of the iacv because I was afraid to damage it. Should I go back and take it apart and clean the spring part of the valve?

The iacv works when you can test the blocked bypass air port or simply observing the engine running from a cold start.

A normal cold start needs a rich fuel/air mixture so the idle is increased to around 1200rpm. This is the iacv raising the idle speed. As the engine warms up the idle speed drops slowly. This the is iacv reducing the high idle speed. As the engine warms up the pcm is constantly monitoring coolant temps and adjusts the iacv to reduce the rpm's. Arriving at idle speed (800-900rpm) means the iacv and the pcm is doing its job. Every time the engine is run. A smooth transition from cold start high idle to warm idle verifies an operating iacv.

[Supra83mkii]

Originally Posted by fdryer

The iacv works when you can test the blocked bypass air port or simply observing the engine running from a cold start.

A normal cold start needs a rich fuel/air mixture so the idle is increased to around 1200rpm. This is the iacv raising the idle speed. As the engine warms up the idle speed drops slowly. This the is iacv reducing the high idle speed. As the engine warms up the pcm is constantly monitoring coolant temps and adjusts the iacv to reduce the rpm's. Arriving at idle speed (800-900rpm) means the iacv and the pcm is doing its job. Every time the engine is run. A smooth transition from cold start high idle to warm idle verifies an operating iacv.

Ok on cold starts the rpm goes to about 1800rpms then within a second or two goes down to about 1200rpms until it warms up staying at about 700 rpms.

[Supra83mkii]

Ok i took two vids of what the engine does.

http://youtube.com/watch?v=AoIcAKd_FFc

And the second

http://youtube.com/watch?v=7PmKxmrVhdE

When the ac is on it usually always stalls unless I apply the accelerator very slowly. This also happens at night when the lights are on. Not on every stop but does happen frequently to cause me concern. Don't want someone at light hitting me cuz my car stalled.

Oldnuc I'll see if I can borrow a scanner from school to check the rpm more accurately.

[fdryer]

Your description of cold start and warm up fits a perfectly working iacv. Don't get hung up on it as you're fixating and developing tunnel vision, not wanting to see what else may be wrong. EFI systems can be very difficult to understand when least understood and attempting to find an answer to something not understood at all.

You haven't addressed the throttle plate; whether or not its completely closed or open according to factory settings as described previously.....................

[Supra83mkii]

Originally Posted by fdryer

Your description of cold start and warm up fits a perfectly working iacv. Don't get hung up on it as you're fixating and developing tunnel vision, not wanting to see what else may be wrong. EFI systems can be very difficult to understand when least understood and attempting to find an answer to something not understood at all.

You haven't addressed the throttle plate; whether or not its completely closed or open according to factory settings as described previously.....................

Yes I didn't test that today because it was cold and raining. I was using the car to pick up my bro from school and used the chance to take the vids. I'll post my findings when I check the bypass air port. I'll let you know the rpms.

The butterfly valve should be slightly opened, Can a visual check be sufficient?

When I took the tb out and cleaned it, I don't believe it had a slight opening. However would the throttle cable put just enough pressure to open it slightly?

Did you see the two vids?

[Supra83mkii]

Originally Posted by Supra83mkii

Ok i took two vids of what the engine does.

http://youtube.com/watch?v=AoIcAKd_FFc

And the second

http://youtube.com/watch?v=7PmKxmrVhdE

When the ac is on it usually always stalls unless I apply the accelerator very slowly. This also happens at night when the lights are on. Not on every stop but does happen frequently to cause me concern. Don't want someone at light hitting me cuz my car stalled.

Oldnuc I'll see if I can borrow a scanner from school to check the rpm more accurately.

Sorry couldn't edit my post. The second vid failed to upload so I reuploaded it. Here is the link.

http://www.youtube.com/watch?v=sJNdr8LBDjo

Here is the first

http://youtube.com/watch?v=AoIcAKd_FFc


I just checked and both are working.

[fdryer]

The videos don't have any meaning and just verify what you're already describing.

If you know and see the throttle 'butterfly' closed then it was meddled with, period. The throttle cable has nothing to do with it to keep it open. Its a mechanical stop screw or nub that the throttle linkage is supposed to sit on when it closes from a spring return. If this throttle body was modified and someone ground off the stop then its probably the reason you're having erratic idle issues. I don't know what else to tell you. Search/Google throttle stop screw to understand how a throttle plate is supposed to be adjusted and set whether its factory set or incorrectly adjusted from anyone not knowing why its done.

Do not attempt to adjust the iacv to make up for a missing throttle plate stop as all you're doing is chasing after a mistake someone already made. if you have to, search for a used throttle body and examine it carefully for a throttle stop. Once set at the factory the butterfly stays slightly open all the time. Electronics kills the engine, not the throttle being closed.

[Supra83mkii]

Originally Posted by fdryer

The videos don't have any meaning and just verify what you're already describing.

If you know and see the throttle 'butterfly' closed then it was meddled with, period. The throttle cable has nothing to do with it to keep it open. Its a mechanical stop screw or nub that the throttle linkage is supposed to sit on when it closes from a spring return. If this throttle body was modified and someone ground off the stop then its probably the reason you're having erratic idle issues. I don't know what else to tell you. Search/Google throttle stop screw to understand how a throttle plate is supposed to be adjusted and set whether its factory set or incorrectly adjusted from anyone not knowing why its done.

Do not attempt to adjust the iacv to make up for a missing throttle plate stop as all you're doing is chasing after a mistake someone already made. if you have to, search for a used throttle body and examine it carefully for a throttle stop. Once set at the factory the butterfly stays slightly open all the time. Electronics kills the engine, not the throttle being closed.

Where is the throttle stop located at? When I took it off at the bottom of the butterfly there is a small rectangular hole. Could there have been a tab there and was filed or broken off? The tb butterfly valve did not have a slight opening when it was off the engine. That's why I assumed the throttle cable applied some pressure to have it slighltly open.

I can check my local junk yard and go take a look at a tb.

[fdryer]

A mechanical stop ensures a throttle plate closes consistently in the same place every time. A mechanical stop would be anything that's drilled and tapped to hold a screw/bolt/whatever so the throttle linkage will have something to back into as the throttle closes. On the end of the spring would be a small bracket/tab/whatever so it sits against the throttle stop screw/bolt/whatever. The adjustment that's made is always at the factory and if made correctly, hidden with a cover or something to prevent anyone from making any adjustment. This setting is the basis for mechanically setting the throttle open just enough to allow the lowest warm engine idle with the bypass air port blocked off. When adjusted correctly, unblocking the hole allows the EFI system to use the iacv to raise the low rpm to 800-900 for electronic idle.

Seeing a closed throttle is the tell tale sign of someone already making mods and may very well be the cause of your erratic idling as the iacv cannot control the idle 100% of the time. The pcm counts on the slight opening and adds some more air (via the bypass port) to fine tune all idle speeds. Look for any grinding marks or compare this throttle body to one from a junk yard.

[Supra83mkii]

Ok sorry it took awhile. But the the wheather wasn't being nice. Its been to windy and dusty to check the idle while plugging the bypass air port.

So I let the engine warm up for about ten minutes then took a short drive and these were the results. It was almost 90 degrees today so I think the engine was fully warmed up.

Engine in park or neutral.

Normal idle about 800rpm. Idle while bypass air port plugged with my finger 400-500 rpm. Engine did not stall. Did several attempts and the engine would not stall. The beginning of the attempt engine would go to 400rpms and would go to 500rpms within a few seconds. Last attempt I used a rag to cover my finger because it was really hot lol and same results. Left it plugged for about 30 seconds or so.

Engine in drive,2,3 and reverse

Normal idle about 700-750 rpm. Idle while bypass air port plugged, engine would go below 400rpms and stall. Every attempt resulted in engine stalling. Did 3 attempts in drive, 2 for 2,3, and for reverse.

So I'm guessing I'll be needing a new iac valve.

[OldNuc]

The IAC should be compensating for the shift from Park/Neutral and it is not. The PCM knows when you shift so that correction is automatic. First thing I would do is grab another IAC and don't adjust it. If you have a JY handy they are all the same on the throttle body engines. If you pick one off of a junk car it will be all set and you will not have to anguish over that issue. The mechanical stop appears to be correctly set.

[fdryer]

Originally Posted by Supra83mkii

.... Engine in park or neutral.....Normal idle about 800rpm. Idle while bypass air port plugged with my finger 400-500 rpm. Engine did not stall. Did several attempts and the engine would not stall. The beginning of the attempt engine would go to 400rpms and would go to 500rpms within a few seconds. Left it plugged for about 30 seconds or so....

There is nothing wrong with this iacv. Thank you for taking the time to verify its operation as described previously.

Originally Posted by Supra83mkii

Engine in drive,2,3 and reverse.....Normal idle about 700-750 rpm. Idle while bypass air port plugged, engine would go below 400rpms and stall. Every attempt resulted in engine stalling. Did 3 attempts in drive, 2 for 2,3, and for reverse. ....So I'm guessing I'll be needing a new iac valve.

This part of your test has never been done by anyone and presents a new wrinkle and twist. I doubt a replacement iacv will fix this issue but that's your decision to try.

What is the fuel pressure?

[Supra83mkii]

Originally Posted by fdryer

There is nothing wrong with this iacv. Thank you for taking the time to verify its operation as described previously.



This part of your test has never been done by anyone and presents a new wrinkle and twist.

What is the fuel pressure?

Haven't done a fuel pressure test yet. Don't have the money to rent it.
And I'll be replacing the fuel filter when funds allow it.

So the iacv is working correctly?

OldNuc

I might still go to a junkyard and see if I can acquire one cheaply.

[fdryer]

Originally Posted by Supra83mkii

Haven't done a fuel pressure test yet. Don't have the money to rent it. ..... And I'll be replacing the fuel filter when funds allow it.......So the iacv is working correctly? ....I might still go to a junkyard and see if I can acquire one cheaply.

• Cold start 1200rpm idle - iacv/pcm
• Transition from cold to warm idle rpm - iacv/pcm
• Warm idle 800-900rpm - iacv/pcm

Performing all the procedures from cold start to warm engine checks verified a correctly operating iacv as its adjusting idle rpm all the time along with the pcm making finer adjustments with the air/fuel mixtures and spark timing since its all electronically controlled. The range from 1800rpm back down to 700rpm is the iacv range of electronic control. The fixed 500-600rpm range is the throttle plate in a partially closed (not completely closed) position. This was needed to be verified to eliminate the throttle body and iacv from causing the drop in rpm.

Autozone may have a fuel pressure gauge for loan. Call.

[OldNuc]

Originally Posted by Supra83mkii

Haven't done a fuel pressure test yet. Don't have the money to rent it.
And I'll be replacing the fuel filter when funds allow it.

So the iacv is working correctly?

OldNuc

I might still go to a junkyard and see if I can acquire one cheaply.

I would as the command to compensate for the transmission drag when in gear is programmed into the PCM. It just changes the idle setpoint to pick up a bit more load and allows the RPM to drop to 750 or so. The correction window is 300 RPM down to 100RPM. It is acting like the open steps are not working every time. The IAC fail is missed steps and if th load correction is less than this you will never get a code. The IAC is responding to gross changes but without a good scanner you have no idea how many steps it is dropping to get there. From your description the response is slow which points at dropping steps. The pintle is moved by a stepper motor.

[Supra83mkii]

Hey guys. So I bought a new iacv and a new fuel filter. And the problem is still there. Rpms still drop and stalls out in gear with ac on. New development, with new iacv the engine will drop rpms while in n or p with ac on, while with the old iacv it did not do that.

Next on my to do list is fuel pressure test as suggested by fdryer.