Replace Throttle Body: Agree? Disagree?

[Caretaker]

I had a chat with my mechanic yesterday. For the past 3 years, I have had intermittent low idle issues where the car rumbles to a stop as the RPMs dive down to 650 at stop signs. The car has never stalled out. "All" carbon has been cleaned out of all the suspect areas, to include doing a slow drip throttle body cleaning. I have also replaced the PCV and intake manifold gasket, upgrading it to the Toyota orange design. Recently, I noticed that my car was idling a bit high when sitting in traffic after the car was fully warmed up by over an hour's worth of driving. The tach was reading about 900 RPMs when it should be at 750. My mechanic suggested that the next step would be to replace the throttle body itself, but that I should simply wait until the car actually begins to stall out. Knowing that our "Corolla-Vibe's" don't have an actual idle air control valve, do you think the next step down the line would be to replace the throttle body?

[circuitsmith]

If the problem hasn't changed in 3 years I would just drive on.
I've noticed that after an extended high speed run the idle will be a little higher for 30 seconds or so.
I also noticed this on older carbureted engines with no active idle speed control.
The idle dropping low when coming to a stop could be a sticky torque converter clutch control.
How many miles since the last trans fluid change?
If over 30k miles it might be worth a shot to change the fluid.

[Caretaker]

thanks; I changed out the fluid in the four speed tranny with World Standard fluid at 59,000 miles and again at 105,000 just a few weeks ago. It is odd; the RPM dive won't happen for days on end. Then one day, it starts, and it will do it for the rest of the day. It isn't very regular at all which is why I figured it was more carbon build up

[vibrologist]

I once had that idle drop with a Mazda Protege. It was a high mileage vehicle. I went through the usual cleanings of TB and IAC valve and all the tune-up jobs. I also replaced a lot of vacuum lines. It pretty much fixed the problem on that car but I don't know exactly which job was doing it.

Did you clean the MAF sensor yet? (I am sure you did). How old are the spark plugs? Is there any corrosion on any electrical contacts including grounding straps etc?

[Caretaker]

I put in fresh Denso plugs at 100,000 miles (over a year ago). I HAVE NOT replaced the ignition coils however; they are the original Denso. Would ignition coils have something to do with engine idle issues? In the Honda, engine stuttering while driving normally points to plugs, the need for a valve adjustment, and lastly the ignition coils. I would think I would have driving issues at speed if one or more of the ignition coils were failing, not just symptoms when I come to a complete stop. The mass air flow sensor has been removed and scrubbed, and this car doesn't have an idle air control valve.

[vibrologist]

You said you "scrubbed" the MAF sensor. If you actually touched the filaments with an object such as a brush you may have damaged it. It is that fragile. the only way to clean the filament is to use a spray cleaner. my favorite is CRC MAF sensor cleaner, nothing else.

I can't advise on the coils. They seem to be very durable. Since the problem comes and goes it may be a good idea to get some life data. That's also something I have no experience with.

Is the valve cover gasket in good shape? If oil gets into the spark plug tube it can lead to misfires.

[jolt]

This may help if you think the throttle body is bad:

https://www.youtube.com/watch?v=0AYq_qk1U7A

Another thing to try is to disconnect the battery for 15 minutes to clear the ECM memory. It will take about 100 miles driving for the ECM to re-set itself after the memory is cleared.

[sideshowalan]

Would ignition coils have something to do with engine idle issues?

A bad ignition coil will throw a code telling you what coil is causing problems.

Ditto on the MAF sensor advice, it may be ruined if you touched the sensor. There is a small wire that you can see from the side and all you do is spray it with electrical cleaner and let it air dry.

[cptnsolo77]

I had a problem with idle dropping for a couple of years and actually stalled once. Cleaned the IAC last August and idle has been steady ever since. I recommend you clean it. Before I FINALLY cleaned the IAC, I changed plugs, cleaned throttle body, MAF sensor and replaced intake gasket. A clean IAC did the trick To clean the IAC you have to remove the throttle body...so the answer would be yes

viewtopic.php?f=17&t=36489&p=476353&hilit=iac#p476353

[Caretaker]

So the answer is yes in that the second generation Vibe doesn't have an idle air control valve, leaving the throttle body itself as the culprit?

[jolt]

If you have eliminated all other possible causes, then yes, it is the throttle body. If you watched the You Tube video I pointed you to and pickup on the fact that the throttle body has a motor in that can wear out, stick, or in other words not respond properly and also has electronics that can go bad; then you can see that the throttle body does not last for ever and does go bad. It is just another check you can do as proof that the throttle body is bad and it could pass this visual test and still be bad.

At roughly $400 for a new throttle body, you want to make sure that it is the problem. You may want to check the salvage yards for a used one at a more reasonable cost. I take it that you have disconnected the battery so that the ECU can try to learn the ideal control again just in case the ECU has gotten some bad data in memory. That does not cost anything and would be worth a try and see if you get lucky.

[Caretaker]

While I have not disconnected the battery this year, I did have the alternator and battery replaced last year, so I'm assuming that in itself performed the reset of the ECU while those parts were being removed/replaced. I did watch the video; it appeared to be for a Camry-Vibe rather than a Corolla-Vibe. Not sure if that would make a difference in that part. And you are right about the cost ($400-$600 on rockauto). I think I'd take it in for another slow drip throttle body cleaning before spending money on the part.

[lannvouivre]

The electronic throttle bodies absolutely can wear out.

The other issue I have seen once before was a faulty accelerator pedal; they use 2-3 different potentiometers to detect the position of the gas pedal. Older cars have the 3rd one as a failsafe. Or maybe it was 3-4? Anyway, I wouldn't expect it to be the gas pedal.

Without the Toyota dealership software, it would be really hard to check the readings of either the TB or the and know which ones are incorrect. You could probably actually just check what the throttle and gas pedal resistance ranges are. The program we used did a full range and showed what it was supposed to be, but it'd probably be fine to just to use the WOT and closed throttle positions for multiple different cars and compare them to yours.