Throttle Body for 97 Tacoma

The boring of TB helps performance by allowing more air volume into the intake, but that alone would not necessarily fix an idle issue. It's the re-machining of the butterfly valve in the TB to close more accurately/tightly that could potentially fix issues due to excess air getting through when the valve is shut. (Maxbore both rebores the TB and manufactures a new butterfly to the newly bored out unit as part of their service).

As stated in the above post my idle issue was not due to the airflow characteristics of my TB or the butterfly within the TB. After much research and trial and error I am leaning toward my issue being electrical (either TPS or ECM related).

 

Not saying it defective but some TPS need to be adjusted for correct idle signal/resistance. Of course I would think its set at the factory but if you have a DMM its worth checking, its just a 10 min job if that. I don't have my FSM handy right now but can check it. If your TSP has slots where the screws go in they can be adjusted but if just 2 holes for the screws no adjustment. I'm not familiar with the 4 cyl.

So along that same line of thinking, perhaps your ECM is unable to control the IAC because of a wiring or contact issue. Pull off the connector, inspect it for corrosion or damage and maybe spray with some contact cleaner, (although since your TPS is new contact issue is unlikely). Also on my V6 and going from memory, if I pull off the IAC conn while the truck is running I get a IAC code so apparently there is some feedback to the ECM.

Next step would be to monitor IAC control signals for proper signal but this is not easy. Its a AC signal and duty cycle controls the opening. However, if you put a DMM on the signal (and you may have to do this with the conn attached because of what I said above, typically using pins to pierce the insulation ) you will see a varying DC signal on the DMM when DMM set to DC because it will average it.

Note that here it says it has a backup bi-metallic strip to default the idle if electrical control is lost, warm idle default is about 1000 rpm.

https://www.toyotanation.com/thread...on-the-toyota-idle-air-control-valve.1676205/

Summary is that you have all new parts as you said but it still does not work so my best guess is that ECM is not able to control it. Its a bit odd that its not throwing a code but on mine it has to be really bad to throw a code. Try pulling off the IAC conn while truck is running to see if anything changes or code thrown.

You may have to take the truck to an electrical automotive mechanic but I have heard they charge by the hour or hopefully someone smarter than me can advise. Pretty much every car I have owned had a problem with IAC valve eventually but always due to carbon buildup. I had a Supra that had a stepper motor to control idle which is more reliable than the duty cycle method but damn stepper motor failed and replacement was $500 15 years ago.

Matter of fact my V6 idle is still not right, it usually settles at about 750 but often is 900 or so. Its annoying but probably not worth spending $400 for new TB. I think you said you are at about 1000 rpms and that is not good and appears to be the default when no IAC signal.

 

Also, on the V6 there is a diagnostic where you short some pins on the diag connector and it runs the idle up to prove that IAC is working. I'll post that for 4 cyl (if 4 cycl has that feature) when I have access to the FSM later today. That would be a good test because it proves beyond a doubt if ECM can control IAC.

 

Have you given the ecu a chance to run all the self-checks (monitors), or are you resetting the ecu and expecting that when you start the truck the next time, it'll sit at 750 rpm after it's warm? Aka, have you given the ecu a chance to "learn" the new tb/iac/tps? The ecu might use some hard coded values until all the monitors are passed.

 

What you are saying about the IAC signal makes sense. I will see if I can find a Toyota specialist shop here, then talk to the service manager to see if they are familiar with this type of issue and decide whether it's worth the cost of fixing. (Since my truck is a manual transmission, the shifting is a little hard if the idle is above 750 RPM which is an annoyance).

As for the other post, I have driven the truck approx. 60 miles since swapping out the Throttle Body with the new OEM unit, so I would imagine that is enough time for the ECU to relearn. Interestingly, the truck has dropped to 750 RPMs on a couple occasions when idling at a stoplight, but then quickly goes back to 1000 RPM. Maybe I just haven't given the ECU ample time to relearn? Would you recommend disconnecting the battery for 20 minutes to force a hard reset?

 

If you can push or pull in towards the firewall on your throttle body where the butterfly valve rotates and it drops your idle back down you could probably get away with shimming your valve center pin rod deal.(sorry if that’s confusing) I’m not sure what they call it but I was able to shim that center pivot point so the butterfly valve recentered in throttle body and allowed it to close enough to cut out the high idle. 1995.5 Tacoma with 315k on it. Cost me $.25 and some time to get the washer to the right thickness on a belt sander.

 

Yes, remove the battery for 5 mins or so - it will learn eventually w/o reset but probably takes longer. So the fact that your idle is not stuck at 1000 probably means the ECU is able to control the IAC - when its learning its seems be a situation where it drops too low and the ecu panics and raises it too much - I have seen this a lot after I clean mine. Also I have noticed if you are driving mostly highway of course it has less of chance to learn the idle. Need lots of stop and go.

 

Yes, I have tried disconnecting the throttle cable and manually rotating the butterfly valve closed while at warm idle, but unfortunately the butterfly will not close any further. (I also lubricated the throttle cable while I had it off for good measure). Since the engine occasionally drops from 1000 RPM down to 750 RPM while driving in stop and go traffic, I'm thinking this is an electrical issue.

I am going to disconnect the battery tonight and let it sit overnight then reconnect in the morning then do some city driving.

 

Wanted to provide a follow up. I was finally able to resolve the idle problem for good, and the engine idles consistently at 750 RPM just like it did the day I drove the truck off the showroom floor in December of 1996. It turned out to be 2 separate issues. The first issue was the worn-out throttle body assembly, which was resolved by replacing the unit (which included a new TPS and IACV). The second issue turned out to be a faulty coolant temperature sensor, which I believe was mentioned by another member earlier in this post. Thank you all for your support in getting this worked out.

 

When you replaced the coolant temp sensor did you use an aftermarket or oem?
I’ve been dealing with everything you’re talking about in your post for about a year in a half now. Super irritating. Cleaned the throttle body and IAC at least a dozen times, new coolant temp sensor, adjusting the shit out of the dash pot. Ready to go ahead and do what you did, get a new throttle body and another coolant temp sensor.

 

always go OEM with sensors on these trucks if you can, Toyotas don't play well with aftermarket sensors and something like that you don't want to have to do twice

 

Update on mine, ended up buying a OEM TPS for $130 that didn’t fix crap. One of my friends was selling a complete throttle body for $150. Grabbed that and an OEM temp sensor, unplugged the ECM fuse while I switched throttle bodies, and problem gone.