Just bought a '97 T100 4x4 5VZ-FE MTX /w 137k. {smiles!} Trying figure out an intermittent but frequent high idle problem. I've noticed that the throttle body appears to close fully, idling at 1100-1600, however if I push on the throttle body rotating-lever-assembly, it rotates just a wee bit more -- and idle drops to 750.

The throttle and cruise cables have plenty of slack, and I've checked, according to the manual, the TPS resistances, the dashpot (what does that do?), and the throttle opener (what does that do?). All are in spec.
Engine off, the throttle assembly closes, but if I push it closed a bit more, it moves and spring back just a little. Maybe that's not a fair test, so I apply vacuum to the throttle opener, and now the assembly closes all the way and can't be rotated that wee bit more.

Does anyone know how the whole throttle body system works? I don't understand the role of the throttle opener and dashpot, and how the mechanical behavior of the lever is affected by vacuum, and other things. No explanation seems to be in the manual. Maybe I just need to pull the TB and adjust the stop screw?

Confused,
-Alex
'97 T100 XC 4x4 MTX
'95 SHO MTX

Thi idle is controlled by a device known as the IAC located on the bottom of the throttle body.Your problem might be caused by a dirty throttle body.DO NOT ADJUST THE STOP SCREW!!!!!!!!!!!!!!!!!!!!!!

No, don't adjust the screw, that's fine by your description.

You've simply got a bind somewhere in the rotating assembly or linkage (ok, cable). Usually you can fix it by simply cleaning and lubricating things. A can of carburetor cleaner and a toothbrush do wonders.

There is a chance it's being caused by some wear, and some piece needs to be replaced. You can carefully locate exactly what piece is sticking by carefully moving things with your finger when it's high idling, to locate exactly what piece is holding the throttle slightly open. It's harder to find than you'd expect.

MTX <img src="/forums/images/graemlins/confused.gif" alt="" />

You might try adjusting the TPS (throttle position sensor). If its slightly out of adjustment the ECU won't get the "IDL" signal consistently, which signals it to drop down into idle mode. Cleaning the throttle body and air flow sensor are always good preventive maintenance and could help too.

The dash pot is on manual transmission trucks and basically just slows the throttle down from being able to slam shut instantly. It supposedly helps reduce emissions.

That idle air control unit basically bypasses air around the throttle body butterfly in conjuction with signals from the computer, temp sensors, etc. until the engine warms up. It also maintains idle after that. Here is some basic good info on the toyota systems.

Autoshop101

Thanks for all the responses, guys.

lawnmowerman: The IAC was my thought, too, as if it were sticking or failing to close a bit more, but then I found this funny behavior of the throttle body arm assembly, which I don't see how can be affected by the IAC.

garym: That's a great site you linked to -- thanks! Unfortunately, Toyota doesn't go into enough detail about the whole TB system. I'm familiar with a simpler TB, TPS, and IAC on my car, but this one has a dashpot and a "throttle opener" under vacuum, and from my test above the presence of vacuum clearly affects the TB's mechanical behavior and I don't understand how or why.

ErikB: Ha ha ... sorry, "MTX" is a term we use in the SHO community meaning Manual Transmission; our trans was labeled by Ford the MTXIV (instead of say R150F). ATX's are slower

Funny business with the IDL signal would make sense, except it looks like in 1997 Toyota changed the TPS from 4-terminal to 3-terminal, and there's no more IDL line!

Thanks for explaining the dashpot.

foxtrapper: A peculiar mechanical bind seems possible (it isn't the cables -- they have slack). I'll try more poking at the rotating assembly, and I guess I'll end up pulling it and going after it with a toothbrush and cleaner like you suggest. At this point that's just a blind guess, because the rotating assembly seems to rotate fully and completely closed with the engine off and vacuum applied to the throttle opener (per the manual), but *doesn't* with the engine on.

It's just weird. Need to smog the vehicle for registration right away, and this isn't going to help pass California emissions <img src="/forums/images/graemlins/confused.gif" alt="" />

-Alex
'97 T100 XC 4x4 MTX
'95 SHO MTX

Well, I removed the throttle body, only mildly dirty, and cleaned it. Out of the car, with vacuum applied to the throttle opener, the mechanism rotates shut completely just as it should. Back in the car, same behavior: high idle, then I push it closed just that little bit more. It's not the throttle opener; I could feel the pin retracting under vacuum, so it's not the throttle opener screw out-of-adjustment.

This is all the obvious stuff, so I'm left confused. Somehow, in the car with the engine running, the TB behaves /differently/ than it does out of the car, even with vacuum applied to the throttle opener. What other things could influence the TB?

-Alex

Do you have air cond? There is an idle up feature with A/C that might be malfunctioning. You might check that since it is an intermittent problem. The idle air control is run by the computer. I can send the IAC test info for a 97 but it may not be the same as a 95.

Man am I interested in the solution to this problem. I have the identical problem in my 1992 22RE engine. I replaced the original dashpot assembly thinking that a burr on the plunger prevented the throttle from returning to the idle stop position. Not so. Problem is still there with new dashpot installed. BTW, the engine seems to run just fine without any dashpot assembly installed (throttle correctly returns to idle stop position)!
Thanks,
James

Gary: Yes, it has A/C, and it may be the IAC, but at the moment I don't think so. It seems to have something to do with the physical movement of the rotating assembly, even when everything seems to be clean and proper. I appreciate the offer the IAC test info, but I have the shop manual for 1997, which is what it is.

James70: At this point, I think we're going to have to replace the whole throttle body. Could also try a new IAC and/or TPS, but I'm reluctant to throw money blindly at new parts when the tests don't point that way. I'm sure the junkyard search engines will find you a TB assembly for the 22RE, too.

I don't think the dashpot would cause that for you, because the dashpot slows the closure of the throttle plate when you let off the gas, but should have no effect once the assembly has almost stopped rotating.

A mystery.

-Alex