3.4 Tacoma High Idle

Hello! Looking for some help here. I have a 98 Taco 3.4 manual 4X4 with around 289 miles on it. The engine idled high and was in need of a timing job so I gave it a full timing job, water pump, valves and cleaned the intake, new knock sensors, new TPC you get the idea the works! After all that the high idle remained so I sent off the throttle body to Maxbore. He did a great job, however, looking at live data when engine is at 180F it still idles from 900 -1,200 and when its cold can be 1,600- 1,800. Kinda at a loss here. I have checked for vacuum leaks with starter fluid, propane, unplugged the brake booster and blocked off the circuit smoke test, etc .with no change .

Now, as you can imagine the IAC is clean inside and out, resistance is perfect at room temp 20-21 and when placing 12v it opens and closes. Now with the engine running I have unplugged the IAC and no change in idle. Checking the connection both the open and closed circuits show as OL from the ECM at operating temp 180F. Now that seems to make sense or does it? Should there be some current to tell the IAC to partially close to lower the idle? When I manually apply 12v to the IAC it opens the idle increases and when I put it on the close circuit it will slow it down and actually kill the engine. The TPC reads 9 which is right.

Now I have seen some test where you jump two connections on the diagnostic port and pull the vacuum off. Not sure what that tells me. Help would be appreciated before throwing parts at it! I've switched out the MAF and ECM with junk yard ones with no change...now granted they both could be bad but the odds of that are low.

 

Junior 747- Just reading the mail and saw your input about a fast idle 3.4. Did you ever get it fixed? what was the solution?

Mine is a 2001 with 285,000 on it. Auto, but same problem - fast idle after about 10 seconds of warm up.

Cleaned the throttle body and IAC and if anything the fast idle went faster after the cleanup.

So far I have located tiny leaks from the fuel injector gaskets and found a general area behind the engine where spraying carb cleaner will increase the RPM, but my home made smoke test shows nothing. I also noticed on the connector from the ECM to the IAC that the open and close lines seem to have nothing on them (no voltage from the ECM while there IS battery voltage on the center wire of the IAC connector. (This is with the IAC connector loose) The open and close lines back to the ECM are good and not shorted to ground. The corresponding pins of the ECM show infinite resistance to ground, but I am not calling the ECM bad until I find out more. This could still be an open collector transistor circuit and they read open anyway with no power applied.

 

If spraying carb cleaner increases rpm then I'd say you found your problem, vacuum leak.
I'd narrow down where it is and fix that before digging deeper.

 

Normally I'd say that would work, but today I found all my spark plugs were not torqued down and my brake booster has a slow leak. It will only hold a 15 inch vacuum and the air plenum can pull 21 inches at idle, so the difference is leaking into the air plenum and possibly causing problems everywhere else since the whole system depends on a solid vacuum source in order to give the ECM good information. That being said, I'm just about to take off the upper and lower plenum looking for bad gaskets or bad hoses. The hoses look original and are pretty crusty, some of them are cracked at the ends. The brake booster hose is too easy to rotate on the metal tubing it is attached to even with the hose clamps from the factory. I'm thinking every one replaces them with regular screw type hose clamps?

 

New hoses should be fine with standard spring type hose clamps. The problem is likely the dry old hose not how tight the clamp is.
I went with full silicone hose replacement, and I picked up an assorted kit of hose clamps to use with them.
These are the hoses I went with:

Upgr8 Universal 4mm/6mm/8mm/12mm Inner Diameter High Performance Silicone Vacuum Hose Kit (Blue)

https://www.amazon.com/dp/B00BQ0SDCU
And clamps:

$18 Spring Band Hose Clamps Assortment Kit Fuel Line Silicone Vacuum Hose Pipe Clamp 8 Size 6-17mm Low Pressure Air Clip Clamp Fasteners Assortment Kit, Pack of 160 (Yellow)

https://www.amazon.com/dp/B077Z68ZDL
And it was enough to replace all the vacuum lines. Just don’t use it on the PCV hose as oil will seep through and rot the silicone

Or you can get new rubber hoses from an auto parts store, I like the silicone though.

 

Had any luck finding that fast idle problem? Neither have I. But I did a smoke test under a slight pressure into the intake manifold and got smoke coming out of the O rings around all the fuel injectors. Who would think that 23 year old O rings would just give up the ghost like that? I have been dreading opening up the intake manifold and just doing fun stuff like rear leaf springs and rear axle bearings, Timing belt, rust removal, brake booster replacement (work in progress) Now I found out that there should be a vac accumulator can under the driver's side fender. I think mine is long gone and the hoses are still there.

Note that your vacuum readings will be 1" lower for each 1000 ft above sea level (Just FYI)

But I did find a pretty good vacuum diagram here : https://www.megazip.net/zapchasti-d...piping-17843409#/il-413332474-4566933-4566933

Please let me know if I can help you in some way. I recently bought my own factory service manual and I'm trying to absorb as much info as I can at 75 years of age! LOL

Good luck!