Ira B

Greetings Folks from Newbeedoo.

Recently purchased an 01 2WD Taco base model, 5 speed, 2.4 liter with 135,000 miles. Vehicle is very clean. Plan this as a very long term keeper.
My background includes a lot of auto and mechanical work but mostly non computer engines.
Vehicle ran slightly rough at low throttle settings while at highway speed 50-65 MPH but was throwing no fault codes.

Planned a through servicing anyways so did ALL fluids, ALL filters (air filter was very grubby) , new stock Denso plugs (the old ones looked just like one would hope for indicating no issues). During this I cleaned and reinstalled all grounds I encountered.
The air filter condition prompted me to clean the MAF and throttle body. Reset the ECU by disconnecting the battery for 2 min.
Engine starts/ runs like new, plenty of power, very smooth.....most of the time. At highway speeds (50-60) at low throttle it sometimes has the slightest hint of stumble...and sometimes not at all. And still no codes.
The dreaded intermittent and un-indicated fault.
Replaced the coolant temp sensor (read they can cause this and what the heck, they are cheap) but no joy.
Have run a few bottles of Lucas injector cleaner through it but no changes.
This issue aside, I freaking LOVE this truck.

Sure I'll sort this out eventually but any troubleshooting hints from you gurus more familiar with this vehicle would be most appreciated and very welcome.

vasinvictor

iTrader: (3)

You didn't mention replaceing the bank 1 sensor 1 o2 sensor. At that mileage, just go ahead an replace it with a direct fit Denso 234-9001, http://urdusa.com/store/Denso-Produc...ml?CDpath=6000. That's a wideband sensor that's why it's more expensive, but you've just gotta bite the bullet. Also keep in mind the front sensor uses the rear sensor to periodically calibrate the front sensor, so your rear sensor need to be in good working condition too. Luckily that's a cheaper narrowband.

234-4189 should be the rear sensor number, although his site leaves that off, for some reason.

Ira B

Really appreciate the reply and advice.

The oxy sensors were on my suspect list but I have never seen one fail intermittently, but I am sure it happens. As you say, at this mileage I may as well just replace them both anyways.

Ira B

Got it sorted out. Issue was the throttle body to intake manifold gasket.
The gasket is very, very thin paper and was leaking only under high manifold vacuum conditions ( highway RPM's with very slight throttle plate opening).
I suspect that because the throttle body is heated by coolant but the upper part of the manifold is cooled by incoming air and the gasket acts as a bit of an insulator, there is a slight difference in expansion that can cause the very thin (why so freaking thin?) gasket to leak under the right conditions.
Like a dumbass who should know better, I had left the used gasket in place when I first cleaned the throttle body.
New gasket in place, problem is gone. Cost was $2.99.