3.4 engine stumbles @ 2500 RPM before full op temp. Ideas?

[Re: 2001 3.4 V6/automatic 4X4 Tacoma @130K-ish mi]

Friends, I have a small stumble / flat spot problem that just appeared. Doing some research, it appears to be a fairly common complaint, but the answers vary quite a bit. Thought I'd ask the 1st Gen experts here.

The symptom: When not yet up to full operating temperature, and either pulling up a medium grade or accelerating onto a highway, the engine falls flat at 2750 RPM, i.e., when I get into the throttle at ~2500, there is no build-up of revs past 2750 or a tranny kick-down. It just holds at 2750. Then, after a few attempts with the throttle, it’ll kick-down and start to accelerate as normal. No CEL / OBD2 codes are thrown. Again, this problem seems (so far) to be only before reaching full operating temp -- and no, I would never a stress a completely cold engine.

As a maintenance baseline, the O2 sensors are only 15K miles old, and 2K miles ago the tranny fluid was serviced (along with all other fluids), I changed the OE-style Denso plugs, added a bottle of Techron (2X a year) and cleaned the K&N air filter. At that time, I also inspected the MAF and throttle body intake system while doing the plugs. Then, as now, the engine starts easily, runs smoothly and otherwise behaves itself — other than this new problem.

Hoping it was something simple, I changed the fuel filter yesterday. On dumping the filter’s contents into a bucket, I can see the filter was doing its job — the fuel was mildly turbid. I was hopeful that was the problem. It wasn’t — I tried it again this morning under the same circumstances: same exact problem as described above. Then when up to full temp, there is no problem, e.g., when the throttle is matted at 2500 RPM or so, revs will climb a bit, then then the tranny kicks-down a gear and, if I keep it matted, it’ll accelerate hard somewhere toward redline.

Anyone been there, done that?

Thanks in advance for any advice!

 

There is a temp sensor the ECU uses to know how much it needs to richen the air/fuel mix when engine is cold. I would ohm test this sensor per the FSM. It's in the throttle body area. This sensor may be reporting incorrectly that the engine is warmed up the ECU, when in fact it isn't yet. This means the ECU may not add enough fuel to compensate for being cold still.

 

Thanks for the input! FSM = factory service manual I assume.

 

Yessir. There might be something already here in the forum about it.
You know what? I think you may be able to just plug in your scanner before engine is warmed up, while it's still cold, see the temp reported by the scanner in the info. It might report incorrectly and be visible right there, easy way to verify that it's reporting correct.

 

Thanks! Okay, so the Engine Coolant Temp sensor might be the culprit ... maaaybe ... but that analysis makes sense as a good place to start. Why not just replace it? I assume this (see attached) is its location on the 3.4?

So, if engine cold, is it really necessary to drain the coolant to replace it, as the factory manual states? Seems a bit much, unless I'm missing something fundamental about air pockets, etc.

While on topic, as suggested, here are two relevant threads I found about the ECT. I assume they relate.

https://www.tacomaworld.com/threads...nt-temp-sensor-location.419900/#post-11959746

https://www.tacomaworld.com/threads...p-sensor-on-2003-tacoma.507315/#post-15890676

Thanks again.

 

Try cleaning the MAF. I went at mine a couple of times with CRC MAF cleaner.

I did a bunch of work getting the shitty CAI to fit together better, as well. She's been running much better. Especially where I was getting a bit of shake at startup.

This was on my '95.

 

i assume they tell you to drain the coolant because of all the things that need to come off to access the sensor. If I recall its beneath the timing belt guard.

 

On my truck there are 2 temp sensors. One for the gauge and one for the ecu. When I swapped the one for the gauge just a tiny amount of coolant spilled out. Luckily for you the one for the ecu is located just behind the upper rad hose and is easy to get at.

 

austinsdad99: thanks. Assuming you mean it's at the location as in the illustration I posted above ...

 

How can you test the ECU Temp gauge? Is there an ohm reading to look out for?

 

It is your truck I would check it first myself rather then just replace .

Why add more unknowns to the mix.

A scanner will give you real time data

 

Friends, FYI, I found this somewhat problem-related quote about a same-year 3.4 4Runner as mine:

"A delayed 2nd to 3rd shift is totally normal when the engine is not warmed up. . . . It's doing exactly what its "supposed" to do. . . ."

Hmm, part of my new problem is, it won't kick-down to accelerate past 2500-2750 RPM (@ modest throttle input) until the temp gauge's needle has begun moving to approximately 1/3 way between null and the normal 9:00 position.

Perhaps a component is failing, because I haven't noticed this temp/throttle/downshift behavior for the last seven years of ownership ... but I guess I can live with it, as long as it doesn't get worse.

 

Yes.

 

A delayed 2-3 shift is totally different from a flat spot. Heck mine does that delayed 2-3 shift in the am if I leave too quickly. Completely normal. Actually mine seemed to get better after changing trans fluid.

 

May not be related but when your temp is low say less than 140ish the trans wont shift out until about 2600-2800ish rpm. That’s the 1-2 shift, the ecu will hold a gear longer to help warm the engine and trans fluids up faster.