I have been experiencing a few different symptoms with my 4runner. It's a 1999 V6 SR5 auto. It is still driving and has never let me down, but I wouldn't trust it on a long trip at this point.

Symptoms:
- RPMs surge when starting from cold. This happens sometime and varies in severity from mild surging to almost killing the engine. This started a could years ago and, in general, the frequency and severity have increased over time.
- Generally has a harder time starting warm than cold (I.e., turns over for longer before actually starting). Always starts, though.
- Occasionally when I stop (parking, at a red light, etc) the engine stops. It then starts again just fine. This has happens approx once a month.
- Occasionally the gas pedal is totally unresponsive (no power when I press it) but it only lasts a few seconds and I get power back after pumping the gas a bit. This has happened twice in the last year.
- A few weeks ago, when I started the engine was very sputtery and jumpy. I barely had the power to make it up the ramp out of my parkade and the rest of the drive I had much less power than normal. Since then, I haven't experience the sputter, but on at least one other occasion I noticed that I had less than normal power, especially when driving up hills.
- I constantly have CEL with P0171, and I think I once had P0120 (but I can't remember for sure and can't find the photo I took), but the latter has not returned since I cleared it.

I took my 4runner into a mechanic I trust and, after an initial inspection, his suggestion was to wait for things to get worse because determining what causing the issue would take a lot of time.

Since have had these issues I have:
- replaced spark plugs (but not wires)
- Cleaned throttle body
- Cleaned MAF sensor
- Tried to clean IAC, but not sure I did a good job of this.

I'm looking for suggestions for what to try to fix this. Any help is appreciated.

I have new O2 sensors which I plan to install soon. Apart from that, my best guess is fuel pump.

I would suspect a failing/failed IAC based on your symptoms. Does the surging go away after the engine gets to operating temp?

Yes, pretty much once the temp is high enough to register on the thermometer the surging stops. That said, the power loss issues don't go away once the engine heats up.

Well if you threw a P0120, and have problems with pedal response, then you're next step should be to replace the TPS and see what happens. They're cheap and easy to get to.

Fuel filter, injectors, fuel pressure regulator should all be considered as well as the fuel pump. FPR's are cheap enough, but you have to take off the fuel rail to get to it so you might as well do both jobs at once, and fuel injectors are gonna run you $300+; so whether you attempt the pump second or last is up to you, dropping the tank is about the same amount of effort as removing the fuel rails. But you should start with a fuel filter cause it's easiest and cheapest.

Wouldn't hurt to poke the injectors with a multimeter or check for vacuum leaks either. Getting to injectors would cost you $20-50 in gaskets and checking for a vacuum leak just needs a can of brake clean and 2 minutes of your time.

I would doubt it's spark related if you're not misfiring and throwing a P0171. If anything you should see a P0172 or somethin that indicates it's too rich as fuel would be burning extremely unevenly.

Not too impressed by the mechanics response, but it is oddly in your best interest. If they don't have a clue you and would have to hunt gremlins it would cost you a small fortune. Still, these are all things he could've and should've advised you on.

Thank you both for the help! I'm going to start with replacing O2 sensors, the IAC, and the TPS, then I'll see how things go from there. I've ordered the parts, so hopefully I can get to this in a few weeks and report back.

Because it's easy and something that can cause intermittent issues I'd change the fuel filter while you are at it.

I have tried a few things with some success so far.

First, I replaced both my O2 sensors and the throttle position sensor. This has seemed to fix my power loss issue, but before long the CEL came back on with P0171.

Second, I replaced my IAC valve. This initially seemed to fix the RPM surging at startup and remove the CEL, but it both have come back now. It does seem like the surging is less severe and common.

I think the next culprit is fuel pressure, but I'm interested in thoughts on how to approach this issue.

Photos below:
1. Downstream O2 sensor
2. Upstream O2 sensor
3. Comparison of old and new TPS
4. Old IAC valve

Attached Images