Rough Idle and Intermittent Stalling 99 runner

Hey Yall

Currently trying to wrap my head around whats causing all this. And I want to change my old ways being a parts canon and flying money into my rig thats not gunna do anything.

This 4runner is new to me and for the most part it runs quite well. But at idle its quite rumbly and the rpms tend to float quite a bit. I’ve investigated my spark plugs which werent too bad lookin and have run a fuel system cleaner through it to see if it would do the trick but not much improvement either.

Got a scan tool to see what it could show me and at idle Im getting a -20% LTFT which seems pretty significant even if under the value to trigger a check engine light. Now when I start crusing around and putting the engine under load that LTFT goes basically to 0 so its mostly in an idle state it does this. My hunch so far for this could be the Fuel Pressure Regulator because one time I started it up after forgetting to plug the vacuum line in for the FPR and it didnt seem to change how it ran.

BUT the one weird thing is every so often ill start it up and the engine rpms drop quite low, and seems to be on the verge of stalling. The cabin lights will also dim with this low pulsing rpm. Last time this happened I pushed the gas pedal and it eventually stabilized and didnt happen again, and furthermore driving this rig around doesnt seem to be an issue, just when its idling. For me it seems weird this would only happen intermittently if it was a bad FPR but im not an expert by any stretch.

Also the oxygen sensor data in the pic attached looked suspect to me because to my knowledge the .5-1v fluctuations are supposed to be pretty smooth?

Would love to hear some thoughts?

 

Your trims are basically all messed up. It should be in the 1-5% range at most. My 2002 is around 0% at all times.

So that means the ECU is trying really hard to fix a RICH condition. Question is what is causing it and only at idle?

Could be bad O2 sensors, fuel injectors, fuel regulator, IAC, MAF...stuck PCV valve...etc.

I'd start with cleaning the MAF and IAC. Make sure the air filter is new. Check the PCV valve.

It shouldn't be a vacuum leak, as that would make for a positive fuel trim (lean) condition. I think the MAF reading you have is correct but am unsure what the proper reading is at idle for a 3400.

 

Yea shouldve mentioned the work Ive done already

-Cleaned MAF
-Cleaned IAC + New Gasket
-Tested PVC and is functional
-Fresh Air Filter
-Fuel Pump and Fuel Filter have been replaced not long ago based on the paperwork that came with the vehicle.

At this point I suspect its either injectors or FPR but Oxygen sensor might be worth looking deeper into. Any good ways of testing for all these components?

 

Found this: His Sensor 1 looks similar to yours. Your sensor 2 doesn't look so hot. I'm thinking Sensor 2 is no good or the cat is bad. Sensor 2 should not be zero and it should maintain a fairly consistent value. From the description in that thread, it is said that the downstream O2 sensor is what drives a lot of the fuel trim (contrary to what the textbooks often say).

I'd replace sensor 2 with one from the dealer or at least Denso from a reputable source (not Amazon or Ebay).

 

Im not seeing that myself in that thread. Seems to be debatable how impactful sensor 2 is on fuel trims where sensor 1 definitely does the heavy lifting. People also seem to think his voltages looked good and they look similar to mine.
One note in there was that it basically will be at 0 volts until you lay on the gas which is what ive seen in mine. So maybe the sensors arent my problem ‍♂️

 

Not sure then. I do not have deep understanding of the O2 or A/F sensors. I thought the downstream (sensor 2) should have some voltage at all times and not be 0v.

One thing for free you can do is use an infrared thermometer to check the cat and make sure it is working. It should be hotter downstream.