Just bought a 94 4Runner with a 3vze for $500, couldn’t pass up that deal. It idles rough but sounds fine after 1500. The previous owner cut the catalytic converter out of the exhaust to “make it sound better”. But he cut it before the downstream O2 sensor. It’s not throwing a code so I’m assuming it’s still picking up some fumes but very little. So is that the problem? If not let me know what it could be.

 

Not having that O2 sensor could definitely cause a rough idle. I believe It helps set the fuel mixture, so if it's not getting the proper reading, could cause a rough idle/running condition.

Others may know better than me though.

 

3.slow21 is saying, I think, that the downstream O2 sensor is missing. (But then he says "it" is picking up some fumes, so I don't know for sure what he is trying to say)

The downstream O2 sensor is there to monitor the effectiveness of the catalytic converter; when ineffective the ECM throws code 27, Butchering the exhaust system can certainly cause a rough idle, but it has nothing to do with the downstream sensor.

Your seller picked $500 for a reason. Maybe he liked you, maybe he was trying to unload a vehicle he couldn't fix. Rough idle can be due to an almost infinite number of causes, but I'd start with verifying ignition timing and plug gap, and eliminating any possible vacuum leaks.

 

Ah, yeah missed the downstream part, sorry!

Like I said, others probably know better than me!

 

I really appreciate the the reply, I’m new to these 3.0’s and car ownership all together. I have a couple of questions. If it threw a code 27 would the check engine light turn on? And if not how do I determine it’s a code 27? The owner stated it ran just fine before he butchered the exhaust system. He’s a close friend so I assume he’s telling the truth. Also, a previous 3.0 owner said it may be a burnt exhaust valve on cylinder six since the idle is rough like that.

 

If he cut out the converter what else did he remove? Does it still has all the factory vac tubing and valves ( egr, afm, etc)? I would get the FSM and start going thru all the systems to confirm correct operation. It is time consuming but that is how I would go at it. Good luck.

 

Downstream O2 sensors are usually only found on California vehicles (you don't state where you are). If there was a code 27 (or any other code), it would light the CEL (check engine light). You read codes by jumpering two pins in the diagnostic connector. A little searching on this site will turn up an easy description of how that's done. So first: when you turn the key, does the CEL come on? If not, the bulb might be burned out (or removed), and you'll never know that you have a code.

Sure, a burned valve on #6 could cause rough idle. Or any other cylinder. That's one of the "infinite" possibilities. If the PO knows (a lot of) something that I don't know, he might be able to make that diagnosis. Otherwise, it's a WAG. If you want to avoid just throwing money at this, you need to get some gear (like a timing light). Or, if all of this is new to you, you might take it to someone (usually called a mechanic) who will go through how she would diagnose it. You have to pay her, but there's no shame in paying for education.

 

I’m not super sure it’s the downstream sensor, but only the cat was cut out of the system, after a sensor on the exhaust and before another. The CEL does turn on when the key is turned. It isn’t on while running. I’m also located in Georgia, USA. But still don’t know the origin of the vehicle.

 

Used exhaust clamps to put back together the cat into the system. Runs not as rough but still idles low. Also when accelerating sometimes. It has hardly no power. And when after revving to about 3500 or so it dies.

ANOTHER UPDATE 6/10/19
replaced spark plugs and wires and she runs fine!!! Thanks to everybody that have advice!