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1994 Toyota 4Runner 3VZ-E unstable idle problems

I have a 1994 Toyota 4Runner 3VZ-E (V6) SR5 with 192K. Recently, it started having problems idling (cold or warm). Symptoms are loping idle from ~200rpm - 1000rpm. Problem comes and goes and does not seem related to the engine temperature. Other than that, the truck runs very well with a smooth acceleration and gas mileage in the low-mid 20s (remarkable for a 4Runner).

Jumping TE1/E1 on the diagnostic connector the ECM shows no codes whatsoever (continuous rapid flashes). I have replaced the the cap/rotor, ignitor, o2 sensor, scoped the MAF (called a VAF by Toyota), scoped the TPS (Throttle Position Sensor), checked the tension on the throttle cable, checked for loose or damaged vacuum lines, and completely cleaned the intake, all intake hoses to the throttle-body, and the throttle-body itself. I have also tried fuel system cleaners and water removers for the last 3 tankfuls with no change whatsoever to the symptoms.

Everything checks out but the problem persists. I have also taken it to 2 ASE-certified master mechanics and they are similarly stumped. So, my suspicion is this is still electrical, but I don’t know what else to check or replace (I have considered replacing the entire throttle-body). Also, in what is possibly a red-herring, these problems started immediately after taking it through a high-pressure automatic car wash. The attendant apparently left the vehicle idling during the wash cycle.

The vehicle is definitely drivable, but the problem worries me as this vehicle is my 9-month pregnant wife’s only transportation to the hospital at the moment.

The next time idle lopes, take the handle of a screwdriver and tap on the Idle Air Control valve. If the idle speed changes when doing this replace the IAC valve.

Tester

What system does this truck use for “cold starts”? Some use an extra fuel injector in the throttle body and perhaps this injector is not closing properly and dripping in some extra fuel. I would check out the cold start circuits and see if you can locate a problem in it.

RE Tester: I tried tapping the ICV (Idle Control Valve) with a wooden ended screwdriver and there was no change to the loping behavior. I also ran through the diagnostic algorithm in the Toyota shop manual and everything was to spec.

RE UncleTurbo: The 3VZ-E uses a cold start injector opposite the throttle body that outputs directly into the plenum of the manifold. It’s controlled by an injector timer switch. I ran the diagnostic algo on the injector itself, it scoped on the high side of normal but was still in spec. The injector timer switch also scoped out normal.

Thank you both for your suggestions. Do you think this could be a problem with the EGR system?

If you believe it could be the EGR, why don’t you disable it temporarily to see what happens.
It could be stuck partially open, though.

I’m having the same issue. It does not begin immediately upon start up. It happens after a minute or two during idle. However, mine eventually goes away once the vehicle warms up to operating temperature. I was assuming it’s a sensor, but I have no idea which one? The vehicle runs fine otherwise and gets “decent” gas mileage. But it’s an annoying issue.

ANY help or ideas would be great!

PROBLEM SOLVED!

Sorry for the tardy response. Took me a couple of days to get back to this. I ran through the diag algo for the EGR system and found the problem. The EGR vacuum control valve was bad. It was bleeding vacuum at idle causing periodic opening of the EGR valve, which as we all know can cause a rough and inconsistent idle. For the time being, I capped the control valve while waiting for the $30 replacement part from Toyota. All is well with the idle now, rock solid at 800rpm and purring like a kitten.

Thanks everyone for your comments and encouragement.

Glad to hear you nailed it.
Thanks for the update.
At least it’s not the EGR valve itself, which would have cost way more than $30!