1993 Toyota 4Runner Rough Idle + 'too lean' and 'too rich'

Engine Performance problem
1993 Toyota 4Runner 6 cyl Four Wheel Drive Manual 200K miles

I have a '93 4runner, 3VZ-E V6 with 4WD and 5-spd manual.

Some history:

Head gaskets were replaced 2X; the 3rd time they replaced the heads with 1997s. Ran great for years after.

I blew ANOTHER head gasket, replaced it (one side only) along with the knock sensor (they broke that off when working on the head) and also 2 fuel injectors (they said they were plugged when working on the head).

Current problem:

Recently it started running like the head gaskets were blown, extremely rough, no power, etc. Although sometimes (usually at freeway speeds) it'll suddenly start running fine. But:

1. No steam/antifreeze in the exhaust.
2. Good compression.
3. OBD codes are 'too rich' and 'too lean'.

After these symptoms started, I failed smog and replaced the EGR valve, smog pump, pre-cat O2 sensor, and the cat converter. I also found a plugged EGR hose and fixed it, passed smog.

Same symptoms.

I replaced the knock sensor (again) since someone suggested it might be defective and it was under warranty.

I gave it a tuneup (replaced plugs/wires/cap/rotor).

So here's what's been suggested:

a. Bad ignition module
b. Plugged fuel injector
c. Car is simply a piece of sh*t

Any ideas? I believe the rich+lean codes are the clue; but haven't been able to find anything to fix.
Do you
have the same problem?
Wednesday, February 4th, 2009 AT 6:06 AM

1 Reply

Hi sbus01,

Thank you for the donation and the history which is helpful to understand the situation.

There have been instances of such symptoms and the cause is a contaminated Mass Air Flow sensor. The contamination gives a false signal to the ECM thus the lean and rich condition.

This is quite a common phenomenon and a special cleaner was formulated to clean the sensor. You can try asking at your local parts outlet for the spray cleaner.
Was this
Wednesday, February 4th, 2009 AT 7:54 AM

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