So,

My truck (88 4runner, 22re, 5 speed) pops up the lean condition check engine code only when I hit highway speeds. Never, not once, while driving around town. It pretty much always does this within 20 minutes or less of hitting 55mph or higher.

Timing, valve clearance and other tuneup stuff has been done with no success.

Otherwise it drives great - never overheats, plenty of power, gets close to 20mpg on the highway which seems awesome for 33's.

Thoughts? Why only at 55 and up?

Thanks for any help!

-andy

 

What code is it throwing?

 

25 air-fuel ratio lean

 

When was the o2 sensor last replaced?

 

I have no idea on that one. I've owned it since March. Certainly wouldn't be a bad idea to replace it. Why would that only trigger at highway speeds?

 

my truck is doing the same thing. 88 pickup. let me know if you figure it out.

 

Might take a look at the plugs, see if it really is running lean (white deposits, worn, rounded electrodes). If it is, the O2 sensor is probably working.

Lean can be caused by clogged injectors. An injector cleaner with polyether amine will be the most effective of the in-the-tank types - Red Line SI-1 Complete Fuel System Cleaner has the most PEA, CRCs Guaranteed to Pass Emissions Test Formula is next best. If the injectors are really clogged, that might not work. You can have a shop run the concentrated stuff into the fuel rails - that will work better than the in-the-tank stuff. For the best results, send the injectors to witchhunter.

Low fuel pressure can also cause code 25 (clogged filter?), as can an exhaust leak (cracked exhaust manifold?).

 

How the o2 sensor works has something to do with the heat cycle and it's expecting to see a certain voltage. If it's bad and out of spec (when the car is warmed up and you are accelerating) it's going to throw a code.

 

sb5walker thanks for the info. what about a seafoam treatment? I have heard good things about it but would it help in this situation?

 

Seafoam is good at removing gunk from the intake, and will clean some loose carbon from piston heads & valves. The symptom of those problems is mainly rough running, especially at idle. Seafoam is not a very effective injector cleaner - not in the same league as PEA.

EDIT: Just thought of another possible cause of a lean code. The signal sent by the O2 sensor is very low voltage - only .45 V when mixture is perfect (stoichiometric - neither too rich nor too lean). Voltage is higher when mixture is too rich - up to almost 1 V; when lean, the sensor volts is low, down to about .1 V. In an older vehicle such as ours, wires and connectors can get corroded and develop resistance, which will degrade the signal (lower the voltage). With such low voltages, it wouldn't take much to lower the volts received by the computer from the O2 sensor, so it's possible that even if the sensor is sending a normal or rich signal, the computer receives a lean signal (below .45 V). So if plugs indicate that the vehicle is not running lean, check and clean up the O2 sensor connector and see if that helps.

 

Good thinking I will try to check it out tomorrow if i have time. Thanks again