October 10th, 2003:

truck still running rich

Last night, after work, I tested a few more parts on the Truck.

The TPS (Throttle Position Sensor) seems to be operating well. I couldn’t test it perfectly as I didn’t have any gauged feelers. But, it shows continuity when it should, and doesn’t when it shouldn’t.

I also tested the MAF (Mass Air Flow) Sensor. It also showed to be within specifications. There were no pictures to show me EXACTLY where to pull what, and I took the screws off the housing instead of just unsnapping the connector the first time. I was surprised when I pulled it out to see lots of large metal connectors that seems to just be crammed in there. I don’t know how these things work, but that is not what I expected to see. I did my best to gently cram them all back in and refit the screws. It drove afterward, so I don’t think I messed anything up. I hope not.

I tested the Oxygen Sensor as well. This one was a bit difficult to do as, apparently, my Volt Meter only samples once per second. However, the Oxygen Sensor is working properly when the voltage shifts from .25ish up to 0.75ish several times per second. I saw that it was fluctuating rapidly, but had no way of seeing if there were any irregularities in its pattern. I’d need an oscilloscope for that. However, when I open the throttle it would read very low and then go back to normal. When I quickly shut the throttle, it would read very high, and then back to normal. This is the way it’s supposed to work. The only thing I could do more is actually detach it from the ECU. By doing this it won’t be able to correct the fuel mixture and by shooting some propane into the intake, or by creating an intake leak I can force rich and lean conditions and make sure it responds appropriately. But that’s a lot of work to do when it seems to be working correctly.

I also, finally, managed to get the ECU to respond to my attempts to read it’s codes. It game me the "all systems normal" indication so there aren’t any codes to view.

I’m not sure what to do next and, to be honest, I’m a bit confused regarding the operation of the ECU. If the O2 sensor is the primary determiner for air/fuel mixture, then why do I even need a MAF sensor? Is this just for when the O2 sensor is broken or too cold to operate? It took a minute or so for my O2 sensor to warm up to operating temperatures. Before then, it showed almost a constant 0.02v. Perhaps the ECU is reading its output, even when it isn’t up to temperature? If this is the case, whenever the sensor cools down, the ECU would sense a rich condition. However, I notice the gas smell even when I know the sensor must be good and hot. So, this can’t really be it, I don’t think. So, if we assume that the ECU is reading the sensor right, and that the O2 sensor is functioning correctly… then how can the car possibly run rich? If the O2 sensor is the primary determiner for fuel mixture, I could have VERY leaky injectors and a HUGE intake leak and everything would still work itself out in the end, right? I mean, once the O2 sensor was hot enough to run, any mixture issues would be immediately corrected. At least, that’s my understanding.

The problems I’m having — rough idle, gas smell with windows open, hot exhaust according to Midas, lack of power, weird power surge around 2000 rpm… maybe — are strange. I notice the rough idle the most after the engine has warmed up enough to be out of the "cold idle" stage, but before it’s been really driven. Say 1-2 minutes after starting and driving the truck. The gas smell is most noticeable when I’m driving it hard, but can also be smelled after it’s been driven for a while and is now driving slowly. I don’t usually smell it much when idling. The lack of power seems to be constant, but that might simply be because the engine isn’t all that powerful. The odd power surge is something that isn’t reproducible and it just "happens" sometimes. And it isn’t really anything. I’ll be accelerating normally, and when I cross the 2000-2500rpm area, at some point in that range, the car will suddenly accelerate faster for a split second, and then it’s back to normal.

I don’t know what to do next. If you know people who know cars, please send them here to see if they can help out. It’s a ’91 Toyota 4Runner V6 (3VZ-E engine) with 76k on it. Thanks.