BajaRunner

So I have had my 1991 3.0L 4x4 for about 3 weeks now and have averaged about 11mpg with 80% highway and 20% stop and go driving. I have 33" tires on stock gears. But, I thought it seemed a bit low still. This weekend I was finally able to spend a little time on it and checking some things.

First thing I did was check for stored codes. It had code 24 and 31 stored. Both VAFM related. I went through the 1993 FSM I have and checked the resistance of the VAFM meter. Checked out just fine, so I cleared the ECM to see if it came back. Drove it around and nope. The connector has a chipped edge right where the snap wire holds it in place, so it might have come loose at some point.

My main question is this... Will stored codes force the ECM to run in OPEN (edited)loop mode? (Not run from sensor feedback). Or does the ECM only run in that mode when the CEL is ON?

The air filter was dirty as heck, so I ordered a new Toyota one. The cap and rotor both looked like thy were on their last legs, and I didn't have time to pull a plug and check to see their condition, but I'm assuming they need to be replaced as well. Maybe I'll get lucky and all it needs is a tune up.

Lastly, the FSM states that the O2 sensor should read 5.1-6.0 ohms (B+ & HC) (sorry, doing this from memory as I dont have one in front of me). However, I measured the O2 resistance at 3.5ohm. The sensor looked new, like within a few months old. I didn't see a Denso stamp on it, or any stamp on it though... So not sure if I should pull it and put in a Denso or not.

Also, if anyone has some manual hubs they'd like to sell, please let me know. I will buy ASAP. Thanks.

scope103

"Closed Loop" is when the sensors (primarily, the O2 Sensor) are giving feedback to the ECM ("in a closed loop") to adjust the injector open-time to adjust the mixture to adjust the amount of un-used oxygen in the exhaust stream. Open loop is when the ECM doesn't have the input from the sensors (mainly, the O2 Sensor is not hot enough yet to produce a signal), and the ECM is setting the mixture "in the blind." Generally, this is only for a minute or two. Closed loop is what you want.

If the O2 sensor failed (code 25 or 26, sometimes 21), the ECM can only used Open Loop. And since it is setting mixture only approximately, bad mileage results.

The VAF is primarily for setting mixture in Open Loop mode (without knowing what's going on in the exhaust, the ECM uses the air flow and temperature to set the injector on-time). SO: codes 24 and 31 are primarily a concern for that minute or two when you're running open loop.

The O2 sensor spec. you give (5.1 to 6.3ohms) is for the heater inside the sensor (which is used to get it up to temp. faster). Your meter isn't really accurate enough to distinguish 3 ohms from 5 ohms (the leads alone probably have about 1.5 ohms resistance), so I wouldn't worry about it. As long as you're not getting a short (less than 1 or 1.5 ohms) or open (more than 50 ohms), the worst that could happen is that it would take another minute to get to Closed Loop.

And 11mpg isn't too good. But I can't comment on your tire size and gearing.

BajaRunner

Wow, thanks for that post. Very imformative.

Hmm, yeah, looks like I need to look elsewhere. I think I can probably gain a few MPG by doing a tuneup (Plugs, cap, rotor, filters). I checked timing, it is right at 10° BTDC and steady. I hooked my smoke machine up to the intake for a good 10 minutes and not a single leak I could find.

Engine runs and pulls strong. Sounds really good too. Makes it tough to really track anything down.

MudHippy

You should test your meter by touching the leads together. A "good" meter will read 0Ω. If it doesn't try twisting the leads at the meter ends to see if you can get the lead plugs to connect better. I always do this with my Fluke to make sure I can get a good zero before using it to check ohms.

But being as you're getting a reading that's too low, not too high, it shouldn't make any difference. Bad connections between the leads and the meter will increase resistance, not lower it. IOW, if you're sure you're getting that reading at the specified temp, I'd trust it. And be doing as the FSM advises by replacing the sensor before doing any further troubleshooting. Regardless of a code. The ECU only throws the code for a voltage malfunction(OX circuit), or an open or short(HT circuit), and doesn't take into account the resistance in the heater circuit. Which is why you're supposed to check it with a meter.

And yes, fail-safe mode "should" trigger the CEL.

BTW, taller tires + higher gearing = better gas mileage (at the expense of torque and HP at the wheels). So it's not that.

BajaRunner

Well to be honest, I have a sub $10 meter that I've used for like 10 years. HA. I didn't even think about the fact that a resolution of 1 ohm is extremely small. It probably is the meter. Maybe I'll go borrow a friends and give it a shot.

If the O2 was a Denso, it would have a "DENSO" on it someplace right?

Also, yes, I didn't incorporate the size difference of the tires. I'm up to about 13mpg with that. I still am shooting for around 16-18 though if possible. What is the life on these fuel filters? Probably should go ahead and change it, but I feel that wouldn't really effect MPG.

MudHippy

I've got a couple that I believe are stock/OEM. They both look identical. Neither has any brand markings.

On the fuel filter, Toyota says:
From page 7 https://web.archive.org/web/20100114.../forms/h22.pdf

I hadn't read that before I replaced the one on my first 3VZ-E. But I don't think it would have stopped me anyway. And I'm not entirely sure I even believe it now. Though it made no noticeable difference when I did. I did pour a bunch of rusty water out of it though. Probably around ¼-½ a cup worth I'd say. How much more it could have held of that without causing problems is a good question. It had ~205K miles on it, and quite obviously had never been touched prior.