GorgeRunner

I've been having a real challenge figuring out why my 90 Toyota pickup is getting such poor gas mileage. I've been testing and replacing potential problems, starting from the cheapest and working my way up, but haven't had much success. 22RE engine is tuned and timed properly, new spark plug wires, thermostat, air filter, O2 sensor and gas cap. Checked for vacuum leaks. No check engine light and motor seems to be running fine, except for the 25% drop in fuel mileage. I've been keeping track of the fuel mileage and it seems to be very slowly getting worse.

I just tested the cold start injector time switch. According to the specs, the STA-STJ readings are well within spec for a cold and hot engine. The STA-Ground is not. It should have 30-90 Ω, but it shows only .4 Ω, which is about a dead short. I assume the switch is defective but I want to be sure because it's not cheap, over $100, so I'm asking for any input or additional information about it before I fork out the cash for a new one. Any other tests or procedures I can do to make sure the switch is bad?

Thanks in advance, all advice is greatly appreciated.

scope103

If you were worried about the Timer Switch affecting mileage, just disconnect the CSI and drive it that way for a tankful. The injector can't switch on if it's disconnected. (That doesn't rule out a LEAKY CSI, but that's even easier to check. Just pull the injector (always leave the fuel line connected, or you will have to replace the crush washers), and start the fuel pump with FP to B+. Does it leak?)

I would be really surprised if STA-Ground was that low. If it was, it would short the starter relay to ground and the starter wouldn't turn. STA-Ground is the internal heater, so it is POSSIBLE it might short, but as soon as the starter relay closed it would go POOF.

GorgeRunner

The timer switch is installed opposite(upside down) from the diagrams so I had the terminals reversed. The switch appears to be within spec, although the STA-Ground and STA-STJ readouts are identical(when engine is cold), about 40Ω, I don't know if that's significant or not . I want to check the cold start injector for leaks but I think I may get a couple gaskets before I remove it. The resistance check of the cold start injector was within spec.

While I had the multimeter out, I checked the VAF. Everything seems to be within specifications, but when I moved the VAF measuring plate, it went from 350Ω closed to 650Ω fully open, which is within spec, but between open and closed, I got some 650kΩ readings as I was moving the measuring plate, not sure what that means. I guess it's OK, I haven't worked on enough of them to know for sure. All I can think of now, if the cold start injector is OK, is the TPS, or maybe one of the fuel injectors is leaking.

This problem is starting to drive me crazy, so thanks for your input, I really appreciate it.

RAD4Runner

Like Scope says ^^^
So you're saying that NOW STJ to GND is 0.4 ohms? Unfortunately, Toyota does not have spec for this ore critical reading. However, low resistance when cold is good, but resistance should go back up when hot.
IF resistance stays low when hot you could get too much fuel in intake manifold during prolonged cranking.

GorgeRunner

STJ-Ground is about .4Ω. If I touch the multimeter leads together, I get .1Ω, so that would be within a margin of error. There are no specs for STJ-Ground, because the sensor is 180° from the shop manual diagrams, I was measuring the wrong post. I assume the STJ-Ground reading of .4Ω shows that the STJ terminal is a ground connection, temperature should not have much of an impact.

I will probably check the sensor again when the engine is warm to make sure my readings are correct. I'll be having a look at the TPS, so the meter will be available.

scope103

STJ grounds the CSI to turn it on. YOUR STJ to GND reads as a short because it is. In your case, the switch is cold enough that you will get CSI when you crank it.

You could test it some more by putting 12v on STA. After a few seconds, STJ will open (preventing the CSI from flooding the engine during prolonged cranking, no matter how cold it is.)

It sounds like it's still in the water bypass (where YOU should leave it), but if it were out you could do more testing! Put the switch in hot water (how hot? I don't know; there's no spec.) Once warm, STJ should open (no CSI at all on a warm engine).

RAD4Runner

STJ-GND is temperature dependent.
Pls read my write-up on how CSI system works.
STJ-to-GND should be low (0.4 is good). This will provide ground to negative side of CSI, to inject added fuel when starting cold.
However, when hot STJ-GND should go high to disable CSI, so hot-start does not get too rich.
IF STJ-GND stays low regardless of temp, it is bad.

RAD4Runner

STJ-GND is temperature dependent.
Pls read my write-up on how CSI system works.
STJ-to-GND should be low (0.4 is good). This will provide ground to negative side of CSI, to inject added fuel when starting cold.
However, when hot STJ-GND should go high to disable CSI, so hot-start does not get too rich.
IF STJ-GND stays low regardless of temp, it is bad.

Like Scope says^^^, test it off the engine. Also see my write-up on test setup.

scope103

Actually, I said to "leave it there." If GorgeRunner's concern is poor mileage, the CSI Timer is unlikely to be the cause of it, and there are much easier ways to test whether the CSI is contributing to the problem. The Timer is wedged into a tiny space, and unless you remove a lot of "stuff" the chance of breaking something (the TVV comes to mind) while removing the Timer switch is pretty high.

If his problem was hard-starting, that would be a different issue, but there are still easier tests to do before digging the switch out of the water bypass.

GorgeRunner

The timer switch is still on the engine, but I did a test while the engine was at operating temperature. STA-STJ is 80.7Ω and STA-Ground is 79Ω when hot, STA-STJ and STA-Ground are 40Ω when cold. All within spec. I did not test the STJ-Ground when hot, but after seeing what you've said about it, I will test it tomorrow along with the cold start injector to see if it is leaking. I checked the TPS today and it looked to be a bit out of adjustment coming out of idle, it tests out OK. I don't think that small adjustment is worth 25% of my gas mileage, but I have to look at all the options.

I still can't believe that an engine that is losing 25% of it's fuel efficiency is not kicking out some kind of code. The CEL came on for a bad O2 sensor without any decrease in mileage and when I lived back in California, this truck, no more than five years old, failed emissions because of a bad gas cap.

Thanks for the information about the STJ-Ground, I'm going to check it out. I did have the two coolant sensors out when I cleaned them about a month ago but I did not test them while they were out.

RAD4Runner

Sorry, I was referring to when Scope said "if it were out you could do more testing..."

Agree, too that CSI will not affect gas mileage unless it's leaking (again, there's a test for this in FSM).

GorgeRunner

I'm going to check the CSI for leaks today.

I'm having trouble pinpointing the source of the poor mileage. I've checked all the obvious causes, at this point, I'm pretty much checking everything. Starting with the most likely problems and working my way down. I don't have many options left.

GorgeRunner

I checked the CSI, it's fine.

I'm not sure what to check next. Checked for fuel and vacuum leaks. Tuneup and timing, new plug wires, O2 sensor, gas cap and air filter. Tested the AFM, coolant temp sensors, TPS and CSI and they are all within spec. Fuel filter replaced 20K ago. The air intake is a little dirty, but no lines are clogged and the throttle plate moves freely. Took the idle speed screw out and cleaned it. With a 25% drop in mileage, I figured that it would be fairly obvious what was wrong, but I guess not. At least the truck is driveable, but it bugs me that it's not running right.

RAD4Runner

Just wondering...
If you have a manual locking hub, did you verify that it is indeed DIS-engaging hub from axles?
In once watched my friend's truck run and saw that his front axle was turning even when he was in 2WD and locking hub handle was in "Unlock" position.

GorgeRunner

It's a 2WD truck, but I did check to see if the emergency brake was dragging and that the front and rear wheels were turning freely.