briholt

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update to solve my fuel issue...
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I'm up to 18 mpg and am going through my posts to update.


Here is what I did:
1) though I wasn't getting any CEL warnings, I checked all computer diagnostic connects (02, tps, anything that conceivably would be related)
2) all codes checked out, I took off my intake plenum to clean it out and to remove the injectors.
3) sent the injectors to RC engineering to have them tested, cleaned and calibrated
4) cleaned the heck out of my plenum. There was about an 1/8 of black gunk through the plenum, including and especially the pipe that carries the exhaust from the EGR valve. that part was completely clogged.
5) cleaned the and tested the egr valve

when I put it all together, I had to shut down the idle screw, which was basically all the way open.


It's only been a week that I've been driving it, but the fuel consumption is about 18 mpg in seattle, minimal highway driving.



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original thread
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1991 22re delux, 70k on engine, 170k on the truck, 5spd, 31 tires, not sure of the gearing, getting 12.5 mpg.

Pretty sure it's the brakes not completely letting go because passenger caliper doesn't completely release, and I've rebuilt the caliper, but I'm also running rich (as evidenced from carbon on the tailpipe). So this thread is about checking the Throttle body, TPS, and the MAF.

I've changes the plugs (old were fine, though warn), plug wires, cap, rotor, I've also checked TPS and set the timing. I can't check the codes because the CEL light isn't working, though I was able to tap into the ECU with a lightbulb probe and I think I got the Knock sensor code. But I'm not sure.


So, I took a look at my throttle body and MAF. More on the MAF in a moment.

The throttle body looked clean, but just inside, not so much:

Clean Throttle body


Not so clean just inside



And here you can see the very back of the plenum, the part in focus, where it's pretty nasty. Anyone think I should assume the injectors need cleaning?




As for the MAF, well, I really wish that I had read all up and down about not taking out those two little screws; I think I diverted disaster because I didn't pull too hard and so as soon as there was resistance on the MAF connector, I stopped and put it back together.

When I checked the Ohm resistance All were in specs, I think. I've put them below.

Link to specs page for MAF (or VAF)

To see my measurements, I've created a table but can't format it here very well, so click this link to see it.

UPDATE: I also did 4crawler's dynamic test, and the maf seems to be working fine


I've got a digital ohm meter, and so, when I test the two test probes together, I basically get zero, which I take to mean that there is zero resistance. As for getting infinity, I get a vertical line on the left side of the display. Am I'm reading that right, that the vertical line means 'infinity?' If so, woo-hoo! MAF is in specs.

Of course, that means I need to figure out, still while I'm running rich.

I took a look at the knock sensor, just to see if was loose, or if the wires looked bad. They looked okay. I've reset the ECU and will give it some time to find any other codes. In the meantime, I'd like to know where the path of the Knock wiring harness goes because though I can see it at the knock sensor, it slips into a wiring plastic tube thing and from there, I can't see where it goes.

for now, I think it's my brake lines, or maybe the master cylinder. I'm getting a vacuum brake bleeder and will do a thorough bleeding soon.

myyota

If your brakes arn`t working properly then start there first, if there dragging that will kill your mileage, and the other thing killing your gas mileage is the 31'' tires, to help things out you need to regear. To clean the throttle body properly you need to remove it, just because it looks clean from the front doesn`t mean it is. What your seeing in the back of the plenum is carbon build up from the EGR, and if that is on your truck it would be a good idea to remove the upper half of the intake and clean it out. If your truck is running rich you might have a bad 02 sensor, if it never been changed them it would probably be a good idea, buy an OEM 02 sensor if you do replace it. Our trucks don`t have a MAF sensor, they have an AFM = air flow meter, there is a big diference between the two.

whokrz

The 31" tires don't kill mileage. With 4.10 gears 31x10.5 and a 1990 22re I averaged 20mpg.
If you decide to do the o2 sensor go to sparkplugs.com they have cheap denso compared to the dealership.
I would work on the brakes first as myyota stated. Normally if they are running rich enough to get 12mpg the exhaust will kinda lightly smoke and smell rich. Do you have this condition?

Kiroshu

Sure you getting 12.5 miles per gallon? Did you calculate correctly? You know your truck has a 17 gallon tank right? Did you Check the Air flow meter for proper resistance? If any of it is out of spec it needs replacing and will significantly reduce your trucks performance. Theres more than one check for the AFM. You hace to check for resistance with the flapper door open and close and among all the terminals. I believe any haynes manual has the steps. Got NGK plugs? The 31" tires will reduce the mileage a bit as well. Go back stock you will get a bit more. You said the MIL light is not working? How do you know? Is there a check engine light on with Key on engine off? If so then its working. O2 sensor? Denso or OEM toyota O2 sensor required for good performance. Vac leaks? Intake manifold leaks? Exhaust manifold leaks? If you have a fault knock sensor you will have performance issues sometimes a well. The computer will play advance and or retard timing when or if it detects a knock. Is the sensor plugged in? You should be able to check the knock sensor for resistance. If its out of spec replace it. If you brakes are dragging that is going to reduce it as well. Fix that first. In these days you never really rebuild calipers generally they are replaced totally. So theres a chance its not totally buttoned up good. I'd inspect and possibly replace with new one. Usually the core charge on them is high so it pays for alot back. Rear brakes dragging? Tire pressure? Right size tires all round? Alignment off badly. Dirty air filter. Fuel filter?

24Runna

For your gearing refer to THIS chart.

For your mileage fill up the tank all the way using the slowest setting, drive about 50 miles like you normally drive, fill up again on the slowest setting, divide gallons filled by miles driven. Now this also is assuming your mileage is correct, but it might considering you have 31's.

Now onto the fun part.
What brand of wires, cap & rotor, plugs did you use?
New airfilter?
What o2 are you using, old or new? (I found that simply having an o2 sensor connected with little or no flow will NOT trigger a CEL) Use sparkplugs.com like Whokrz said.
Vacuum leaks? (Test with butane)
Other brakes dragging?
Leaks in the intake or exhaust?
Tires properly inflated?

For the VAFM, did you take the top cover off and test the resistance then? Like Kiroshu said there a few other test to run.

Good Luck!

whokrz

There is a trouble shooting guide in the FSM. Sounds like you may have already come across it but just in case http://ncttora.com/fsm/1993/engine/3troubles.pdf
and the index http://ncttora.com/fsm/1993/engine.html
Hope you get your milage back up.

shaeff

If you've got a knock sensor code, that'll kill your mileage right there. When you get that, the ECU retards timing and enriches fuel mixture in order to prevent detonation/pre-ignition.

Fix the knock sensor issue. It's usually the wiring that's bad, so it should be an easy fix.

briholt

thanks for all the replies.
I should have said that I've already replaced the o2 sensor (denso) with no change.

briholt

smells rich, doesn't smoke.

briholt

No vac leaks as tested by spraying carb cleaner around the vac hoses. Fuel filter, is a good question. I haven't checked it because the truck runs very strong, no hesitation, no intermittent firing.

briholt

good link, thank you.

for what it's worth fellas, I know how to calculate MPG. the word 'per' means 'divided by'.

I never said whether my speedo was off, and so that's a really good point that I should have said the following: When I drive by a radar driven speed sign, the kind that reflects back your current speed, it matches my speedo exactly.

Is that not a good indicator that the odo and speedo are working accurately?

Of course, another way would be fore me to just reset the odo and drive a known distance. I'm happy to try that.


wires and plugs are ngk
cap and rotor OEM
no new air filter, but It's pretty clean. I might upgrade to a K&N
O2 sensor is new and a denso
no vac leaks
No leaks in the intake or exhaust that I can find
tires are inflated, though I should check them again.
regarding the brakes, the tire doesn't fully rotate around in one full circle after jacking it up. I think that given that I onlyl drive short distances, it seems like the most resistance that a dragging brake would have is on initial starting from stop, or going up inclines. I do a lot of each.




I didn't take the cover off as it's not necessary to test the resistance to do so. You just unplug the connector (NOT those little two screws) and you can check the sensor there.

Also, as I said the post (I'm not sure if Kiroshu read it that closely--it's cool, sometimes that happens) I did both the static test and the dynamic test.

briholt

Right. This is one that I'm wanting to confirm. Since the CEL is out of commission, I have to go an extra step and expose the ECU under the kick pannel and stick a probe into the correct wire. It's a hassle, so I try avoiding it.

But upon inspecting the Knock sensor (in the block) it looks clean and tight, as does the connector and the wire. But here is where I could use some help: Where does the wire travel from the knock sensor? Mine goes up under the plenum into a bundle of wires that are covered and it's hard to see where it goes. In other words, where is the other end of the knock sensor connect to?

shaeff

The knock sensor wire is coax. It has an outer shield that's grounded, and the inner signal wire goes to the ECU. Generally, Toyota uses a clear insulation on the wire for the knock sensors, but I've never messed with it on these trucks, so don't take that as gospel. As I said, it's usually the wiring that fails and causes the signal wire to short to ground, thus, giving you code 52.

briholt

Interesting that it's a clear insulation. Mine is stone black. (not grime black).

i know the engine was rebuilt 70K ago, perhaps they had a better sheilded wire.

If it goes to teh ECU, should it not have it's own connector to disconnect, just to test if it works or not?

thanks for the reply.

briholt

Gearing is 4.100, at least according to that chart and the stamp on the inside of my door: G292.

I took a 1.6 mile trip according to my odometer, and when I checked google maps to see if the distance was the same 1.6, it sure was.

So I'm thinking that even though I have 31's my speedometer is working properly. i'm assuming that the mechanism that keeps track of speed as well as keeping track of distance is the same.

shaeff

My apologies for not being more specific. In the engine bay it's always covered, and you'd never tell it's clear. Up near the ECU, it should be clear. It won't have it's very own connector, it should be grouped in with a number of other wires on a large connector.

briholt

Thanks Shaef. but I thought it was an easy swap of the wiring for the knock sensor. Swapping what you described seems like you'd have to disconnect a bunch of other components. I must be misunderstanding something. maybe it's not such a big deal to track down all those other components?

shaeff

Generally, you won't be swapping out the wire, per se, but totally replacing it. When you totally replace the wire, (with coax) you can route it any way you please, and splice it in near the ECU connector. I wouldn't try to remove the wire from the loom, that would be tedious and take forever!

briholt

Got it and thanks!

briholt

turns out that I was wrong in calculating my MPG. I was wrong because I assumed that driving by a radar driven speed sign would be accurate enough; it's not. since 31" tires only change you mpg by 10%. They probably aren't sensitive enough to detect difference at that speed.


But that still doesn't explain the poor mileage, and I think I might have found a significant issue: the Previous Owner (PO) didn't put the aisin hub back together correctly on the passenger side.

The issue was that they didn't thread the clutch gear with the spring, as seen in this here pic borrowed from ih9mud. the spring was just sitting there:






I'm going to go fuel up and to start a new calculation of mpg.