Gamefreakgc

Hey all,

I've been getting bad gas milage on my 91 Toyota Pickup (no surprise). It has a 3VZ-E engine. I know it doesn't get good gas mileage but mine is plain terrible. Was getting 14 MPG highway so I figured the money I put into fixing it would pay itself off and then some.

Recently, I got a muffler job and put a Magnaflow straight-through exhaust on with 2 1/4 in piping on. MPG went from 14 to 17 . However, when they were done I got error codes 25 and 41: engine running lean and TPS. The TPS was from me, just put a new one on after my old one went bad (thanks to those who helped me on that one). I didn't reset the codes from the TPS fix so not worried about that one. I've checked the sensor with my ohm meter and works great, no issues there. It's 25 that's got me wondering, the lean air/fuel ratio. The mechanic read off the diagnostics for me which I remember him mentioning the Mass Airflow Sensor. He said the engine is running lean and pouring on gas to compensate (you can smell gas coming from exhaust).

Anyone have any suggestions on where to go from here? The solutions I've seen so far has been the MAF sensor or a possible issue with the ECU. I don't even know where my ECU is. I've replaced the TPS and O2 sensors. I'd rather not just buy a bunch of sensors only to find that's not the problem, some are $200+, but then again I'd really like to save some money at the pump, especially when unburned gas is coming out the exhaust. Any ideas?

MudHippy

1. You don't have a Mass Air Flow sensor. You have a Volume Air Flow Meter.

2. The smell of unburned fuel coming from the exhaust, IF ANYTHING, means the engine is running RICH. To say that's caused by the engine running lean is grade A(as in Asinine) stupidity. But it could also be because you removed the catalytic convertor from the system(which I assume you didn't...but maybe you did). Anyway, I wouldn't worry about it right now. As it has nothing to do with code 25.

3. The most likely reason you would get a code 25(Air–Fuel Ratio Lean Malfunction) after installing an aftermarket exhaust is because you have have created a leak in the system somewhere before the O2 sensor that needs to be fixed. Meaning, there's fresh air being drawn into, and mixing with/diluting, the exhaust gasses before reaching the O2 sensor, thereby causing it to sense that the exhaust gasses are "too lean". Which would most likely be caused by an exhaust gasket leaking(such as the gasket for the O2 sensor itself). But it could also be caused by a variety of different things. Namely:

http://personal.utulsa.edu/~nathan-b...85diagnosi.pdf

Gamefreakgc

Yes, I realize that. I was expecting to pull a code 26. The muffler shop said the ECU was overcompensating by pouring on the gas. Cat was not touched, only piping and muffler (it's new, only 10k miles).

Maybe I should clarify a bit more. I've had the same issue before and after the exhaust job, so I don't think they messed anything up. Prior to replacing the O2 sensor it ran rich (another shop said the same thing), afterwards it cleared another CEL but still ran rich. Two mechanics at different shops have confirmed there's too much gas in the exhaust. Changed fuel filter a while back as well as poured an injector cleaner in the tank a few months ago.

TNRabbit

1. How long since the O2 sensor was replaced?

2. Check for vacuum leaks. BEST way is with a propane or MAPP gas torch (UNLIT!!!); pass it around ALL vacuum hoses at idle & listen to a very slight idle up; that will reveal any leak. It's the only way to really tell because you can't see every square inch of the MILES of vacuum hose on the 3vze.

Also check the intake hose for cracks. They're famous for unnoticed cracks in the recesses of the accordion sections.

ANY unmetered air leak can cause a lean burn condition that the O2 sensor picks up. If you have over 100,000 miles on the O2, replace it.

Gamefreakgc

1. 2 months, about 2k miles ago.

2. I don't have a torch... is there any other way to test it? Last time it was in the shop (for other issues...) they replaced two bad vacuum hoses. I've replaced two on my own. I've tried examining them all and if questionable, I replace it.

Just had the intake hose off for the TPS fix. It's on there snug with no cracks that I can see. How big does a crack need to be to make a difference? It was sound as far as my untrained eyes can tell. Not a big deal to take it off again to check.

I should say that cylinder 6 has bad compression (110 compared to 160~ on the rest), common 3VZ-E issue. I could see how that could possibly bring unburned gas into the exhaust but doesn't explain why it's running rich in the first place.

scope103

If air is leaking into the exhaust, the O2 sensor senses the "extra" oxygen and signals that the engine is running lean (when it isn't). Trying to fix that, the ECM adds more fuel, making it even richer (giving the gas smell). But the O2 sensor is still seeing the oxygen leaked in after the cylinder, and continues to signal "lean." The ECM pushes it as far as it can trying to fix the (non-existent) lean condition, gives up, and throws the code 25 (not 26).

A vacuum leak can give a true lean condition (air is added after the VAF, the fuel is not set correctly for the actual air), but a true lean shouldn't smell like gas.

For an easy first test, get a meter with a bar-graph (or an old fashioned needle meter that will read down to 1v) and monitor your o2 sensor at the Ox1 diagnostic port. You should see the voltage switch about 8 times in 10 seconds. If you don't, the o2 sensor is not working, or you are really, really lean.

Gamefreakgc

All I have is an electronic multi meter (digital display), but it does read volts.

So basically, if cylinder 6 is alowing oxygen into the exhaust, I'll need a engine rebuilt or replacement? Or is there a way to override the O2 settings?

Anything else I can check first to help narrow the causes?

Gamefreakgc

Not to bring back a thread from the dead, but I'm working on it today and took the VAF off.

*EDIT*

Never mind, I steadied my hand and checked it again. VAF is working just fine.

Also, to follow up, I don't have a blowtorch so couldn't check for air leaks. I do not suspect any issues with the O2 sensor since replacing it a few months back did not make a difference (has new seal).

I'm going to do the FSM Heater Oxygen Sensor check, anyone know what a PCV hose is? I'll need to disconnect it at some point but I don't know which one it is.

plattey

Hi, new here and have been reading around and found this thread. Gamefreakgc, I have an '89 with the same poor mpg problem and it gets consistently 14mpg at highway speed. Just blipping the throttle in the driveway, I can see black smoke come out the exhaust (too rich). To answer your question, the pcv hose is on the driver's side valve cover and taps into the large intake hose just before the throttle body. It vents oil vapors from the engine into the intake.
Sounds like there's a laundry list of things that can cause the overly rich condition. I'll update the thread with what I've done and the results.

TNRabbit

PCV is NOT on the driver's side; it's under the plenum on the passenger side.

plattey

Ooops, I apologize.

Gamefreakgc

Thanks for the help guys, I'll take a look at it. I'm still yet to do the diagnosis, it's a two man job.

It's not running as rich as yours plattey, since I don't have any smoke. It seems to be doing a bit better since I cleaned out the VAF sensor, less of a "gas" smell coming from the exhaust when the engine is warmed up. Still smells like gas when it's cold. The CEL is back on again so I'll be working on it again soon.

Gamefreakgc

I don't know if anyone cares but here's what I've done so far (drives me nuts when I search for something and find someone with the same problem, only to find they never posted how they fixed it!):

I've done the CEL 25/26 diagnosis as outlined in the FSM. Nothing came up as defective (measured voltage, was consistently around 8 or more per 10 seconds). Didn't need to remove PCV hose since everything was working as specificed. VAF is good, measured circuits. O2 sensor is new. Going to check the resistance on the Engine Coolant Temperature sensor once it's cold to see if that's the culprit. I may replace anyway... it's cheap and I still have the original one on, 279k miles later. Haven't found an issue yet but haven't checked everything yet either.

btu44

You may want to try this procedure:

http://www.autoshop101.com/forms/h27.pdf

Go to page 14 and try: To Use the VF Terminal to Confirm Air/Fuel Ratio.

Actually there's alot of good info on this site thats not in the FSM.

Gamefreakgc

Thanks btu44, will try that.

Upon further inspection, I've noticed that the truck runs rich when the engine is COLD. Once it warms up, it runs normal (as normal as my nose can tell!). Going to check some things tonight.

UPDATED: Noticed that the bindings on the air intake hose from filter to throttle body was slightly loose. It had been there so long, grooves had formed in the plastic and made a weak seal. Changed the position for a more snug fit.

Also did a "mod" on the air intake by removing the plastic screen from behind the headlight and the pipe that leads to the air chamber as recommended by some users here. Hopeing that will increase air intake. Will have to wait until the weekend to check the coolant sensor since it got too dark for me to check it before it was cool. Will also check the Air/Fuel ratio using the link you provided btu44 then.

CaratFive

This thread is the story of my life right now. Chasing that code 25.

sub 5k mi o2 sensor

New vacuum lines

almost dies on me when idling (have to give it gas)

Did a headgasket job, but problem existed prior to it.

plattey

Quick update: Checked o2 sensor voltage at diagnostic port and voltage was changing like someone said it should here on the forum. Noticed an exhaust leak at the o2 sensor bung and fixed that. Air'd up the tires to rated psi and got 15.8mpg with a mix of hwy and interstate driving....that's just about average for these trucks huh? Also forgot to mention the PO put a cherry bomb on it.

Gamefreakgc

Check the Throttle Position Sensor. Mine was busted but didn't show the TPS check engine light. It controls your idle, mine would fall to 400 RPM and sputter real bad. You'll need an ohmeter and a FSM to do it right since it's a $100 part to replace, and just putting a new one on doesn't fix the issue.

Plattey, how did you find the leak? Was it real obvious?

*UPDATE*

Checked the engine coolant temerature sensor (ECT, the one that goes to the ECM) when I noticed it was loose. To my surprise, the "guts" of the sensor just pulled right out leaving an empty shell! Guess that's my problem! Someone must have messed it up real bad, it is so damaged I can't get it out. Will try with more time tomorrow.

plattey

yes, the leak was very obvious: carbon color around the leak, puffing noise coming from the blown o2 sensor gasket, and when I put my had close to it, I could feel the exhaust gas blowing out of the leak.

CaratFive

yeah, that sounds about familiar. :/

FSM and OHMetere ready to get to work. I'll report back soon.