Hello everyone. I have a '96 4Runner SR5, 4wd, 3.4 v-6. I bought it used several months ago. It ran great and got 17 mpg in the city. Then something changed. I noticed that the sound of the engine would change after it got warmed up. It would go from a smooth whoosh when cold to a clattery tractor-like growl when warm. Then it became progressively less powerful and now gets less than 10 mpg. It still runs great for about a block after a cold start. It also runs better in cool, dry weather. One rainy day last week, it would barely move. It now idles at about 500-600 rpm when warm, and it surges and stumbles. It seems that it runs richer and richer as it warms up.

No CEL ever came on (it does work), and there were no stored trouble codes. A mechanic friend first thought it could be the torque converter, because we could only get the revs up to about 1500 rpm by power-braking it. We had it rebuilt. No change. We replaced the fuel filter, checked the fuel pressure, cleaned the MAF sensor, checked for vacuum leaks, checked the catalytic converter, etc., etc., etc. No joy. A major tuneup was done about a year ago, with new plugs and wires, timing belt, etc.

The only thing we can come up with now is to replace the O2 sensors. Is it possible for them to go bad without a CEL? I'm out of ideas at this point, and any thoughts will be greatly appreciated.

Thanks!

 

It could be possible, But check your wires for arcing, and check the plugs.

I had a similar problem, My wires were arcing and one plug the porcelyn was cracked.

 

Might as well check/replace the standard stuff:

Plugs
Plug Wires
MAF
Air Filter
TPS
O2 sensor

...and reset your ECU by pulling the EFI fuse under the hood. The fact that is has gradually gotten worse suggests to me the MAF is dirty/bad, might check there first.

 

I think im going to use this a quote in my sig you say it a lot. but 90% of the time it works

 

I have been doing it alot lately, but hey it's free and effective... The factory ECU isn't always the shiniest penny in the jar.

 

and that is the truth

 

I cleaned the MAF with CRC Electronics Cleaner twice. No difference. Is there a way to test it? Would it not trigger a CEL? I did reboot the computer. I did it the way you suggest after lurking and reading one of your posts. Thanks for that tip!

 

ruth

make sure you use denso plugs too... just thought id throw that out there.

if you do replace the o2 sensor ( i dont think thats your problem...) replace with only with a denso from toyota, dont use others

check if you have good spark too. might be the coils are going. have you checked timing?

 

change the PCV valve.


Also these fine people on this forum recommended me to trying seafoam. It helped. Pull the pcv valve...pull the valve out of the hose. Have someone to get in and crank it while giving half throttle. Then slowly pour the seafoam into the hose. Wait until it starts smoking out of the exhaust. kill it. Then crank it back up in about 10 min.


make sure all the vaccum lines are connected securely.

Could also be one of your coil packs getting hot and getting weak.


I dunno though.

 

Oh yeah...my o2 sensor went bad while only giving a CEL light which reported coolant system failure. The o2 sensor can go bad without giving the appropriate warning.

 

side notes...

Half throttle isn't required nor a good idea. Being under the hood with the engine running at 3000 RPM isn't very safe. Idle is fine... if the engine wants to die, stick your thumb over the hole until you can start pouring the Seafoam in.


You'll get better results if you let it sit longer. I suggest 2 hours as a minimum, overnight is the easiest thing to do.

 

my vote is for o2 sensor...they make dramatic statements in mpg when they fart out...

 

Yeah, that's why we went in that direction. The fuel mileage dropped by 50%.

 

Did you get a CEL?

 

I done what I had to do to get the seafoam in. I could cover the hose with my thumb and take it off and try to get the seamfoam in and it would die. What can I say? Putting your head under the hood with it running at all is just as dangerous.

Anywho. Good point on the seafoam. I didn't think of letting it sit longer. I was trying to hurry up and get it done. Might have had even better results if i done so.

 

The MF sensor is probably not the one that's bad. Replace the A/F sensor between the engine and the cat. They should be replaced every 70,000 miles at least. Not cheap but makes a big difference if it's bad or acting up. Mike

 

fwiw, that sensor on a 96 isn't a wideband A/F sensor, it's an O2 sensor. Still expensive, but not like an A/F on later rigs.

 

"It would go from a smooth whoosh when cold to a clattery tractor-like growl when warm"


Do you know how many miles on the timing belt? Almost sounds like the belt slipped a couple of teeth.

 

Just a thought, but have you tried taking an Ohm meter to the fuel injectors?

 

The timing belt was replaced a little over a year ago; it has about 15,000 miles on it. My mechanic checked it out and it was fine.
I'm not sure if he did that or not. I left it with him for a day and he checked everything he could think of.

I replaced the PCV valve and air filter today, and ran Seafoam through the PCV tube and dumped the rest into the crankcase. Definitely don't restart the vehicle inside the garage! I haven't seen a vehicle smoke that much since the rotor seals went out on my Mazda RX-7. I rebooted the comp again and it seems to be running better, but not right.

I ordered both O2 sensors from the Toyota dealer, along with plugs and wires. They'll go on tomorrow; my fingers are crossed.