Lankan

Hey guys, I'm new to this forum as I just bought a 94 Runner about two weeks ago. Having some issues with getting the smog passed any help would be much appreciated! Here's a bit of history on the problem:

1) After 1st smog test: The technician said that there's a "lean condition", and it could be any of the following: EGR system (he said it's most likely this), incorrect timing (at the time this couldn't be checked as my lower timing cover was not there), or finally the cat. I had passed "HC" & "CO" with flying colors, but "NO" was 709 (max allowed is 356). I started by installing a lower timing cover, having the timing set to exactly 10*BTDC.

2) 2nd smog test: "NO" reading was 471, so failed again. This time the technician said that if I go to a repair center and they say the cat needs to be replaced, that means they're trying to rip me off, as the measured "O2" levels were zero, and the cat is doing it's job just fine.

3) Went to a friends garage, his Dad took a look, checked the EGR with a vacuum meter and checked EGR vacuum modulator and said that it's working fine. He then said that the timing was way off (even though I had someone else set it previously at 10*BTDC, and the truck was running excellent with good power). He said he's resetting the timing (re****ing it) so that it will pass smog, and that he will readjust it after the test. I use 87 octane gas, so he said to run it till the tank is empty, and then top it up with 91 octane and then go for the test. Do you guys think I should try this out, as I'm thinking that all he's doing is fooling the smog test, while there's something still wrong with the truck. The other thing is, the way he has set it now, it doesn't run well at all (like there's a constant miss at idle), and no power during acceleration.

Since I bought the truck about two weeks ago, the check engine light comes on after 50-100 miles even if I reset it by disconnecting the battery. Re-checked the code this morning, and now I'm sure it's code 25, which can be many things. The FSM says the symptom is Air–FuelRatio Lean Malfunction (same thing that was told to me by the technician after the 1st smog test), and the possible causes are:

• Engine ground bolt loose
• Open in E1 circuit
• Open in injector circuit
• Fuel line pressure (injector blockage,
etc.)
• Open or short in heated oxygen
sensor circuit
• Heated oxygen sensor
• Ignition system
• Engine coolant temp. sensor
• Volume air flow meter (Air intake)
• ECM

From reading this thread:

https://www.yotatech.com/forums/f116...levels-140347/

it seems like the 3-way cat actually does filter "NO", as it seemed to have solved that guys problem. What should I do, and where should I start?? I just want to get this smog out of the way and concentrate on more important stuff on the truck! Here's a picture of the 2nd smog test in case it makes more sense to some of you to pin-point the problem... thanks!

sdhunter

I recently went thru a high NOX smog issue on my 94 4runner. Timing and a new O2 sensor did the trick. High NOX is caused by high combustion chamber temps. Timing advanced too much and lean conditions cause this.

EGR helps cool the temps, if that is fine, and the O2 sensor has been in place for an unknown amount of time, I would try that.

Good luck!

Lankan

which O2 sensor did you replace... the one before or after the cat?

sdhunter

I only have one O2 sensor upstream from the cat.

Lankan

That's interesting! I have the same year vehicle... howcome mine has two??

MaK92-4RnR

94 1/2 - 95 have 2 o2s

Lankan

Any way to determine which of the two sensors may be defective? Almost feel like taking it in to a smog specialist who said he can diagnose the problem for $75. Assuming that will be my best bet instead of guessing and replacing stuff right??

Dirt Driver

The front sensor is the only one that affects fuel mixture. The rear one is just there so the ECU knows the cat is working.

I hope you didn't put 91 gas in it already because that's not going to help. Also, the smog tech will check ignition timing, so there's no way to cheat there outside of setting it at the upper or lower tolerance limit. You shouldn't have a problem passing smog with ignition set it where it should be.

How many miles are on the rig? The first thing I suspect when a code 25 pops up is the O2 sensor. There is a way to test the sensor outlined in the service manual, but it's kind of involved. If you replace it, be sure to get a Denso, not a Bosch. Bosch makes good sensors, but a lot of Toyotas from this era don't like them.

Lankan

Didn't put in the 91 gas yet, but I'm empty now so need to refill. Are you saying to put in 87 gas?? It's got 287k miles. I'll check the FSM for how to try and find if the sensor is bad. Thanks for all the replies!

rworegon

ChefYota4x4 had the same issue with high NOX in Cali.....he passed today...see page 8 and 9 for what he did.
https://www.yotatech.com/forums/f116...nvo-th-252467/

My vote: new front sensor and new CAT. The sensor may test ok and still be lazy.

Dirt Driver

Yes, I'm saying to keep putting 87 gas in it. You could try 91 if you want; I just don't think it's going to make a positive difference. Since you've only had the 4Runner for two weeks and it has that many miles on it, it's anyone's guess how many are on the sensor. It should have been changed several times by now. If it says Toyota on it, then it's either original or from a dealer.

Lankan

I don't mind trying the 91 gas. It's not going to have a negative effect is it?? Will check the sensor. Thanks!

Lankan

Just checked the sensors, and both of them say Toyota on them! So I will go ahead and order one Denso replacement and replace the one that's before the cat. Thanks!

Lankan

Wanted to update you guys on my progress so far. Didn't want to replace anything without being sure that it was the problem (as many others had done this and still failed), so read many many threads to figure out what the possibilities might be.

Since a couple of guys were suspecting the O2 sensor, there was a procedure in the FSM to check this. The main issue was finding a road where I could go at 30-60mph with the RPM @2000 for 90 seconds. I did this last Saturday, but used a lower gear (the one below "D") as the road I went to wasn't long enough (without having to stop). No error code showed up. Then yesterday I took off the EGR and it was clean as ever. Was a bit skeptical about the gear I used to test the O2 sensor, so afterwards drove a bit further from my house to find a road where I could keep in in "D" for 90 seconds. I had been frustrated all this time as my 4Runner ran fine,with no issues at idle or other sypmtoms associated with a bad O2 sensor. Only issue was that it's mpg was bad... about 200-220 per tank on the highway and about 160-170 in town. Towards the end of the 90 second test (at about 80 seconds) the check engine light lit up! I know it sounds funny, but I was overjoyed, as I had finally confirmed that it was the O2!!

A big thanks to all of you that helped me with this, and hope that this thread will help someone else in the future.

I was wondering though "Dirt Driver" said:

"The front sensor is the only one that affects fuel mixture. The rear one is just there so the ECU knows the cat is working"

Is there any way that the downstream sensor can be bad? Has anyone had to replace their downstream sensors?

Dirt Driver

Problems related to the rear sensor are supposed to trip a code 27. Mine has 226k+ on it and I don't plan on replacing it until that happens. The front sensor only has about 20k.

Lankan

Thanks! Until the part arrives, should I go ahead with 1 or 2 seafoam treatments using the brake booster? How about in the fuel tank as well, but if I do add it to the fuel, is it okay to fo in for a smog check with the seafoam still in the tank? Was thinking of adding seafoam to the engine oil as well, running it about 100 miles, and switching over to synthetic (as some have suggested this), but not sure as I've always believed that it's better to use mineral oil (I use Castrol GTX) and change it and the filter every 3k. Any thoughts if I should or shouldn't go synthetic?

Dirt Driver

Skip the SeaFoam altogether. ALWAYS. Also, factory-recommended oil change intervals are fine. 3000mi is excessive for EFI engines unless they are just totally clapped-out. Oil color does not indicate how well it's working. Keep using mineral oil; there's little to gain by using synthetic now.

Lankan

Will do, thanks again for the advice!

Lankan

Will be getting my new Denso O2 sensor today (I hope). Once installed, do I need to reset the computer by removing the battery terminals or removing the fuse for the EFI circuit?

About what engine oil to use, since my heads have been rebuilt recently and new pistons and rings have been installed (no receipts or proof, it's what the seller told me). But when looking at it from the outside, the heads do look like they have been replaced recently, should I stay with the stock recommnded oil... 10-30? It's going to be a VERY hot summer in Palm Springs and the truck will rarely see cool temps. It does have 288k miles on it though.

Dirt Driver

I didn't reset my ECU when the O2 was replaced. I don't think there's a reason to unless there is an O2 code in memory.

Outside temps aren't going to affect oil temperature much at all as long as the engine is running at normal temp, especially since the 3VZE already has an oil cooler. I would still use a mineral oil in the stock viscosity. Xw-40 would be suitable if you really want something heavier.

Hey, check out that new 5-year member banner under my screen name! EDIT: Oh, funny, it was there before I posted, and gone after.