Toyota Tacoma CEL Issue - Long

[IROC]

Since there seems to be a lot of Toyota Tacoma owners on here... I've got a 2001 Tacoma DoubleCab with the 3.4L V6 with 117k miles. About 15 months ago the check engine light comes on. No driveability issues. I mention it to a buddy here at work with the same truck and he bets it is the O2 sensor, but cautions me to only use the Denso brand and not a Bosch as he says the Bosch will continue to throw error codes for some reason. I'm skeptical.

Anyway, I get the code read at Adavance Auto and they verify it's the O2 sensor, but they didn't have the Denso one in stock - only the Bosch. I end up buying the Bosch and installing it. Reset the computer and everything is fine for about a month and then the CEL comes on again. No driveability issues. I assume it is the Bosch O2 sensor and just keep resetting the computer every month or two to clear the CEL light because it annoys me just like Sheldon on "Big Bang Theory".

Fast forward to about 4 months ago. I got the local Toyota dealer to change the timing belt (I know - but it was cheap). I asked them to "read the codes" and make sure everything was OK relative to my occasional CEL issue and they did and said it was fine - no issues. Not long after that, the CEL came on again, but now I had some driveability issues. Mainly a loss of power at highway speeds, some mild pinging and a drop of about 4 mpg in gas mileage (I check gas mileage with every tank of gas). If I reset the CEL, everything is fine for a day or two and then it comes back.

I take the truck and get the codes read again and the guy says "O2 sensor". I ask him what it actually says and he shows it to me and it says, "System too lean - bank one". I asked him how he knows that means "O2 sensor" and he said that's what it always is. I do some checking on the interwebs and come to find out that code is typically the MAF sensor or a vacuum leak. So, this past weekend I remove and clean the MAF sensor and check the entire truck over for vacuum leaks and find nothing.

I put the whole thing back together and it runs better than it has in years until this morning (three days later) and the CEL comes on again.

Anyone have any ideas?

[kaisen]

We'd certainly need to know more to be of real help.

A lean code at the front sensor could be a sensor error of course. You could check the voltage and see, or just swap another one. Or, they may have set the timing slightly off when they did the belt. Or, you really could be running lean -- clogged injector(s), MAF sensor thinking it was getting less air than it really was, faulty knock sensor dialing back timing, vacuum leak, carbon build up on a valve where it doesn't seat 100%, lots, and lots of little possibilities.

The rear sensor, bank two, is the real policeman of catalyst performance and it isn't throwing a code (right?) so the cat is doing its job and, generally, AFR is at Lambda.

Toyotas are funny. Some code readers won't get the whole story. I'd suggest letting a good Toyota technician have at it. It's likely something really simple. But, it still needs to be repaired so you don't do any damage.

How are YOU resetting the codes?

[onewhippedpuppy]

Your MAF cleaning leading to improved performance is interesting, I suspect that your MAF might be going bad. I've read of many instances where a bad MAF will show up as a bad O2 sensor, and if the MAF was good why did performance improve post cleaning? I've also read of a bad MAF temporarily improving with a cleaning, then defaulting back to bad behavior in a short period of time.

[kaisen]

Matt's right. But the MAF is an expensive sensor to just "try". So make sure you TEST IT to see if it is performing properly.

[IROC]

Originally Posted by kaisen

We'd certainly need to know more to be of real help.

A lean code at the front sensor could be a sensor error of course. You could check the voltage and see, or just swap another one. Or, they may have set the timing slightly off when they did the belt. Or, you really could be running lean -- clogged injector(s), MAF sensor thinking it was getting less air than it really was, faulty knock sensor dialing back timing, vacuum leak, carbon build up on a valve where it doesn't seat 100%, lots, and lots of little possibilities.

One of my first thoughts was a slight cam timing issue related to the belt change, but the problem is intermittent (and cam timing wouldn't be). The truck runs too good most of the time for this to a big problem, but something is definitely awry. If it's a clogged injector, etc., it somehow decides to fix itself on occasion.

The rear sensor, bank two, is the real policeman of catalyst performance and it isn't throwing a code (right?) so the cat is doing its job and, generally, AFR is at Lambda.

Appears to be bank one for sure, but I wouldn't put money on it. I'm pretty sure the code reader said "P0171". That's unfortunate as the A/F sensor is the cheap one.

Toyotas are funny. Some code readers won't get the whole story. I'd suggest letting a good Toyota technician have at it. It's likely something really simple. But, it still needs to be repaired so you don't do any damage.

I'm considering this...

How are YOU resetting the codes?

Unplugging the EFI fuse for 30 seconds.

[IROC]

Originally Posted by kaisen

Matt's right. But the MAF is an expensive sensor to just "try". So make sure you TEST IT to see if it is performing properly.

Thanks, guys. The MAF sensor was definitely "dirty" as the MAF cleaner removed some sort of contaminant that returned the wires to a nice, bright appearance, but I don't know how "dirty" is "dirty". I do not have a K&N filter, so filter oil is not the issue.

Is there a way to test the MAF sensor? I have a Chilton manual (and haven't even looked at it), but will try that.

[onewhippedpuppy]

I think you can check resistance across certain terminals on the MAF, but I'm not sure.

Just look at simple cause and effect before you get too deep. You had the issue pre-timing belt change. You cleaned the MAF and it provided a temporary gain before defaulting back to the the prior issues. If your cam timing was off a MAF cleaning would not make the issue go away.

Do you notice any changes simply unplugging the MAF and driving? That's an easy way to determine if your MAF is totally shot, it might not be as effective if the issue is intermittent.

[trekkor]

Best investment if you do your own diagnostics is an OBDII reader.
Spend the $200 for a good one.

The one I bought has live data monitoring.

As far as 'Bank One' is concerned, there should be an O2 sensor before and after the cat.
The OBDII code should be specific.

After the exhaust is hot, you can compare the data to bank two and pinpoint the faulty sensor.


KT

[kaisen]

Originally Posted by trekkor

Best investment if you do your own diagnostics is an OBDII reader.
Spend the $200 for a good one.

The one I bought has live data monitoring.

Absolutley. It's a must-have tool. You can buy a reader-only for under $100, but a better one with some data-capture, graphing, live monitoring would be better. I bought my ~$300 Acctron used on eBay for $75 a couple years ago.

As far as 'Bank One' is concerned, there should be an O2 sensor before and after the cat.

After the exhaust is hot, you can compare the data to bank two and pinpoint the faulty sensor.

The readings for bank one (pre cat) and bank two (post cat) SHOULD be different, as the cat should be burning things clean. The two sensors perform different functions, but the computer uses the data in concert to make adjustments.

Since you're getting a bank one code, it is telling you a little more about what the engine is actually doing rather than how clean things are post-cat. If you're getting a lean code, it has to be either too little fuel or too much air. The MAF reads how much air is coming in so if it "thinks" it's getting x volume but is actually getting 1.1x volume, the system will be lean. Or if the fuel injectors aren't giving enough fuel, then it will be lean. An air leak downstream of the MAF will add air to the system, hence 1.1x versus x. If the EGR diaphragm is bad it could add some air downstream of the MAF, but it would/should throw its own code.

As far as the timing thing, you said you were asymptomatic before the timing belt, and actually had your first driveability issues after the change. That's why it was significant to me.

Simply unplugging the ECM for 30 minutes ALSO resets all of the data maps the engine uses. It defaults rather than using what it has "learned". So it may run differently for a while. That's why resetting a CEL with a scan tool is preferred.

[IROC]

Originally Posted by kaisen

As far as the timing thing, you said you were asymptomatic before the timing belt, and actually had your first driveability issues after the change. That's why it was significant to me.

I thought the same thing and went over the engine with a fine-toothed comb last Sunday (when I cleaned the MAF sensor) and couldn't find anything awry.

Simply unplugging the ECM for 30 minutes ALSO resets all of the data maps the engine uses. It defaults rather than using what it has "learned". So it may run differently for a while. That's why resetting a CEL with a scan tool is preferred.

Yeah, it runs like crap for the first minute or so after I reset. Then it runs wonderful right up until the CEL comes back on (anywhere from a day or two to a month).

One other point, the CEL only ever comes on at low speed, very light engine loads. Usually coasting, etc. Don't know if that is relevant.

[1990C4S]

Lean on one bank can be an intake leak....

[kaisen]

Originally Posted by 1990C4S

Lean on one bank can be an intake leak....

one bank is different than "Sensor, Bank One"

But you're exactly right, as I pointed out, any air leak downstream of the MAF could make the system lean. That could be the joint just behind the MAF, the tubing or elbows running to the intake. The throttle body gasket. The intake gasket. Or any other point in the system between the MAF and the intake valve

[IROC]

I've ordered a scan tool that also displays parameters like fuel trims, O2 sensor output, etc., so hopefully I will be able to get smarter on this.

[yel911]

Go to tundra solutions.com, under the Tacoma section. If there is an issue, someone's already fixed it!!!

[IROC]

Originally Posted by yel911

Go to tundra solutions.com, under the Tacoma section. If there is an issue, someone's already fixed it!!!

I have done a lot of reading and there is some useful info out there. That's what led to my cleaning of my MAF sensor. I've driven the truck all week (since cleaning the sensor on Sunday) and even though the CEL is back on, it is running fine.

Scan tool is supposed to be here today...

[yel911]

I have done a lot of reading and there is some useful info out there.

Honestly, I would have thought somebody on there would have had the same issue. Let us know how you make out.

[onewhippedpuppy]

I assume you've given everything a good once over? The corrugated plastic and rubber intake hoses and boots like to crack on the corrugations, it can be very difficult to see without removing and flexing the part.

But I'm still voting on the MAF.

[syncroid]

I've only had my CEL come on once. My truck has 113K on it. It was the A/F ratio sensor that was the culprit. My feeling on yours is also the MAF. The cam timing isn't the problem. Toyota makes timing belt replacement so easy a monkey could probably do it and not get it wrong. The best way to look for an intake leak is to smoke test it.
BTW, I'm selling a OBD II generic scan tool. F.S. OBD II Scanner

[IROC]

Originally Posted by onewhippedpuppy

I assume you've given everything a good once over? The corrugated plastic and rubber intake hoses and boots like to crack on the corrugations, it can be very difficult to see without removing and flexing the part.

But I'm still voting on the MAF.

I looked over everything pretty well. The intake hoses and boots are easy to remove, so that might be worth it.

The only check in my Haynes manual for the MAF was a single resistance check across two of the five terminals. I haven't done that.

What I am trying to avoid is throwing parts at this thing until it's fixed, plus maybe I'll learn something. It is running fine at this moment - even with the CEL light on - so I have the luxury of time to some extent.

[IROC]

Update: Finally got my scan tool hooked up and it is only showing one code - the P0420 (catalytic system efficiency low or something like that). Reading on the inter webs, the water is still a little muddy, but the factory service manual says that a P0420 is either 1. Exhaust leak, 2. Bad A/F sensor or 3. Bad cat. My exhaust looks like new. The truck has 119k miles (a little more than I previously thought) and many people claim that the rear O2 sensor (A/F sensor) is a prime culprit at this mileage.

I am going to keep an eye on it and when the CEL comes on again, I'll be able to see what the code is real time. The truck is running much better, so I'm assuming my MAF cleaning was both needed and successful. The scan tool also has the ability to read the O2 sensor output, but I haven't figured out how to do that yet..