Toyota Tacoma O2 sensor puzzle

2006 Toyota Tacoma had an engine put in it and then it started setting codes for lean exhaust, and after replacement of the O2's and air/fuel sensors it finally settled into setting P0138, and P0158. One is B1S2 voltage high and B2S2 voltage low. Bank #1 downstream sensor voltage was going to .1v or lower and staying there, while the bank #2 downstream sensor would go to .8 and stay there. Bank 1 A/F sensor voltage was 3.1v, indicating rich,. and bank #2 A/F sensor 3.45 volts indicating lean.

Now before I say what was wrong, see if you can figure it out or what could you do try and prove what was wrong with the truck.

FWIW fuel trims were adding fuel to bank #2, and taking fuel away from bank #1

Think this through before reading the answer below.

Toyota Tacoma Puzzle Answer

If you take note of the highlights and make comparisons you would see that B1S1 (bank one, sensor one, upstream) shows rich, while B1S2 the downstream sensor on the same bank shows that its lean. There are a couple reasons that could cause that to occur, one would be an exhaust leak between the sensors and checking for that none was found.

Let's look at the other bank now. B2S1 is showing that it's lean, while B2S2 is showing that it's rich. Now unless there is a sensor or circuit issue that really doesn't make any sense.

So how could we rule out a sensor issue from the front seat of the car? A hard acceleration drives the system rich, and both upstream and downstream sensors reported the condition correctly. A closed throttle decel then results in a fuel cut-off or extremely lean and all of the sensors reported that correctly as well. So at that point one or more could still be innacurate, but they are definitely working for the most part. (Rich has the upstream sensor voltage dropping to 2.4v while the downstream sensor reports .8v or more, lean the upstream sensor goes to 4.99 and the downstream drops to .1v)

We find bank to bank fuel trim issues from time to time, but fully expect to see both sensors on either side reporting the same condition and the fuel trim data showing that the PCM is trying to correct for it. The data seen in the fuel trims was really confusing, while it appeared to be making corrections, it was going the wrong direction according to the upstream sensors. Bank #1 was rich and fuel trim was adding fuel instead of taking it away, bank #2 was lean and the fuel trims was taking fuel away.

This truck was demonstrating a condition that proves the downstream O2 sensors have more authority than the upstream air/fuel or O2 sensors have. Combine that with bank #2 downstream sensor reporting more fuel than the upstream sensor was reporting and the only thing that makes sense is someone has cross connected the down stream sensors sometime in the past. They even secured the harnesses in place on the brackets on the transmission just like they are supposed to be making it difficult to notice visually.

We stopped in a parking lot and disconnected the downstream sensors, released the harness from the brackets and re-routed them across the top of the transmission to the opposite sides and re-connected them and then road tested again. Now the downstream sensors were on the correct sides and the system functioned correctly. The troubling part there is the shop that originally put the engine in didn't have anything to do with that part of the wiring harness that was above the transmission, they didn't cause the problem. But both they and the second shop made the same mistake, they "assumed" the that problem was related to the engine replacement instead of just troubleshooting it. By starting with that assumption in place they were blinded to all the possible causes and got trapped. Between them they each had fought this for a week, and with a disciplined approach it was diagnosed and corrected in about a half an hour.

Now for the kicker, the codes would set as pending within a few minutes of driving, and if you shut the engine down and then restarted the truck would light the MIL (check engine light) in a few minutes after that. That pending code and subsequent MIL would prevent many of the other tests the computer has to run from being performed. That means the truck could easily still have other issues that can cause the MIL to come on, but there is no way to know until the computer gets to run the tests and that of course explains why a tech can do a good repair only to have the light come back on a few days or a week later and it really is a different problem even though to a customer the light coming back on looks like "its doing the same thing".

 2002 Acura RSX 2.0l

The customer reported that the car starts, runs for a few seconds and then shuts off. The Check Engine light is on. Pulling codes revealed a P0341 CMP/CKP (camshaft/crankshaft) synchronization error. The computer constantly looks at the camshaft sensor waveforms, and compares them to the crankshaft sensor wave form to make sure that the camshafts are in time. In the event of a mechanical failure and the waveforms get out of sync, the computer shuts the engine down to try and prevent further damage. The customer reported that several people have already looked at this, including removing the valve cover to inspect the timing ,marks and failed to find the cause of the problem.

Since the car runs for a few moments that's plenty of time to get a compression waveform.


The cursors mark the two compression peaks, so between them is all four strokes of the engine starting with decompression, then the exhaust, intake, and the next compression stroke. By measuring the time in between the cursors, you can divide by four and then mark the individual strokes of the engine and see if the camshaft is in time or not.



The total time between the cursors was 99.2ms, divided by 4, that's just about 24.9ms, so the one cursor gets moved to first display the decompression stroke which is when the crankshaft turns from TDC (top dead center) to BDC (bottom dead center) and then the second one gets moved to show the exhaust stroke as the crankshaft turns back to TDC.

See the rise in pressure at the end of the exhaust stroke? That means the cylinder pressure was rising when the exhaust valve should have still been open during the overlap of the intake valves and exhaust valves. You can see the pressure start to drop just before TDC, that's the intake valve opening.

So when did the exhaust valve open, and how far out of time is the exhaust camshaft?

The waveform is created by two full revolutions of the crankshaft, so that's 720 degrees of rotation that took place in 99 ms. Dividing 720 by 99  gives us just over seven degrees of rotation per ms.

The point where the waveform stops dropping on the decompression stroke shows us when the exhaust valve opens. By placing a cursor at that point, and multiplying the time elapsed from TDC you can see that the exhaust valve opened up about 56 degrees after TDC, that's about 90 degrees early! That also explains why the exhaust valve closes to soon.

So it's confirmed that no matter what we see with the timing marks when we get this apart, the camshaft is out of time, and the computer is correct in setting the P0341.

Questions?

It’s nice to get an easy one once in a while amid all of the random failures that we usually have to fight through. But even an easy one, or should I say easier one demands both attention to detail and discipline to make sure that you haven’t missed anything significant in the early stages of the diagnostic routine.

Another shop brought us a 2001 Chevrolet Impala with a 3.8l engine, they reported that they had replaced the ignition switch but the car still wouldn’t start. Well, yea that car uses the pass lock anti-theft system, so when the ignition switch and lock cylinder are replaced, the system doesn’t recognize the voltage signal from the new pass lock sensor and the BCM reacts as if there is a theft being attempted. Attaching a scan tool and accessing the BCM codes there was a B2960 found for the passlock sensor signal valid but incorrect. That’s an easy fix and the system can be retrained with a re-flash routine in about ten minutes, or it can also be done with three ten minute key-crank-on cycles.

The rest of the story on this car was it was abandoned and impounded so it hadn’t run in a couple of years, which is why they had to replace the lock cylinder and key. Once the pass lock system was retrained the car was now able to crank and it fired but it stalled right away. With practice a technician can hear the way an engine tries to start and how it dies and actually recognize if a problem is specific to one or more cylinders, he/she can often tell of the car is too rich or too lean and if it lost spark or fuel to all of the cylinders. This one sounded rich, and it appeared to not be losing spark. Rich often allows for one or two cylinders to fire while cranking but it’s random, and opening the throttle normally allows more air in than the engine is getting fuel and the engine then clears up and starts running. The problem was this one wouldn’t clear out, in fact it would seem like it briefly cleared out and then got worse and would even kick back against the starter. At this point it seemed that I might just be discovering why someone walked away from this. Since the tech II was hooked up it made sense to check the essential scan data. Both the intake air temperature sensor signal and the coolant sensor signals reflected the ambient temperature of eighty degrees, the map sensor showed ninety-seven kilopascals which is close enough to normal ambient pressure, the cam and crank rpm signals matched when the engine was trying to start but the MAF sensor was jumping to thirty-five, then fifty- eight and up to seventy-six grams of air per second when it would stall and the engine just barely to one thousand rpm.  So that was clearly wrong, the sensor was way over reporting, but even then the engine just didn’t react to the throttle the way it should be expected to. Fortunately with the MAP sensor in the system, the MAF can be disconnected and then see how the system reacts. By disconnecting the MAF the engine now would start and idle, but it would not rev up when opening the throttle, in fact it sounded very labored. Looking at the MAP signal it was reporting about eighty kilopascals. That meant there was very little vacuum being generated, opening the throttle even the slightest amount had the MAP reading showing the same ninety-seven  kilopascals that was seen before cranking the engine.

One other important note was the fuel trims were taking away about twenty five percent of the fuel pulse. That meant that the computer was at least trying to compensate for the system being too rich. When you have a speed density system, which is what this became with the MAF disconnected the computer looks at the high manifold pressure as an engine that is working real hard to accelerate, so it calculates a long pulse width. But in this the airflow to match really wasn’t there so that ends up being too much fuel and the trims have to react. At this point there really was only two possibilities for the condition, either the exhaust was restricted or the camshaft in the engine is out of time with the crankshaft. Fortunately one simple test can provide the answer to which one it is and that’s do a running compression test with a pressure transducer and measure the valve opening events.

The first capture is cylinder number three running with only cursors measuring the entire cycle. That allows the time elapsed between the compression peaks to be measured, and then divided by four to give each piston stroke.

The second capture shows cursors set for the exhaust stroke, the pressure in the cylinder should go below atmospheric pressure before the piston gets to the bottom of what would normally be the power stroke of the engine, then the exhaust valve open at just about forty degrees before bottom dead center and since the cylinder is in a vacuum, the pressure rises because the exhaust is at, or slightly above atmospheric pressure. This second capture proves that the camshaft is in time.

 

Now all we need to do is speed the engine up. Notice how you no longer see the compression peaks, they are clear off the top of the screen, at the same time the exhaust stroke plateau has risen as well. The horizontal cursor allows that pressure peak to be measured and its over forty psi. That’s way too much back pressure and explains why the car won’t run above an idle.

 

At this point the discovered repairs need to be performed before any more diagnostics could be performed. There may easily be other issues that require attention, but sometimes you don’t have any choice because problems like these two will make proving anything else almost impossible IMO.

Hyundai Sonata Transmission 2

 Here are the current waveforms for the Hyundai Sonata that prove that the circuit is good to the  internal power splice for the overdrive and second gear solenoids, and the failure is between that point and the harness pin at the transaxle connector for the overdrive solenoid ground control. (yellow/white)  At the same time, these captures also prove that the circuit is going open and not grounding. This first capture is normal operation. The red trace is from a current probe that is attached to the common power input for the solenoids. The green trace is the overdrive solenoid circuit that is randomly failing. The tan is the second gear solenoid command and the blue is the overdrive solenoid.

 One of the first things you should notice are the three rises in current on the red trace. There are three solenoids that are actually powered up by that supply, so you see the PCM confirming all three solenoid circuits. Look at the duty cycles displayed on the tan and blue traces. If you were looking at those circuits with a volt meter, it would average that and display about six volts which is what the transmission shop saw. Notice how the PCM stops pulsing the solenoids and pulls them to ground?  You can see the corresponding rise in current when that occurs in the red trace. Now notice how the current rise on the green trace lines up with the grounding of the overdrive solenoid on the blue trace?
These are the key elements that we need to pay attention to when a failure occurs.

So here in the next captures are the circuit failure. I've already mentioned that the circuit is going open, or you could look at it as very high resistance. Try and figure out what you see in these and we can discuss them a little later.

 

HYUNDAI SONATA P0765

 

This Hyundai was sent to me to diagnose a random failure of the overdrive solenoid

circuit. The scope connections are: The blue trace is the overdrive solenoid circuit

at the PCM, (yellow/white) the green trace is the overdrive solenoid circuit at the transaxle.

The tan trace is the second gear solenoid connection at the transaxle, which shares

the same power input as the overdrive solenoid to the transaxle.

The red trace is a low amps probe at the PCM.

 

The circuit is clearly failing inside the transaxle, but one question should be answered yet.

Is it going open, or is it grounding? What changes in the connections could be made to

prove that fault before any disassembly is done?

 

 

 

 

 

This is my comment to an article that was shared today on a forum that I frequent.

 

You can find the article here:

 

http://www.courier-journal.com/article/20130503/BETTERLIFE04/305030115

 

Before anyone runs and signs up for training to become an automotive technician because it looks like you can make a great income, you need to learn the facts. While mathematically it may be possible for a select few to generate a decent in...come the reality is politics inside the shiops usually come into play and will have a much greater impact on someone's ability to make a living fixing cars. One of the biggest challenges in a dealership is flat rate and how it works against the technician more than it does for him/her. The idea that flat rate is supposed to reward a talented, hard working technician gets trumped by warranty times that are often as little as half of the customer pay times. Imaging doing a major repair like a head gasket on a given engine, the customer pay labor time might be six hours, while warranty time for the exact same job is only two and a half to three hours. It's the same job, but there is nothing flat about the times that the technician gets pad. The young technician will hear a lot of excuses for why its done like that, but they have to put up with it or they won't get to last as a tech.

Then we have the electronics and especially the diagnostics portion of the job. Most manufacturers pay around eighteen minutes to perform a basic diagnostic routine. That time includes getting the repair order and punching onto it with the time clock, finding the car, roadtesting and verifying the reported symptom all while using the scan tool to retrieve the trouble codes and examine the data produced by the modules on the vehicle. The tech then needs to research the system involved and check for TSB's and today for software updates. Then its actuallytime to test the components and/or the wiring in the system to identify the failure. Once the problem is found, the technician still has to document the findings and access any required parts and set about doing the repair.

All of that in .3 hours, or eighteen minutes.

It would be fair to say that this is a rough draft of what it really takes to be that top diagnostic technician who by the story is supposed to make 80K a year. The problem is that it takes decades to achieve this level of proficiency and with the ever changing complexity of today's cars the need to study never stops. There is always something new to have to learn and the pressure to have to be fast at actually doing the work drives most of our young peope right back out of the trade way before they reach their full potential as technicians.

Before you go and try to become a mechanic/technician talk to the peope who are in the trade working as mechanics/technicians first. If you love cars and you like solving very complicated technical puzzles then this can be a rewarding career. Remember you will have to live on that median income mentioned in the article and still buy the $30,000 - $50,000 that you will have to invest in your own tools while you are learning the trade which will take some fifteen to twenty years. There is no finish line in this trade, you will only be as good as the billable hours that you can produce which normally will peak while you are in your forties.