Monitor details: a useful Techstream feature I had overlooked

In a part of the Repair Manual where I had never really looked before, I noticed that Techstream is able to display data collected during the ECM's internal "readiness monitor" tests.

The monitor function gets talked about on PriusChat, but usually only in the context that an emissions inspection can't be passed if the internal checks haven't all been "completed" since the last time any codes were cleared or the battery was disconnected. They give a pass/fail indication to the emissions inspector.

What I hadn't seen mentioned was that the ECM saves not just the pass/fail status, but the actual test values from these automatic internal tests. You can see them in Techstream from the "engine control and ECT" page by clicking "Monitor" and then clicking the "Details" magnifying-glass icon next to a particular completed test.

These values are a nice addition to the pure pass/fail monitor result, because they can show something getting closer to a fail result before a failure gets flagged.

For example, I have never had a catalyst-efficiency code and that is still officially passing, but it looks close enough to the minimum that I had probably better start sending some extra pennies to the piggy bank.



On the other hand, the internal tests of EGR and VVT (intake camshaft timing adjustment) seem to look pretty good. The EGR figure is how much the intake manifold pressure goes up when the EGR valve is opened during the automated test. Has to be less than one kilopascal to fail; mine's still showing over ten. (For this test, there's not really a 'max' limit, which is probably what you'd figure any time the shown 'max' value reminds you of some 2ⁱ−1 number like 65535.)



The test value for "VVT stuck" is the number of seconds to change the timing by a certain amount. Fail is 9½ seconds or more; for this test there's probably no such thing as "too fast", so the 'min' of 0.00 is probably standing in for "we don't care about min". The test value of 0.00 seems like it would be hard to improve on. (It surely doesn't mean literally zero seconds, but even "rounds to zero at hundredths of a second" seems pretty good.)

Standard, non-Techstream scan tools should also be able to retrieve these values, but will only show them by hex codes, and won't know what measurement unit or scaling to show. That information is in the repair manual for use with those tools.

 

I imagine anybody who wanted to quantify that, rather than speculating, would eventually obtain a thermocouple or two, a small drill bit and some silicone, and devise a test protocol to find out.

 

I forgot to add: if the curious person had heard Westheimer's observation ("a coupla months in the laboratory can save you a coupla hours in the library"), the person might start by taking a pencil and an old envelope into a library, and asking a librarian for a reference on heat exchanger calculations. The pencil and the back of the envelope could then be used to ask "for a heat exchanger of such-and-such size, water/glycol on one side at 85 ℃, exhaust gas at such-and-such temp on the other side, such-and-such flow rate, what is the maximum gas temperature reduction to be expected when the exchanger's in perfect condition?"

The follow-up question would be "ok, even if that total amount of temperature reduction were completely lost, would that be enough, or even nearly in the ballpark of enough, to explain such-and-such particular effect that I'm speculating about?"

Probably only if the answer were "yes", or at least "not obviously no", would the person go on to obtain thermocouples and drill bits. If the back of the envelope said "no" or "not by a country mile", the person would probably just move on to speculate about other things.

The library part would still involve a little honest work to supply plausible numbers for the such-and-suches. On the other hand, the library might also have some published papers specifically on car engine cooled EGR, which would probably have plausible numbers already in them.

 

So here's the thing with the way the manual is organized: it has many sections giving details of diagnostic trouble codes, one section for each code. The section for each engine code starts with a "Monitor Description" that explains the self-test that the ECM uses to set that code.

It turns out that P0420 is the trouble code you would get if the catalyst efficiency monitor test ever failed. So the P0420 section in the manual is where you go to learn how the monitor test works. But of course the display of the monitor test in Techstream doesn't tell you "this monitor test goes with the P0420 code", and if the test hasn't failed yet, you don't have a P0420 code, so finding the explanation of the monitor in the manual is a little puzzling if you don't know the trick. You have to first look through the DTC chart to find the trouble code(s) that might be associated with whatever that monitor tests, and then turn to the sections for those codes to find the description of how the test works.

The catalyst efficiency monitor works like this: the ECM waits until the engine is warmed up and you are driving kind of steadily for a while between 47 and 75 MPH. Then the ECM deliberately makes the mixture a little too lean for a while, meaning there is still unused oxygen in the exhaust, which gets stored up in the catalyst. After several seconds of that, the ECM deliberately makes the mixture too rich, so there is unburned fuel in the exhaust, which uses up the oxygen stored in the catalyst. How long it takes for the downstream O₂ sensor to show the final oxygen content dropping shows how much oxygen the catalyst was able to store, with more being better.

Yours may have been 0.253-0.254 the last couple of months, but did you happen to check the value right after the brand new cat was installed? Maybe it wasn't a whole lot higher to begin with. Is it the official Toyota part or an aftermarket?

I was a little concerned seeing mine at 0.251 in the original post. But that was a year and a half ago and I think it was still 0.251 the last time I checked, so it's possible it just doesn't decline very quickly. (But then, I haven't been driving a lot in that time.)