- Jan 14, 2018
- Reaction score
- The Seasonally Frozen Wastelands
Excellent detail. "Not running", but key-on. (KOEO, Key On Engine Off) Otherwise the computer is "dead" and you don't get any info on the scan tool. I write-down all the codes along with a quick description, and whether they're current codes, history codes, "Pending", "Failed this ignition cycle", etc. If there's LOTS of codes, I'm not opposed to clearing them and seeing what comes back. (Instantly, or during the test-drive, or during extended driving.)First check for trouble codes, stored, pending or current. Then look at live data on a cold engine that is not running.. Make sure the coolant temp is near ambient. Make sure the MAP is reading close to the barometric pressure.
If equipped, an intake air temp sensor would also read near-ambient temperature, or somewhat higher if the sun is beating down on the hood raising the air-cleaner temperature.
Typically shows a small voltage. Above "zero", but not much above. If you slowly depress the throttle, the voltage should smoothly/proportionally increase with no drops or spikes in the voltage. At WFO, the scan tool may indicate that the computer is in "clear flood" mode.TPS should read Zero.
KOER--Key On, Engine Running.Then start the engine.
At idle, engine cold, I'd expect zero knock sensor "counts". You'd have to put the engine under some load to get signal from the Knock Sensor--which would be something to look for during the test-drive after you've warmed-up the engine and looked at the stuff listed below.
Electrical system voltage shown on the scan tool should be reasonable. You'll likely have data showing an automatic transmission is in Park/Neutral vs. in gear. There may be data showing whether the A/C is commanded "on".
MAP = Manifold Absolute Pressure.The MAP sensor should read around 20 in/hg vacuum, some scanners read it as absolute pressure. Baro being 30 in/hg or so and 20 in/hg vacuum being around 10 in/hg absolute pressure.
For old guys like me, that are used to "vacuum gauges", the MAP sensor signal is "backwards". And just as you explained--whatever the MAP shows for pressure, is subtracted from barometric pressure. The result is the "vacuum" us codgers are used to dealing with.
I guess some scan tools convert the pressure to a vacuum level for you. None of mine did that.
I'd have said "switching rapidly" rather than "switching steadily". But the concept is the same. The sensor better swing from "low" to "high" and back. Depending on the vehicle and the scan tool, you may have a display of "cross counts"; the speed that the O2 sensor goes from high to low, or low to high. Older vehicles may be in the single-digits at idle, newer vehicles tend to have higher cross-counts; but in any event the higher the number, the better. O2 sensors get "lazy" with age, they don't switch rapidly. Low cross-counts or a graph showing slow switching may be indicating an O2 sensor at the end of it's service life.As the engine starts to warm up the CTS should rise and the short term fuel trims will become active. A range of -10 to +10 with some fluctuation is pretty common during initial warm up before the long terms become active. The front 02 sensors (Bank 1 & Bank 2, Sensor 1) voltage should have a nice swing to them from around 0.100 to 0.800 volts or so and be switching steadily.
The scan tools I'm used to display the number, not the percentage. As with so many "digital" things, there's a 0--256 scale with 128 as "neutral", lower numbers indicate lean-command (computer reducing fuel compared to 128) higher numbers indicate rich-command (computer increasing fuel compared to 128) While the scale is technically 0--256; in actual use it's unlikely to see the extremes of the scale, things tend to be in the range of 100 to 150.As the engine warms up beyond 140F, the long term fuel trim will enable and the long terms should start to follow the short terms. I would expect to see them in the -10 to +10 range as well with Zero being ideal.
If I'm right about percentages vs. numbers, -10(%) (less than) 128 would be roughly 13 numbers--115. +10(%) (more than) 128 would be 141. I like to see things within +/- 5% on stock vehicles, but sometimes that's not achievable. Yes, the quality of the fuel (quantity of alcohol, and other factors) matters in terms of rich/lean command.
Thanks for that. Easy to remember, easy to test for.At idle the MAF should read around 1 gm/sec for every liter of engine displacement.
Yes. And I think this covers the sensors that input data to the computer; except for the Vehicle Speed sensor which you'll have to test-drive to check, and the various sensors related to ABS, traction control, transmission, etc.As you accelerate the engine the MAF reading should smoothly increase. If you snap the throttle it should rise sharply.
Depending on the computer/vehicle, you'll also be looking for various computer outputs. There may (or may not) be data relating to spark advance/retard, EGR operation, AIR diverter valve operation, fuel flow through the injectors, electric cooling fan(s) commanded "On", etc. Newer vehicles' computers have more information available than older, less-sophisticated computers provide.
Overall, very well done. Thank you.
We haven't talked about "Bi-Directional Control" but I don't have time now. I have to accomplish stuff before sundown.