Smog tech wont test car because computer says EVAP system not ready

Tiny
JNR311
  • MEMBER
  • 2001 GMC SIERRA
  • 6.0L
  • V8
  • 4WD
  • AUTOMATIC
  • 172,000 MILES
He gave me a paper that says how to drive your car to run a drive trace, but there are a bunch of pre conditioning requirements that I don't understand. MIL must be off, barometric pressure exceeds 75 KPa, all sstart up IAT&ECT is between 45f & 85 f, IAT is not greater than 2f greater than ETC, ECT is not mote than 12f greater than IAT, the TPS is between 9&35%, the evap solenoid is at 50% PWM within 65 seconds of engine run time
Do you
have the same problem?
Yes
No
Friday, November 29th, 2013 AT 4:20 PM

3 Replies

Tiny
CARADIODOC
  • EXPERT
"MIL must be off": That's the "malfunction indicator lamp". We typically call that the "Check Engine" light. There's over a thousand potential fault codes the computer can set related to things it monitors. About half of those relate to things that could adversely affect emissions. Those are the codes that turn on the Check Engine light. The rest do not. If the light is off, nothing has been detected that could increase emissions. Those self-tests replace much of the manual testing that must be done at the test centers.

"barometric pressure exceeds 75 KPa": The MAP sensor measures intake manifold vacuum which is an indicator of load on the engine. After you turn on the ignition switch, but before the engine is cranking or running, its reading represents barometric pressure. That value is used by the Engine Computer in the fuel metering calculations. When driving into real high altitude areas, the MAP sensor may go out of range. The Check Engine light may turn on, and the engine will run poorly. A lot of people used to drive to a dealership, stop the engine, go inside to make an appointment, then go out and restart the engine, and the light is off. By restarting the engine, the MAP sensor took a new barometric pressure reading and is measuring manifold vacuum around that number correctly. All this requirement is saying is you need to be at a normal altitude. Some self-tests won't run at higher levels.

"IAT&ECT is between 45f & 85 f": Intake air temperature and engine coolant temperature are between 45 and 85 degrees. Some self-tests won't run if it's real hot or cold. Normal coolant temperature is 195 degrees, so some of the self-tests run shortly after the engine is started but before it starts to warm up. Some people think every test will run eventually if they just drive long enough, but coolant temperature will exceed 85 degrees within a few minutes of starting the engine, so all the driving in the world won't cause some tests to run.

"IAT is not greater than 2f greater than ETC": The Engine Computer constantly compares various sensor readings and operating conditions to each other to determine when there's a problem. In this case, it knows that after the engine has been off for at least six hours, the coolant temperature sensor and the intake air temperature sensor had better be reading the same temperature. That will only be true when the engine is first started. No two sensors are ever exactly alike. The computer will learn the characteristics of every sensor over time, but when people start randomly replacing various parts in a misguided attempt at solving a problem, the computer can become confused. Until it sorts everything out and learns the response of the new parts, the engine may not run properly or new fault codes might be set. Experienced mechanics diagnose the probable cause of a problem first, then replace just the one suspect part. The computer will learn its characteristic mush faster when it has only one new variable to work with.

If the IAT and ECT don't read the same temperature at engine start-up, the computer has to figure out which one is wrong. Without a reliable reference, it won't run self-tests that compare something to that failed circuit.

"ECT is not mote than 12f greater than IAT": Same as above, but in this case, this a valid condition. It just means the engine hasn't fully cooled down yet. The computer knows how long it has been since the engine was stopped, so it knows when it's okay to run some self-tests. Those tests won't run until then. As long as both temperature sensors report voltages within their acceptable ranges, no fault codes will be set, but if one reading is incorrect, it can prevent some self-tests from running. Those are the types of things a mechanic has to look for, and obviously, he needs the car to have been off for a sufficient amount of time.

"the TPS is between 9&35%": "TPS" is the throttle position sensor. Basically the accelerator pedal stays between slightly above idle to about one third of the way to the floor. There's usually a time factor too, as in you must be cruising at a steady, slower highway speed for five minutes, for example. If you fully release the accelerator pedal, or hit wide-open-throttle for even an instant, the timer resets and has to reach the specified time again before some tests will be performed. At that point you may feel something from the engine that gets your attention, but it rarely lasts more than few seconds. The computer is commanding something to take place so it can see how some sensors respond.

"the evap solenoid is at 50% PWM within 65 seconds of engine run time": Once a number of conditions are met, the solenoid / valve will cycle on and off a couple of times per second. When the valve is open, stored fuel vapors collected from the fuel tank will be drawn into the engine and burned. You don't want that occurring yet when the engine is cold or when you'll feel it, like at idle. If the solenoid starts switching within a minute, you know a number of requirements were met meaning those related circuits and systems are working. There's no need to perform the same tests manually.

Some people also think they're going to magically reset something by disconnecting the battery cable for a minute. What that does is erase all the stored data from the previously-passed tests. Now they have to start all over with another drive cycle and hope all the tests run. You can erase fault codes after repairs are completed by removing the battery cable, but if you erase them with a scanner, that other data doesn't get erased.
Was this
answer
helpful?
Yes
No
Friday, November 29th, 2013 AT 5:46 PM
Tiny
JNR311
  • MEMBER
Can I run the test in my driveway by idling the car at a certain rpm for 5 minutes instead of driving it. It says let idle 5 minutes, making sure it is in closed loop, then drive at steady speed for 5 minutes between 30&40 mph, then stop and let idle for 2 minutes
Was this
answer
helpful?
Yes
No
Saturday, November 30th, 2013 AT 1:14 PM
Tiny
CARADIODOC
  • EXPERT
Every model is different and there's no way to remember them all. The only time the requirements need to be followed is when you're specifically trying to get all the monitors to run. Otherwise they just run on their own when the various conditions are met. Those conditions typically do not have to be performed in a specific order. Any test can run in any order when it's conditions are met.

As for "closed loop", all that means is along with all the other sensors and operating conditions, the readings from the oxygen sensors are added to the list of variables for the fuel metering calculations. Those readings are used to fine-tune the mixture. Oxygen sensors do not work well below 600 degrees, but there's no way to measure the exhaust temperature, so instead, the assumption is made that the sensors are up to 600 degrees when the coolant temperature reaches a certain point, typically around 160 degrees. The coolant temperature will stay above that during prolonged idle but the exhaust temperature will drop below that. That's why they added electric heaters to the oxygen sensors. I don't know what they're testing for during a prolonged idle, but that would be typical of a police car stopped in traffic or sitting in a traffic jam, both plausible conditions that need to be handled, emissions-wise.
Was this
answer
helpful?
Yes
No
Saturday, November 30th, 2013 AT 1:28 PM

Please login or register to post a reply.

Recommended Guides